Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)

Cátia Pinho; Francisco Rodrigues; Ana  Maia Tavares; Carla Rodrigues; Cláudio  Emanuel Rodrigues; António Teixeira

doi:10.3390/app10114024

Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)

Pinho, Cátia;Rodrigues, Francisco;Tavares, Ana Maia;Rodrigues, Carla;Rodrigues, Cláudio Emanuel;Teixeira, António 2020-06-10 00:00:00 applied sciences Review Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited) 1 , 2 , 3 1 , 2 , 3 1 , 2 , 3 3 Cátia Pinho , Francisco Rodrigues , Ana Maia Tavares , Carla Rodrigues , 4 1 , 2 , Cláudio Emanuel Rodrigues and António Teixeira * Instituto de Telecomunicações (IT), University of Aveiro, 3810-193 Aveiro, Portugal; catiap@ua.pt (C.P.); francisco@picadvanced.com (F.R.); ana@picadvanced.com (A.M.T.) Department of Electronics, Telecommunications and Informatics (DETI), University of Aveiro, 3810-193 Aveiro, Portugal PICadvanced, PCI—Creative Science Park Via do Conhecimento, Edifício Central, 3830-352 Ílhavo, Portugal; carla.rodrigues@picadvanced.com Altice Labs, R. José Ferreira Pinto Basto, 3810-106 Aveiro, Portugal; claudio-e-rodrigues@alticelabs.com * Correspondence: teixeira@ua.pt Received: 31 March 2020; Accepted: 3 June 2020; Published: 10 June 2020 Abstract: The development of photonic integrated circuits (PIC) for access network applications, such as passive optical networks (PON), constitutes a very attractive ecosystem due to PON’s potential mass market. The implementation of PIC solutions in this context is expected to facilitate the possibility of increasing the complexity and functionalities of devices at a potentially lower cost. We present a review addressing the prominent access network market requirements and the main restrictions stemming from its speciﬁc ﬁeld of application. Higher focus is given to PON devices for the optical network unit (ONU) and the implications of designing a device ready for market by discussing its various perspectives in terms of technology and cost. The discussed PIC solutions/approaches in this paper are mainly based on indium phosphide (InP) technology, due to its monolithic integration capabilities. A comprehensive set of guidelines considering the current technology limitations, beneﬁts, and processes are presented. Additionally, key current approaches and eorts are analyzed for PON next generations, such as next-generation PON 2 (NGPON2) and high-speed PON (HSP). Keywords: photonic integrated circuits (PIC); access networks; passive optical network (PON); next-generation passive optical network 2 (NGPON2); high-speed PON (HSP); super-PON 1. Introduction Telecommunication network infrastructures can be described by three main network segments, i.e., core or backbone, metro/regional, and access networks, as depicted Figure 1. Long-distance communications with high aggregate trac are attained with core and metro networks, while access networks entail service provider to end-user ﬁber-optic telecommunications technology. These systems are subject to signiﬁcant growth in capacity requirements with operators looking for feasible cost-eective solutions. The potentiation of cost-eective solutions can be reached with the improvement of current network technologies, as they become widely used, e.g., coherent systems in future optical access networks [1,2]. Appl. Sci. 2020, 10, 4024; doi:10.3390/app10114024 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 2 of 19 Appl. Sci. 2020, 10, x 2 of 19 Figure Figure 1. 1. Diagram Diagram of a te of a telecommunications lecommunications netw network ork infrastructure, based infrastructure, basedin [3]. in [3]. Photonic integrated circuits (PIC) are considered an evolving technology oering a sustainable Photonic integrated circuits (PIC) are considered an evolving technology offering a sustainable (ecient and cost-eective) alternative to data transmission interfaces [4]. The implementation of (efficient and cost-effective) alternative to data transmission interfaces [4]. The implementation of integrated photonics can thus enable savings and new functionalities, increasing the transmission integrated photonics can thus enable savings and new functionalities, increasing the transmission capacity capacity [5 [5].]. In the a In the access ccess network context, network context, PIC can offer compelling per PIC can oer compelling performance formance improv improvements ements compared to traditional bulk technology [6], like bi-directional optical sub assembly (BOSA). Upgrades compared to traditional bulk technology [6], like bi-directional optical sub assembly (BOSA). in Upgra terms des of in weight terms o and f we volume, ight and r vol educed ume, re power duce consumption, d power consump high tion, h mechanical igh mechanic and thermal al and tstability hermal , and stabil assembly ity, andsimpliﬁcation assembly simplif for high ication subsystem for high integration subsystem i aren expected tegration under are expected under PIC PIC solutions [6–9]. solutions [6–9]. By combining the need of providing enhanced passive optical network (PON) technologies and taking By combinin advantage g the need of of photonic providing en integrated hanced solutions passive optical net beneﬁts, a compr work (PO ehensive N) t description echnologies ofand PIC taking advantage of photonic integrated solutions benefits, a comprehensive description of PIC design challenges for the optical network unit (ONU) side of next-generation PON is delivered design challenges for the optical network unit (ONU) side of next-generation PON is delivered in this in this study (see optical access network branch in Figure 1). The paper is organized into four study (see optical access network branch in Figure 1). The paper is organized into four additional additional sections. The historical PIC evolution addressing current developments and expectations sections. The historical PIC evolution addressing current developments and expectations are are described in Section 2. Section 3 addresses the PON standards and next-generation PON timeline. described in Section 2. Section 3 addresses the PON standards and next-generation PON timeline. The implementation of PIC solutions for PON is introduced in Section 4, discussing the technology The implementation of PIC solutions for PON is introduced in Section 4, discussing the technology impact on reach (Section 4.1), the adopted band and wavelength selection criteria (Section 4.2), photonic impact on reach (Section 4.1), the adopted band and wavelength selection criteria (Section 4.2), packaging (Section 4.3), and control complexity, power dissipation and form factor issues (Section 4.4). photonic packaging (Section 4.3), and control complexity, power dissipation and form factor issues The study is concluded in Section 5. (Section 4.4). The study is concluded in Section 5. 2. PIC Evolution 2. PIC Evolution Photonic integrated solutions are a promising technology for the 21st-century optical systems, Photonic integrated solutions are a promising technology for the 21st-century optical systems, under the current demand for ﬂexibility/reconﬁgurability in optical communication networks [5,10]. under the current demand for flexibility/reconfigurability in optical communication networks [5] Dierentiated examples of PIC solutions include the implementation of photonic integrated optical [10]. Differentiated examples of PIC solutions include the implementation of photonic integrated transforms for data compression applications in dierent integrated platforms, e.g., indium phosphide optical transforms for data compression applications in different integrated platforms, e.g., indium (InP) [11,12], silicon nitride (SiN) [13], and new organic-inorganic hybrid materials [14]; SiN-integrated phosphide (InP) [11,12], silicon nitride (SiN) [13], and new organic-inorganic hybrid materials [14]; photonic dispersion compensator enabling extended reach pulse-amplitude modulation with four SiN-integrated photonic dispersion compensator enabling extended reach pulse-amplitude amplitude levels (PAM-4) transmission as a low-cost interface for data center interconnects (DCI) [15]; modulation with four amplitude levels (PAM-4) transmission as a low-cost interface for data center development of state-of-the-art sub-hertz fundamental linewidth (<1 Hz) photonic integrated Brillouin interconnects (DCI) [15]; development of state-of-the-art sub-hertz fundamental linewidth (< 1 Hz) laser, narrow enough to move demanding scientiﬁc applications to the chip-scale [16–18]; and a new photonic integrated Brillouin laser, narrow enough to move demanding scientific applications to the soft-packaging ﬂexible platform approach for photonic integrated processors, based on the spatial chip-scale [16–18]; and a new soft-packaging flexible platform approach for photonic integrated light modulation operation principle [19,20]. Moreover, new concepts of PIC-to-PIC interconnects processors, based on the spatial light modulation operation principle [19,20]. Moreover, new concepts with integrated fan-in/fan-out multicore ﬁber applications for optical dense networks have been of PIC-to-PIC interconnects with integrated fan-in/fan-out multicore fiber applications for optical Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 3 of 19 reported [21]. Photonic integration appears as a dominant technology in high bandwidth optical communication systems [9], oering increased valuable solutions in several innovative ﬁelds, such as bio-photonics [22], sensing [23], and space technology [24]. Nevertheless, PIC technology is still more expensive than standard microelectronics, which can restrict its application into some niche markets [5]. PICs are the equivalent of electronic integrated circuits (EIC) in the optical domain. As an alternative to transistors and other electronic components, PICs contain optical elements, such as modulators, detectors, attenuators, multiplexers, optical ampliﬁers, and lasers embedded in a single chip using a waveguide (WG) architecture [19]. The photonic integration technology is going through a similar evolution path as microelectronic integration, however, with a time delay of about 20 to 30 years [25]. Historically, the invention of the transistor in 1948 [26,27] launched microelectronic integration developments; analogously, the invention of the semiconductor laser in 1969 [28] was the breakthrough of photonic integration technology. Transistor and laser technologies were primarily implemented as discrete components. Microelectronics integration technology started with Kilby in 1959 [29] and maturated with the complementary metal-oxide-semiconductor (CMOS) [30] in the 1970s. A ﬁrst PIC comprising a laser integrated with a modulator was reported in 1987 [31], marking the start of PIC exponential development, typically associated with Moore’s law [25]. Moore’s law forecasted a double in the number of components per integrated circuit about every two years, with a new technology generation introduced approximately every three years and an expected compound annual growth rate (CAGR) under the dozens [32]. Optical communications evolution has brought the advent of improved PIC, presenting an economic and sustainable alternative to data transmission [4]. PIC technology oers compelling performance advances in terms of small weight and volume, low power consumption, high mechanical and thermal stability, and the ease for assembling a substantial number of complex systems [9]. Consequently, the current PIC-related contexts are evolving at a good rate with an overall worldwide concertation set of eorts aiming to reach a sustainable PIC ecosystem model [33,34]. From the 2000s when the ﬁrst consortia started to consolidate the concepts around photonics multi-project wafer (MPW), the way for groundbreaking photonic integration developments was paved. State-of-the-art PIC-based consortia, such as JePPIX [35], TriPleX [36], and ePIX [37], pioneered the eort in Europe and globally, followed by private and public-private arrangements, like AIM photonics [38], Compoundtek silicon photonics, and others [39]. They are working to replicate in optics what happened in the 80s–90s with electronics, e.g., the Marconi GaAs MPW runs and others [40]. The traction gathered by these consortia allowed a fast development of integrated optics simulators and of the process design software (PDS) market, supported by software suites running process design kits (PDK) from almost all available fabs. The development of a generic common language for the photonic design was an important breakthrough that allowed integrated photonics to be regarded as a promising sort-term technology. During these developments, many integrated and hybrid solutions emerged [41–43]; nonetheless, the majority were bulky, expensive, and typically non-replicable, making these approaches unfeasible to be integrated into further complex systems and subsystems. For instance, devices such as optical ampliﬁers [44], lasers [16,45], and modulators [46], are not supplied by the dierent integrated platforms due to limitations of semiconductor integration technology. Thus, it restricted the investment of the dierent labs and research institutes in the development of their concepts/testing to validate their contributions/solutions while waiting for the necessary developments of the technology. In the 90s, few basic building blocks (BB) were made available in a packaged version, e.g., the semiconductor optical ampliﬁer (SOA) based Mach-Zehnder interferometer (MZI) [47], though with an expensive associated cost of several tens of thousands of euros. Dierent materials are suitable to produce PICs, nonetheless, only a few have been selected by actors in the ecosystem and, therefore, have gained traction into foundries, software suppliers, design houses, and fabless companies [48]. Current main photonic integration platform technologies Appl. Appl. Sci. Sci. 2020 2020 , 10,, x10 , x Appl. Sci. 2020, 10, x 4 of4 19 of 19 4 of 19 inteinte gragra tedt ed ph otonic photonic pla pla tform tinte form to gra realize toted realize ph PICs otonic PICs is pla dr isiven tdr form iven by to by the realize the functiona fu PICs nctiona l isre ldr quire reiven quire m by ents m the ents of fu the ofnctiona the devices devices l re quire ments of the devices to be to deve be deve loplop ed eand d and the the unto derly un be derly deve ingi ng av lop a av ieladabi and llaeb compone l ethe compone underly ntsn (ac itsng t(ac iv av e/p tiv aie/p laassive balessive compone ) fo)r fo thre th differen nets differen (activ t e/p plat t plat aforms ssive forms ) for the different platforms Appl. Sci. 2020, 10, x Appl. Sci. 2020, 10, x 4 of 19 4 of 19 [33[,55 33,5 ,556],5. 6] An . An overview overview of the [of 33 the , 55 ke,5 y ke 6] chy. ar An chac ar overview teri acteri stics st ics be of tw be the een tw ke een the y the ch diar ffe di acrffe eteri ntre st av ntics ai avlab be ailab tw le int een le eint grat the egrat ed diffe ed plat r eplat nt form av form ai lab le integrated platform Appl. Sci. 2020, 10, x 4 of 19 technolo technolo giesgies is provided is provided intechnolo Table in Table 1 gies [33 1 [33 ,56 is ,5 ,provided 567],5. 7] BB. BB com in com p Table onents, ponents, 1 [33 su,ch 56 su ,5 as ch7] la as . sers, BB lasers, com optical poptical onents, am apl m su ifpl iers, chif iers, as and la and sers, optical amplifiers, and integrated photonic platform integra to trealize ed photonic PICs ispla drtform iven by to realize the functiona PICs isl re drquire iven m byents the of fu nctiona the devices l require ments of the devices Appl. Sci. 2020, 10, 4024 4 of 19 detec detec tors, tors, pose pose de sign design ch allenge detec challenge tors, s for s pose for their their de im sign pim lem p chlem eallenge nta ention tation s in for in a their SiaN Si ‐based im N‐p based lem monoli e nmonoli tation thic thic in in a te in Si gra te Ngra t‐ed based t ed monolithic integrated to be developed and the un toderly be deve inglop avaeidla and ble compone the underly ntsi ng (ac tav ivae/p ilaabssive le compone ) for then ts differen (active/p t plat assive forms ) fo r the different platforms integrated photonic platform to realize PICs is driven by the functional requirements of the devices platplat form form ; thi; sthi constrain s constrain can plat can be form be overcom overcom ; thise constrain with e with hy br hy can id br and ibe d and overcom /or /heterogeneous or heterogeneous e with hybr pro id pro cand esses cesses /or, as heterogeneous , pre as pre sente sente d d processes, as presented [33,55,56]. An overview of[3 the 3,55 ke ,5y6] ch . An arac overview teristics be of tw theeen ke ythe ch di arffe acteri rentst av icsai be lab twleeen int ethe grat died ffe plat rentform avai lable integrated platform to be developed and the underlying available components (active/passive) for the different platforms in Table in Table 1. Si 1.mil Simil arly, arl hybr y, hybr id/he in id/he Table terogen t erogen 1. Siemil ous eoa rl us processes y, processes hybrid/he are are templo erogen emplo yeeodyus efor d processes for lasers lasers and are and optic emplo optic al am ayle am plif d for plif ier lasier sers s and optical amplifiers technologies is provided technolo in Table gies 1 [33 is ,56 provided ,57]. BB com in Table ponents, 1 [33 su ,56 ch,5 as 7]. la BB sers, com optical ponents, am su plifchiers, as la and sers, optical amplifiers, and Appl. Appl. Sci. Sci. 2020 2020 , 10,, x10 , x 4 of4 19 of 19 [33,55,56]. An overview of the key characteristics between the different available integrated platform oering access BBs to BBs ingeneric Si in‐ ba Si‐sba eds e pr techno d ocesses techno logy. logy. BBs and in Si manufacturing ‐based technology. thr ough multi-project wafer (MPW) runs are detectors, pose design chdetec allenge tors, s for pose their de sign imp lch emallenge entation s for in their a SiN im‐based plem emonoli ntationthic in in a te Sigra N‐based ted monolithic integrated technologies is provided in Table 1 [33,56,57]. BB components, such as lasers, optical amplifiers, and Appl. Sci. 2020, 10, x Appl. Sci. 2020, 10, x 4 of 19 4 of 19 inteinte gragra tsilicon edt ed ph otonic photonic (Si) pla [ 49 pla tform plat ,50 tform form ], toindium realize to; thi realize s constrain PICs phosphide PICs is dr is can iven dr plat iven be form by (InP) overcom by the ; thi the fus[nctiona fu 51 constrain e nctiona with –53l ], hy rel and quire can br reiquire d be and m silicon ents overcom m/or ents of heterogeneous nitride the ofe the with devices devices hy (SiN) br ipro d and c [esses 48/or ,54 ,heterogeneous as ]. pre The sente selection d processes, as presented detectors, pose design challenges for their implementation in a SiN‐based monolithic integrated Table Table 1. Key 1. Key fea tfe ure ature s from s from thre Table thre e availab e1. availab Keyle fephotonic laet ure photonic s from integ integ thre ration er ation availab platfor platfor lem photonic s,m i.s,e. ,i. indiu e. ,integ indiu mr ation phosphide m phosphide platfor m s, i.e., indium phosphide to be to deve be deve loplop ed eand d and the Appl. the un in derly un Sci. Table derly 2020 ing 1. , 10 i ng av Si , xmil a av ilaaabirl Appl. llaey,b compone Appl. lhybr eSci. compone Sci. 2020 inid/he Table 2020 , n 10ts t,,erogen n x 10 (ac ts 1. , x t(ac Si ivmil e/p etiv ous ae/p arlssive processes y, assive hybr ) fo)rid/he fo th rare e th differen terogen eemplo differen ety oplat eus dt plat for forms processes la forms sers and are optic emplo aly am ed 4 plif of for 19 ier la sers s and optic 4 aofl 4 am 19 of plif 19 iers of the integrated photonic platform platto form realize ; this constrain PICs is can driven be overcom by the e with fu nctional hybrid andr/equir or heterogeneous ements of pro the cesses, as presented integrated photonic pla inte tform grated to ph realize otonic PICs pla tis form driven to realize by the PICs functiona is driven l require by the ments functiona of the ldevices require ments of the devices (InP), (InP), sili con silicon (Si), (S and i), and sili con silicon ni (InP), t rni idter si i(SiN) dliecon (SiN) monolithic (Si), monolithic and sili technologies. con technologies. nitride (SiN) monolithic technologies. BBs in Si‐based technology. BBs in Si‐based technology. [33[,55 33,5 ,556],5. 6] An . An overview overview of the of the key ke chyar chac arteri acteri stics st ics betw beeen tween the the diffe dirffe entre av nt ai avlab ailab le int le eint grat egrat ed ed plat plat form form to be developed and the to be un deve derlylop inge dav and aila the ble un compone derly in Table ing nts av 1. (ac aSiitlmil iv abe/p lea rlcompone ay, ssive hybr ) fo id/he nr ts th (ac et erogen differen tive/peaotssive us plat processes )forms for th e differen are emplo t plat yeforms d for la sers and optical amplifiers devices to be developed and the underlying available components (active/passive) for the dierent integrated photonic inte plainte tgra form gra ted to t ed ph realize otonic photonic PICs pla pla tform ist form dr iven to realize to by realize the PICs fu PICs nctiona is dr isiven dr l re iven quire by by them the fu ents nctiona fu of nctiona thel re devices lquire require m ents ments of the of the devices devices technolo technolo giesgies is provided is provided in Table in Table 1 [33 1 [33 ,56,5 ,567],5. 7] BB. BB com com ponents, ponents, such su as ch la assers, lasers, optical optical am aplmifpl iers, ifiers, and and [33,55,56]. An overview [33 of ,55 the ,56] ke . An y ch overview aracterist ofics the BBs be ke tw in yeen ch Si ‐ar the baac s edi teri dffe techno strics ent be av logy. tw ailab een l ethe int di egrat ffered ent plat avaiform lable integrated platform Build Build inging blo blo ck c(BB) k (BB) InP Build InP ing blocSi k (BB) Si SiN InP SiN Si SiN platforms [33,55,56]. An overview of the key characteristics between the dierent available integrated to be developed andto the be to un deve bederly deve loping lop ed av eand da and i lthe ab lthe eun compone derly underly ingni ng av ts (ac a av ilataiv billaee/p b compone lea ssive compone ) fo nts r n th (ac tse t(ac differen ive/p tive/p assive ta plat ssive ) fo forms )r fo thre th differen e differen t plat t plat forms forms Table 1. Key features from Table three availab 1. Key lfe e aphotonic tures from integ thre ration e availab platfor le photonic ms, i.e., indiu integm ration phosphide platfor ms, i.e., indium phosphide detec detec tors, tors, pose pose de sign design ch allenge challenge s for s for their their im pim lem plem enta ention tation in in a SiaN Si‐based N‐based monoli monoli thicthic in teingra tegra tedt ed technologies is provided technolo in Table gies 1is [33 provided ,56,57]. BB in Table comp 1onents, [33,56 ,5 su7]ch . BB as com lasers, ponents, optical su am chpl asif iers, lasers, and optical amplifiers, and PasPas sivesive components components ✓✓ Pas ✓✓ s ive components ✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓✓ [33,55,5(InP), 6]. An si overview licon (S[3i),3 [,and 55 3 of 3,5 , 55 the s6] ili,5.con ke 6] An .(InP), y An ni overview ch tr iar overview d sieac li (SiN) con teri of (S st monolithic i), ics the of and be the ketw sy ke ili een ch con y technologies. ar ch the ni ac ar tteri r ac di idteri effe st (SiN) ics rst ent ics be monolithic av tw beai een tw lab een the le int technologies. the dieffe grat dirffe ent ed re av nt plat ai avlab form ailab le int le eint grat egrat ed ed plat plat form form platplat form form platform ; thi; sthi constrain s constrain technologies can can be be overcom overcom is provided e with e with hyin br hyiT d br able and id and /1 or[ /heterogeneous 33 or ,heterogeneous 56,57]. BB pro components, pro cesses cesses , as, pre as pre ssuch ente sente d asd lasers, optical ampliﬁers, Table 1. Key features from three available photonic integration platforms, i.e., indium phosphide detectors, pose design detec challenge tors, pose s for de their sign im challenge plemensta for tion their in a im Sip Nl‐em based enta tion monoli in thic a Si N in‐tebased grated monoli thic integrated technologies is provided technolo technolo in Table gies Lase gies Lase is rs 1 provided [33 is rs provided ,56,57] .in BB ✓✓✓ Table in ✓✓✓ com Table 1p onents, Lase [33 1 [33 ,56 rs ⚪ ,5 , 56 ⚪ su 7] H,5.ch 7] BB H .as BB com la com sers, ⚪ ✓✓✓ ponents, ⚪ H p optical onents, H su ach su m ⚪ as ch pl H if la as iers, sers, lasers, and optical ⚪ optical H am aplmifpl iers, ifiers, and and in Table in Table 1. Si 1.mil Simil arly, arl hybr y, hybr id/he id/he terogen terogen eous eo us processes processes are are emplo emplo yedy efor d for lasers lasers and and optic optic al am al am plifplif iersier s (InP), silicon (Si), and silicon nitride (SiN) monolithic technologies. platform and ; thi det s constrain ectors, plat can pose form be; design thi overcom s constrain e challenges with can hy be bri d overcom for and/their or eheterogeneous with implement hybrid and pro ation /or cesses heterogeneous in , as a pre SiN-based sente pro d cesses monolithic , as presente integrated d Building block (BB) Build InP ing blockSi (BB) InP SiN Si SiN detectors, pose design detec detec chtors, allenge tors, pose spose for de sign de their sign ch im allenge ch plallenge emesn ta for stion for their in their im a Si pim N lem p‐based lem enta ention ta monoli tion in in athic SiaN Si in‐based N te‐gra based t ed monoli monoli thicthic in teingra tegra tedt ed BBsBBs in Si in‐ ba Si‐sba eds etechno d techno logy. logy. Modulators Modulators ✓✓✓ ✓✓✓ Modulators ✓✓ ✓✓ ✓✓✓ ✓ ✓ ✓✓ ✓ in Table 1. Similarly, in hybr Table id/he 1.t erogen Similarleoy,us hybr processes id/heterogen are emplo eousy processes ed for lasers are and emplo optic yeadl for am la plif sers ier sand optical amplifiers platform; this constrain can be overcome with hybrid and/or heterogeneous processes, as presented in platform; this constrain platplat form canform be ; thi overcom ; sthi constrain s constrain e with can hy can be Build br be overcom id ing overcom and blo /ore cwith heterogeneous ke (BB) with hy br hyid br and iInP d pro and / orc /esses heterogeneous or heterogeneous , as Si pre sente pro d pro SiN cesses c esses , as, pre as pre sente sente d d Passive components Pas ✓✓ sive components ✓✓ ✓✓✓ ✓✓ ✓✓ ✓✓✓ BBs in Si‐based techno BBs logy. in Si‐based technology. SwiSwi tches tches ✓✓ ✓✓ Swi tches ✓✓ ✓✓ ✓✓ ✓ ✓ ✓✓ ✓ Table 1. Similarly in Table , hybrid 1. Simil/aheter rly,in hybr Table in ogeneous Table id/he 1. Si t1. erogen mil Sipr mil arlocesses y, eaorl us hybr y, processes hybr id/he ar id/he eterogen employed tare erogen emplo eous eo us processes ye for dprocesses for lasers la sers are are and emplo and emplo optic optical yedya efor ld am for la ampliﬁers plif sers lasers ier and s and optic optic BBs al am al am plifplif iersier s Table Table 1. Key 1. Key fea tfe ure ature s from s from thre thre e availab e availab le photonic le photonic integ integ ration ration platfor platfor ms,m i.s,e. ,i. indiu e., indiu m phosphide m phosphide Lasers ✓✓✓ Lasers⚪ H ✓✓✓ ⚪ H ⚪ H ⚪ H Passive components ✓✓ ✓✓ ✓✓✓ Optical Optical am aplif mplif iersie rs ✓✓✓ ✓✓✓ Optical am ⚪ plif ⚪ H ie rs H ⚪ ✓✓✓ ⚪ H H ⚪ H ⚪ H (InP), (InP), sili con silicon (Si), (S and i),BBs and sili in con sili Si con ni‐ba t rni isdeterd i(SiN) d techno e (SiN) BBs monolithic logy. BBs monolithic in Si in ‐ ba Si technologies. ‐sba etechnologies. ds etechno d techno logy. logy. in Si-based technology. Table 1. Key features from Table thre 1. eKey availab feature le photonic s from thre integ e availab ration leplatfor photonic ms, i.integ e., indiu ration m platfor phosphide ms, i.e., indium phosphide Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓ ✓✓ ✓ Lasers ✓✓✓ ⚪ H ⚪ H Detec Detec torstors ✓✓✓ ✓✓✓ Detec tors ✓✓ ✓✓ ⚪ ✓✓✓ ⚪ H H ✓✓ ⚪ H (InP), silicon (Si), and sili (InP), con ni sitlircon ide (SiN) (Si), and monolithic silicon ni technologies. tride (SiN) monolithic technologies. Switches ✓✓ Switches✓✓ ✓✓ ✓ ✓✓ ✓ Build Build ing Table ing blo blo c1.k Key c(BB) k (BB) fe ature sInP Table from InP Table thre 1. Key 1.e Key availab fe Sia tfe Si ure al teure s photonic from s from SiN thre SiN Modulators thre integ e availab e ravailab ation le platfor photonic le photonic ms, ✓✓✓ integ i.e integ ., rindiu ation ration m platfor phosphide ✓✓ platfor m s,m i.s, e. ,i. indiu e., ✓ indiu m phosphide m phosphide Footprin Footprin t t ✓✓ ✓✓ Footprin ✓✓✓ ✓✓✓ t ✓✓ ✓ ✓ ✓✓✓ ✓ Table 1. Key features from three available photonic integration platforms, i.e., indium phosphide (InP), (InP), silicon (Si), and s(InP), ilicon (InP), si ni li tcon si rilidcon e(S (SiN) i), (S and i), monolithic and sili con silicon ni ttechnologies. rni idter i(SiN) de (SiN) monolithic monolithic technologies. technologies. Optical amplifiers ✓✓✓ Optical amplif ⚪i eH rs ✓✓✓ ⚪ H ⚪ H ⚪ H Building block (BB) BuildInP ing block (BB) Si InP SiNSwi tches Si SiN ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓✓ PasPas sivesive components components Chip Chip cost cost ✓ ✓Chip cost ✓✓ ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓✓ silicon (Si), and silicon nitride (SiN) monolithic technologies. Detectors ✓✓✓Detec tors ✓✓ ✓✓✓ ✓✓✓ ⚪ H ✓✓ ⚪ H Optical amplifiers ⚪ H ⚪ H Lase Lase rs rs ✓✓✓ ✓✓✓ Passive components Pas CMOS sCMOS ive ✓✓ components compa compa t⚪ ib✓✓ ilt⚪ iitH b yil it Hy ✓✓ CMOS ⚪ ✓✓✓ ⚪ H compa H ✓✓ ✓✓ ti✓✓ b il ity ✓✓✓ ✓ ✓ ✓✓ ✓ ⚪⚪ ⚪⚪ ⚪⚪ Building block (BB)Build Build ing InP ing blo blo ck c(BB) k (BB) Si InPInP SiN Si Si SiNSiN Footprint ✓✓ Detec Footprin tors ✓✓✓ 1t 1 ✓✓✓ ✓✓ ✓ ✓✓✓ ✓✓ 1 ✓ Building Block (BB) InP Si SiN ⚪ H Modulators Modulators ✓✓✓ ✓✓✓ ✓✓ ✓✓ ✓ ✓ ⚪ ⚪ /✓✓ /✓✓ ⚪ /✓✓ Lasers ✓✓✓ Lase rs ⚪ H ✓✓✓ ⚪ H ⚪ H ⚪ H Passive componentsPas Pas sive ✓✓ sive components components ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓✓ ✓✓ Low Low ‐cost‐cost pa ck paack gianggi ng ⚪ Low ⚪ ‐cost packaging ⚪ 2 2 2 Chip cost ✓ Chip cost ✓✓ ✓✓ ✓ ✓✓ ✓✓ Footprint ✓✓ ✓✓✓ ✓ SwiSwi tches tches ✓✓ ✓✓ ✓✓ ✓✓ ✓ ✓ Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓ ✓✓ ✓ Passive components 33 33 333 Lasers ✓✓✓ Lase Lase rs rs ⚪ H ✓✓✓ ✓✓✓ ⚪ H ⚪⚪ H H ⚪⚪ H H CMOS compatibility ⚪⚪ CMOS compa✓✓ tibilit y ⚪⚪ ✓ ✓✓ ✓ Chip cost ✓ ✓✓ ✓✓ Optical Optical Perform am Perform aplif mplif aience: rsaie nce: rs ✓✓✓ ✓✓✓ ✓✓✓ ✓✓✓ very very good; Perform good; ⚪ ✓✓ ⚪ ✓✓ H a nce: good H good ✓✓✓ ; ✓ ⚪ ; ✓ mod ⚪ H very mod eH s tgood; ;e ⚪ st; ⚪ chal ✓✓ chal leng good leng ing;ing ; ⚪⚪ ✓; ⚪⚪ mod very es very t; challeng ⚪ challeng challeng ing.ing ing. ; ⚪⚪ very challenging. Swi Lasers tches ✓✓ Swi tches 333 ✓✓ ✓✓ ✓ ✓✓ ✓ H H Modulators Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓✓✓ ✓ ✓✓ ✓✓ ✓ ✓ 1 1 ⚪ /✓✓ ⚪ /✓✓ H: hybrid/hete H: hybrid/hete rogeneous rogeneous processe processe H: hybrid/hete s tos over to over corme ogeneous co me the the desig processe desig n chnalleng chs alleng to es over asso esco asso me ciated cthe iated with desig with the n c the monolithic halleng monolithic es asso ciated with the monolithic CMOS compatibility ⚪⚪ ✓✓ ✓ Modulators Detec Detec torstors ✓✓✓ ✓✓✓ 333 ✓✓ ✓✓ ⚪⚪ H H 33 3 Optical amplifiers Optical ✓✓✓ am plifie⚪ rs H ✓✓✓ ⚪ H ⚪ H ⚪ H Low‐cost packaging Low‐cost packaging ✓✓ ✓✓ ⚪ ⚪ Switches Swi ✓✓ Swi tches tches ✓✓ ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓ ✓ 1 1 1 2 2 2 2 2 integ integ rated rated tec htec nol honol gy.ogy. End End ‐fire integ ‐fire cou r ated cou pling p tec ling (lo hnol w (lo reflect owgy. reflect iEnd on); ion); ‐ fire vertical cou vertical pling cou cou p(lo ling w pl ireflect (medium ng (medium ion); reflection). vertical reflection). cou p ling (medium reflection). ⚪ /✓✓ Switches 33 33 3 Footprin Footprin t t ✓✓ ✓✓ ✓✓✓ ✓✓✓ ✓ ✓ Detectors ✓✓✓ Detec tors ✓✓ ✓✓✓ ⚪ H ✓✓ ⚪ H Low‐cost packaging ⚪ ✓✓ Optical amplifiers Optical Optical ✓✓✓ am aplif mplif iers ⚪ ie rs H ✓✓✓ ✓✓✓ ⚪ H ⚪⚪ H H ⚪⚪ H H Performance: ✓✓✓ very good; Perform ✓✓ ance: good ✓✓✓ ; ✓ mod verye good; st; ⚪ ✓✓ chal leng good ing ; ;✓ ⚪⚪ mod every st; ⚪ challeng challeng inging . ; ⚪⚪ very challenging. Optical ampliﬁers 333 H H Chip Chip cost cost ✓ ✓ ✓✓ ✓✓ ✓✓ ✓✓ Footprint ✓✓ Footprin t ✓✓✓ ✓✓ ✓ ✓✓✓ ✓ Sever Sever al adv al adv anta anta gesg and es and /orSever /limi or limi tat al ions tadv ations aca nta n ca gbe n es be high and high lighted /or lighted limi tfrom at ions from the ca the n referre be referre high d tech lighted d tech nologies. nologies. from Hi the gh Hi referre gh d technologies. High Detectors ✓✓✓ Detec Detec t orstors ✓✓ ✓✓✓ ✓✓✓ ⚪ H ✓✓ ✓✓ ⚪⚪ H H H: hybrid/heterogeneous processe H: hybrid/hete s to overco rogeneous me the desig processe n challeng s to over es asso come ci ated the desig withn the ch alleng monolithic es asso ciated with the monolithic Performance: ✓✓✓ very good; ✓✓ good; ✓ modest; ⚪ challenging; ⚪⚪ very challenging. Detectors 333 33 CMOS CMOS gain gain compa , spee compa , spee d,ti band d, iltiit band y illow ity low loss lo ar ssgain e ar three e, three spee of✓✓ d, the of✓✓ and the ma low ma in at in lo tr at ss ✓ ibtr u ar ✓ ib tes eu three tof es phot of of phot onic theonic ma devices’ in devices’ attr ib improved u improved tes of phot performa H performa onic devices’ nce.nce. improved performance. Chip cost ⚪⚪ ⚪⚪ Chip ✓ cost ✓✓ ✓ ✓✓ ✓✓ ✓✓ 1 1 2 2 integrated technology. Footprin End‐integ fire t cou rated pling tec h(lo nol Footprin ✓✓ woFootprin reflect gy. End ti on);‐tfire ✓✓✓ vertical coup ling ✓✓ cou ✓✓ (lo pw li✓ ng reflect (medium ✓✓✓ i✓✓✓ on); reflection). vertical ✓ cou ✓ pl ing (medium reflection). H: hybrid/heterogeneous processes to overcome the design challenges associated with the monolithic 1 1 As As InP InP is ais direct a direct ba nba dga ndpga As mat p InP mat erial, is erial, ait direct presen it presen batsn the dtsga the best p mat best ave rial, ai avlab ai it lab le presen ga le iga n perform in ts perform the best ance, a av nce, however, ailab however, le ga iwith n perform with ance, however, with ⚪ ⚪ /✓✓ /✓✓ CMOS compatibility CMOS ⚪⚪ compa tibil✓✓ ity ⚪⚪ ✓ ✓✓ ✓ Footprint 33 1 333 2 3 Low Low ‐cost‐cost pa ck paack gianggi ng ⚪ Chip ⚪ cost integ rated technol Chip ✓ oChip ✓✓ gy. cost ✓✓ End cost ‐fire ✓✓ cou pling ✓ (lo ✓✓✓ w reflect ✓✓ ion); ✓✓ vertical ✓✓ cou ✓✓ pling (medium reflection). higher higher assoc assoc iateiate d loss d loss es du eshigher edu toe el to eassoc celtreic 2ctariate li2 cdoping ad l doping loss es from du from ecurren to curren eletc tinj ritc einj action l doping ection which which from require curren require s the t sinj the incl ection incl usiu owhich n si on requires the inclusion 1 1 Several advantages and/or Sever limiatlat adv ions anta cang es be and high /or lighted limitat from ions the can referre be high dlighted technologies. from the Hi gh referre d technologies. High ⚪ /✓✓ ⚪ /✓✓ Chip cost 3 33 33 Low‐cost packaging Low CMOS‐cost ⚪ compa packtaibgilinitgyCMOS CMOS ⚪⚪ ⚪ compa compa ✓✓ ti biltiitb✓✓ yil ity ⚪⚪ ⚪⚪ ✓✓ ✓ ✓✓ ✓✓ ✓ ✓ of optic of optic al am al am plifplif iersier [5s5] [5. 5] of Reg . optic Reg ardaing ard l ing am sy plif stems systems ier‐son [5‐‐on 5] ch.‐ip ch Reg (SOC) ipa (SOC) rding com sy com pstems lexity, plexity, ‐on InP ‐ch InP ip plat (SOC) plat form form ha com sha aps lexity, good a good InP platform has a good 2 2 gain, speed, and low loss ar gain e three , spee of d, the and ma low in at lotr ssib aruet es three of phot of the onic ma devices’ in attrib improved utes of phot performa onic devices’ nce. improved performance. Perform Perform ance: ance: ✓✓✓ ✓✓✓ very very good; good; ✓✓ ✓✓ good good ; ✓; ✓ mod mod esSever t;e ⚪ st; ⚪ a chal l adv chal leng aleng nta ingg;ing es ⚪⚪ ; and ⚪⚪ /very or very limi challeng t challeng ations ing ca .ing n .be highlighted from the referred technologies. High CMOS compatibility 33 3 1 1 1 mama turit turit y wi y twi h chips th chips comprising comprising maturit upy up to wi hundreds to th hundreds chips comprising of components. of components. ⚪ up/✓✓ to S hundreds ili Scon ilicon ‐ba‐s ba of e⚪ ds components. ep⚪ dh /otonic p✓✓ h/otonic ✓✓ proce proce Silisses con sses are ‐ba are sed photonic processes are As InP is a direct bandgaAs p mat InPe rial, is a direct it presen bantsd the gap best mat eav rial, ailab it lpresen e gain ts perform the best ance, avai however, lable gain with perform ance, however, with Low‐cost packagingLow Low ‐cost ⚪ ‐cost pa ck paack gianggi ng ⚪⚪ ✓✓ ✓✓ ✓✓ H: hybrid/hete H: hybrid/hete rogeneous rogeneous processe processe s tos over to over come co me thegain the desig desig , spee n chnalleng d, ch and alleng es low asso es asso lo ciss ated c ar iated ewith three with the of the monolithic the monolithic main at tributes of photonic devices’ improved performance. 1 2 Performance: ✓✓✓ very Perform good; ance: ✓✓ ✓✓✓ good ; very ✓ mod good; es t;✓✓ ⚪ chal good leng ; ✓ ing mod ; ⚪⚪ est; ⚪ very chal 2 challeng lenginging ; ⚪⚪ . very 2 2challenging. Low-cost packaging 33 complementary complementary metal metal ‐ox‐ide ox complementary ide ‐sem‐sem iconduc iconduc tor metal tor (CMOS (CMOS ‐oxide )‐co‐)sem ‐mpat compat iconduc /ible, 33 ible, altor lowi al lowi (CMOS ng ng its it)in‐sc t oeinmpat gra tegra tion ible, tion to al be tolowi be run ng run it s integration to be run 1higher 1 associated losses du higher e to el 2assoc ec2triicate al doping d losses from due to curren electtr iinj cael ction doping which from require currenst the inje incl ction us which ion requires the inclusion integ integ rated rated tec htec nol honol gy.ogy. End End ‐fire‐fire cou cou pling pling (low (lo reflect w reflect ion); ion); vertical vertical cou cou pling pl i(medium ng (medium reflection). reflection). As InP is a direct bandgap material, it presents the best available gain performance, however, with H: hybrid/heterogeneous H: processe hybrid/hete s to over rogeneous come the processe designs to ch alleng overcoes me asso thec idesig ated nwith challeng the monolithic es associated with the monolithic in ain CMO a CMO S fou S fou ndry ndry wh wh ichinic provides ha provides CMOS we fou we lln‐controlled dry ll‐controlled which and provides and more more we rapi lrapi l‐dcontrolled sc dal scab aliab lit yiand l ito ty the to more the fa brica rapi fabrica dti osc ntial onab ility to the fabrication of optical amplifiers [55]of . Reg optic ardaing l am sy plif stems iers ‐[5 on5]‐ch . Reg ip (SOC) arding com systems plexity, ‐on ‐InP chip plat (SOC) form com hasp lexity, a good InP platform has a good Performance: ✓✓✓ Perform very Perform good; ance: a✓✓ nce: ✓✓✓ ✓✓✓ good very ; ✓ very good; mod good; e s✓✓ t; ⚪ ✓✓ good chal good leng ; ✓; ing ✓ mod ; mod ⚪⚪ est;e ⚪ st ;very ⚪ chal chal challeng lengleng ing;ing ing ⚪⚪ ;. ⚪⚪ very very challeng challeng ing.ing . Performance: 1 333 very good; 1 higher 33 assoc good; 2 iate3 d loss modest; es due to 2 elecchallenging; trical doping from curren very t injchallenging. ection which requires the inclusion integrated technology. integ End‐rfire ated cou tecphling nol o(lo gy. w reflect End‐fire ion); cou p vertical ling (lo cou w reflect pling i(medium on); vertical reflection). coupling (medium reflection). environment environment [42] [4. 2] Never . Never thenvironment etles hesles , as s, com as com p [4 onents 2] ponents . Never in siin thli esi con les licon ssubs , as subs com trate trpate sonents ussua usllua yin llha si y vliha econ v higher e subs higher dimensions trate dimensions s usuall y ha ve higher dimensions Sever Sever al adv al adv anta anta gesg and es ma H: and turit /or hybrid/hete /limi or y wi limi tat thions t chips atrogeneous ions ca comprising n ca H: be n hybrid/hete H: ma processe be high hybrid/hete turit high lighted up ylighted s to wi to rogeneous over t h hundreds from r ogeneous chips co from me the processe comprising the the referre of processe desig referre components. s d nto cstech over h dup to alleng tech over nologies. to co me hundreds nologies. es co Sili me asso the con the desig cHi i‐ated ba desig gh Hi of sne gh with ccomponents. dhn p alleng chh the otonic alleng es monolithic asso es proce asso Scili iated con csses iated with ‐ba are with s the e d the p monolithic hotonic monolithic proce sses are H: hybrid/heterogeneous processes to overcome the design challenges associated with the monolithic integrated of optical amplifiers [55]. Regarding systems‐on‐chip (SOC) complexity, InP platform has a good than than in InP in InP an dan sinc d sinc e sieli si con lithan con photonic photonic in InPs an lasck d la sinc lickght lieght si soliurce con source sphotonic and s and am apl sm la ifpl ck iers, if liiers, ght InP InP so‐based urce ‐based s platforms and platforms ampl if priers, o pr viod InP vei d‐ebased platforms provide 1 2 1 1 1 2 2 2 gain gain , spee , spee d, and d, technology and low low loss lo . ar complementary sse End-ﬁr integ ar three e three rated of e coupling the of tec the hma nol metal ma in ogy. (low at in‐integ tr ox at End ibinteg complementary ide rtr eﬂection); urib‐tated fire es‐u sem r tated of es cou tec i phot of conduc htec p nol phot ling honic vertical onol gy. (lo onic tor ometal w devices’ gy. End reflect (CMOS devices’ coupling End ‐‐fire ox‐iide fire on); improved cou )‐‐ c improved sem cou o p (medium mpat ling vertical piling conduc (lo ible, performa w (lo cou performa reflect rw eﬂection). al tor p reflect llowi ing (CMOS ion); nce. (medium ng ion); nce. it vertical s ) ‐ vertical in co reflection). tmpat e gra cou tcou ible, pion ling p l to ial (medium ng lowi be (medium run ng reflection). its reflection). integra tion to be run maturity with chips comprising up to hundreds of components. Silicon‐based photonic processes are Several advantages and Sever /ora limi l adv tataions ntag es ca nand be/ or high limi lighted tations from can the be high referre lighted d tech from nologies. the referre High d technologies. High a hiag hi hegrh pot er pot enteiantl ia for l for volume volume a hi gschaling esc r aling pot and ent and ia inl tegr for integr avolume tioantio [2n 5] sc [2. aling 5] Despite . Despite and the in the tegr high high a tpr ion ocessin pr [2ocessin 5]. Despite g tech g tech nology the nology high processing technology As As InP InP is ais direct a direct ba nba dga ndin pga mat ap CMO mat erial, erial, S itfou presen itn presen dryts wh the tsicin the hbest aprovides best CMO av ai av Slab ai fou we lab le nga llldry e‐controlled iga n perform iwh n perform ich provides and ance, a nce, more however, however, we rapi ll‐controlled d with sc with al abi l iand ty to more the fa rapi brica d tsc ioal n ability to the fabrication gain, speed, and low lo gain ss ar , spee e three d, and of the low ma loin ss at artrecomplementary ib three utes of of the phot ma onic in metal at devices’ trib‐ox utide es improved of‐sem phot iconduc onic performa devices’ tor (CMOS nce. improved )‐compat performa ible, allowi nce. ng its integration to be run mama turit turit y ofy si oflicon silicon photon photon ics, maics, turit as an asy an ind of ind si irlicon ect ir eba ct photon ndga bandga p ics, ma p ma tas eria an telria na ind l tiv na ireetiv ct ga eba iga nndga is in difficult isp difficult mate to ria realize to l na realize tiv eun ga der un inder is difficult to realize under Several advantages Sever andSever /or al limi adv al adv taat nta ions anta ges ca g and esn and be /or high /limi or limi lighted tations tations from can ca be n the be high referre high lighted lighted d tech from nologies. from the the referre Hi referre ghd tech d tech nologies. nologies. Hi gh Hi gh environment [42]. Nevertenvironment heless, as com [4p2] onents . Never int si heliles con s, subs as com trate ponents s usua lliny siha liv con e higher substr dimensions ates usually have higher dimensions higher higher assoc assoc iateiate d loss d loss es du es edu toe el toe celtreicctarli cdoping al doping from from curren curren t injt einj ction ection which which require require s the s the incl incl usiu on si on As InP is a direct banAs dga InP p mat is ae rial, direct it presen bandgatsp the mat in abest e rial, CMO av itai S presen lab foulen dry ga ts ithe n wh perform best ich provides avaai nce, lab lhowever, e we gailnl‐ controlled perform with ance, and however, more rapi with d sc alability to the fabrication Several advantages and/or limitations can be highlighted from the referred technologies. High thisthi pla s pla tform. tform. The The pla pla tform tform thi fe s at pla feures attform. ures are are The in‐ between in pla ‐between tform InP fe InP at and ures and ul are tra ul‐tra low in‐‐low between los slos SisN Si InP techno N and techno logies, ultra logies, ‐low with with los s SiN technologies, with gain, speed, and lowgain logain ss, ar spee ,e spee three d, and d, of and low the low ma loss in lo ar ss ate tr ar three ib e u three tes of of the of phot the ma onic ma in at in tr devices’ at ibtruib tes u tof es improved phot of phot onic onic performa devices’ devices’ nce. improved improved performa performa nce.nce. of optic of optic al am al am plifplif iersier [5s5] [5than . 5] Reg . Reg in ard InP aing rd an ing sy d stems sinc systems e ‐si on li‐‐con on ch than ‐ip ch photonic (SOC) ip in InP (SOC) an com s la d com ck sinc p lexity, lipght elexity, si liso con InP urce InP photonic plat s and plat form aform m s pl ha lack if sha iers, alis ght good a InP good so ‐urce based s and platforms amplif iers, prov InP ide ‐based platforms provide higher associated losshigher es due assoc to eleiate ctridc aloss l doping es du efrom environment to el curren ectricalt doping inj [42] ection . Never from which th curren eles require s, as t inj com se ction the ponents incl which us iin on si require licon subs s thetr incl atesu sus ioua n lly have higher dimensions some some activ acetiv dee tde ecttieoctni oelnem elem some entsents but ac but tiv noe no li de ght liteght ct soiource so n urce el em solutions, so ents lutions, but and no and li me ght me di so adl urce ioptic al optic so al lutions, losses al losses (am and (am id me InP idd InP iand al optic and al losses (amid InP and gain, speed, and low loss are three of the main attributes of photonic devices’ improved performance. As InP is a direct baAs nd As ga InP pInP is mat ais edirect rial, a direct it ba presen nba dga ndpts ga mat the p mat e best rial, erial, av it presen ai itlab presen lets ga the ts in the perform best best av ai av alab nce, ailab le however, ga le iga n perform in perform witha nce, ance, however, however, with with mama turit turit y wi y twi h chips th chips comprising comprising a high eup r pot up to ehundreds to nt hundreds ial for volume of acomponents. of hi components. g hsceraling pote and Sntiliia Scon lili in for con tegr ‐ba volume ‐sba aetdio s epndh [2 otonic psc h5]otonic aling . Despite proce and proce sses in the sses tegr are high aare tio pr n ocessin [25]. Despite g tech nology the high processing technology of optical amplifiers of [5 5] optic . Reg ala am rding plif sy ierstems s [55]‐.on Reg than ‐chaip rd in ing (SOC) InP sy an stems com d sinc p‐lexity, on e si‐ch licon ip InP (SOC) photonic plat form com s la p ha lexity, cks li aght good InP source platsform and aha mspl aif iers, good InP ‐based platforms provide SiN) Si N) [55] [5. Re 5]. gRe ard gaing rding wa wa velevSin eN) le gtnh [5 g properties, t5] h .properties, Regarding Si/S Si wa iO /Sv‐ iO an ele‐ dn an g InP d th InP ‐properties, ba‐sba eds pl eda pl tf Si oart/S m foisrOm are ‐ san are main d InP main ly‐ba use lys euse dd in pld anear in tf onear r‐ms‐ are mainly used in near‐ higher associated loss higher eshigher du assoc e to assoc eliate ecitate dri closs adl loss doping es du es edu to from e el toe cel curren treicctarli cdoping ta inj l doping ection from which from curren curren require t injt einj sction the ection incl which which usio require n require s the s the incl incl usiu on si on complementary complementary As InP metal is metal a dir ‐ox‐ma ide ect oxide turit ‐sem bandgap ‐sem yiconduc ofi conduc silicon tor material, tor photon (CMOS (CMOS ma ics, )‐turit c it oas )‐mpat pr c oan ympat esents of ind ible, silicon iible, r eal ctthe lowi ba al photon lowi ndga ng best ng itp ics, s available ma itin sas t tein e gra an ria te gra ind tl ion natiion tiv r gain to ect e be to ga ba be irun ndga performance, n isrun difficult p materia tol realize nahowever tive ga unider n is , difficult with to realize under a higher potential for volume scaling and integration [25]. Despite the high processing technology maturity with chips comprising maturity wi up th to chips hundreds comprising of components. up to hundreds Silicon of‐ ba components. sed photonic Sili proce con‐ba sses se dare ph otonic processes are infrinfr ared ared (NIR) (NIR) , wh , wh ile ile sili si con liinfr con nitride ared nitride (NIR) can ca operate ,n wh operate ile from sili from con visib nitride visib le to le ca NIR ton NIR operate [48] [4. 8] .from visible to NIR [48]. of optical amplifiersof [5 optic of 5] .optic Reg al am aard l am plif ingplif ier sysier stems [5s5] [5. ‐5] Reg on . ‐Reg ch ard ipaing rd (SOC) ing sy stems sy com stems ‐pon lexity, ‐‐on ch‐ip ch InP (SOC) ip (SOC) plat com form com plexity, ha plexity, s a InP good InP plat plat form form ha sha as good a good this platform. The platform this fe pla atures tform. are The in‐ between platform InP feat ures and ul are tra in‐low ‐between loss Si InP N techno and ul logies, tra‐low with los s SiN technologies, with in ain CMO a CMO S fou S fou ndry ndry wh wh ichic provides h provides we we ll‐controlled ll‐controlled and and more more rapi rapi d sc dal scab aliab lityil ito ty the to the fabrica fabrica tionti on complementary higher associated metalcomplementary ‐oxide‐losses semiconduc due metal tor to‐ox (CMOS electrical idema ‐sem turit )‐cioconduc mpat ydoping of siible, tor licon (CMOS al fr photon lowi omng )‐curr c ics, it ompat s as in ent t an eible, gra injection ind t ion al irlowi e ct to ba be ngndga which run its in p tma egra requir teria tion l na es totiv be the e run gainclusion in is difficult to realize under Ultra Ultra ‐low‐low los slo Si ssN Si pNlan plar an liar Ultra ght light w‐low aw vea vcirc loes scirc uits SiN uits (P pLC) l an (PLC) ar wit li ght wit h pw hrop apvrag op e circ at agioat uits nio los n (P slos LC) below s below wit 0. h1 p 0.dB r1op /dB m ag /hav at mio hav en los e s below 0.1 dB/m have maturity with chips ma comprising ma turit turit y wi y tup wi h chips t to h chips hundreds comprising comprising of components. up up to hundreds to hundreds Silicon of components. ‐of ba components. sed photonic Sili Scon proce ilicon ‐basses ‐sba eds are epdh otonic photonic proce proce ssessses are are some active detection elem some ents ac but tiv no e de light tect so ionurce elem soents lutions, but no and li ght med so iaurce l optic soalutions, l losses and (am ime d InP dia and l optic al losses (amid InP and environment environment [42] [4. 2] Never . Never thetles hesles , as s, com as com ponents ponents in siinli si con licon subs subs trate trate s ussua usllua y llha y vha e v higher e higher dimensions dimensions in a CMOS foundry in wh aic CMO h provides S foun dry wel l‐wh controlled ich thi provides s pla and tform. more wel lThe ‐ controlled rapi pla d tsc form al ab and ife li at tmore yures to the rapi are fa d inbrica ‐sc between altab ioinli ty InP to the and fa ulbrica tra‐low tion loss SiN technologies, with of optical ampliﬁers [55]. Regarding systems-on-chip (SOC) complexity, InP platform © © has a good © been been demonstrated demonstrated by by Un been Un ivers ivers idemonstrated ty iof ty Ca of liCa folirnia fo by rnia , Sa Un , nt Sa ivers ant Ba a irBa ty ba rrof baa (UCS rCa a li(UCS foB) rnia B) and , and Sa Li nt oaLi n iBa oXnri X brease r arear se (UCS ch ar ch [5B) 8] [5 ,and 8], LioniX research [58], complementary metal complementary ‐complementary oxide‐semiconduc metal metal tor‐ox (CMOS ‐ide oxide ‐sem‐sem )‐iconduc coimpat conduc tor ible, tor (CMOS al (CMOS lowi)ng‐co) it‐mpat cso mpat intible, egra ible, talion lowi al to lowi ng be ng it run s itins t eingra tegra tiontion to be to be run run than than in InP in InP an dan sinc d sinc e sieli si con Si licon N) photonic [5 photonic 5]. Resg la asrd ck laing lickght li wa ght so veurce Si so leN) nurce gs [5 tand h5] s properties, and . Re amg apl am rd ifpl iers, ing if Si iers, wa /S InP iO vInP ‐e‐ based le an‐nbased dg tInP h platforms properties, ‐ platforms based pl pra oSi t pr fvo/S irodm ivO ei sd‐ are an e d main InP‐ly ba use sedd pl inat near form‐s are mainly used in near‐ environment [42]. Never environment theless, as [4 com 2]. p Never onents th einles some sisli, con as ac com tiv subs ep de onents trtate ectsi ous n in ua e si lem llliycon ents ha vsubs ebut higher tr no ate lis ght dimensions usua sollurce y ha so v elutions, higher and dimensions medial optical losses (amid InP and orders orders of magnitude of magnitude lowe lowe orders r than r than of ot h magnitude ot erh reported er reported lowe pla pl trf oathan rtm fosrm . ot As s.h As ear re reported as re ulst,u high lt, pl high ‐ape tfo‐rpe rfomrrsma fo. rAs ma nc ean cpa re e sspa suilvst,es i high ve ‐performance passive maturity with chips comprising up to hundreds of components. Silicon-based photonic processes in a CMOS foundryin wh ain CMO ic ah CMO provides S fou S fou ndry we ndry lwh l‐controlled wh ichic provides h provides and we more we ll‐controlled l lrapi ‐controlled d sc aland ab and i limore ty more to rapi the rapi dfa brica sc dal scab taliioab ln it yil ito ty the to the fabrica fabrica tionti on a hiag hi hegrh pot er pot enteiantl ia for l for volume infr volume ared scaling sc (NIR) aling and , wh and inile tegr in sitegr liacon tinfr ioant io nitride ared [2n5] [2 .(NIR) 5] Despite ca . Despite n ,operate wh the ile the si high from li con high pr visib nitride ocessin processin le to ca g NIR ntech goperate tech nology [48]nology . from visible to NIR [48]. than in InP and since than silicon in photonic InP and sinc s lack e si lilight con so Si photonic urce N) [5s 5] and .s Re la agck mapl rd light ifing iers, so wa urce InP vele‐sbased nand gth a properties, platforms mplifiers, pr InP Sio/Sv‐ibased iO de‐ an dplatforms InP‐base d pr pl ovaitdfoer ms are mainly used in near‐ components components are are en abled enabled bycomponents by thi sthi pla s pla tform, t form, are thou en thou abled gh gh electr by electr thi ical s ical pla pump tpump form, for thou for ga iga ngh is in electr una is una vical aivlaai bl pump laebl due e due for to ga th toe ith n is e unavailable due to the environment [42]. Never environment environment theless, as [4 com 2] [4. 2] Never p. onents Never thet les in he ssi les , lias scon , com as subs com ponents tr ponents ates in us si ua inli llsi con yli ha con subs v esubs higher trate trate s us dimensions sua usllua y llha y vha e v higher e higher dimensions dimensions mama turit turit y ar of ey si of complementary licon silicon photon photon ics,ics, as Ultra an as metal-oxide-semiconductor ‐an ind low ind ir eloct isr seba ct Si ndga ba N ndga plp an ma ar pUltra ma tlieght ria te‐lria w low na alv tiv na elo circ etiv s sga (CMOS)-compatible, eSi uits iga n N is i pn(P ldifficult an isLC) difficult ar wit light to h w p realize toarop v realize e ag circ at unuits io allowing der un n der los (PLC) s below wit itsh 0. integration p1r op dBag /mat hav ion elos to s below be 0.1 dB/m have infrared (NIR), while silicon nitride can operate from visible to NIR [48]. a higher potential fora volume higher pot scaling entia and l for in volume tegratio scnaling [25]. and Despite integr the at io high n [2 pr 5]ocessin . Despite g tech the nology high pr ocessing technology insu insu latilng ati ng nat nat ureu rofe of SiN/Si SiN/Si insu O glas O la tiglas ng s WG snat WG su r[e55] s of [55] . Howe Si. N/Si Howe ver, O ver, glas the sthe de WG mo des mo ns [55] tns ra. ttiHowe roanti oof n ver, of optic optic the al gai ade l mo gai n usin ns n tusin rga tiogn of optical gain using © © than in InP and sincethan sithan licon in InP in photonic InP an dan sinc ds la sinc eck sie lili si con ght licon photonic source photonic s and s las ck a la m lick pl ght ifliiers, ght source so InP urce s‐ based and s and am platforms aplmifpl iers, ifiers, InP pr InP o‐based vid‐based e platforms platforms pro pr viodvei de been demonstrated by Un been ivers demonstrated ity of California by , Un Sant ivers a Ba ityrb of ar aCa (UCS lifornia B) and , Sa nt Liao nBa iXrb are rase (UCS arch B) [5 8] and , LioniX research [58], thisthi pla s pla tform. tform. The The pla pla tform tform feat feures atures are are in‐ between in‐between InP InP and and ultra ul‐tra low‐low los slos SisN Si techno N techno logies, logies, with with maturit run y of in silicon a CMOS photon maturit foundry ics,y as of an silicon ind which i rphoton ect ba pndga rics, ovides as p ma an Ultra tind well-contr eria‐low irl ena ct lo tiv ba ssendga Si ga olled Nin pp is ma lan difficult and tar eria light lmor na to wtiv arealize evee rapid ga circ in un uits is scalability der difficult (P LC) wit to realize h to prop the un agat der fabrication io n loss below 0.1 dB/m have erbium erbium ‐dope ‐dope d WG d WG distrib distrib erbium uteudte Br d‐ ag dope Brgag rg d eflec rWG eflec tor distrib tor (D BR (Du)BR te and )d and Br distrib ag distrib g ru eflec teudte tor feedb d feedb (DBR ack)a c(DFB) and k (DFB) distrib laser lau ser arrays te darrays feedb ack (DFB) laser arrays a higher potential for a hi volume ag hi hegrh pot e rsc pot ealing nteiantl ia and for l for volume in tegr volume a tsc ioaling n sc [2 aling 5] and . Despite and integr in tegr the atio ahigh ntio [2n5] pr [2. 5] ocessin Despite . Despite g the tech the high nology high pr ocessin processin g tech g tech nology nology some some activ acetiv dee tde ecttieoctni oelnem eorders lem entsents but of but no magnitude no light light source so lowe urce so orders rlutions, so than lutions, of ot and magnitude he and r me reported me diadl ioptic lowe al pl optic aart lf than losses oarlm losses sot . (am As he (am r iadreported re InP ids uInP land t, high and pla t‐fpe orrm fosr.ma Asn cae re pa suslsti,v high e ‐performance passive this platform. The plathi tform s pla fe tform. atures The are pla in‐tbetween formbeen feat InP ures demonstrated and are ul intra ‐between ‐low by los Un InP sivers Si and Ni ty techno uloftra Ca‐logies, low lifo rnia los with s, Si SaN nt techno a Barblogies, ara (UCS with B) and LioniX research [58], environment [42]. Nevertheless, as components in silicon substrates usually have higher dimensions maturity of silicon photon mama turit turit ics, y of as y si of an licon si ind licon photon ire ct photon baics, ndga ics, asp an as ma an ind te ria ind irelct ina r eba ct tiv ndga ba e ga ndga p in ma is p difficult ma teria telria nal tiv to na realize etiv gae iga n is iun n difficult is der difficult to realize to realize un der under SiN) Si N) [55] [5. Re 5]. gRe ard gaing rding wa wa vecomponents levn ele gtnhg properties, th properties, are en Si abled /S SiiO /S ‐ components by iO an ‐ thi dan InP sd pla InP ‐ba tform, ‐s ba eare ds pl e en dthou a pl tabled foartgh m fosr by electr m are s thi are main ical s main pla ly pump t form, use ly use d for in thou d near in ga gh near in‐ is electr ‐unaical vaila pump ble due for to ga thine is unavailable due to the some active detectionsome elem ents activ but e de no tect liight on eso lem urce ents orders so but lutions, no of magnitude li and ght so me urce dia lowe lso optic lutions, r athan l losses and oth (am me er d reported id ia lInP optic and apl l losses atform (am s. As id InP a re and sult , high‐performance passive Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: Appl. Sci. 2020, 10, x; doi: www.mdpi.com/ www.mdpi.com/ journal/applsci journal/applsci www.mdpi.com/ journal/applsci than in InP and since silicon photonics lack light sources and ampliﬁers, InP-based platforms provide this platform. The pla thitsthi form pla s pla t form. fetat form. ures The The are pla in pla tform ‐between tform feat feures at InP ures are and are in ul‐ between in tra‐between ‐low los InP s InP Si and N and techno ultra ul‐tra low logies, ‐low los slos with SisN Si techno N techno logies, logies, with with infrinfr ared ared (NIR) (NIR) , wh , wh ile ile sili si con insu licon nitride la tnitride ing ca nat n ca uoperate rne operate of Si from N/Si insu from O visib l aglas visib ting les to lnat WG e NIR tou srNIR e [[4 55] of8] [4 ..Si 8] Howe N/Si . Over, glas the s WG demo s [55] nst.r aHowe tion of ver, optic thea lde gai mo n ns usin tratgi on of optical gain using SiN) [55]. Regarding wa SiN) ve le [5n5]g.t Re h properties, garding wa Siv/S eleiO ncomponents g‐ tan h d properties, InP‐ba are sed Si en pl/Sabled aitO fo‐ ran m by sd are InP thi main s‐ ba pla setly d form, pl use atd fthou o in rm near sgh are ‐electr mainical ly use pump d in for near ga‐ in is unavailable due to the some active detectiosome n esome lem acents tiv acetiv but dee t de no ectt ielioct ght ni oe lnso em eurce lem entsents so but lutions, but no no light and light so me urce sodurce i aso l optic lutions, solutions, al losses and and me (am me diiaddl ioptic InP al optic and al losses a l losses (am (am id InP id InP and and Ultra a Ultra higher ‐low‐low los slo potential Si ssN Si pNerbium lan plar an for liarght ‐dope light volume wadw v eWG a vcirc e circ uits distrib scaling uits (P erbium LC) u (P teLC) d and wit Br‐dope wit h ag integration pghr dop rp eWG rflec ag opat ag tor distrib ioat n(D io los nBR [ 25 uslos te )below ]. sd and below Br Despite ag distrib 0.g1 0.dB re1flec u/dB m te the d tor /hav m feedb high hav (D e BR ea c)pr k and (DFB) ocessing distrib laser ute technolog arrays d feedb ack y(DFB) laser arrays insulating nature of SiN/SiO glass WGs [55]. However, the demonstration of optical gain using infrared (NIR), while infr silicon ared nitride (NIR) ,ca wh n ile operate silicon from nitride visib calen to operate NIR [4 from 8]. visible to NIR [48]. SiN) [55]. RegardingSi wa N) Siv N) [5 ele5] [5 n.g Re 5]th. g Re properties, ard gaing rding wa wa v Siele /S vn eiO le gt‐ nhan g properties, th d properties, InP‐ba©se Si d © /S pl SiiaO /S tf‐ ioO an rm‐ dan s InP are d InP ‐ ba main ‐sba edly s pl e duse a pl tfodart in m fo srnear m are s are ‐main main ly use ly use d ind near in near ‐ ‐ been been demonstrated demonstrated by by Un Un ivers ivers ity iof ty Ca of liCa folirnia fornia , Sa, nt Saant Ba a rBa barrbaa (UCS ra (UCS B) B) and and Li oLi nioXni Xrese rearsech ar ch [58] [5, 8], Ultra maturity ‐low losof s Sisilicon N plUltra anarphotonics, ‐lilow ght w loasvse Si circ N as puits lan anerbium ar (P indir LC) light ‐wit ect w dope ahv bandgap epd rcirc op WG ag uits at distrib io (Pn material LC) los u te swit d below hBr pag native rop 0. g 1ag r edB flec atio /gain m tor n hav los (D is se BR below di ) and cult 0. 1distrib dB to/m ru ealize hav ted efeedb under ack (DFB) laser arrays infr Appl. ared Sci. (NIR) 2020, ,10 wh , x;ile doi: infr si infr liared con ared Appl. (NIR) nitride (NIR) Sci., 2020 wh ca, nwh ile , 10 operate ile si , x;li doi: si con licon from nitride nitride visib can lca e operate to n operate NIR [4 from 8] from . visib visib www.mdpi.com/ le to le NIR to NIR [48] j[4 ournal/applsci . 8]. www.mdpi.com/ journal/applsci orders orders of magnitude of magnitude lowe lowe r than r than oth ot erh reported er reported pla pl tfoartm fosrm . As s. As a re as re ulst,u high lt, high ‐pe‐rpe forrma forma ncen cpa e spa sivsesi ve © © been demonstrated by been Un demonstrated iversity of Ca liby fo rnia Univers , Santity a Ba of rCa barliaf o(UCS rnia, B) Sa and nta Ba LirobnairXa (UCS resear B)ch and [58] Li , oniX research [58], this platform. The platform features are in-between InP and ultra-low loss SiN technologies, with Ultra‐low loss SiN pUltra lanUltra ar‐ low light ‐low lows aslov Si ses N circ Si pNluits an plar an (P liar LC) ght light w wit aw vhea p vcirc erop circ uits aguits at (PioLC) n (P los LC) wit s below wit h phrop p r0.ag op 1 at ag dB ioat /nm io los n hav slos below es below 0.1 0.dB 1 /dB m /hav m hav e e components components are are en abled enabled by by thi sthi pla s pla tform, tform, thou thou gh gh electr electr icalical pump pump for for ga iga n is in una is una vaivlaaibllaebl due e due to th toe th e Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci orders of magnitudeorders lower of than magnitude other reported lower pl than atf oot rm hes.r As reported a resu lpl t, ahigh tform‐pe s. rAs for ma a re nscu e ltpa , high ssive‐ performance passive © © © some activebeen detection demonstrated elements been bybeen Un demonstrated but ivers demonstrated no ity light of Ca by lisour f oby Un rnia Un ivers ce , ivers Sa solutions, int tya iof tyBa Ca of rb aliCa rfaoli and rnia (UCS fornia , Sa medial B) , nt Sa and ant Ba a Li rBa optical boanrrbiaXa (UCS ra re (UCS se losses B) ar ch B) and [5 and (amid 8] Li, o Li nioXni InP Xrese rearsech ar ch [58] [5, 8], insu insu latilng ati ng nat nat ureu rofe of SiN/Si SiN/Si O glas O glas s WG s WG s [55] s [55] . Howe . Howe ver,ver, the the de mo demo nstns rattiroanti oof n of optic optic al gai al gai n usin n usin g g components are enabled components by this pla are tform, enabled thou by gh thi electr s plaical tform, pump thou for gh ga electr in is ical una pump vailabl for e due ga ito n isth una e vailable due to the orders of magnitude orders lowe orders rof than magnitude of magnitude other reported lowe lowe r than rpl than at fot orh m ot ersh .reported eAs r reported a res u pllta ,pl t fhigh oartm fo‐srpe m . As sr.f o As rama re an s re ucelst ,upa high lt,s shigh iv‐pe e ‐rpe forrma forma ncen cpa e spa sivsesi ve erbium erbium ‐and dope ‐dope d SiN) WG d WG [distrib 55 distrib ]. Regar uteudte Br d ding ag Brgag rwavelength geflec reflec tor tor (D BR (D)BR pr and ) operties, and distrib distrib uteSi udte /feedb SiO- d feedb aand cka c(DFB) k InP-based (DFB) laser laser arrays arrays platforms are mainly used in insulating nature of insu SiN/Si latO ing glas natsu WG re ofs [Si 55] N/Si . Howe O glas ver, s WG thes de [55] mo. ns Howe tratiover, n of the optic demo al gai nstnra usin tion gof optical gain using components are enabled components components by this pla are are t en form, abled enabled thou bygh by thi electr sthi pla s pla tical form, tform, pump thou thou for gh gh ga electr ielectr n isical una ical pump v ai pump la bl for e for due ga iga n to is in th una ise una vaivlaaibllaebl due e due to th toe th e erbium‐doped WG distrib erbium ute‐dope d Brdag WG g re flec distrib tor u(D teBR d Br ) and agg distrib reflector ute (D d BR feedb ) and ack distrib (DFB) ula teser d feedb arrays ac k (DFB) laser arrays near-infrared (NIR), while silicon nitride can operate from visible to NIR [48]. Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: insulating nature ofinsu Siinsu N/Si latilng aOt i ng glas nat nat usr eWG u rofe sof Si [55] N/Si SiN/Si . O Howe glas Owww.mdpi.com/ glas ver, swww.mdpi.com/ WG s the WG s [de 55] s [mo journal/applsci 55] . Howe jns ournal/applsci . Howe tratver, ionver, of the optic the de mo de almo ns gai tns rn att irusin oanti oof gn of optic optic al gai al gai n usin n usin g g Ultra-low loss SiN planar lightwave circuits (PLC) with propagation loss below 0.1 dB/m have erbium‐doped WG erbium distrib erbium u‐dope ted‐dope Br d ag WG dg WG r edistrib flec distrib toru te (DudBR te Br d) ag Br and gag r gedistrib flec reflec toru tor te (Dd BR (D feedb )BR and ) aand c kdistrib (DFB) distrib u telaudser te feedb d arrays feedb acka c(DFB) k (DFB) laser laser arrays arrays Appl. Sci. 2020, 10, x; doi: Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci www.mdpi.com/ journal/applsci been demonstrated by University of California, Santa Barbara (UCSB) and LioniX research [58], orders Appl. Sci. 2020, 10, x; doi: Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: www.mdpi.com/journal/applsci www.mdpi.com/ www.mdpi.com/ journal/applsci journal/applsci of magnitude lower than other reported platforms. As a result, high-performance passive components are enabled by this platform, though electrical pump for gain is unavailable due to the insulating nature of SiN/SiO glass WGs [55]. However, the demonstration of optical gain using erbium-doped WG distributed Bragg reﬂector (DBR) and distributed feedback (DFB) laser arrays integrated within ultra-low loss SiN [59] shows potential for the platform’s active component integration. Silicon is Appl. Sci. 2020, 10, 4024 5 of 19 among the most mature photonic integrated platforms, leveraging from existing technologies like silicon electronics and micro electro-mechanical systems (MEMS) industries and enabling foundry services robustness and product commercialization. InP platforms can be seen as one of the options that stem out from the alternatives since it integrates monolithically high-quality lasers, receivers, ampliﬁers, and passives [33,35,60]. Si and SiN are more prone to be used for passive and ﬁltering components, with the need to recur to hybrid packaging [61] when incorporating lasing and gain. Some of the available foundries/players under the InP, Si, and SiN integrated technologies are described below: i. InP: Fraunhofer Heinrich-Hertz-Institut–Fraunhofer HHI (Berlin, Germany); Smart Photonics (Eindhoven, The Netherlands); Global Communication Semiconductors, LLC (Torrance, CA, USA); and fabless solution providers, e.g., the INTENGENT III/V Photonics Company (Ontario, Canada), VLC Photonics (Valencia, Spain), Bright Photonics (Eindhoven, The Netherlands), and PICadvanced (Aveiro, Portugal); ii. Si: Acacia Communications, Inc. (Maynard, MA, USA); Luxtera Inc. (Carlsbad, CA, USA); Intel Corporation (Mountain View, CA, USA); Cisco Systems Inc. (San José, CA, USA); Mellanox Technologies (Sunnyvale, CA, Israel/USA); Finisar Corporation (Sunnyvale, CA, USA); Hamamatsu Photonics K.K. (Shimokanzo, Iwata City, Japan); International Business Machines Corporation (IBM, Armonk, NY, USA); GlobalFoundries Inc. (Santa Clara, CA, USA); NeoPhotonics Corporation (San José, CA, USA); Oclaro Inc. (San José, CA, USA); CompoundTek (Singapore); and the American Institute for Manufacturing Integrated Photonics (AIM Photonics, Albany, NY, USA); iii. SiN: Ligentec (Ecublens, Switzerland); Lionix (Enschede, Netherlands); Institute of Microelectronics of Barcelona IBM-CNM, “Silicon Nitride Technology,” (Barcelona, Spain). Furthermore, several reference institutions with state-of-the-art research in integrated photonics contributed to the technology developments, namely the University of California, Santa Barbara (UCSB) with the available California NanoSystems Institute (CNSI) Nanostructure Cleanroom Facility (NCF); the Kavli Nanoscience Institute (KNI) laboratory at the California Institute of Technology (Caltech), the Fraunhofer Heinrich-Hertz-Institut (HHI); the ePIXfab, INTEC department–Photonics Research Group at Ghent University; the Nanophotonics Technology Center (NTC) at the Universitat Politècnica de València (UPV); etc. Beyond the generic availability of design/fabrication, it is necessary to further develop and invest in packaging technologies to reach simpler, standardized, and ecient approaches, thus making it accessible for users and/or companies. Currently, the technology-associated limitations are mainly related to the lack of general design rules that ease the encapsulation and allow successful testing of the manufactured photonic chips [6]. This last integration step (packaging) is imperative for labs and companies that are trying to enter/grow in the integrated photonic technology and market. A consortium model aiming to establish PIC packaging processes under European commission Horizon 2020 funding is currently being conducted [62] to facilitate the development of this PIC technologic branch through the development of standard technologies to provide access to aordable, mature, and scalable packaging solutions [63]. 3. Passive Optical Networks From the 80s onwards, PONs [64] have evolved greatly with the simpliﬁcation of network management, pushing network trac multiplexing closer to the user with the minimization of stranded capacity [65]. Since then, PON architecture remained one of the most predominant access network options. PONs have been under mass deployment, with more than 900 million broadband subscribers reported in 2017 [66]. This infrastructure has subsidized PONs establishment relevance for a long time [65]. PON basic technology includes: (i) the equipment at the edge of Metro/Access, i.e., the optical line terminal (OLT); (ii) the user end terminal, i.e., the optical networks unit (ONU); and (iii) the optical Appl. Sci. 2020, 10, x 6 of 19 Appl. Sci. 2020, 10, 4024 6 of 19 i.e., the optical line terminal (OLT); (ii) the user end terminal, i.e., the optical networks unit (ONU); distribution and (iii) the opti network cal distri (ODN) butithat on network ( connectsO the DN) tha OLT to t connects the the ONUs (see OLT to the ONUs (see optica optical access network branch l access in Figur network bran e 1). ch in Figure 1). PON PON emer emerg ginging technic technical ra equir l reements quiremen arets ar verye ver unique, y un being ique, being the the surviving solutions surviving particularly solutions sensitive particulato rly price sensit and ive t related o price and re costs, both lated capital costs, bot expense h capit (CAPEX) al expen and se (CAPEX operating ) and expense operat (OPEX) ing expense [67]. Nevertheless, (OPEX) [67]. Neverthele PON posesss, several PON pose advantages, s severasuch l advant as the agesimpliﬁcation s, such as the s of impli infrastr ficat uctur ion oe f infr management, astructure as mana it isgement based ,on as it a fully is base passive d on a ODN fully p [65 ass ].ive OD A completely N [65]. A complet passive ODN ely pamade ssive O ofDburied N made o glass f buried with glass with cabinets to ensure connections and splitting anticipates long-term expectation of 20 years cabinets to ensure connections and splitting anticipates long-term expectation of 20 years or even mor or even more e. Its architectur . Its architect e derives ure der from ives thefr technical om the te appr chnica oach l approa to be deployed ch to be deployed a at the PON t the PON ends. ends. Several Several technologies are competing to be implemented in the access space, with the most common technologies are competing to be implemented in the access space, with the most common based on based a passive on a passive splitter splitter tree topology tree topology . The OL . The OLT T (interface (interface from the from operator the operator side) feeds side) one feedtr s one unk trun ﬁber k fiber that splits into several arms, each arm is divided into another set of arms, and so on, generating that splits into several arms, each arm is divided into another set of arms, and so on, generating a tr a tree ee strstructure, wh ucture, which, ich, d depending epending on on the the technolog technology,ycan , can range from low range from low (8 (8 to to 16) 16) to hi to high gh (64 (64 to to 256) 256) terminal counts. terminal counts. Optical Optical access access stan standar dardization dization h hasas ev evolved olved r rapidly apidly, w , with ith IE IEEE EE 802.3 802.3 and and International International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) Q2 groups playing Telecommunication Union - Telecommunication Standardization Sector (ITU-T) Q2 groups playing dominant rdominant ro oles in the prles in ocess the proc [68]. Theess most [68]. The common most co technologies mmon tar echnologi e Gbit-capable es are G PON bit-c (GPON) apable PON and Ethernet (GPON) and Ethernet PON (EPON) [69], typically supporting up to 64 users in the same OLT trunk fiber. At PON (EPON) [69], typically supporting up to 64 users in the same OLT trunk ﬁber. At the end users the end users lays an ONU (the user terminal for the fiber). For each end technology, a different ONU lays an ONU (the user terminal for the ﬁber). For each end technology, a dierent ONU is typically is typically required. Currently, it is very common to have combined interfaces that allow multiple required. Currently, it is very common to have combined interfaces that allow multiple technologies to technologies to be handled within the same device, the so-called “combo” solutions [70]. Figure 2 be handled within the same device, the so-called “combo” solutions [70]. Figure 2 illustrates some of illustrates some of the main existing characteristics of the ITU-T related technologies. the main existing characteristics of the ITU-T related technologies. Figure 2. Recent International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Figure 2. Recent International Telecommunication Union-Telecommunication Standardization Sector (ITU- based optical technologies standards evolution. T) based optical technologies standards evolution. Some operators are engaging in the 10 G-capable symmetric PON (XGSPON) and 10 G-capable Some operators are engaging in the 10 G-capable symmetric PON (XGSPON) and 10 G-capable EPON (10GEPON). Moreover, a number of more aggressive and disrupting operators are investing in EPON (10GEPON). Moreover, a number of more aggressive and disrupting operators are investing deploying NGPON2, merging the traditional concepts of time-multiplexed single-channel per direction in deploying NGPON2, merging the traditional concepts of time-multiplexed single-channel per PON combined with wavelength division multiplexing (WDM) [71]. This approach allows for further direction PON combined with wavelength division multiplexing (WDM) [71]. This approach allows developing network ﬂexibility concepts and the smooth evolution of oered data rates [70], i.e., up to for further developing network flexibility concepts and the smooth evolution of offered data rates 40 or 80 Gbit/s aggregated rates. An example of a PON with coexisting technologies supported by the [70], i.e., up to 40 or 80 Gbit/s aggregated rates. An example of a PON with coexisting technologies current ITU-T standard series [71] is depicted in Figure 3. supported by the current ITU-T standard series [71] is depicted in Figure 3. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 7 of 19 Appl. Sci. 2020, 10, x 7 of 19 Figure 3. Diagram describing a PON architecture with three coexisting technologies. Figure 3. Diagram describing a PON architecture with three coexisting technologies. New trends are emerging and triggering discussions in the fora, mostly focused on either an New trends are emerging and triggering discussions in the fora, mostly focused on either an architecture innovation, like the Super-PON, which is on the standardization path in IEEE and ITU-T, architecture innovation, like the Super-PON, which is on the standardization path in IEEE and ITU-T, or on the eorts to move to high-speed PON (HSP) towards higher data rate technologies, such as or on the efforts to move to high-speed PON (HSP) towards higher data rate technologies, such as 25 25 Gbit/s and 50 Gbit/s [72]. Gbit/s and 50 Gbit/s [72]. Current low-cost laser and receiver technologies face some challenges to deliver higher data rates Current low-cost laser and receiver technologies face some challenges to deliver higher data without increasing packaging or electronic costs. Namely, the current methods used for interconnecting rates without increasing packaging or electronic costs. Namely, the current methods used for devices with drivers and boards are typically based on wire bonding and ﬂex printed circuit board interconnecting devices with drivers and boards are typically based on wire bonding and flex printed (f-PCB). The latter has limited high-frequency performance and tough mechanical design challenges to circuit board (f-PCB). The latter has limited high-frequency performance and tough mechanical meet the right soldering robustness, with an expected high impact on the electrical performance of design challenges to meet the right soldering robustness, with an expected high impact on the assemblies and sub-assemblies. electrical performance of assemblies and sub-assemblies. Among PON future technologies, high interest has been given to the research/development Among PON future technologies, high interest has been given to the research/development of of long-reach/high-speed transmission, where optical ampliﬁers, electrical domain digital signal long-reach/high-speed transmission, where optical amplifiers, electrical domain digital signal processing (DSP) to enhance the link budget, and/or transceiver technology are predicted to be key processing (DSP) to enhance the link budget, and/or transceiver technology are predicted to be key factors [73]. Generically, for N subscribers, a PON requires a single transceiver at the central oce, factors [73]. Generically, for N subscribers, a PON requires a single transceiver at the central office, which results in a total of N + 1 transceivers overall [67]. For NGPON2, the OLT transceiver should which results in a total of N + 1 transceivers overall [67]. For NGPON2, the OLT transceiver should contain four transmitters (Tx) and four receivers (Rx) with an internal wavelength mux/demux optical contain four transmitters (Tx) and four receivers (Rx) with an internal wavelength mux/demux module. In the ONU, a 10 Gbps transceiver with both tunable Tx and Rx is required [70]. By employing optical module. In the ONU, a 10 Gbps transceiver with both tunable Tx and Rx is required [70]. By wavelength-selective branching devices in ODN, losses can be diminished, nevertheless at the cost of employing wavelength-selective branching devices in ODN, losses can be diminished, nevertheless wavelength transparency waiving. Furthermore, by increasing the number of wavelengths, a high at the cost of wavelength transparency waiving. Furthermore, by increasing the number of number of transceivers at the OLT is expected, raising power consumption, port density, and cost [73]. wavelengths, a high number of transceivers at the OLT is expected, raising power consumption, port Similar to what happened with integrated circuits’ contribution to electronics development, PIC density, and cost [73]. Similar to what happened with integrated circuits’ contribution to electronics technology is one of the most attractive solutions to tackle the referred issues. Thus, integrated development, PIC technology is one of the most attractive solutions to tackle the referred issues. Thus, photonics can have a key role in the deployment of more ﬂexible PON networks, as they can integrated photonics can have a key role in the deployment of more flexible PON networks, as they incorporate dierent optical components with various functions in a potentially cost-eective way can incorporate different optical components with various functions in a potentially cost-effective without increasing the assembly process complexity [7,8]. way without increasing the assembly process complexity [7,8]. 4. PIC for Next-Generation PON 4. PIC for Next-Generation PON Photonic integration can provide advantageous solutions to PON by limiting the number of Photonic integration can provide advantageous solutions to PON by limiting the number of optical and electrical interconnections and thus overcoming some of the bulk packaging restrictions, optical and electrical interconnections and thus overcoming some of the bulk packaging restrictions, such as bandwidth, power dissipation, and cost [65,74]. Nevertheless, complexities can arise with such as bandwidth, power dissipation, and cost [65,74]. Nevertheless, complexities can arise with the the use of this technology, namely ﬁber interfacing diculties, number of electrical connections, use of this technology, namely fiber interfacing difficulties, number of electrical connections, and and possible high-density of components per device, which demands a high power dissipation possible high-density of components per device, which demands a high power dissipation Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 8 of 19 Appl. Sci. 2020, 10, x 8 of 19 management [70]. The development of PIC solutions for access networks has already been reported, management [70]. The development of PIC solutions for access networks has already been reported, e.g., with an InP monolithic transceiver PIC for NGPON2 applications [75], the proposed layout e.g., with an InP monolithic transceiver PIC for NGPON2 applications [75], the proposed layout has has the potential to be implemented as an OLT and with ﬁlter redesigning as an ONU. A photonic the potential to be implemented as an OLT and with filter redesigning as an ONU. A photonic integrated apparatus for tunable multi-wavelength transmission was patented in 2017 [76], the time and integrated apparatus for tunable multi-wavelength transmission was patented in 2017 [76], the time wavelength division multiplexing (TWDM)-PON transmitter system proposed is tailored to support and wavelength division multiplexing (TWDM)-PON transmitter system proposed is tailored to current and next-generation access technologies. Monolithically integrated dual electro-absorption support current and next-generation access technologies. Monolithically integrated dual electro- modulated laser (DEML) solutions with application in PON and claims of small footprint and low-cost absorption modulated laser (DEML) solutions with application in PON and claims of small footprint external-modulator-free Tx can likewise be found in the literature [46]. More recently, a theoretical and low-cost external-modulator-free Tx can likewise be found in the literature [46]. More recently, analysis of polarization-independent receiver (PI-Rx) based in a 3 3 symmetric coupler with a theoretical analysis of polarization-independent receiver (PI-Rx) based in a 3 × 3 symmetric coupler applications for access networks was reported [77]. An example of a BOSA transceiver solution with applications for access networks was reported [77]. An example of a BOSA transceiver solution implemented in ONU side of a PON with a representation of a 10 Gbit small form-factor pluggable implemented in ONU side of a PON with a representation of a 10 Gbit small form-factor pluggable (XFP) evolution from discrete optical component to PIC technology is depicted in Figure 4. (XFP) evolution from discrete optical component to PIC technology is depicted in Figure 4. Figure 4. Diagram describing the evolution of photonic integrated circuits (PIC) for PON, speciﬁcally Figure 4. Diagram describing the evolution of photonic integrated circuits (PIC) for PON, specifically the implementation of a bi-directional optical sub assembly (BOSA) transceiver solution in PON the implementation of a bi-directional optical sub assembly (BOSA) transceiver solution in PON ONU ONU side, i.e., XFP with discrete optical component packaging (current PON products) and with PIC side, i.e., XFP with discrete optical component packaging (current PON products) and with PIC approach technology (next-generation products). approach technology (next-generation products). Several steps and guidelines for PIC implementation in PON are addressed in this section, along Several steps and guidelines for PIC implementation in PON are addressed in this section, along with the technology impact on reach (Section 4.1); the adopted band and wavelength selection criteria with the technology impact on reach (Section 4.1); the adopted band and wavelength selection criteria (Section 4.2); the integrated photonics packaging (Section 4.3); and the issues related with the control (Section 4.2); the integrated photonics packaging (Section 4.3); and the issues related with the control of complexity, power dissipation, and form factor (Section 4.4). of complexity, power dissipation, and form factor (Section 4.4). 4.1. Technology and Impact on Reach 4.1. Technology and Impact on Reach A major requirement for future PON technologies is the need for coexistence with current deployed A major requirement for future PON technologies is the need for coexistence with current PON systems [65,70]. For example, GEPON has already been deployed on a scale of millions, thus deployed PON systems [65,70]. For example, GEPON has already been deployed on a scale of any new PON technology must be capable of coexisting with this large base [65]. A PON framework millions, thus any new PON technology must be capable of coexisting with this large base [65]. A of three coexisting technologies, i.e., GPON, XGSPON, and NGPON2, is depicted in Figure 3 from PON framework of three coexisting technologies, i.e., GPON, XGSPON, and NGPON2, is depicted Section 3. Here, the same ODN conveys several technologies allowing the migration and evolution of in Figure 3 from Section 3. Here, the same ODN conveys several technologies allowing the migration the network without forcing clients to shift technology. This approach leads to reduced CAPEX and and evolution of the network without forcing clients to shift technology. This approach leads to supports pay-as-you-go approaches since it allows keeping using the legacy operating technologies reduced CAPEX and supports pay-as-you-go approaches since it allows keeping using the legacy untouched, opening an evolution path for other technologies to follow [7]. operating technologies untouched, opening an evolution path for other technologies to follow [7]. The longer the required reach, the more dicult it gets to ﬁnd the laser technology able to support The longer the required reach, the more difficult it gets to find the laser technology able to the demanded distance. As an example, for a 10 Gbit/s with a short-reach like 10 km, almost all support the demanded distance. As an example, for a 10 Gbit/s with a short-reach like 10 km, almost standard laser technologies can be used. If addressing the 2.5 Gbit/s market, a medium-reach like all standard laser technologies can be used. If addressing the 2.5 Gbit/s market, a medium-reach like 40 km and further are still not a major limiting factor to the laser choice. However, if requiring a 40 km and further are still not a major limiting factor to the laser choice. However, if requiring a common GPON ODN which ranges 20 km including splitters and the handling of dispersion penalty, common GPON ODN which ranges 20 km including splitters and the handling of dispersion penalty, it poses challenges to most of the 10 Gbit/s solutions. To achieve that target, the laser must be designed it poses challenges to most of the 10 Gbit/s solutions. To achieve that target, the laser must be designed to perform a low penalty, typically bellow 1 dB to 2 dB, at the required distance. To meet those specs, to perform a low penalty, typically bellow 1 dB to 2 dB, at the required distance. To meet those specs, Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 9 of 19 Appl. Sci. 2020, 10, x 9 of 19 specially designed direct modulated lasers (DML) are required, or even the migration to external specially designed direct modulated lasers (DML) are required, or even the migration to external modulated laser (EML) technologies [7,8]. modulated laser (EML) technologies [7,8]. Furthermore, in most recent standards, ODN should support 128 users and a 20 km reach. These Furthermore, in most recent standards, ODN should support 128 users and a 20 km reach. These requirements have the penalty of a 21-dB attenuation stemming from splitting losses plus 4 dB due requirements have the penalty of a 21-dB attenuation stemming from splitting losses plus 4 dB due to fiber losses and around 3 dB insertion loss (IL) of co-existence elements, leading to an ODN total to ﬁber losses and around 3 dB insertion loss (IL) of co-existence elements, leading to an ODN total loss greater than 28 dB. If we target 10 Gbit/s with avalanche photodiode (APD) receivers allowing a loss greater than 28 dB. If we target 10 Gbit/s with avalanche photodiode (APD) receivers allowing −26 dBm to −30 dBm receiver sensitivity, it leads to a minimum laser output power of 4 dBm. A a 26 dBm to 30 dBm receiver sensitivity, it leads to a minimum laser output power of 4 dBm. possible alternative to overcome this requirement is to use an amplification stage in the path which A possible alternative to overcome this requirement is to use an ampliﬁcation stage in the path which removes the passivity of the ODN, however, it reduces the potential usage of EML-based solutions removes the passivity of the ODN, however, it reduces the potential usage of EML-based solutions since its typical power does not exceed the 0 dBm. since its typical power does not exceed the 0 dBm. 4.2. Band and Wavelength Selection Criteria 4.2. Band and Wavelength Selection Criteria To improve reach, several techniques are available and many stem from the laser (e.g., low To improve reach, several techniques are available and many stem from the laser (e.g., low dynamic dynamic chirp or frequency modulation efficiency), laser driving (pre-chirping or equalization), and chirp or frequency modulation eciency), laser driving (pre-chirping or equalization), and wavelength wavelength selection (e.g., O-band instead of C and L bands) [2,70]. O-Band has been used for selection (e.g., O-band instead of C and L bands) [2,70]. O-Band has been used for supporting upstream supporting upstream and was adopted by some of the standards like GPON and XGSPON. O-band, and was adopted by some of the standards like GPON and XGSPON. O-band, besides presenting besides presenting low or null dispersion, only introduces slightly higher attenuation than C-band. low or null dispersion, only introduces slightly higher attenuation than C-band. These facts greatly These facts greatly reduce the pressure on the O-band components industry, impacting positively on reduce the pressure on the O-band components industry, impacting positively on the cost and yield. the cost and yield. The current spectral allocation from O to L-band is depicted in Figure 5. The current spectral allocation from O to L-band is depicted in Figure 5. Figure 5. Spectrum allocation in ITU-T based standards for PON and trends for channel spacing. Figure 5. Spectrum allocation in ITU-T based standards for PON and trends for channel spacing. If a certain technology is deployed in greenﬁeld, a viable approach is to implement standards that If a certain technology is deployed in greenfield, a viable approach is to implement standards are not back-compatible, i.e., does not support legacy technologies. In this scenario, the use of O-band that are not back-compatible, i.e., does not support legacy technologies. In this scenario, the use of is an option for future PON. The drawback of this option is that the operator has to guarantee that the O-band is an option for future PON. The drawback of this option is that the operator has to guarantee PON evolution is on top of the new technology deployed. To cope with the maturity of the technology that the PON evolution is on top of the new technology deployed. To cope with the maturity of the and promote an easier and simpliﬁed entry into the market, initial PON technologies, like GPON, technology and promote an easier and simplified entry into the market, initial PON technologies, like were set in large bandwidths for better yield in laser choice with the upstream in the O-band [7,67]. GPON, were set in large bandwidths for better yield in laser choice with the upstream in the O-band The wavelength range of 20 nm to 30 nm per trac direction was reserved for this technology, which [7,67]. The wavelength range of 20 nm to 30 nm per traffic direction was reserved for this technology, allowed it to easily mature while still being competitive. This strategy proved to be solid since it which allowed it to easily mature while still being competitive. This strategy proved to be solid since resulted in the current GPON BOSA to sit in the few dollars range (as of today). it resulted in the current GPON BOSA to sit in the few dollars range (as of today). Furthermore, PON technology evolution can be achieved by data rate multiplication and more Furthermore, PON technology evolution can be achieved by data rate multiplication and more ecient use of the bands to promote further network evolution scenarios. Following this path, XGSPON efficient use of the bands to promote further network evolution scenarios. Following this path, Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 10 of 19 Appl. Sci. 2020, 10, x 10 of 19 standard was conceived with slightly smaller bandwidths for both upstream and downstream, which XGSPON standard was conceived with slightly smaller bandwidths for both upstream and resulted in a four-times higher bitrate than GPON [70,78]. downstream, which resulted in a four-times higher bitrate than GPON [70,78]. To further exploit the available bands (see Figure 5), dense wavelength division multiplexing To further exploit the available bands (see Figure 5), dense wavelength division multiplexing (DWDM) (DWDM) can can be a route t be a route tooexplor explore. Th e. Thee possibilit possibility y of of including including t tunability unability al allows lows network network active active transformation and management. Even considering the associated price to pay, it is an important transformation and management. Even considering the associated price to pay, it is an important solution solution under undethe r th re equir reqement uirement of tig of htly tigcontr htly cont olledrol lasers. led la Furthermor sers. Furthermore, w e, with this capability ith this cap , a pr abilit ojected y, a OPEX project re eduction d OPEX red is expected, uction is since expeclients cted, since can seamlessly clients can migrate seamles to sly any miof grat the e tavailable o any of t wavelengths he available while wavelen thegnetwork ths while gr the network grow ows smoothly and s s systematically moothly and .systematically. The The most most evident evident bar barrier rier for for the the spr spre ead ad of of this te this technology chnology i is s t the he laser laser and and rece receiver iver re requir quirement ements, s, which which sho should uld be at be attained tained un under der a pot a potential ential low cost low cost and sim and ult simultaneously aneously have a have high gr a high ade ograde f stabilit of y stability [7,8]. Figur [7,8 e]. 5 presents the Figure 5 presents bands for the bands the different for the dist eps o erent fsteps the netwo of thernet k evo work lution evolution, , i.e., from i.e., the from legacy the legacy technologies technologies wide bwide ands to the ne bands to xt ge the next neration t generation ighter bandw tighter bandwidths. idths. A A represent representation ation of of the the typi typical cal distribution distributi of on of lasers, lase stemming rs, stemmin fromga from a single wafer single wafe run, is depicted r run, is in depict Figur ed e 6in F A. Each igure of 6A the . E lasers ach of t collected he laser fr som colthe lectwafer ed from will thhave e waf its er emission will have wavelength its emission w anda thus velengt they h can and t behused us they in can be a more speciﬁc used in/ a more tighter wavelength specific/tight range. er waT velen o accomplish gth range. To this featur accom e,p alish t tuning his pr feat ocess ure, is a rtu equir ning pr ed, e.g., oces temperatur s is require, ed curr , e.g ent, ., te etc. mpe Each ratur wafer e, curr run ent, etc. E may present ach wafer dierent run statistics may present due to different material deposition statistics due to ma doping and terial prdeposi ocessing, tionsee doping Figurand e 6B. pr Nonetheless, ocessing, see the Figu wafer re 6B. unifor Nonetheless, th mity wavelength e wafer map uniformit is in general y wavelen scar gcely th ma reported p is in gene by the ral sc foundries, arcely re some ported by t works h suggest e foundr these ies, s claims ome works s [79,80].uT gge ight st contr these claims ol of the doping [79,80]. T process ight control of the doping may reduce this variation prand ocess m greatly ay r impr educe t ove h the is v yield ariat and ion temperatur and greatley contr improve the ol mechanisms. yield and te Themperature co ﬁner the requir ntrol mechan ed tuning, isms. The fin the more complex er the requir the laser ed tuning selection , th pr e more ocess. The complex t qualiﬁcation he laser of se a lect laser ion to proces operate s. The in a q certain ualific wavelength ation of a la range ser tor esults operatfr e om in a setting certain its wave maximum length allowed range res wavelength ults from set attiinitial ng its max temperatur imum al es.lowed The br wavelen oad requir gth at ements initia of l tthe emp technology eratures. Th ine broa termsd of req the uirclient ements of locations the technol may requir ogy in e terms of industrial the cl temperatur ient locat e ranges ions ma ( y req 40 to uir 85 e ind C),uwhich strial tbring emperextra ature rrange equisites s (−4to 0 tthe o 85 laser °C), whic wavelength h bring ext choice ra requ or thermal isites tocontr the lol asmechanisms. er wavelengthDir cho ectly ice or modulated thermal clasers ontrol have mecha good nisms. Directly optical output modula power ted la and sers areha easy ve good opti to drive and cal output modulate. power However and are ea , thesy to dri inherent ve high and wavelength modulate. Ho ﬂuctuation wever, the in when herent high w modulatedaor velen theginher th fluctua ent chirp tion when modula can result in pr ted opagation or the inherent chi limitations. rp Laser can redesign sult in propagat /control e ion limitation orts are being s.made Laser to de minimize sign/control these effo e rts are bei ects and n curr g m ently ade to mi , thereni ar miz e alr e these eady some effectdevices s and cuin rrent the ly, market there are able alto ready cope so with me dev some ices of in the the m most arket stringent able to cope with some of standard requirements the most [81], e.g., stringent the NGPON2. standard re W quire ith an ments [81], e.g., the external cavity, EMLs NGPO result N2. Wi in a th much an externa lowerl ca chirp, vity, EMLs resul however, having t in a typically much lower a mor chirp, howeve e complex tunability r, having ty mechanism pically a more c when omplex t compar ued nab to ilit the y mechan simpleis curr m when com ent or thermal pared tuning to the si of mpl theeDML current or therma [7]. l tuning of the DML [7]. Figure 6. Representation of a generic statistical distribution of lasers over their initial wavelength () in Figure 6. Representation of a generic statistical distribution of lasers over their initial wavelength (λ) a set of three wafer runs. Dierent wafers may result in dierent central s. (A): distribution of initial in a set of three wafer runs. Different wafers may result in different central λs. (A): distribution of of wafer #2; (B): example of an initial s distribution for three dierent wafers. initial λ of wafer #2; (B): example of an initial λs distribution for three different wafers. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 11 of 19 Appl. Sci. 2020, 10, x 11 of 19 4.3. Integrated Photonics Packaging 4.3. Integrated Photonics Packaging Photonic packaging technology has achieved a higher signiﬁcance under optical communication Photonic packaging technology has achieved a higher significance under optical communication systems’ recent developments [82] by covering the optical and electronic connections in/out of the systems’ recent developments [82] by covering the optical and electronic connections in/out of the PIC. Packaging process developments are of great importance for the next generation of optical PIC. Packaging process developments are of great importance for the next generation of optical components [6]. Being one of the most complex segments of the integrated component viability, components [6]. Being one of the most complex segments of the integrated component viability, it it can pose several challenges, such as limitations in terms of volume, cost, RF performance, power can pose several challenges, such as limitations in terms of volume, cost, RF performance, power dissipation, and its high-volume manufacturing capability [83], which can highly impact the cost of dissipation, and its high-volume manufacturing capability [83], which can highly impact the cost of the solution. the solution. Currently, packaging is seen as one of the most signiﬁcant bottlenecks in the development of Currently, packaging is seen as one of the most significant bottlenecks in the development of commercially relevant PIC devices [60,84]. The packaging design ﬂow is divided into three main commercially relevant PIC devices [60,84]. The packaging design flow is divided into three main areas: the optical design, the electrical design, and the thermal management of the module. A ﬁrst areas: the optical design, the electrical design, and the thermal management of the module. A first open-access PIC assembly and packaging pilot line of Europe’s leading industrial and research open-access PIC assembly and packaging pilot line of Europe’s leading industrial and research organization teams (i.e., PIXAPP) are dedicating eorts to the development of a photonic packaging organization teams (i.e., PIXAPP) are dedicating efforts to the development of a photonic packaging platform to gather state-of-the-art technologies and procedures [62]. This highly interdisciplinary platform to gather state-of-the-art technologies and procedures [62]. This highly interdisciplinary team provides single-point access to PIC assembly and packaging for users. The project key objectives team provides single-point access to PIC assembly and packaging for users. The project key objectives include: (i) custom solutions through standard packaging technologies; (ii) training and education as a include: (i) custom solutions through standard packaging technologies; (ii) training and education as future workforce; and (iii) link the PIC ecosystem through the development of packaging standards a future workforce; and (iii) link the PIC ecosystem through the development of packaging standards and roadmaps [17]. Considering the typical wavelength gain band of an active device, ranging from and roadmaps [17]. Considering the typical wavelength gain band of an active device, ranging from 20 nm to 30 nm for the O- to L-bands, and the technology wavelength map presented in Figure 7, 20 nm to 30 nm for the O- to L-bands, and the technology wavelength map presented in Figure 7, several approaches may be required to merge in a single device the upstream/downstream signals. several approaches may be required to merge in a single device the upstream/downstream signals. There are several network standards, like XGSPON, which require upstream/downstream wavelengths There are several network standards, like XGSPON, which require upstream/downstream spaced by more than 200 nm. A 200 nm spacing brings higher challenges and may require hybrid wavelengths spaced by more than 200 nm. A 200 nm spacing brings higher challenges and may integration, which poses additional packaging challenges [7]. require hybrid integration, which poses additional packaging challenges [7]. Figure 7. Diagram describing transmitter and receiver technologies with potential use in access Figure 7. Diagram describing transmitter and receiver technologies with potential use in access networks. Typical gain and lasing media proﬁles are presented, with 20 nm to 60 nm bandwidths for networks. Typical gain and lasing media profiles are presented, with 20 nm to 60 nm bandwidths for common current materials, PINs with dierent doping materials, and APDs. (A): transmitter/(pre-) common current materials, PINs with different doping materials, and APDs. (A): transmitter/(pre-) ampliﬁcation side; (B): receiver side. amplification side; (B): receiver side. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 12 of 19 In the same technological space, NGPON2 requires only 60 nm between the upstream and downstream, thus a fully monolithic approach may be achievable by changing slightly the doping or applying regrowth. Figure 7 provides a representation of the gain curve for a generic material doping, the abstraction of a laser for the transmitter side, and three types of available receiver techniques/devices. On the transmitter side tunability limitation of a laser is depicted and corroborated by the limited gain media in which the laser light is generated, see Figure 7A. This constraint results in 20 nm to 30 nm tunability range, considering laser operation with signiﬁcant output power. Simple tuning based on thermal eects usually results in an available tuning range of 3 nm to 4 nm. A combination of a tunable device with an external cavity supported by a convenient control mechanism results in a wider tuning range. On the receiver side several techniques are described, such as a positive-intrinsic-negative photodiode (PIN), APD, and coherent [7,8], see Figure 7B. The PIN is one of the cheapest and simplest detecting techniques presenting a wide light wavelength detection range. Its proﬁle can be changed by the type of materials and doping used. PINs can be easily integrated into e.g., InP PIC, which is already available for bandwidths exceeding >35 GHz and operational wavelength ranges covering the common ﬁber telecommunication bands [33,60,85]. APDs are tougher to integrate, however, from the system point of view, they result in improved sensitivity due to their intrinsic avalanche gain. The referred receiving technologies are wide wavelength band, however, with a coherent detection scheme tunable reception can be achieved. Depending on the arrangement, a dierent number of associated PINs (from dual-polarization dierential with 8 PINs to quasi-coherent single photodetector) simultaneous gain and ﬁltering (stemming from the local oscillator beating with the signal) can be implemented [7,77,86]. To achieve the optical interface between the PIC and the ﬁber, there are several challenges to be surpassed, such as the mode adaptation between the WG in the PIC and the cylinder-shaped ﬁber core [87]. Spot size convert (SSC) modules in the PIC, lensed ﬁbers connected through V-grooves, holders, and other techniques are used to achieve consistent solutions. The PIC packaging interfacing of the electrical connections between the PIC and the PCB is usually carried out by wire-bonding [34]. As bandwidth grows, the trend is to replace the bonding wires by ﬂip-chip solutions to reach higher thermal and electrical performance. Furthermore, packaging should be a gateway to solve power dissipation, which is expected to increase signiﬁcantly with density [7]. Packaging solutions based on holders with high thermal conductivity and inherent thermal monitoring is an attractive solution to overcome this constraint. 4.4. Control Complexity, Power Dissipation and Form Factor The dierent PON standards and technologies have their own requirements in terms of complexity control, especially for tunable devices. When wavelength control is not essential, simpler actuation systems with lower power are required [88]. Nonetheless, once the wavelength spectrum becomes more crowded, the options have to be restricted and the requirements for more complex control increase [7]. A summary of the technology and expected power for its optoelectronic interfaces versus the complexity and requirements for a certain interface is depicted in Figure 8. A general sweep, from low data rate with large wavelength range and short-reach to higher data rate with tighter wavelength range and longer reaches, are also presented in the Figure 8. Additionally, the technologies with an approximate relative power requirement classiﬁcation (from low to high power) are also identiﬁed. The x-axis represents the optical requirements, ranging from low data rate and short-reach to long-reach DWDM and high data rate. The red arrows describe the complexity steps from the system requirements. Appl. Sci. 2020, 10, 4024 13 of 19 Appl. Sci. 2020, 10, x 13 of 19 Figure 8. Diagram of requirements in terms of control, complexity, and power for different Figure 8. Diagram of requirements in terms of control, complexity, and power for dierent technologies. technologies. Potential form factor solutions able to cope with the required power dissipation are also presented in thePotenti far right al f oform f Figuraector 8. Technology solutions a evolution ble to cope wi poses incr th the requi easing challenges red power dissi in the contr patol ion a complexity re also and presented in t required power he far right of Fig . As the demanded ure 8. Tec power hnology ev grows,o alchange ution poses in the in form creasing ch factor is allenges needed. in the control Form factors have complexity and two major characteristics: required power. As minimum the dem volume anded and pmaximum ower grow dissipated s, a change power in th[e form 89]. factor is needed. An appr Form oximate factors have two major chart of the average char pricing acteri for stics: minimum GPON, XGSPON, volume and a NGPON2 nd maximum dissip evolution versus ated power [89]. time is presented in Figure 9. GPON faced a strong price drop with its increased manufacturability, An approximate chart of the average pricing for GPON, XGSPON, and NGPON2 evolution pushed by the worldwide broad adoption of the technology by the operators. XGSPON is following versus time is presented in Figure 9. GPON faced a strong price drop with its increased the same steps, which for the user can be the motto to have 10 Gbit solutions sooner. Technically, manufacturability, pushed by the worldwide broad adoption of the technology by the operators. XGSPON generically adopted the physical layer characteristics of IEEE 802.3av 10GEPON systems, XGSPON is following the same steps, which for the user can be the motto to have 10 Gbit solutions e.g., diering in the burst mode timing requirements [70,78]. Moreover, to jump-start the market, sooner. Technically, XGSPON generically adopted the physical layer characteristics of IEEE 802.3av initial implementations of XGSPON systems relaxed the timing requirements to fully adopt 10GEPON 10GEPON systems, e.g., differing in the burst mode timing requirements [70,78]. Moreover, to jump- standards [70]. With this rapid price erosion model, the proﬁt margins in the supply chain depreciate start the market, initial implementations of XGSPON systems relaxed the timing requirements to very fast. As NGPON2 is based on the limited tunability of DWDM lasers and receivers, it poses fully adopt 10GEPON standards [70]. With this rapid price erosion model, the profit margins in the some challenges for its adoption by manufacturers, vendors, and operators. This fact has delayed its supply chain depreciate very fast. As NGPON2 is based on the limited tunability of DWDM lasers deployment process. NGPON2 technology diers from the traditional model since it goes out of the and receivers, it poses some challenges for its adoption by manufacturers, vendors, and operators. O-band, making the laser control and manufacturability more stringent. Thus, it results in a potentially This fact has delayed its deployment process. NGPON2 technology differs from the traditional model slower cost reduction and more sustainability from the supply chain point of view [7]. since it goes out of the O-band, making the laser control and manufacturability more stringent. Thus, it results in a potentially slower cost reduction and more sustainability from the supply chain point of view [7]. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 14 of 19 Appl. Sci. 2020, 10, x 14 of 19 Figure 9. Representation of average price evolution of PON technology for GPON, XGSPON, Figure 9. Representation of average price evolution of PON technology for GPON, XGSPON, and and NGPON2 bulk type BOSA (not PIC based). Pricing was empirically collected along the years from NGPON2 bulk type BOSA (not PIC based). Pricing was empirically collected along the years from several providers and averaged in the graph. several providers and averaged in the graph. The indicative pricing evolution of the traditional bulk type BOSA for the three technologies is The indicative pricing evolution of the traditional bulk type BOSA for the three technologies is depicted in Figure 9. Early, broad and late adoption notes refer to perception and predictions estimated depicted in Figure 9. Early, broad and late adoption notes refer to perception and predictions by the authors given the present standing of the technology. Pricing is indicative and may not represent estimated by the authors given the present standing of the technology. Pricing is indicative and may all realities/contexts, and is based on, per vendor and per order, 100 pieces at the early adoption stage, not represent all realities/contexts, and is based on, per vendor and per order, 100 pieces at the early 100k pieces at broad adoption, and 1M pieces at late adoption stages, see Figure 9. adoption stage, 100k pieces at broad adoption, and 1M pieces at late adoption stages, see Figure 9. A reduction in physical volume, number of sub-assemblies, and calibration steps are expected A reduction in physical volume, number of sub-assemblies, and calibration steps are expected with the migration to PICs. These steps may greatly reduce the price of the subassemblies based on PICs with the migration to PICs. These steps may greatly reduce the price of the subassemblies based on when compared with traditional bulk BOSA [7,8]. As identiﬁed above, by having upstream/downstream PICs when compared with traditional bulk BOSA [7,8]. As identified above, by having on approximately the same wavelength band, NGPON2 can be a great applicant for monolithic PIC. upstream/downstream on approximately the same wavelength band, NGPON2 can be a great In such a case, ONU and OLT can coexist in the same PIC platform without further processing. applicant for monolithic PIC. In such a case, ONU and OLT can coexist in the same PIC platform XGSPON, besides being already stressed in price, may require O-band and L-band integration, which without further processing. XGSPON, besides being already stressed in price, may require O-band can entail extra processing and packaging steps, thus increasing the limitations of PIC viability in this and L-band integration, which can entail extra processing and packaging steps, thus increasing the wavelength range. DWDM is also included in the wavelength plan of NGPON2 and is already an limitations of PIC viability in this wavelength range. DWDM is also included in the wavelength plan ITU-T standard being used for implementing several telecommunication systems, which also poses a of NGPON2 and is already an ITU-T standard being used for implementing several good opportunity for the use of the PIC technology. telecommunication systems, which also poses a good opportunity for the use of the PIC technology. 5. Conclusions 5. Conclusions PICs are a promising technology with great potential in several ﬁelds including telecom, sensing, PICs are a promising technology with great potential in several fields including telecom, sensing, and bio-photonics. In this work, we have reviewed the PON technology standards and evaluated the and bio-photonics. In this work, we have reviewed the PON technology standards and evaluated the challenges and beneﬁts of meeting its requirements with the prospective usage of PIC. Depending challenges and benefits of meeting its requirements with the prospective usage of PIC. Depending on on the requirements, the type of materials, and components, a tailored approach has to be carefully the requirements, the type of materials, and components, a tailored approach has to be carefully speciﬁed. The dierent materials used in the available solutions for implementing PIC result in dierent specified. The different materials used in the available solutions for implementing PIC result in behaviors, steps, processes, and capabilities. The main constraint of PIC is still the limited gain band different behaviors, steps, processes, and capabilities. The main constraint of PIC is still the limited of the active devices which are in its core. This may imply advanced processing and/or packaging gain band of the active devices which are in its core. This may imply advanced processing and/or packaging techniques to be able to cope with the existing standards, e.g., regrowth and hybrid Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 15 of 19 techniques to be able to cope with the existing standards, e.g., regrowth and hybrid packaging. After carefully choosing the standard to be followed and the packaging approach, its control complexity also has to be cautiously considered to guarantee that the solution is feasible and adapted. Speciﬁcally, for PON the most stringent requirements are the cost and robustness of the proposed solution. We have discussed several of the challenges to be considered when equating the introduction of PIC in the next-generation PONs, like productization, wideband wavelength range (O, C, and L-bands), laser control, and tunability mechanisms. PON standards have dierent ﬂavors that result in quite dierent requirements, especially regarding the wavelength ranges to be covered by the two trac directions, i.e., the upstream and downstream. For instance, GPON and XGSPON have speciﬁcations of >200 nm spacing between upstream and downstream, while NGPON2 requires around 60 nm for the same parameter. Due to technological inherent limitations of some technologies, hybrid integration may be required, e.g., XGSPON and GPON. Smaller bands will ease this process and therefore potentiate monolithic PIC implementation, simplifying packaging and deployment, e.g., NGPON2. PICs have typically two major interfaces, electrical and optical, each with dierent requirements, techniques, and available materials to be used. Packaging and technical considerations such as size, power, RF compliance, and sealing are relevant. In a nutshell, the manufacturability is anticipated to be one of the most critical steps for the technology to be successful, especially in PON. In our vision, the use of PICs in PON and other subsystems is very close to being an eective reality with prospective advantages. Author Contributions: This is a review paper, each author contributed to all chapters. C.P. and A.T. prepared the manuscript, i.e., writing, review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the European Regional Development Fund (FEDER), through the Competitiveness and Internationalization Operational Programme (COMPETE 2020) of the Portugal 2020 framework [Project Virtual Fiber Box with Nr. 033910 (POCI-01-0247-FEDER-033910)]; plugPON (POCI-01-0247-FEDER-047221); FEDER through the Regional Operational Programme of Centre (CENTRO 2020) of the Portugal 2020 framework [Project HeatIT with Nr. 017942 (CENTRO-01-0247-FEDER-017942)]; and IT (UID/EEA/50008/2019). Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Teixeira, A.; Shahpari, A.; Ferreira, R.; Guiomar, F.P.; Reis, J.D. Coherent Access. In Proceedings of the OFC 2016—Optical Fiber Communication Conference, Anaheim, CA, USA, 20–24 March 2016; p. M3C.5. 2. Shahpari, A.; Ferreira, R.M.; Luis, R.S.; Vujicic, Z.; Guiomar, F.P.; Reis, J.D.; Teixeira, A.L. Coherent Access: A Review. J. Lightwave Technol. 2017, 35, 1050–1058. [CrossRef] 3. Micolta, J.C.V. 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applied sciences Review Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited) 1 , 2 , 3 1 , 2 , 3 1 , 2 , 3 3 Cátia Pinho , Francisco Rodrigues , Ana Maia Tavares , Carla Rodrigues , 4 1 , 2 , Cláudio Emanuel Rodrigues and António Teixeira * Instituto de Telecomunicações (IT), University of Aveiro, 3810-193 Aveiro, Portugal; catiap@ua.pt (C.P.); francisco@picadvanced.com (F.R.); ana@picadvanced.com (A.M.T.) Department of Electronics, Telecommunications and Informatics (DETI), University of Aveiro, 3810-193 Aveiro, Portugal PICadvanced, PCI—Creative Science Park Via do Conhecimento, Edifício Central, 3830-352 Ílhavo, Portugal; carla.rodrigues@picadvanced.com Altice Labs, R. José Ferreira Pinto Basto, 3810-106 Aveiro, Portugal; claudio-e-rodrigues@alticelabs.com * Correspondence: teixeira@ua.pt Received: 31 March 2020; Accepted: 3 June 2020; Published: 10 June 2020 Abstract: The development of photonic integrated circuits (PIC) for access network applications, such as passive optical networks (PON), constitutes a very attractive ecosystem due to PON’s potential mass market. The implementation of PIC solutions in this context is expected to facilitate the possibility of increasing the complexity and functionalities of devices at a potentially lower cost. We present a review addressing the prominent access network market requirements and the main restrictions stemming from its speciﬁc ﬁeld of application. Higher focus is given to PON devices for the optical network unit (ONU) and the implications of designing a device ready for market by discussing its various perspectives in terms of technology and cost. The discussed PIC solutions/approaches in this paper are mainly based on indium phosphide (InP) technology, due to its monolithic integration capabilities. A comprehensive set of guidelines considering the current technology limitations, beneﬁts, and processes are presented. Additionally, key current approaches and eorts are analyzed for PON next generations, such as next-generation PON 2 (NGPON2) and high-speed PON (HSP). Keywords: photonic integrated circuits (PIC); access networks; passive optical network (PON); next-generation passive optical network 2 (NGPON2); high-speed PON (HSP); super-PON 1. Introduction Telecommunication network infrastructures can be described by three main network segments, i.e., core or backbone, metro/regional, and access networks, as depicted Figure 1. Long-distance communications with high aggregate trac are attained with core and metro networks, while access networks entail service provider to end-user ﬁber-optic telecommunications technology. These systems are subject to signiﬁcant growth in capacity requirements with operators looking for feasible cost-eective solutions. The potentiation of cost-eective solutions can be reached with the improvement of current network technologies, as they become widely used, e.g., coherent systems in future optical access networks [1,2]. Appl. Sci. 2020, 10, 4024; doi:10.3390/app10114024 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 2 of 19 Appl. Sci. 2020, 10, x 2 of 19 Figure Figure 1. 1. Diagram Diagram of a te of a telecommunications lecommunications netw network ork infrastructure, based infrastructure, basedin [3]. in [3]. Photonic integrated circuits (PIC) are considered an evolving technology oering a sustainable Photonic integrated circuits (PIC) are considered an evolving technology offering a sustainable (ecient and cost-eective) alternative to data transmission interfaces [4]. The implementation of (efficient and cost-effective) alternative to data transmission interfaces [4]. The implementation of integrated photonics can thus enable savings and new functionalities, increasing the transmission integrated photonics can thus enable savings and new functionalities, increasing the transmission capacity capacity [5 [5].]. In the a In the access ccess network context, network context, PIC can offer compelling per PIC can oer compelling performance formance improv improvements ements compared to traditional bulk technology [6], like bi-directional optical sub assembly (BOSA). Upgrades compared to traditional bulk technology [6], like bi-directional optical sub assembly (BOSA). in Upgra terms des of in weight terms o and f we volume, ight and r vol educed ume, re power duce consumption, d power consump high tion, h mechanical igh mechanic and thermal al and tstability hermal , and stabil assembly ity, andsimpliﬁcation assembly simplif for high ication subsystem for high integration subsystem i aren expected tegration under are expected under PIC PIC solutions [6–9]. solutions [6–9]. By combining the need of providing enhanced passive optical network (PON) technologies and taking By combinin advantage g the need of of photonic providing en integrated hanced solutions passive optical net beneﬁts, a compr work (PO ehensive N) t description echnologies ofand PIC taking advantage of photonic integrated solutions benefits, a comprehensive description of PIC design challenges for the optical network unit (ONU) side of next-generation PON is delivered design challenges for the optical network unit (ONU) side of next-generation PON is delivered in this in this study (see optical access network branch in Figure 1). The paper is organized into four study (see optical access network branch in Figure 1). The paper is organized into four additional additional sections. The historical PIC evolution addressing current developments and expectations sections. The historical PIC evolution addressing current developments and expectations are are described in Section 2. Section 3 addresses the PON standards and next-generation PON timeline. described in Section 2. Section 3 addresses the PON standards and next-generation PON timeline. The implementation of PIC solutions for PON is introduced in Section 4, discussing the technology The implementation of PIC solutions for PON is introduced in Section 4, discussing the technology impact on reach (Section 4.1), the adopted band and wavelength selection criteria (Section 4.2), photonic impact on reach (Section 4.1), the adopted band and wavelength selection criteria (Section 4.2), packaging (Section 4.3), and control complexity, power dissipation and form factor issues (Section 4.4). photonic packaging (Section 4.3), and control complexity, power dissipation and form factor issues The study is concluded in Section 5. (Section 4.4). The study is concluded in Section 5. 2. PIC Evolution 2. PIC Evolution Photonic integrated solutions are a promising technology for the 21st-century optical systems, Photonic integrated solutions are a promising technology for the 21st-century optical systems, under the current demand for ﬂexibility/reconﬁgurability in optical communication networks [5,10]. under the current demand for flexibility/reconfigurability in optical communication networks [5] Dierentiated examples of PIC solutions include the implementation of photonic integrated optical [10]. Differentiated examples of PIC solutions include the implementation of photonic integrated transforms for data compression applications in dierent integrated platforms, e.g., indium phosphide optical transforms for data compression applications in different integrated platforms, e.g., indium (InP) [11,12], silicon nitride (SiN) [13], and new organic-inorganic hybrid materials [14]; SiN-integrated phosphide (InP) [11,12], silicon nitride (SiN) [13], and new organic-inorganic hybrid materials [14]; photonic dispersion compensator enabling extended reach pulse-amplitude modulation with four SiN-integrated photonic dispersion compensator enabling extended reach pulse-amplitude amplitude levels (PAM-4) transmission as a low-cost interface for data center interconnects (DCI) [15]; modulation with four amplitude levels (PAM-4) transmission as a low-cost interface for data center development of state-of-the-art sub-hertz fundamental linewidth (<1 Hz) photonic integrated Brillouin interconnects (DCI) [15]; development of state-of-the-art sub-hertz fundamental linewidth (< 1 Hz) laser, narrow enough to move demanding scientiﬁc applications to the chip-scale [16–18]; and a new photonic integrated Brillouin laser, narrow enough to move demanding scientific applications to the soft-packaging ﬂexible platform approach for photonic integrated processors, based on the spatial chip-scale [16–18]; and a new soft-packaging flexible platform approach for photonic integrated light modulation operation principle [19,20]. Moreover, new concepts of PIC-to-PIC interconnects processors, based on the spatial light modulation operation principle [19,20]. Moreover, new concepts with integrated fan-in/fan-out multicore ﬁber applications for optical dense networks have been of PIC-to-PIC interconnects with integrated fan-in/fan-out multicore fiber applications for optical Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 3 of 19 reported [21]. Photonic integration appears as a dominant technology in high bandwidth optical communication systems [9], oering increased valuable solutions in several innovative ﬁelds, such as bio-photonics [22], sensing [23], and space technology [24]. Nevertheless, PIC technology is still more expensive than standard microelectronics, which can restrict its application into some niche markets [5]. PICs are the equivalent of electronic integrated circuits (EIC) in the optical domain. As an alternative to transistors and other electronic components, PICs contain optical elements, such as modulators, detectors, attenuators, multiplexers, optical ampliﬁers, and lasers embedded in a single chip using a waveguide (WG) architecture [19]. The photonic integration technology is going through a similar evolution path as microelectronic integration, however, with a time delay of about 20 to 30 years [25]. Historically, the invention of the transistor in 1948 [26,27] launched microelectronic integration developments; analogously, the invention of the semiconductor laser in 1969 [28] was the breakthrough of photonic integration technology. Transistor and laser technologies were primarily implemented as discrete components. Microelectronics integration technology started with Kilby in 1959 [29] and maturated with the complementary metal-oxide-semiconductor (CMOS) [30] in the 1970s. A ﬁrst PIC comprising a laser integrated with a modulator was reported in 1987 [31], marking the start of PIC exponential development, typically associated with Moore’s law [25]. Moore’s law forecasted a double in the number of components per integrated circuit about every two years, with a new technology generation introduced approximately every three years and an expected compound annual growth rate (CAGR) under the dozens [32]. Optical communications evolution has brought the advent of improved PIC, presenting an economic and sustainable alternative to data transmission [4]. PIC technology oers compelling performance advances in terms of small weight and volume, low power consumption, high mechanical and thermal stability, and the ease for assembling a substantial number of complex systems [9]. Consequently, the current PIC-related contexts are evolving at a good rate with an overall worldwide concertation set of eorts aiming to reach a sustainable PIC ecosystem model [33,34]. From the 2000s when the ﬁrst consortia started to consolidate the concepts around photonics multi-project wafer (MPW), the way for groundbreaking photonic integration developments was paved. State-of-the-art PIC-based consortia, such as JePPIX [35], TriPleX [36], and ePIX [37], pioneered the eort in Europe and globally, followed by private and public-private arrangements, like AIM photonics [38], Compoundtek silicon photonics, and others [39]. They are working to replicate in optics what happened in the 80s–90s with electronics, e.g., the Marconi GaAs MPW runs and others [40]. The traction gathered by these consortia allowed a fast development of integrated optics simulators and of the process design software (PDS) market, supported by software suites running process design kits (PDK) from almost all available fabs. The development of a generic common language for the photonic design was an important breakthrough that allowed integrated photonics to be regarded as a promising sort-term technology. During these developments, many integrated and hybrid solutions emerged [41–43]; nonetheless, the majority were bulky, expensive, and typically non-replicable, making these approaches unfeasible to be integrated into further complex systems and subsystems. For instance, devices such as optical ampliﬁers [44], lasers [16,45], and modulators [46], are not supplied by the dierent integrated platforms due to limitations of semiconductor integration technology. Thus, it restricted the investment of the dierent labs and research institutes in the development of their concepts/testing to validate their contributions/solutions while waiting for the necessary developments of the technology. In the 90s, few basic building blocks (BB) were made available in a packaged version, e.g., the semiconductor optical ampliﬁer (SOA) based Mach-Zehnder interferometer (MZI) [47], though with an expensive associated cost of several tens of thousands of euros. Dierent materials are suitable to produce PICs, nonetheless, only a few have been selected by actors in the ecosystem and, therefore, have gained traction into foundries, software suppliers, design houses, and fabless companies [48]. Current main photonic integration platform technologies Appl. Appl. Sci. Sci. 2020 2020 , 10,, x10 , x Appl. Sci. 2020, 10, x 4 of4 19 of 19 4 of 19 inteinte gragra tedt ed ph otonic photonic pla pla tform tinte form to gra realize toted realize ph PICs otonic PICs is pla dr isiven tdr form iven by to by the realize the functiona fu PICs nctiona l isre ldr quire reiven quire m by ents m the ents of fu the ofnctiona the devices devices l re quire ments of the devices to be to deve be deve loplop ed eand d and the the unto derly un be derly deve ingi ng av lop a av ieladabi and llaeb compone l ethe compone underly ntsn (ac itsng t(ac iv av e/p tiv aie/p laassive balessive compone ) fo)r fo thre th differen nets differen (activ t e/p plat t plat aforms ssive forms ) for the different platforms Appl. Sci. 2020, 10, x Appl. Sci. 2020, 10, x 4 of 19 4 of 19 [33[,55 33,5 ,556],5. 6] An . An overview overview of the [of 33 the , 55 ke,5 y ke 6] chy. ar An chac ar overview teri acteri stics st ics be of tw be the een tw ke een the y the ch diar ffe di acrffe eteri ntre st av ntics ai avlab be ailab tw le int een le eint grat the egrat ed diffe ed plat r eplat nt form av form ai lab le integrated platform Appl. Sci. 2020, 10, x 4 of 19 technolo technolo giesgies is provided is provided intechnolo Table in Table 1 gies [33 1 [33 ,56 is ,5 ,provided 567],5. 7] BB. BB com in com p Table onents, ponents, 1 [33 su,ch 56 su ,5 as ch7] la as . sers, BB lasers, com optical poptical onents, am apl m su ifpl iers, chif iers, as and la and sers, optical amplifiers, and integrated photonic platform integra to trealize ed photonic PICs ispla drtform iven by to realize the functiona PICs isl re drquire iven m byents the of fu nctiona the devices l require ments of the devices Appl. Sci. 2020, 10, 4024 4 of 19 detec detec tors, tors, pose pose de sign design ch allenge detec challenge tors, s for s pose for their their de im sign pim lem p chlem eallenge nta ention tation s in for in a their SiaN Si ‐based im N‐p based lem monoli e nmonoli tation thic thic in in a te in Si gra te Ngra t‐ed based t ed monolithic integrated to be developed and the un toderly be deve inglop avaeidla and ble compone the underly ntsi ng (ac tav ivae/p ilaabssive le compone ) for then ts differen (active/p t plat assive forms ) fo r the different platforms integrated photonic platform to realize PICs is driven by the functional requirements of the devices platplat form form ; thi; sthi constrain s constrain can plat can be form be overcom overcom ; thise constrain with e with hy br hy can id br and ibe d and overcom /or /heterogeneous or heterogeneous e with hybr pro id pro cand esses cesses /or, as heterogeneous , pre as pre sente sente d d processes, as presented [33,55,56]. An overview of[3 the 3,55 ke ,5y6] ch . An arac overview teristics be of tw theeen ke ythe ch di arffe acteri rentst av icsai be lab twleeen int ethe grat died ffe plat rentform avai lable integrated platform to be developed and the underlying available components (active/passive) for the different platforms in Table in Table 1. Si 1.mil Simil arly, arl hybr y, hybr id/he in id/he Table terogen t erogen 1. Siemil ous eoa rl us processes y, processes hybrid/he are are templo erogen emplo yeeodyus efor d processes for lasers lasers and are and optic emplo optic al am ayle am plif d for plif ier lasier sers s and optical amplifiers technologies is provided technolo in Table gies 1 [33 is ,56 provided ,57]. BB com in Table ponents, 1 [33 su ,56 ch,5 as 7]. la BB sers, com optical ponents, am su plifchiers, as la and sers, optical amplifiers, and Appl. Appl. Sci. Sci. 2020 2020 , 10,, x10 , x 4 of4 19 of 19 [33,55,56]. An overview of the key characteristics between the different available integrated platform oering access BBs to BBs ingeneric Si in‐ ba Si‐sba eds e pr techno d ocesses techno logy. logy. BBs and in Si manufacturing ‐based technology. thr ough multi-project wafer (MPW) runs are detectors, pose design chdetec allenge tors, s for pose their de sign imp lch emallenge entation s for in their a SiN im‐based plem emonoli ntationthic in in a te Sigra N‐based ted monolithic integrated technologies is provided in Table 1 [33,56,57]. BB components, such as lasers, optical amplifiers, and Appl. Sci. 2020, 10, x Appl. Sci. 2020, 10, x 4 of 19 4 of 19 inteinte gragra tsilicon edt ed ph otonic photonic (Si) pla [ 49 pla tform plat ,50 tform form ], toindium realize to; thi realize s constrain PICs phosphide PICs is dr is can iven dr plat iven be form by (InP) overcom by the ; thi the fus[nctiona fu 51 constrain e nctiona with –53l ], hy rel and quire can br reiquire d be and m silicon ents overcom m/or ents of heterogeneous nitride the ofe the with devices devices hy (SiN) br ipro d and c [esses 48/or ,54 ,heterogeneous as ]. pre The sente selection d processes, as presented detectors, pose design challenges for their implementation in a SiN‐based monolithic integrated Table Table 1. Key 1. Key fea tfe ure ature s from s from thre Table thre e availab e1. availab Keyle fephotonic laet ure photonic s from integ integ thre ration er ation availab platfor platfor lem photonic s,m i.s,e. ,i. indiu e. ,integ indiu mr ation phosphide m phosphide platfor m s, i.e., indium phosphide to be to deve be deve loplop ed eand d and the Appl. the un in derly un Sci. Table derly 2020 ing 1. , 10 i ng av Si , xmil a av ilaaabirl Appl. llaey,b compone Appl. lhybr eSci. compone Sci. 2020 inid/he Table 2020 , n 10ts t,,erogen n x 10 (ac ts 1. , x t(ac Si ivmil e/p etiv ous ae/p arlssive processes y, assive hybr ) fo)rid/he fo th rare e th differen terogen eemplo differen ety oplat eus dt plat for forms processes la forms sers and are optic emplo aly am ed 4 plif of for 19 ier la sers s and optic 4 aofl 4 am 19 of plif 19 iers of the integrated photonic platform platto form realize ; this constrain PICs is can driven be overcom by the e with fu nctional hybrid andr/equir or heterogeneous ements of pro the cesses, as presented integrated photonic pla inte tform grated to ph realize otonic PICs pla tis form driven to realize by the PICs functiona is driven l require by the ments functiona of the ldevices require ments of the devices (InP), (InP), sili con silicon (Si), (S and i), and sili con silicon ni (InP), t rni idter si i(SiN) dliecon (SiN) monolithic (Si), monolithic and sili technologies. con technologies. nitride (SiN) monolithic technologies. BBs in Si‐based technology. BBs in Si‐based technology. [33[,55 33,5 ,556],5. 6] An . An overview overview of the of the key ke chyar chac arteri acteri stics st ics betw beeen tween the the diffe dirffe entre av nt ai avlab ailab le int le eint grat egrat ed ed plat plat form form to be developed and the to be un deve derlylop inge dav and aila the ble un compone derly in Table ing nts av 1. (ac aSiitlmil iv abe/p lea rlcompone ay, ssive hybr ) fo id/he nr ts th (ac et erogen differen tive/peaotssive us plat processes )forms for th e differen are emplo t plat yeforms d for la sers and optical amplifiers devices to be developed and the underlying available components (active/passive) for the dierent integrated photonic inte plainte tgra form gra ted to t ed ph realize otonic photonic PICs pla pla tform ist form dr iven to realize to by realize the PICs fu PICs nctiona is dr isiven dr l re iven quire by by them the fu ents nctiona fu of nctiona thel re devices lquire require m ents ments of the of the devices devices technolo technolo giesgies is provided is provided in Table in Table 1 [33 1 [33 ,56,5 ,567],5. 7] BB. BB com com ponents, ponents, such su as ch la assers, lasers, optical optical am aplmifpl iers, ifiers, and and [33,55,56]. An overview [33 of ,55 the ,56] ke . An y ch overview aracterist ofics the BBs be ke tw in yeen ch Si ‐ar the baac s edi teri dffe techno strics ent be av logy. tw ailab een l ethe int di egrat ffered ent plat avaiform lable integrated platform Build Build inging blo blo ck c(BB) k (BB) InP Build InP ing blocSi k (BB) Si SiN InP SiN Si SiN platforms [33,55,56]. An overview of the key characteristics between the dierent available integrated to be developed andto the be to un deve bederly deve loping lop ed av eand da and i lthe ab lthe eun compone derly underly ingni ng av ts (ac a av ilataiv billaee/p b compone lea ssive compone ) fo nts r n th (ac tse t(ac differen ive/p tive/p assive ta plat ssive ) fo forms )r fo thre th differen e differen t plat t plat forms forms Table 1. Key features from Table three availab 1. Key lfe e aphotonic tures from integ thre ration e availab platfor le photonic ms, i.e., indiu integm ration phosphide platfor ms, i.e., indium phosphide detec detec tors, tors, pose pose de sign design ch allenge challenge s for s for their their im pim lem plem enta ention tation in in a SiaN Si‐based N‐based monoli monoli thicthic in teingra tegra tedt ed technologies is provided technolo in Table gies 1is [33 provided ,56,57]. BB in Table comp 1onents, [33,56 ,5 su7]ch . BB as com lasers, ponents, optical su am chpl asif iers, lasers, and optical amplifiers, and PasPas sivesive components components ✓✓ Pas ✓✓ s ive components ✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓✓ [33,55,5(InP), 6]. An si overview licon (S[3i),3 [,and 55 3 of 3,5 , 55 the s6] ili,5.con ke 6] An .(InP), y An ni overview ch tr iar overview d sieac li (SiN) con teri of (S st monolithic i), ics the of and be the ketw sy ke ili een ch con y technologies. ar ch the ni ac ar tteri r ac di idteri effe st (SiN) ics rst ent ics be monolithic av tw beai een tw lab een the le int technologies. the dieffe grat dirffe ent ed re av nt plat ai avlab form ailab le int le eint grat egrat ed ed plat plat form form platplat form form platform ; thi; sthi constrain s constrain technologies can can be be overcom overcom is provided e with e with hyin br hyiT d br able and id and /1 or[ /heterogeneous 33 or ,heterogeneous 56,57]. BB pro components, pro cesses cesses , as, pre as pre ssuch ente sente d asd lasers, optical ampliﬁers, Table 1. Key features from three available photonic integration platforms, i.e., indium phosphide detectors, pose design detec challenge tors, pose s for de their sign im challenge plemensta for tion their in a im Sip Nl‐em based enta tion monoli in thic a Si N in‐tebased grated monoli thic integrated technologies is provided technolo technolo in Table gies Lase gies Lase is rs 1 provided [33 is rs provided ,56,57] .in BB ✓✓✓ Table in ✓✓✓ com Table 1p onents, Lase [33 1 [33 ,56 rs ⚪ ,5 , 56 ⚪ su 7] H,5.ch 7] BB H .as BB com la com sers, ⚪ ✓✓✓ ponents, ⚪ H p optical onents, H su ach su m ⚪ as ch pl H if la as iers, sers, lasers, and optical ⚪ optical H am aplmifpl iers, ifiers, and and in Table in Table 1. Si 1.mil Simil arly, arl hybr y, hybr id/he id/he terogen terogen eous eo us processes processes are are emplo emplo yedy efor d for lasers lasers and and optic optic al am al am plifplif iersier s (InP), silicon (Si), and silicon nitride (SiN) monolithic technologies. platform and ; thi det s constrain ectors, plat can pose form be; design thi overcom s constrain e challenges with can hy be bri d overcom for and/their or eheterogeneous with implement hybrid and pro ation /or cesses heterogeneous in , as a pre SiN-based sente pro d cesses monolithic , as presente integrated d Building block (BB) Build InP ing blockSi (BB) InP SiN Si SiN detectors, pose design detec detec chtors, allenge tors, pose spose for de sign de their sign ch im allenge ch plallenge emesn ta for stion for their in their im a Si pim N lem p‐based lem enta ention ta monoli tion in in athic SiaN Si in‐based N te‐gra based t ed monoli monoli thicthic in teingra tegra tedt ed BBsBBs in Si in‐ ba Si‐sba eds etechno d techno logy. logy. Modulators Modulators ✓✓✓ ✓✓✓ Modulators ✓✓ ✓✓ ✓✓✓ ✓ ✓ ✓✓ ✓ in Table 1. Similarly, in hybr Table id/he 1.t erogen Similarleoy,us hybr processes id/heterogen are emplo eousy processes ed for lasers are and emplo optic yeadl for am la plif sers ier sand optical amplifiers platform; this constrain can be overcome with hybrid and/or heterogeneous processes, as presented in platform; this constrain platplat form canform be ; thi overcom ; sthi constrain s constrain e with can hy can be Build br be overcom id ing overcom and blo /ore cwith heterogeneous ke (BB) with hy br hyid br and iInP d pro and / orc /esses heterogeneous or heterogeneous , as Si pre sente pro d pro SiN cesses c esses , as, pre as pre sente sente d d Passive components Pas ✓✓ sive components ✓✓ ✓✓✓ ✓✓ ✓✓ ✓✓✓ BBs in Si‐based techno BBs logy. in Si‐based technology. SwiSwi tches tches ✓✓ ✓✓ Swi tches ✓✓ ✓✓ ✓✓ ✓ ✓ ✓✓ ✓ Table 1. Similarly in Table , hybrid 1. Simil/aheter rly,in hybr Table in ogeneous Table id/he 1. Si t1. erogen mil Sipr mil arlocesses y, eaorl us hybr y, processes hybr id/he ar id/he eterogen employed tare erogen emplo eous eo us processes ye for dprocesses for lasers la sers are are and emplo and emplo optic optical yedya efor ld am for la ampliﬁers plif sers lasers ier and s and optic optic BBs al am al am plifplif iersier s Table Table 1. Key 1. Key fea tfe ure ature s from s from thre thre e availab e availab le photonic le photonic integ integ ration ration platfor platfor ms,m i.s,e. ,i. indiu e., indiu m phosphide m phosphide Lasers ✓✓✓ Lasers⚪ H ✓✓✓ ⚪ H ⚪ H ⚪ H Passive components ✓✓ ✓✓ ✓✓✓ Optical Optical am aplif mplif iersie rs ✓✓✓ ✓✓✓ Optical am ⚪ plif ⚪ H ie rs H ⚪ ✓✓✓ ⚪ H H ⚪ H ⚪ H (InP), (InP), sili con silicon (Si), (S and i),BBs and sili in con sili Si con ni‐ba t rni isdeterd i(SiN) d techno e (SiN) BBs monolithic logy. BBs monolithic in Si in ‐ ba Si technologies. ‐sba etechnologies. ds etechno d techno logy. logy. in Si-based technology. Table 1. Key features from Table thre 1. eKey availab feature le photonic s from thre integ e availab ration leplatfor photonic ms, i.integ e., indiu ration m platfor phosphide ms, i.e., indium phosphide Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓ ✓✓ ✓ Lasers ✓✓✓ ⚪ H ⚪ H Detec Detec torstors ✓✓✓ ✓✓✓ Detec tors ✓✓ ✓✓ ⚪ ✓✓✓ ⚪ H H ✓✓ ⚪ H (InP), silicon (Si), and sili (InP), con ni sitlircon ide (SiN) (Si), and monolithic silicon ni technologies. tride (SiN) monolithic technologies. Switches ✓✓ Switches✓✓ ✓✓ ✓ ✓✓ ✓ Build Build ing Table ing blo blo c1.k Key c(BB) k (BB) fe ature sInP Table from InP Table thre 1. Key 1.e Key availab fe Sia tfe Si ure al teure s photonic from s from SiN thre SiN Modulators thre integ e availab e ravailab ation le platfor photonic le photonic ms, ✓✓✓ integ i.e integ ., rindiu ation ration m platfor phosphide ✓✓ platfor m s,m i.s, e. ,i. indiu e., ✓ indiu m phosphide m phosphide Footprin Footprin t t ✓✓ ✓✓ Footprin ✓✓✓ ✓✓✓ t ✓✓ ✓ ✓ ✓✓✓ ✓ Table 1. Key features from three available photonic integration platforms, i.e., indium phosphide (InP), (InP), silicon (Si), and s(InP), ilicon (InP), si ni li tcon si rilidcon e(S (SiN) i), (S and i), monolithic and sili con silicon ni ttechnologies. rni idter i(SiN) de (SiN) monolithic monolithic technologies. technologies. Optical amplifiers ✓✓✓ Optical amplif ⚪i eH rs ✓✓✓ ⚪ H ⚪ H ⚪ H Building block (BB) BuildInP ing block (BB) Si InP SiNSwi tches Si SiN ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓✓ PasPas sivesive components components Chip Chip cost cost ✓ ✓Chip cost ✓✓ ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓✓ silicon (Si), and silicon nitride (SiN) monolithic technologies. Detectors ✓✓✓Detec tors ✓✓ ✓✓✓ ✓✓✓ ⚪ H ✓✓ ⚪ H Optical amplifiers ⚪ H ⚪ H Lase Lase rs rs ✓✓✓ ✓✓✓ Passive components Pas CMOS sCMOS ive ✓✓ components compa compa t⚪ ib✓✓ ilt⚪ iitH b yil it Hy ✓✓ CMOS ⚪ ✓✓✓ ⚪ H compa H ✓✓ ✓✓ ti✓✓ b il ity ✓✓✓ ✓ ✓ ✓✓ ✓ ⚪⚪ ⚪⚪ ⚪⚪ Building block (BB)Build Build ing InP ing blo blo ck c(BB) k (BB) Si InPInP SiN Si Si SiNSiN Footprint ✓✓ Detec Footprin tors ✓✓✓ 1t 1 ✓✓✓ ✓✓ ✓ ✓✓✓ ✓✓ 1 ✓ Building Block (BB) InP Si SiN ⚪ H Modulators Modulators ✓✓✓ ✓✓✓ ✓✓ ✓✓ ✓ ✓ ⚪ ⚪ /✓✓ /✓✓ ⚪ /✓✓ Lasers ✓✓✓ Lase rs ⚪ H ✓✓✓ ⚪ H ⚪ H ⚪ H Passive componentsPas Pas sive ✓✓ sive components components ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓ ✓✓ ✓✓ ✓✓ ✓✓✓ ✓✓✓ ✓✓ Low Low ‐cost‐cost pa ck paack gianggi ng ⚪ Low ⚪ ‐cost packaging ⚪ 2 2 2 Chip cost ✓ Chip cost ✓✓ ✓✓ ✓ ✓✓ ✓✓ Footprint ✓✓ ✓✓✓ ✓ SwiSwi tches tches ✓✓ ✓✓ ✓✓ ✓✓ ✓ ✓ Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓ ✓✓ ✓ Passive components 33 33 333 Lasers ✓✓✓ Lase Lase rs rs ⚪ H ✓✓✓ ✓✓✓ ⚪ H ⚪⚪ H H ⚪⚪ H H CMOS compatibility ⚪⚪ CMOS compa✓✓ tibilit y ⚪⚪ ✓ ✓✓ ✓ Chip cost ✓ ✓✓ ✓✓ Optical Optical Perform am Perform aplif mplif aience: rsaie nce: rs ✓✓✓ ✓✓✓ ✓✓✓ ✓✓✓ very very good; Perform good; ⚪ ✓✓ ⚪ ✓✓ H a nce: good H good ✓✓✓ ; ✓ ⚪ ; ✓ mod ⚪ H very mod eH s tgood; ;e ⚪ st; ⚪ chal ✓✓ chal leng good leng ing;ing ; ⚪⚪ ✓; ⚪⚪ mod very es very t; challeng ⚪ challeng challeng ing.ing ing. ; ⚪⚪ very challenging. Swi Lasers tches ✓✓ Swi tches 333 ✓✓ ✓✓ ✓ ✓✓ ✓ H H Modulators Modulators ✓✓✓ Modulators ✓✓ ✓✓✓ ✓✓✓ ✓ ✓✓ ✓✓ ✓ ✓ 1 1 ⚪ /✓✓ ⚪ /✓✓ H: hybrid/hete H: hybrid/hete rogeneous rogeneous processe processe H: hybrid/hete s tos over to over corme ogeneous co me the the desig processe desig n chnalleng chs alleng to es over asso esco asso me ciated cthe iated with desig with the n c the monolithic halleng monolithic es asso ciated with the monolithic CMOS compatibility ⚪⚪ ✓✓ ✓ Modulators Detec Detec torstors ✓✓✓ ✓✓✓ 333 ✓✓ ✓✓ ⚪⚪ H H 33 3 Optical amplifiers Optical ✓✓✓ am plifie⚪ rs H ✓✓✓ ⚪ H ⚪ H ⚪ H Low‐cost packaging Low‐cost packaging ✓✓ ✓✓ ⚪ ⚪ Switches Swi ✓✓ Swi tches tches ✓✓ ✓✓ ✓✓ ✓ ✓✓ ✓✓ ✓ ✓ 1 1 1 2 2 2 2 2 integ integ rated rated tec htec nol honol gy.ogy. End End ‐fire integ ‐fire cou r ated cou pling p tec ling (lo hnol w (lo reflect owgy. reflect iEnd on); ion); ‐ fire vertical cou vertical pling cou cou p(lo ling w pl ireflect (medium ng (medium ion); reflection). vertical reflection). cou p ling (medium reflection). ⚪ /✓✓ Switches 33 33 3 Footprin Footprin t t ✓✓ ✓✓ ✓✓✓ ✓✓✓ ✓ ✓ Detectors ✓✓✓ Detec tors ✓✓ ✓✓✓ ⚪ H ✓✓ ⚪ H Low‐cost packaging ⚪ ✓✓ Optical amplifiers Optical Optical ✓✓✓ am aplif mplif iers ⚪ ie rs H ✓✓✓ ✓✓✓ ⚪ H ⚪⚪ H H ⚪⚪ H H Performance: ✓✓✓ very good; Perform ✓✓ ance: good ✓✓✓ ; ✓ mod verye good; st; ⚪ ✓✓ chal leng good ing ; ;✓ ⚪⚪ mod every st; ⚪ challeng challeng inging . ; ⚪⚪ very challenging. Optical ampliﬁers 333 H H Chip Chip cost cost ✓ ✓ ✓✓ ✓✓ ✓✓ ✓✓ Footprint ✓✓ Footprin t ✓✓✓ ✓✓ ✓ ✓✓✓ ✓ Sever Sever al adv al adv anta anta gesg and es and /orSever /limi or limi tat al ions tadv ations aca nta n ca gbe n es be high and high lighted /or lighted limi tfrom at ions from the ca the n referre be referre high d tech lighted d tech nologies. nologies. from Hi the gh Hi referre gh d technologies. High Detectors ✓✓✓ Detec Detec t orstors ✓✓ ✓✓✓ ✓✓✓ ⚪ H ✓✓ ✓✓ ⚪⚪ H H H: hybrid/heterogeneous processe H: hybrid/hete s to overco rogeneous me the desig processe n challeng s to over es asso come ci ated the desig withn the ch alleng monolithic es asso ciated with the monolithic Performance: ✓✓✓ very good; ✓✓ good; ✓ modest; ⚪ challenging; ⚪⚪ very challenging. Detectors 333 33 CMOS CMOS gain gain compa , spee compa , spee d,ti band d, iltiit band y illow ity low loss lo ar ssgain e ar three e, three spee of✓✓ d, the of✓✓ and the ma low ma in at in lo tr at ss ✓ ibtr u ar ✓ ib tes eu three tof es phot of of phot onic theonic ma devices’ in devices’ attr ib improved u improved tes of phot performa H performa onic devices’ nce.nce. improved performance. Chip cost ⚪⚪ ⚪⚪ Chip ✓ cost ✓✓ ✓ ✓✓ ✓✓ ✓✓ 1 1 2 2 integrated technology. Footprin End‐integ fire t cou rated pling tec h(lo nol Footprin ✓✓ woFootprin reflect gy. End ti on);‐tfire ✓✓✓ vertical coup ling ✓✓ cou ✓✓ (lo pw li✓ ng reflect (medium ✓✓✓ i✓✓✓ on); reflection). vertical ✓ cou ✓ pl ing (medium reflection). H: hybrid/heterogeneous processes to overcome the design challenges associated with the monolithic 1 1 As As InP InP is ais direct a direct ba nba dga ndpga As mat p InP mat erial, is erial, ait direct presen it presen batsn the dtsga the best p mat best ave rial, ai avlab ai it lab le presen ga le iga n perform in ts perform the best ance, a av nce, however, ailab however, le ga iwith n perform with ance, however, with ⚪ ⚪ /✓✓ /✓✓ CMOS compatibility CMOS ⚪⚪ compa tibil✓✓ ity ⚪⚪ ✓ ✓✓ ✓ Footprint 33 1 333 2 3 Low Low ‐cost‐cost pa ck paack gianggi ng ⚪ Chip ⚪ cost integ rated technol Chip ✓ oChip ✓✓ gy. cost ✓✓ End cost ‐fire ✓✓ cou pling ✓ (lo ✓✓✓ w reflect ✓✓ ion); ✓✓ vertical ✓✓ cou ✓✓ pling (medium reflection). higher higher assoc assoc iateiate d loss d loss es du eshigher edu toe el to eassoc celtreic 2ctariate li2 cdoping ad l doping loss es from du from ecurren to curren eletc tinj ritc einj action l doping ection which which from require curren require s the t sinj the incl ection incl usiu owhich n si on requires the inclusion 1 1 Several advantages and/or Sever limiatlat adv ions anta cang es be and high /or lighted limitat from ions the can referre be high dlighted technologies. from the Hi gh referre d technologies. High ⚪ /✓✓ ⚪ /✓✓ Chip cost 3 33 33 Low‐cost packaging Low CMOS‐cost ⚪ compa packtaibgilinitgyCMOS CMOS ⚪⚪ ⚪ compa compa ✓✓ ti biltiitb✓✓ yil ity ⚪⚪ ⚪⚪ ✓✓ ✓ ✓✓ ✓✓ ✓ ✓ of optic of optic al am al am plifplif iersier [5s5] [5. 5] of Reg . optic Reg ardaing ard l ing am sy plif stems systems ier‐son [5‐‐on 5] ch.‐ip ch Reg (SOC) ipa (SOC) rding com sy com pstems lexity, plexity, ‐on InP ‐ch InP ip plat (SOC) plat form form ha com sha aps lexity, good a good InP platform has a good 2 2 gain, speed, and low loss ar gain e three , spee of d, the and ma low in at lotr ssib aruet es three of phot of the onic ma devices’ in attrib improved utes of phot performa onic devices’ nce. improved performance. Perform Perform ance: ance: ✓✓✓ ✓✓✓ very very good; good; ✓✓ ✓✓ good good ; ✓; ✓ mod mod esSever t;e ⚪ st; ⚪ a chal l adv chal leng aleng nta ingg;ing es ⚪⚪ ; and ⚪⚪ /very or very limi challeng t challeng ations ing ca .ing n .be highlighted from the referred technologies. High CMOS compatibility 33 3 1 1 1 mama turit turit y wi y twi h chips th chips comprising comprising maturit upy up to wi hundreds to th hundreds chips comprising of components. of components. ⚪ up/✓✓ to S hundreds ili Scon ilicon ‐ba‐s ba of e⚪ ds components. ep⚪ dh /otonic p✓✓ h/otonic ✓✓ proce proce Silisses con sses are ‐ba are sed photonic processes are As InP is a direct bandgaAs p mat InPe rial, is a direct it presen bantsd the gap best mat eav rial, ailab it lpresen e gain ts perform the best ance, avai however, lable gain with perform ance, however, with Low‐cost packagingLow Low ‐cost ⚪ ‐cost pa ck paack gianggi ng ⚪⚪ ✓✓ ✓✓ ✓✓ H: hybrid/hete H: hybrid/hete rogeneous rogeneous processe processe s tos over to over come co me thegain the desig desig , spee n chnalleng d, ch and alleng es low asso es asso lo ciss ated c ar iated ewith three with the of the monolithic the monolithic main at tributes of photonic devices’ improved performance. 1 2 Performance: ✓✓✓ very Perform good; ance: ✓✓ ✓✓✓ good ; very ✓ mod good; es t;✓✓ ⚪ chal good leng ; ✓ ing mod ; ⚪⚪ est; ⚪ very chal 2 challeng lenginging ; ⚪⚪ . very 2 2challenging. Low-cost packaging 33 complementary complementary metal metal ‐ox‐ide ox complementary ide ‐sem‐sem iconduc iconduc tor metal tor (CMOS (CMOS ‐oxide )‐co‐)sem ‐mpat compat iconduc /ible, 33 ible, altor lowi al lowi (CMOS ng ng its it)in‐sc t oeinmpat gra tegra tion ible, tion to al be tolowi be run ng run it s integration to be run 1higher 1 associated losses du higher e to el 2assoc ec2triicate al doping d losses from due to curren electtr iinj cael ction doping which from require currenst the inje incl ction us which ion requires the inclusion integ integ rated rated tec htec nol honol gy.ogy. End End ‐fire‐fire cou cou pling pling (low (lo reflect w reflect ion); ion); vertical vertical cou cou pling pl i(medium ng (medium reflection). reflection). As InP is a direct bandgap material, it presents the best available gain performance, however, with H: hybrid/heterogeneous H: processe hybrid/hete s to over rogeneous come the processe designs to ch alleng overcoes me asso thec idesig ated nwith challeng the monolithic es associated with the monolithic in ain CMO a CMO S fou S fou ndry ndry wh wh ichinic provides ha provides CMOS we fou we lln‐controlled dry ll‐controlled which and provides and more more we rapi lrapi l‐dcontrolled sc dal scab aliab lit yiand l ito ty the to more the fa brica rapi fabrica dti osc ntial onab ility to the fabrication of optical amplifiers [55]of . Reg optic ardaing l am sy plif stems iers ‐[5 on5]‐ch . Reg ip (SOC) arding com systems plexity, ‐on ‐InP chip plat (SOC) form com hasp lexity, a good InP platform has a good Performance: ✓✓✓ Perform very Perform good; ance: a✓✓ nce: ✓✓✓ ✓✓✓ good very ; ✓ very good; mod good; e s✓✓ t; ⚪ ✓✓ good chal good leng ; ✓; ing ✓ mod ; mod ⚪⚪ est;e ⚪ st ;very ⚪ chal chal challeng lengleng ing;ing ing ⚪⚪ ;. ⚪⚪ very very challeng challeng ing.ing . Performance: 1 333 very good; 1 higher 33 assoc good; 2 iate3 d loss modest; es due to 2 elecchallenging; trical doping from curren very t injchallenging. ection which requires the inclusion integrated technology. integ End‐rfire ated cou tecphling nol o(lo gy. w reflect End‐fire ion); cou p vertical ling (lo cou w reflect pling i(medium on); vertical reflection). coupling (medium reflection). environment environment [42] [4. 2] Never . Never thenvironment etles hesles , as s, com as com p [4 onents 2] ponents . Never in siin thli esi con les licon ssubs , as subs com trate trpate sonents ussua usllua yin llha si y vliha econ v higher e subs higher dimensions trate dimensions s usuall y ha ve higher dimensions Sever Sever al adv al adv anta anta gesg and es ma H: and turit /or hybrid/hete /limi or y wi limi tat thions t chips atrogeneous ions ca comprising n ca H: be n hybrid/hete H: ma processe be high hybrid/hete turit high lighted up ylighted s to wi to rogeneous over t h hundreds from r ogeneous chips co from me the processe comprising the the referre of processe desig referre components. s d nto cstech over h dup to alleng tech over nologies. to co me hundreds nologies. es co Sili me asso the con the desig cHi i‐ated ba desig gh Hi of sne gh with ccomponents. dhn p alleng chh the otonic alleng es monolithic asso es proce asso Scili iated con csses iated with ‐ba are with s the e d the p monolithic hotonic monolithic proce sses are H: hybrid/heterogeneous processes to overcome the design challenges associated with the monolithic integrated of optical amplifiers [55]. Regarding systems‐on‐chip (SOC) complexity, InP platform has a good than than in InP in InP an dan sinc d sinc e sieli si con lithan con photonic photonic in InPs an lasck d la sinc lickght lieght si soliurce con source sphotonic and s and am apl sm la ifpl ck iers, if liiers, ght InP InP so‐based urce ‐based s platforms and platforms ampl if priers, o pr viod InP vei d‐ebased platforms provide 1 2 1 1 1 2 2 2 gain gain , spee , spee d, and d, technology and low low loss lo . ar complementary sse End-ﬁr integ ar three e three rated of e coupling the of tec the hma nol metal ma in ogy. (low at in‐integ tr ox at End ibinteg complementary ide rtr eﬂection); urib‐tated fire es‐u sem r tated of es cou tec i phot of conduc htec p nol phot ling honic vertical onol gy. (lo onic tor ometal w devices’ gy. End reflect (CMOS devices’ coupling End ‐‐fire ox‐iide fire on); improved cou )‐‐ c improved sem cou o p (medium mpat ling vertical piling conduc (lo ible, performa w (lo cou performa reflect rw eﬂection). al tor p reflect llowi ing (CMOS ion); nce. (medium ng ion); nce. it vertical s ) ‐ vertical in co reflection). tmpat e gra cou tcou ible, pion ling p l to ial (medium ng lowi be (medium run ng reflection). its reflection). integra tion to be run maturity with chips comprising up to hundreds of components. Silicon‐based photonic processes are Several advantages and Sever /ora limi l adv tataions ntag es ca nand be/ or high limi lighted tations from can the be high referre lighted d tech from nologies. the referre High d technologies. High a hiag hi hegrh pot er pot enteiantl ia for l for volume volume a hi gschaling esc r aling pot and ent and ia inl tegr for integr avolume tioantio [2n 5] sc [2. aling 5] Despite . Despite and the in the tegr high high a tpr ion ocessin pr [2ocessin 5]. Despite g tech g tech nology the nology high processing technology As As InP InP is ais direct a direct ba nba dga ndin pga mat ap CMO mat erial, erial, S itfou presen itn presen dryts wh the tsicin the hbest aprovides best CMO av ai av Slab ai fou we lab le nga llldry e‐controlled iga n perform iwh n perform ich provides and ance, a nce, more however, however, we rapi ll‐controlled d with sc with al abi l iand ty to more the fa rapi brica d tsc ioal n ability to the fabrication gain, speed, and low lo gain ss ar , spee e three d, and of the low ma loin ss at artrecomplementary ib three utes of of the phot ma onic in metal at devices’ trib‐ox utide es improved of‐sem phot iconduc onic performa devices’ tor (CMOS nce. improved )‐compat performa ible, allowi nce. ng its integration to be run mama turit turit y ofy si oflicon silicon photon photon ics, maics, turit as an asy an ind of ind si irlicon ect ir eba ct photon ndga bandga p ics, ma p ma tas eria an telria na ind l tiv na ireetiv ct ga eba iga nndga is in difficult isp difficult mate to ria realize to l na realize tiv eun ga der un inder is difficult to realize under Several advantages Sever andSever /or al limi adv al adv taat nta ions anta ges ca g and esn and be /or high /limi or limi lighted tations tations from can ca be n the be high referre high lighted lighted d tech from nologies. from the the referre Hi referre ghd tech d tech nologies. nologies. Hi gh Hi gh environment [42]. Nevertenvironment heless, as com [4p2] onents . Never int si heliles con s, subs as com trate ponents s usua lliny siha liv con e higher substr dimensions ates usually have higher dimensions higher higher assoc assoc iateiate d loss d loss es du es edu toe el toe celtreicctarli cdoping al doping from from curren curren t injt einj ction ection which which require require s the s the incl incl usiu on si on As InP is a direct banAs dga InP p mat is ae rial, direct it presen bandgatsp the mat in abest e rial, CMO av itai S presen lab foulen dry ga ts ithe n wh perform best ich provides avaai nce, lab lhowever, e we gailnl‐ controlled perform with ance, and however, more rapi with d sc alability to the fabrication Several advantages and/or limitations can be highlighted from the referred technologies. High thisthi pla s pla tform. tform. The The pla pla tform tform thi fe s at pla feures attform. ures are are The in‐ between in pla ‐between tform InP fe InP at and ures and ul are tra ul‐tra low in‐‐low between los slos SisN Si InP techno N and techno logies, ultra logies, ‐low with with los s SiN technologies, with gain, speed, and lowgain logain ss, ar spee ,e spee three d, and d, of and low the low ma loss in lo ar ss ate tr ar three ib e u three tes of of the of phot the ma onic ma in at in tr devices’ at ibtruib tes u tof es improved phot of phot onic onic performa devices’ devices’ nce. improved improved performa performa nce.nce. of optic of optic al am al am plifplif iersier [5s5] [5than . 5] Reg . Reg in ard InP aing rd an ing sy d stems sinc systems e ‐si on li‐‐con on ch than ‐ip ch photonic (SOC) ip in InP (SOC) an com s la d com ck sinc p lexity, lipght elexity, si liso con InP urce InP photonic plat s and plat form aform m s pl ha lack if sha iers, alis ght good a InP good so ‐urce based s and platforms amplif iers, prov InP ide ‐based platforms provide higher associated losshigher es due assoc to eleiate ctridc aloss l doping es du efrom environment to el curren ectricalt doping inj [42] ection . Never from which th curren eles require s, as t inj com se ction the ponents incl which us iin on si require licon subs s thetr incl atesu sus ioua n lly have higher dimensions some some activ acetiv dee tde ecttieoctni oelnem elem some entsents but ac but tiv noe no li de ght liteght ct soiource so n urce el em solutions, so ents lutions, but and no and li me ght me di so adl urce ioptic al optic so al lutions, losses al losses (am and (am id me InP idd InP iand al optic and al losses (amid InP and gain, speed, and low loss are three of the main attributes of photonic devices’ improved performance. As InP is a direct baAs nd As ga InP pInP is mat ais edirect rial, a direct it ba presen nba dga ndpts ga mat the p mat e best rial, erial, av it presen ai itlab presen lets ga the ts in the perform best best av ai av alab nce, ailab le however, ga le iga n perform in perform witha nce, ance, however, however, with with mama turit turit y wi y twi h chips th chips comprising comprising a high eup r pot up to ehundreds to nt hundreds ial for volume of acomponents. of hi components. g hsceraling pote and Sntiliia Scon lili in for con tegr ‐ba volume ‐sba aetdio s epndh [2 otonic psc h5]otonic aling . Despite proce and proce sses in the sses tegr are high aare tio pr n ocessin [25]. Despite g tech nology the high processing technology of optical amplifiers of [5 5] optic . Reg ala am rding plif sy ierstems s [55]‐.on Reg than ‐chaip rd in ing (SOC) InP sy an stems com d sinc p‐lexity, on e si‐ch licon ip InP (SOC) photonic plat form com s la p ha lexity, cks li aght good InP source platsform and aha mspl aif iers, good InP ‐based platforms provide SiN) Si N) [55] [5. Re 5]. gRe ard gaing rding wa wa velevSin eN) le gtnh [5 g properties, t5] h .properties, Regarding Si/S Si wa iO /Sv‐ iO an ele‐ dn an g InP d th InP ‐properties, ba‐sba eds pl eda pl tf Si oart/S m foisrOm are ‐ san are main d InP main ly‐ba use lys euse dd in pld anear in tf onear r‐ms‐ are mainly used in near‐ higher associated loss higher eshigher du assoc e to assoc eliate ecitate dri closs adl loss doping es du es edu to from e el toe cel curren treicctarli cdoping ta inj l doping ection from which from curren curren require t injt einj sction the ection incl which which usio require n require s the s the incl incl usiu on si on complementary complementary As InP metal is metal a dir ‐ox‐ma ide ect oxide turit ‐sem bandgap ‐sem yiconduc ofi conduc silicon tor material, tor photon (CMOS (CMOS ma ics, )‐turit c it oas )‐mpat pr c oan ympat esents of ind ible, silicon iible, r eal ctthe lowi ba al photon lowi ndga ng best ng itp ics, s available ma itin sas t tein e gra an ria te gra ind tl ion natiion tiv r gain to ect e be to ga ba be irun ndga performance, n isrun difficult p materia tol realize nahowever tive ga unider n is , difficult with to realize under a higher potential for volume scaling and integration [25]. Despite the high processing technology maturity with chips comprising maturity wi up th to chips hundreds comprising of components. up to hundreds Silicon of‐ ba components. sed photonic Sili proce con‐ba sses se dare ph otonic processes are infrinfr ared ared (NIR) (NIR) , wh , wh ile ile sili si con liinfr con nitride ared nitride (NIR) can ca operate ,n wh operate ile from sili from con visib nitride visib le to le ca NIR ton NIR operate [48] [4. 8] .from visible to NIR [48]. of optical amplifiersof [5 optic of 5] .optic Reg al am aard l am plif ingplif ier sysier stems [5s5] [5. ‐5] Reg on . ‐Reg ch ard ipaing rd (SOC) ing sy stems sy com stems ‐pon lexity, ‐‐on ch‐ip ch InP (SOC) ip (SOC) plat com form com plexity, ha plexity, s a InP good InP plat plat form form ha sha as good a good this platform. The platform this fe pla atures tform. are The in‐ between platform InP feat ures and ul are tra in‐low ‐between loss Si InP N techno and ul logies, tra‐low with los s SiN technologies, with in ain CMO a CMO S fou S fou ndry ndry wh wh ichic provides h provides we we ll‐controlled ll‐controlled and and more more rapi rapi d sc dal scab aliab lityil ito ty the to the fabrica fabrica tionti on complementary higher associated metalcomplementary ‐oxide‐losses semiconduc due metal tor to‐ox (CMOS electrical idema ‐sem turit )‐cioconduc mpat ydoping of siible, tor licon (CMOS al fr photon lowi omng )‐curr c ics, it ompat s as in ent t an eible, gra injection ind t ion al irlowi e ct to ba be ngndga which run its in p tma egra requir teria tion l na es totiv be the e run gainclusion in is difficult to realize under Ultra Ultra ‐low‐low los slo Si ssN Si pNlan plar an liar Ultra ght light w‐low aw vea vcirc loes scirc uits SiN uits (P pLC) l an (PLC) ar wit li ght wit h pw hrop apvrag op e circ at agioat uits nio los n (P slos LC) below s below wit 0. h1 p 0.dB r1op /dB m ag /hav at mio hav en los e s below 0.1 dB/m have maturity with chips ma comprising ma turit turit y wi y tup wi h chips t to h chips hundreds comprising comprising of components. up up to hundreds to hundreds Silicon of components. ‐of ba components. sed photonic Sili Scon proce ilicon ‐basses ‐sba eds are epdh otonic photonic proce proce ssessses are are some active detection elem some ents ac but tiv no e de light tect so ionurce elem soents lutions, but no and li ght med so iaurce l optic soalutions, l losses and (am ime d InP dia and l optic al losses (amid InP and environment environment [42] [4. 2] Never . Never thetles hesles , as s, com as com ponents ponents in siinli si con licon subs subs trate trate s ussua usllua y llha y vha e v higher e higher dimensions dimensions in a CMOS foundry in wh aic CMO h provides S foun dry wel l‐wh controlled ich thi provides s pla and tform. more wel lThe ‐ controlled rapi pla d tsc form al ab and ife li at tmore yures to the rapi are fa d inbrica ‐sc between altab ioinli ty InP to the and fa ulbrica tra‐low tion loss SiN technologies, with of optical ampliﬁers [55]. Regarding systems-on-chip (SOC) complexity, InP platform © © has a good © been been demonstrated demonstrated by by Un been Un ivers ivers idemonstrated ty iof ty Ca of liCa folirnia fo by rnia , Sa Un , nt Sa ivers ant Ba a irBa ty ba rrof baa (UCS rCa a li(UCS foB) rnia B) and , and Sa Li nt oaLi n iBa oXnri X brease r arear se (UCS ch ar ch [5B) 8] [5 ,and 8], LioniX research [58], complementary metal complementary ‐complementary oxide‐semiconduc metal metal tor‐ox (CMOS ‐ide oxide ‐sem‐sem )‐iconduc coimpat conduc tor ible, tor (CMOS al (CMOS lowi)ng‐co) it‐mpat cso mpat intible, egra ible, talion lowi al to lowi ng be ng it run s itins t eingra tegra tiontion to be to be run run than than in InP in InP an dan sinc d sinc e sieli si con Si licon N) photonic [5 photonic 5]. Resg la asrd ck laing lickght li wa ght so veurce Si so leN) nurce gs [5 tand h5] s properties, and . Re amg apl am rd ifpl iers, ing if Si iers, wa /S InP iO vInP ‐e‐ based le an‐nbased dg tInP h platforms properties, ‐ platforms based pl pra oSi t pr fvo/S irodm ivO ei sd‐ are an e d main InP‐ly ba use sedd pl inat near form‐s are mainly used in near‐ environment [42]. Never environment theless, as [4 com 2]. p Never onents th einles some sisli, con as ac com tiv subs ep de onents trtate ectsi ous n in ua e si lem llliycon ents ha vsubs ebut higher tr no ate lis ght dimensions usua sollurce y ha so v elutions, higher and dimensions medial optical losses (amid InP and orders orders of magnitude of magnitude lowe lowe orders r than r than of ot h magnitude ot erh reported er reported lowe pla pl trf oathan rtm fosrm . ot As s.h As ear re reported as re ulst,u high lt, pl high ‐ape tfo‐rpe rfomrrsma fo. rAs ma nc ean cpa re e sspa suilvst,es i high ve ‐performance passive maturity with chips comprising up to hundreds of components. Silicon-based photonic processes in a CMOS foundryin wh ain CMO ic ah CMO provides S fou S fou ndry we ndry lwh l‐controlled wh ichic provides h provides and we more we ll‐controlled l lrapi ‐controlled d sc aland ab and i limore ty more to rapi the rapi dfa brica sc dal scab taliioab ln it yil ito ty the to the fabrica fabrica tionti on a hiag hi hegrh pot er pot enteiantl ia for l for volume infr volume ared scaling sc (NIR) aling and , wh and inile tegr in sitegr liacon tinfr ioant io nitride ared [2n5] [2 .(NIR) 5] Despite ca . Despite n ,operate wh the ile the si high from li con high pr visib nitride ocessin processin le to ca g NIR ntech goperate tech nology [48]nology . from visible to NIR [48]. than in InP and since than silicon in photonic InP and sinc s lack e si lilight con so Si photonic urce N) [5s 5] and .s Re la agck mapl rd light ifing iers, so wa urce InP vele‐sbased nand gth a properties, platforms mplifiers, pr InP Sio/Sv‐ibased iO de‐ an dplatforms InP‐base d pr pl ovaitdfoer ms are mainly used in near‐ components components are are en abled enabled bycomponents by thi sthi pla s pla tform, t form, are thou en thou abled gh gh electr by electr thi ical s ical pla pump tpump form, for thou for ga iga ngh is in electr una is una vical aivlaai bl pump laebl due e due for to ga th toe ith n is e unavailable due to the environment [42]. Never environment environment theless, as [4 com 2] [4. 2] Never p. onents Never thet les in he ssi les , lias scon , com as subs com ponents tr ponents ates in us si ua inli llsi con yli ha con subs v esubs higher trate trate s us dimensions sua usllua y llha y vha e v higher e higher dimensions dimensions mama turit turit y ar of ey si of complementary licon silicon photon photon ics,ics, as Ultra an as metal-oxide-semiconductor ‐an ind low ind ir eloct isr seba ct Si ndga ba N ndga plp an ma ar pUltra ma tlieght ria te‐lria w low na alv tiv na elo circ etiv s sga (CMOS)-compatible, eSi uits iga n N is i pn(P ldifficult an isLC) difficult ar wit light to h w p realize toarop v realize e ag circ at unuits io allowing der un n der los (PLC) s below wit itsh 0. integration p1r op dBag /mat hav ion elos to s below be 0.1 dB/m have infrared (NIR), while silicon nitride can operate from visible to NIR [48]. a higher potential fora volume higher pot scaling entia and l for in volume tegratio scnaling [25]. and Despite integr the at io high n [2 pr 5]ocessin . Despite g tech the nology high pr ocessing technology insu insu latilng ati ng nat nat ureu rofe of SiN/Si SiN/Si insu O glas O la tiglas ng s WG snat WG su r[e55] s of [55] . Howe Si. N/Si Howe ver, O ver, glas the sthe de WG mo des mo ns [55] tns ra. ttiHowe roanti oof n ver, of optic optic the al gai ade l mo gai n usin ns n tusin rga tiogn of optical gain using © © than in InP and sincethan sithan licon in InP in photonic InP an dan sinc ds la sinc eck sie lili si con ght licon photonic source photonic s and s las ck a la m lick pl ght ifliiers, ght source so InP urce s‐ based and s and am platforms aplmifpl iers, ifiers, InP pr InP o‐based vid‐based e platforms platforms pro pr viodvei de been demonstrated by Un been ivers demonstrated ity of California by , Un Sant ivers a Ba ityrb of ar aCa (UCS lifornia B) and , Sa nt Liao nBa iXrb are rase (UCS arch B) [5 8] and , LioniX research [58], thisthi pla s pla tform. tform. The The pla pla tform tform feat feures atures are are in‐ between in‐between InP InP and and ultra ul‐tra low‐low los slos SisN Si techno N techno logies, logies, with with maturit run y of in silicon a CMOS photon maturit foundry ics,y as of an silicon ind which i rphoton ect ba pndga rics, ovides as p ma an Ultra tind well-contr eria‐low irl ena ct lo tiv ba ssendga Si ga olled Nin pp is ma lan difficult and tar eria light lmor na to wtiv arealize evee rapid ga circ in un uits is scalability der difficult (P LC) wit to realize h to prop the un agat der fabrication io n loss below 0.1 dB/m have erbium erbium ‐dope ‐dope d WG d WG distrib distrib erbium uteudte Br d‐ ag dope Brgag rg d eflec rWG eflec tor distrib tor (D BR (Du)BR te and )d and Br distrib ag distrib g ru eflec teudte tor feedb d feedb (DBR ack)a c(DFB) and k (DFB) distrib laser lau ser arrays te darrays feedb ack (DFB) laser arrays a higher potential for a hi volume ag hi hegrh pot e rsc pot ealing nteiantl ia and for l for volume in tegr volume a tsc ioaling n sc [2 aling 5] and . Despite and integr in tegr the atio ahigh ntio [2n5] pr [2. 5] ocessin Despite . Despite g the tech the high nology high pr ocessin processin g tech g tech nology nology some some activ acetiv dee tde ecttieoctni oelnem eorders lem entsents but of but no magnitude no light light source so lowe urce so orders rlutions, so than lutions, of ot and magnitude he and r me reported me diadl ioptic lowe al pl optic aart lf than losses oarlm losses sot . (am As he (am r iadreported re InP ids uInP land t, high and pla t‐fpe orrm fosr.ma Asn cae re pa suslsti,v high e ‐performance passive this platform. The plathi tform s pla fe tform. atures The are pla in‐tbetween formbeen feat InP ures demonstrated and are ul intra ‐between ‐low by los Un InP sivers Si and Ni ty techno uloftra Ca‐logies, low lifo rnia los with s, Si SaN nt techno a Barblogies, ara (UCS with B) and LioniX research [58], environment [42]. Nevertheless, as components in silicon substrates usually have higher dimensions maturity of silicon photon mama turit turit ics, y of as y si of an licon si ind licon photon ire ct photon baics, ndga ics, asp an as ma an ind te ria ind irelct ina r eba ct tiv ndga ba e ga ndga p in ma is p difficult ma teria telria nal tiv to na realize etiv gae iga n is iun n difficult is der difficult to realize to realize un der under SiN) Si N) [55] [5. Re 5]. gRe ard gaing rding wa wa vecomponents levn ele gtnhg properties, th properties, are en Si abled /S SiiO /S ‐ components by iO an ‐ thi dan InP sd pla InP ‐ba tform, ‐s ba eare ds pl e en dthou a pl tabled foartgh m fosr by electr m are s thi are main ical s main pla ly pump t form, use ly use d for in thou d near in ga gh near in‐ is electr ‐unaical vaila pump ble due for to ga thine is unavailable due to the some active detectionsome elem ents activ but e de no tect liight on eso lem urce ents orders so but lutions, no of magnitude li and ght so me urce dia lowe lso optic lutions, r athan l losses and oth (am me er d reported id ia lInP optic and apl l losses atform (am s. As id InP a re and sult , high‐performance passive Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: Appl. Sci. 2020, 10, x; doi: www.mdpi.com/ www.mdpi.com/ journal/applsci journal/applsci www.mdpi.com/ journal/applsci than in InP and since silicon photonics lack light sources and ampliﬁers, InP-based platforms provide this platform. The pla thitsthi form pla s pla t form. fetat form. ures The The are pla in pla tform ‐between tform feat feures at InP ures are and are in ul‐ between in tra‐between ‐low los InP s InP Si and N and techno ultra ul‐tra low logies, ‐low los slos with SisN Si techno N techno logies, logies, with with infrinfr ared ared (NIR) (NIR) , wh , wh ile ile sili si con insu licon nitride la tnitride ing ca nat n ca uoperate rne operate of Si from N/Si insu from O visib l aglas visib ting les to lnat WG e NIR tou srNIR e [[4 55] of8] [4 ..Si 8] Howe N/Si . Over, glas the s WG demo s [55] nst.r aHowe tion of ver, optic thea lde gai mo n ns usin tratgi on of optical gain using SiN) [55]. Regarding wa SiN) ve le [5n5]g.t Re h properties, garding wa Siv/S eleiO ncomponents g‐ tan h d properties, InP‐ba are sed Si en pl/Sabled aitO fo‐ ran m by sd are InP thi main s‐ ba pla setly d form, pl use atd fthou o in rm near sgh are ‐electr mainical ly use pump d in for near ga‐ in is unavailable due to the some active detectiosome n esome lem acents tiv acetiv but dee t de no ectt ielioct ght ni oe lnso em eurce lem entsents so but lutions, but no no light and light so me urce sodurce i aso l optic lutions, solutions, al losses and and me (am me diiaddl ioptic InP al optic and al losses a l losses (am (am id InP id InP and and Ultra a Ultra higher ‐low‐low los slo potential Si ssN Si pNerbium lan plar an for liarght ‐dope light volume wadw v eWG a vcirc e circ uits distrib scaling uits (P erbium LC) u (P teLC) d and wit Br‐dope wit h ag integration pghr dop rp eWG rflec ag opat ag tor distrib ioat n(D io los nBR [ 25 uslos te )below ]. sd and below Br Despite ag distrib 0.g1 0.dB re1flec u/dB m te the d tor /hav m feedb high hav (D e BR ea c)pr k and (DFB) ocessing distrib laser ute technolog arrays d feedb ack y(DFB) laser arrays insulating nature of SiN/SiO glass WGs [55]. However, the demonstration of optical gain using infrared (NIR), while infr silicon ared nitride (NIR) ,ca wh n ile operate silicon from nitride visib calen to operate NIR [4 from 8]. visible to NIR [48]. SiN) [55]. RegardingSi wa N) Siv N) [5 ele5] [5 n.g Re 5]th. g Re properties, ard gaing rding wa wa v Siele /S vn eiO le gt‐ nhan g properties, th d properties, InP‐ba©se Si d © /S pl SiiaO /S tf‐ ioO an rm‐ dan s InP are d InP ‐ ba main ‐sba edly s pl e duse a pl tfodart in m fo srnear m are s are ‐main main ly use ly use d ind near in near ‐ ‐ been been demonstrated demonstrated by by Un Un ivers ivers ity iof ty Ca of liCa folirnia fornia , Sa, nt Saant Ba a rBa barrbaa (UCS ra (UCS B) B) and and Li oLi nioXni Xrese rearsech ar ch [58] [5, 8], Ultra maturity ‐low losof s Sisilicon N plUltra anarphotonics, ‐lilow ght w loasvse Si circ N as puits lan anerbium ar (P indir LC) light ‐wit ect w dope ahv bandgap epd rcirc op WG ag uits at distrib io (Pn material LC) los u te swit d below hBr pag native rop 0. g 1ag r edB flec atio /gain m tor n hav los (D is se BR below di ) and cult 0. 1distrib dB to/m ru ealize hav ted efeedb under ack (DFB) laser arrays infr Appl. ared Sci. (NIR) 2020, ,10 wh , x;ile doi: infr si infr liared con ared Appl. (NIR) nitride (NIR) Sci., 2020 wh ca, nwh ile , 10 operate ile si , x;li doi: si con licon from nitride nitride visib can lca e operate to n operate NIR [4 from 8] from . visib visib www.mdpi.com/ le to le NIR to NIR [48] j[4 ournal/applsci . 8]. www.mdpi.com/ journal/applsci orders orders of magnitude of magnitude lowe lowe r than r than oth ot erh reported er reported pla pl tfoartm fosrm . As s. As a re as re ulst,u high lt, high ‐pe‐rpe forrma forma ncen cpa e spa sivsesi ve © © been demonstrated by been Un demonstrated iversity of Ca liby fo rnia Univers , Santity a Ba of rCa barliaf o(UCS rnia, B) Sa and nta Ba LirobnairXa (UCS resear B)ch and [58] Li , oniX research [58], this platform. The platform features are in-between InP and ultra-low loss SiN technologies, with Ultra‐low loss SiN pUltra lanUltra ar‐ low light ‐low lows aslov Si ses N circ Si pNluits an plar an (P liar LC) ght light w wit aw vhea p vcirc erop circ uits aguits at (PioLC) n (P los LC) wit s below wit h phrop p r0.ag op 1 at ag dB ioat /nm io los n hav slos below es below 0.1 0.dB 1 /dB m /hav m hav e e components components are are en abled enabled by by thi sthi pla s pla tform, tform, thou thou gh gh electr electr icalical pump pump for for ga iga n is in una is una vaivlaaibllaebl due e due to th toe th e Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci orders of magnitudeorders lower of than magnitude other reported lower pl than atf oot rm hes.r As reported a resu lpl t, ahigh tform‐pe s. rAs for ma a re nscu e ltpa , high ssive‐ performance passive © © © some activebeen detection demonstrated elements been bybeen Un demonstrated but ivers demonstrated no ity light of Ca by lisour f oby Un rnia Un ivers ce , ivers Sa solutions, int tya iof tyBa Ca of rb aliCa rfaoli and rnia (UCS fornia , Sa medial B) , nt Sa and ant Ba a Li rBa optical boanrrbiaXa (UCS ra re (UCS se losses B) ar ch B) and [5 and (amid 8] Li, o Li nioXni InP Xrese rearsech ar ch [58] [5, 8], insu insu latilng ati ng nat nat ureu rofe of SiN/Si SiN/Si O glas O glas s WG s WG s [55] s [55] . Howe . Howe ver,ver, the the de mo demo nstns rattiroanti oof n of optic optic al gai al gai n usin n usin g g components are enabled components by this pla are tform, enabled thou by gh thi electr s plaical tform, pump thou for gh ga electr in is ical una pump vailabl for e due ga ito n isth una e vailable due to the orders of magnitude orders lowe orders rof than magnitude of magnitude other reported lowe lowe r than rpl than at fot orh m ot ersh .reported eAs r reported a res u pllta ,pl t fhigh oartm fo‐srpe m . As sr.f o As rama re an s re ucelst ,upa high lt,s shigh iv‐pe e ‐rpe forrma forma ncen cpa e spa sivsesi ve erbium erbium ‐and dope ‐dope d SiN) WG d WG [distrib 55 distrib ]. Regar uteudte Br d ding ag Brgag rwavelength geflec reflec tor tor (D BR (D)BR pr and ) operties, and distrib distrib uteSi udte /feedb SiO- d feedb aand cka c(DFB) k InP-based (DFB) laser laser arrays arrays platforms are mainly used in insulating nature of insu SiN/Si latO ing glas natsu WG re ofs [Si 55] N/Si . Howe O glas ver, s WG thes de [55] mo. ns Howe tratiover, n of the optic demo al gai nstnra usin tion gof optical gain using components are enabled components components by this pla are are t en form, abled enabled thou bygh by thi electr sthi pla s pla tical form, tform, pump thou thou for gh gh ga electr ielectr n isical una ical pump v ai pump la bl for e for due ga iga n to is in th una ise una vaivlaaibllaebl due e due to th toe th e erbium‐doped WG distrib erbium ute‐dope d Brdag WG g re flec distrib tor u(D teBR d Br ) and agg distrib reflector ute (D d BR feedb ) and ack distrib (DFB) ula teser d feedb arrays ac k (DFB) laser arrays near-infrared (NIR), while silicon nitride can operate from visible to NIR [48]. Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: insulating nature ofinsu Siinsu N/Si latilng aOt i ng glas nat nat usr eWG u rofe sof Si [55] N/Si SiN/Si . O Howe glas Owww.mdpi.com/ glas ver, swww.mdpi.com/ WG s the WG s [de 55] s [mo journal/applsci 55] . Howe jns ournal/applsci . Howe tratver, ionver, of the optic the de mo de almo ns gai tns rn att irusin oanti oof gn of optic optic al gai al gai n usin n usin g g Ultra-low loss SiN planar lightwave circuits (PLC) with propagation loss below 0.1 dB/m have erbium‐doped WG erbium distrib erbium u‐dope ted‐dope Br d ag WG dg WG r edistrib flec distrib toru te (DudBR te Br d) ag Br and gag r gedistrib flec reflec toru tor te (Dd BR (D feedb )BR and ) aand c kdistrib (DFB) distrib u telaudser te feedb d arrays feedb acka c(DFB) k (DFB) laser laser arrays arrays Appl. Sci. 2020, 10, x; doi: Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci www.mdpi.com/ journal/applsci been demonstrated by University of California, Santa Barbara (UCSB) and LioniX research [58], orders Appl. Sci. 2020, 10, x; doi: Appl. Appl. Sci. Sci. 2020 2020 , 10,, x; 10 doi: , x; doi: www.mdpi.com/journal/applsci www.mdpi.com/ www.mdpi.com/ journal/applsci journal/applsci of magnitude lower than other reported platforms. As a result, high-performance passive components are enabled by this platform, though electrical pump for gain is unavailable due to the insulating nature of SiN/SiO glass WGs [55]. However, the demonstration of optical gain using erbium-doped WG distributed Bragg reﬂector (DBR) and distributed feedback (DFB) laser arrays integrated within ultra-low loss SiN [59] shows potential for the platform’s active component integration. Silicon is Appl. Sci. 2020, 10, 4024 5 of 19 among the most mature photonic integrated platforms, leveraging from existing technologies like silicon electronics and micro electro-mechanical systems (MEMS) industries and enabling foundry services robustness and product commercialization. InP platforms can be seen as one of the options that stem out from the alternatives since it integrates monolithically high-quality lasers, receivers, ampliﬁers, and passives [33,35,60]. Si and SiN are more prone to be used for passive and ﬁltering components, with the need to recur to hybrid packaging [61] when incorporating lasing and gain. Some of the available foundries/players under the InP, Si, and SiN integrated technologies are described below: i. InP: Fraunhofer Heinrich-Hertz-Institut–Fraunhofer HHI (Berlin, Germany); Smart Photonics (Eindhoven, The Netherlands); Global Communication Semiconductors, LLC (Torrance, CA, USA); and fabless solution providers, e.g., the INTENGENT III/V Photonics Company (Ontario, Canada), VLC Photonics (Valencia, Spain), Bright Photonics (Eindhoven, The Netherlands), and PICadvanced (Aveiro, Portugal); ii. Si: Acacia Communications, Inc. (Maynard, MA, USA); Luxtera Inc. (Carlsbad, CA, USA); Intel Corporation (Mountain View, CA, USA); Cisco Systems Inc. (San José, CA, USA); Mellanox Technologies (Sunnyvale, CA, Israel/USA); Finisar Corporation (Sunnyvale, CA, USA); Hamamatsu Photonics K.K. (Shimokanzo, Iwata City, Japan); International Business Machines Corporation (IBM, Armonk, NY, USA); GlobalFoundries Inc. (Santa Clara, CA, USA); NeoPhotonics Corporation (San José, CA, USA); Oclaro Inc. (San José, CA, USA); CompoundTek (Singapore); and the American Institute for Manufacturing Integrated Photonics (AIM Photonics, Albany, NY, USA); iii. SiN: Ligentec (Ecublens, Switzerland); Lionix (Enschede, Netherlands); Institute of Microelectronics of Barcelona IBM-CNM, “Silicon Nitride Technology,” (Barcelona, Spain). Furthermore, several reference institutions with state-of-the-art research in integrated photonics contributed to the technology developments, namely the University of California, Santa Barbara (UCSB) with the available California NanoSystems Institute (CNSI) Nanostructure Cleanroom Facility (NCF); the Kavli Nanoscience Institute (KNI) laboratory at the California Institute of Technology (Caltech), the Fraunhofer Heinrich-Hertz-Institut (HHI); the ePIXfab, INTEC department–Photonics Research Group at Ghent University; the Nanophotonics Technology Center (NTC) at the Universitat Politècnica de València (UPV); etc. Beyond the generic availability of design/fabrication, it is necessary to further develop and invest in packaging technologies to reach simpler, standardized, and ecient approaches, thus making it accessible for users and/or companies. Currently, the technology-associated limitations are mainly related to the lack of general design rules that ease the encapsulation and allow successful testing of the manufactured photonic chips [6]. This last integration step (packaging) is imperative for labs and companies that are trying to enter/grow in the integrated photonic technology and market. A consortium model aiming to establish PIC packaging processes under European commission Horizon 2020 funding is currently being conducted [62] to facilitate the development of this PIC technologic branch through the development of standard technologies to provide access to aordable, mature, and scalable packaging solutions [63]. 3. Passive Optical Networks From the 80s onwards, PONs [64] have evolved greatly with the simpliﬁcation of network management, pushing network trac multiplexing closer to the user with the minimization of stranded capacity [65]. Since then, PON architecture remained one of the most predominant access network options. PONs have been under mass deployment, with more than 900 million broadband subscribers reported in 2017 [66]. This infrastructure has subsidized PONs establishment relevance for a long time [65]. PON basic technology includes: (i) the equipment at the edge of Metro/Access, i.e., the optical line terminal (OLT); (ii) the user end terminal, i.e., the optical networks unit (ONU); and (iii) the optical Appl. Sci. 2020, 10, x 6 of 19 Appl. Sci. 2020, 10, 4024 6 of 19 i.e., the optical line terminal (OLT); (ii) the user end terminal, i.e., the optical networks unit (ONU); distribution and (iii) the opti network cal distri (ODN) butithat on network ( connectsO the DN) tha OLT to t connects the the ONUs (see OLT to the ONUs (see optica optical access network branch l access in Figur network bran e 1). ch in Figure 1). PON PON emer emerg ginging technic technical ra equir l reements quiremen arets ar verye ver unique, y un being ique, being the the surviving solutions surviving particularly solutions sensitive particulato rly price sensit and ive t related o price and re costs, both lated capital costs, bot expense h capit (CAPEX) al expen and se (CAPEX operating ) and expense operat (OPEX) ing expense [67]. Nevertheless, (OPEX) [67]. Neverthele PON posesss, several PON pose advantages, s severasuch l advant as the agesimpliﬁcation s, such as the s of impli infrastr ficat uctur ion oe f infr management, astructure as mana it isgement based ,on as it a fully is base passive d on a ODN fully p [65 ass ].ive OD A completely N [65]. A complet passive ODN ely pamade ssive O ofDburied N made o glass f buried with glass with cabinets to ensure connections and splitting anticipates long-term expectation of 20 years cabinets to ensure connections and splitting anticipates long-term expectation of 20 years or even mor or even more e. Its architectur . Its architect e derives ure der from ives thefr technical om the te appr chnica oach l approa to be deployed ch to be deployed a at the PON t the PON ends. ends. Several Several technologies are competing to be implemented in the access space, with the most common technologies are competing to be implemented in the access space, with the most common based on based a passive on a passive splitter splitter tree topology tree topology . The OL . The OLT T (interface (interface from the from operator the operator side) feeds side) one feedtr s one unk trun ﬁber k fiber that splits into several arms, each arm is divided into another set of arms, and so on, generating that splits into several arms, each arm is divided into another set of arms, and so on, generating a tr a tree ee strstructure, wh ucture, which, ich, d depending epending on on the the technolog technology,ycan , can range from low range from low (8 (8 to to 16) 16) to hi to high gh (64 (64 to to 256) 256) terminal counts. terminal counts. Optical Optical access access stan standar dardization dization h hasas ev evolved olved r rapidly apidly, w , with ith IE IEEE EE 802.3 802.3 and and International International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) Q2 groups playing Telecommunication Union - Telecommunication Standardization Sector (ITU-T) Q2 groups playing dominant rdominant ro oles in the prles in ocess the proc [68]. Theess most [68]. The common most co technologies mmon tar echnologi e Gbit-capable es are G PON bit-c (GPON) apable PON and Ethernet (GPON) and Ethernet PON (EPON) [69], typically supporting up to 64 users in the same OLT trunk fiber. At PON (EPON) [69], typically supporting up to 64 users in the same OLT trunk ﬁber. At the end users the end users lays an ONU (the user terminal for the fiber). For each end technology, a different ONU lays an ONU (the user terminal for the ﬁber). For each end technology, a dierent ONU is typically is typically required. Currently, it is very common to have combined interfaces that allow multiple required. Currently, it is very common to have combined interfaces that allow multiple technologies to technologies to be handled within the same device, the so-called “combo” solutions [70]. Figure 2 be handled within the same device, the so-called “combo” solutions [70]. Figure 2 illustrates some of illustrates some of the main existing characteristics of the ITU-T related technologies. the main existing characteristics of the ITU-T related technologies. Figure 2. Recent International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Figure 2. Recent International Telecommunication Union-Telecommunication Standardization Sector (ITU- based optical technologies standards evolution. T) based optical technologies standards evolution. Some operators are engaging in the 10 G-capable symmetric PON (XGSPON) and 10 G-capable Some operators are engaging in the 10 G-capable symmetric PON (XGSPON) and 10 G-capable EPON (10GEPON). Moreover, a number of more aggressive and disrupting operators are investing in EPON (10GEPON). Moreover, a number of more aggressive and disrupting operators are investing deploying NGPON2, merging the traditional concepts of time-multiplexed single-channel per direction in deploying NGPON2, merging the traditional concepts of time-multiplexed single-channel per PON combined with wavelength division multiplexing (WDM) [71]. This approach allows for further direction PON combined with wavelength division multiplexing (WDM) [71]. This approach allows developing network ﬂexibility concepts and the smooth evolution of oered data rates [70], i.e., up to for further developing network flexibility concepts and the smooth evolution of offered data rates 40 or 80 Gbit/s aggregated rates. An example of a PON with coexisting technologies supported by the [70], i.e., up to 40 or 80 Gbit/s aggregated rates. An example of a PON with coexisting technologies current ITU-T standard series [71] is depicted in Figure 3. supported by the current ITU-T standard series [71] is depicted in Figure 3. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 7 of 19 Appl. Sci. 2020, 10, x 7 of 19 Figure 3. Diagram describing a PON architecture with three coexisting technologies. Figure 3. Diagram describing a PON architecture with three coexisting technologies. New trends are emerging and triggering discussions in the fora, mostly focused on either an New trends are emerging and triggering discussions in the fora, mostly focused on either an architecture innovation, like the Super-PON, which is on the standardization path in IEEE and ITU-T, architecture innovation, like the Super-PON, which is on the standardization path in IEEE and ITU-T, or on the eorts to move to high-speed PON (HSP) towards higher data rate technologies, such as or on the efforts to move to high-speed PON (HSP) towards higher data rate technologies, such as 25 25 Gbit/s and 50 Gbit/s [72]. Gbit/s and 50 Gbit/s [72]. Current low-cost laser and receiver technologies face some challenges to deliver higher data rates Current low-cost laser and receiver technologies face some challenges to deliver higher data without increasing packaging or electronic costs. Namely, the current methods used for interconnecting rates without increasing packaging or electronic costs. Namely, the current methods used for devices with drivers and boards are typically based on wire bonding and ﬂex printed circuit board interconnecting devices with drivers and boards are typically based on wire bonding and flex printed (f-PCB). The latter has limited high-frequency performance and tough mechanical design challenges to circuit board (f-PCB). The latter has limited high-frequency performance and tough mechanical meet the right soldering robustness, with an expected high impact on the electrical performance of design challenges to meet the right soldering robustness, with an expected high impact on the assemblies and sub-assemblies. electrical performance of assemblies and sub-assemblies. Among PON future technologies, high interest has been given to the research/development Among PON future technologies, high interest has been given to the research/development of of long-reach/high-speed transmission, where optical ampliﬁers, electrical domain digital signal long-reach/high-speed transmission, where optical amplifiers, electrical domain digital signal processing (DSP) to enhance the link budget, and/or transceiver technology are predicted to be key processing (DSP) to enhance the link budget, and/or transceiver technology are predicted to be key factors [73]. Generically, for N subscribers, a PON requires a single transceiver at the central oce, factors [73]. Generically, for N subscribers, a PON requires a single transceiver at the central office, which results in a total of N + 1 transceivers overall [67]. For NGPON2, the OLT transceiver should which results in a total of N + 1 transceivers overall [67]. For NGPON2, the OLT transceiver should contain four transmitters (Tx) and four receivers (Rx) with an internal wavelength mux/demux optical contain four transmitters (Tx) and four receivers (Rx) with an internal wavelength mux/demux module. In the ONU, a 10 Gbps transceiver with both tunable Tx and Rx is required [70]. By employing optical module. In the ONU, a 10 Gbps transceiver with both tunable Tx and Rx is required [70]. By wavelength-selective branching devices in ODN, losses can be diminished, nevertheless at the cost of employing wavelength-selective branching devices in ODN, losses can be diminished, nevertheless wavelength transparency waiving. Furthermore, by increasing the number of wavelengths, a high at the cost of wavelength transparency waiving. Furthermore, by increasing the number of number of transceivers at the OLT is expected, raising power consumption, port density, and cost [73]. wavelengths, a high number of transceivers at the OLT is expected, raising power consumption, port Similar to what happened with integrated circuits’ contribution to electronics development, PIC density, and cost [73]. Similar to what happened with integrated circuits’ contribution to electronics technology is one of the most attractive solutions to tackle the referred issues. Thus, integrated development, PIC technology is one of the most attractive solutions to tackle the referred issues. Thus, photonics can have a key role in the deployment of more ﬂexible PON networks, as they can integrated photonics can have a key role in the deployment of more flexible PON networks, as they incorporate dierent optical components with various functions in a potentially cost-eective way can incorporate different optical components with various functions in a potentially cost-effective without increasing the assembly process complexity [7,8]. way without increasing the assembly process complexity [7,8]. 4. PIC for Next-Generation PON 4. PIC for Next-Generation PON Photonic integration can provide advantageous solutions to PON by limiting the number of Photonic integration can provide advantageous solutions to PON by limiting the number of optical and electrical interconnections and thus overcoming some of the bulk packaging restrictions, optical and electrical interconnections and thus overcoming some of the bulk packaging restrictions, such as bandwidth, power dissipation, and cost [65,74]. Nevertheless, complexities can arise with such as bandwidth, power dissipation, and cost [65,74]. Nevertheless, complexities can arise with the the use of this technology, namely ﬁber interfacing diculties, number of electrical connections, use of this technology, namely fiber interfacing difficulties, number of electrical connections, and and possible high-density of components per device, which demands a high power dissipation possible high-density of components per device, which demands a high power dissipation Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 8 of 19 Appl. Sci. 2020, 10, x 8 of 19 management [70]. The development of PIC solutions for access networks has already been reported, management [70]. The development of PIC solutions for access networks has already been reported, e.g., with an InP monolithic transceiver PIC for NGPON2 applications [75], the proposed layout e.g., with an InP monolithic transceiver PIC for NGPON2 applications [75], the proposed layout has has the potential to be implemented as an OLT and with ﬁlter redesigning as an ONU. A photonic the potential to be implemented as an OLT and with filter redesigning as an ONU. A photonic integrated apparatus for tunable multi-wavelength transmission was patented in 2017 [76], the time and integrated apparatus for tunable multi-wavelength transmission was patented in 2017 [76], the time wavelength division multiplexing (TWDM)-PON transmitter system proposed is tailored to support and wavelength division multiplexing (TWDM)-PON transmitter system proposed is tailored to current and next-generation access technologies. Monolithically integrated dual electro-absorption support current and next-generation access technologies. Monolithically integrated dual electro- modulated laser (DEML) solutions with application in PON and claims of small footprint and low-cost absorption modulated laser (DEML) solutions with application in PON and claims of small footprint external-modulator-free Tx can likewise be found in the literature [46]. More recently, a theoretical and low-cost external-modulator-free Tx can likewise be found in the literature [46]. More recently, analysis of polarization-independent receiver (PI-Rx) based in a 3 3 symmetric coupler with a theoretical analysis of polarization-independent receiver (PI-Rx) based in a 3 × 3 symmetric coupler applications for access networks was reported [77]. An example of a BOSA transceiver solution with applications for access networks was reported [77]. An example of a BOSA transceiver solution implemented in ONU side of a PON with a representation of a 10 Gbit small form-factor pluggable implemented in ONU side of a PON with a representation of a 10 Gbit small form-factor pluggable (XFP) evolution from discrete optical component to PIC technology is depicted in Figure 4. (XFP) evolution from discrete optical component to PIC technology is depicted in Figure 4. Figure 4. Diagram describing the evolution of photonic integrated circuits (PIC) for PON, speciﬁcally Figure 4. Diagram describing the evolution of photonic integrated circuits (PIC) for PON, specifically the implementation of a bi-directional optical sub assembly (BOSA) transceiver solution in PON the implementation of a bi-directional optical sub assembly (BOSA) transceiver solution in PON ONU ONU side, i.e., XFP with discrete optical component packaging (current PON products) and with PIC side, i.e., XFP with discrete optical component packaging (current PON products) and with PIC approach technology (next-generation products). approach technology (next-generation products). Several steps and guidelines for PIC implementation in PON are addressed in this section, along Several steps and guidelines for PIC implementation in PON are addressed in this section, along with the technology impact on reach (Section 4.1); the adopted band and wavelength selection criteria with the technology impact on reach (Section 4.1); the adopted band and wavelength selection criteria (Section 4.2); the integrated photonics packaging (Section 4.3); and the issues related with the control (Section 4.2); the integrated photonics packaging (Section 4.3); and the issues related with the control of complexity, power dissipation, and form factor (Section 4.4). of complexity, power dissipation, and form factor (Section 4.4). 4.1. Technology and Impact on Reach 4.1. Technology and Impact on Reach A major requirement for future PON technologies is the need for coexistence with current deployed A major requirement for future PON technologies is the need for coexistence with current PON systems [65,70]. For example, GEPON has already been deployed on a scale of millions, thus deployed PON systems [65,70]. For example, GEPON has already been deployed on a scale of any new PON technology must be capable of coexisting with this large base [65]. A PON framework millions, thus any new PON technology must be capable of coexisting with this large base [65]. A of three coexisting technologies, i.e., GPON, XGSPON, and NGPON2, is depicted in Figure 3 from PON framework of three coexisting technologies, i.e., GPON, XGSPON, and NGPON2, is depicted Section 3. Here, the same ODN conveys several technologies allowing the migration and evolution of in Figure 3 from Section 3. Here, the same ODN conveys several technologies allowing the migration the network without forcing clients to shift technology. This approach leads to reduced CAPEX and and evolution of the network without forcing clients to shift technology. This approach leads to supports pay-as-you-go approaches since it allows keeping using the legacy operating technologies reduced CAPEX and supports pay-as-you-go approaches since it allows keeping using the legacy untouched, opening an evolution path for other technologies to follow [7]. operating technologies untouched, opening an evolution path for other technologies to follow [7]. The longer the required reach, the more dicult it gets to ﬁnd the laser technology able to support The longer the required reach, the more difficult it gets to find the laser technology able to the demanded distance. As an example, for a 10 Gbit/s with a short-reach like 10 km, almost all support the demanded distance. As an example, for a 10 Gbit/s with a short-reach like 10 km, almost standard laser technologies can be used. If addressing the 2.5 Gbit/s market, a medium-reach like all standard laser technologies can be used. If addressing the 2.5 Gbit/s market, a medium-reach like 40 km and further are still not a major limiting factor to the laser choice. However, if requiring a 40 km and further are still not a major limiting factor to the laser choice. However, if requiring a common GPON ODN which ranges 20 km including splitters and the handling of dispersion penalty, common GPON ODN which ranges 20 km including splitters and the handling of dispersion penalty, it poses challenges to most of the 10 Gbit/s solutions. To achieve that target, the laser must be designed it poses challenges to most of the 10 Gbit/s solutions. To achieve that target, the laser must be designed to perform a low penalty, typically bellow 1 dB to 2 dB, at the required distance. To meet those specs, to perform a low penalty, typically bellow 1 dB to 2 dB, at the required distance. To meet those specs, Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 9 of 19 Appl. Sci. 2020, 10, x 9 of 19 specially designed direct modulated lasers (DML) are required, or even the migration to external specially designed direct modulated lasers (DML) are required, or even the migration to external modulated laser (EML) technologies [7,8]. modulated laser (EML) technologies [7,8]. Furthermore, in most recent standards, ODN should support 128 users and a 20 km reach. These Furthermore, in most recent standards, ODN should support 128 users and a 20 km reach. These requirements have the penalty of a 21-dB attenuation stemming from splitting losses plus 4 dB due requirements have the penalty of a 21-dB attenuation stemming from splitting losses plus 4 dB due to fiber losses and around 3 dB insertion loss (IL) of co-existence elements, leading to an ODN total to ﬁber losses and around 3 dB insertion loss (IL) of co-existence elements, leading to an ODN total loss greater than 28 dB. If we target 10 Gbit/s with avalanche photodiode (APD) receivers allowing a loss greater than 28 dB. If we target 10 Gbit/s with avalanche photodiode (APD) receivers allowing −26 dBm to −30 dBm receiver sensitivity, it leads to a minimum laser output power of 4 dBm. A a 26 dBm to 30 dBm receiver sensitivity, it leads to a minimum laser output power of 4 dBm. possible alternative to overcome this requirement is to use an amplification stage in the path which A possible alternative to overcome this requirement is to use an ampliﬁcation stage in the path which removes the passivity of the ODN, however, it reduces the potential usage of EML-based solutions removes the passivity of the ODN, however, it reduces the potential usage of EML-based solutions since its typical power does not exceed the 0 dBm. since its typical power does not exceed the 0 dBm. 4.2. Band and Wavelength Selection Criteria 4.2. Band and Wavelength Selection Criteria To improve reach, several techniques are available and many stem from the laser (e.g., low To improve reach, several techniques are available and many stem from the laser (e.g., low dynamic dynamic chirp or frequency modulation efficiency), laser driving (pre-chirping or equalization), and chirp or frequency modulation eciency), laser driving (pre-chirping or equalization), and wavelength wavelength selection (e.g., O-band instead of C and L bands) [2,70]. O-Band has been used for selection (e.g., O-band instead of C and L bands) [2,70]. O-Band has been used for supporting upstream supporting upstream and was adopted by some of the standards like GPON and XGSPON. O-band, and was adopted by some of the standards like GPON and XGSPON. O-band, besides presenting besides presenting low or null dispersion, only introduces slightly higher attenuation than C-band. low or null dispersion, only introduces slightly higher attenuation than C-band. These facts greatly These facts greatly reduce the pressure on the O-band components industry, impacting positively on reduce the pressure on the O-band components industry, impacting positively on the cost and yield. the cost and yield. The current spectral allocation from O to L-band is depicted in Figure 5. The current spectral allocation from O to L-band is depicted in Figure 5. Figure 5. Spectrum allocation in ITU-T based standards for PON and trends for channel spacing. Figure 5. Spectrum allocation in ITU-T based standards for PON and trends for channel spacing. If a certain technology is deployed in greenﬁeld, a viable approach is to implement standards that If a certain technology is deployed in greenfield, a viable approach is to implement standards are not back-compatible, i.e., does not support legacy technologies. In this scenario, the use of O-band that are not back-compatible, i.e., does not support legacy technologies. In this scenario, the use of is an option for future PON. The drawback of this option is that the operator has to guarantee that the O-band is an option for future PON. The drawback of this option is that the operator has to guarantee PON evolution is on top of the new technology deployed. To cope with the maturity of the technology that the PON evolution is on top of the new technology deployed. To cope with the maturity of the and promote an easier and simpliﬁed entry into the market, initial PON technologies, like GPON, technology and promote an easier and simplified entry into the market, initial PON technologies, like were set in large bandwidths for better yield in laser choice with the upstream in the O-band [7,67]. GPON, were set in large bandwidths for better yield in laser choice with the upstream in the O-band The wavelength range of 20 nm to 30 nm per trac direction was reserved for this technology, which [7,67]. The wavelength range of 20 nm to 30 nm per traffic direction was reserved for this technology, allowed it to easily mature while still being competitive. This strategy proved to be solid since it which allowed it to easily mature while still being competitive. This strategy proved to be solid since resulted in the current GPON BOSA to sit in the few dollars range (as of today). it resulted in the current GPON BOSA to sit in the few dollars range (as of today). Furthermore, PON technology evolution can be achieved by data rate multiplication and more Furthermore, PON technology evolution can be achieved by data rate multiplication and more ecient use of the bands to promote further network evolution scenarios. Following this path, XGSPON efficient use of the bands to promote further network evolution scenarios. Following this path, Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 10 of 19 Appl. Sci. 2020, 10, x 10 of 19 standard was conceived with slightly smaller bandwidths for both upstream and downstream, which XGSPON standard was conceived with slightly smaller bandwidths for both upstream and resulted in a four-times higher bitrate than GPON [70,78]. downstream, which resulted in a four-times higher bitrate than GPON [70,78]. To further exploit the available bands (see Figure 5), dense wavelength division multiplexing To further exploit the available bands (see Figure 5), dense wavelength division multiplexing (DWDM) (DWDM) can can be a route t be a route tooexplor explore. Th e. Thee possibilit possibility y of of including including t tunability unability al allows lows network network active active transformation and management. Even considering the associated price to pay, it is an important transformation and management. Even considering the associated price to pay, it is an important solution solution under undethe r th re equir reqement uirement of tig of htly tigcontr htly cont olledrol lasers. led la Furthermor sers. Furthermore, w e, with this capability ith this cap , a pr abilit ojected y, a OPEX project re eduction d OPEX red is expected, uction is since expeclients cted, since can seamlessly clients can migrate seamles to sly any miof grat the e tavailable o any of t wavelengths he available while wavelen thegnetwork ths while gr the network grow ows smoothly and s s systematically moothly and .systematically. The The most most evident evident bar barrier rier for for the the spr spre ead ad of of this te this technology chnology i is s t the he laser laser and and rece receiver iver re requir quirement ements, s, which which sho should uld be at be attained tained un under der a pot a potential ential low cost low cost and sim and ult simultaneously aneously have a have high gr a high ade ograde f stabilit of y stability [7,8]. Figur [7,8 e]. 5 presents the Figure 5 presents bands for the bands the different for the dist eps o erent fsteps the netwo of thernet k evo work lution evolution, , i.e., from i.e., the from legacy the legacy technologies technologies wide bwide ands to the ne bands to xt ge the next neration t generation ighter bandw tighter bandwidths. idths. A A represent representation ation of of the the typi typical cal distribution distributi of on of lasers, lase stemming rs, stemmin fromga from a single wafer single wafe run, is depicted r run, is in depict Figur ed e 6in F A. Each igure of 6A the . E lasers ach of t collected he laser fr som colthe lectwafer ed from will thhave e waf its er emission will have wavelength its emission w anda thus velengt they h can and t behused us they in can be a more speciﬁc used in/ a more tighter wavelength specific/tight range. er waT velen o accomplish gth range. To this featur accom e,p alish t tuning his pr feat ocess ure, is a rtu equir ning pr ed, e.g., oces temperatur s is require, ed curr , e.g ent, ., te etc. mpe Each ratur wafer e, curr run ent, etc. E may present ach wafer dierent run statistics may present due to different material deposition statistics due to ma doping and terial prdeposi ocessing, tionsee doping Figurand e 6B. pr Nonetheless, ocessing, see the Figu wafer re 6B. unifor Nonetheless, th mity wavelength e wafer map uniformit is in general y wavelen scar gcely th ma reported p is in gene by the ral sc foundries, arcely re some ported by t works h suggest e foundr these ies, s claims ome works s [79,80].uT gge ight st contr these claims ol of the doping [79,80]. T process ight control of the doping may reduce this variation prand ocess m greatly ay r impr educe t ove h the is v yield ariat and ion temperatur and greatley contr improve the ol mechanisms. yield and te Themperature co ﬁner the requir ntrol mechan ed tuning, isms. The fin the more complex er the requir the laser ed tuning selection , th pr e more ocess. The complex t qualiﬁcation he laser of se a lect laser ion to proces operate s. The in a q certain ualific wavelength ation of a la range ser tor esults operatfr e om in a setting certain its wave maximum length allowed range res wavelength ults from set attiinitial ng its max temperatur imum al es.lowed The br wavelen oad requir gth at ements initia of l tthe emp technology eratures. Th ine broa termsd of req the uirclient ements of locations the technol may requir ogy in e terms of industrial the cl temperatur ient locat e ranges ions ma ( y req 40 to uir 85 e ind C),uwhich strial tbring emperextra ature rrange equisites s (−4to 0 tthe o 85 laser °C), whic wavelength h bring ext choice ra requ or thermal isites tocontr the lol asmechanisms. er wavelengthDir cho ectly ice or modulated thermal clasers ontrol have mecha good nisms. Directly optical output modula power ted la and sers areha easy ve good opti to drive and cal output modulate. power However and are ea , thesy to dri inherent ve high and wavelength modulate. Ho ﬂuctuation wever, the in when herent high w modulatedaor velen theginher th fluctua ent chirp tion when modula can result in pr ted opagation or the inherent chi limitations. rp Laser can redesign sult in propagat /control e ion limitation orts are being s.made Laser to de minimize sign/control these effo e rts are bei ects and n curr g m ently ade to mi , thereni ar miz e alr e these eady some effectdevices s and cuin rrent the ly, market there are able alto ready cope so with me dev some ices of in the the m most arket stringent able to cope with some of standard requirements the most [81], e.g., stringent the NGPON2. standard re W quire ith an ments [81], e.g., the external cavity, EMLs NGPO result N2. Wi in a th much an externa lowerl ca chirp, vity, EMLs resul however, having t in a typically much lower a mor chirp, howeve e complex tunability r, having ty mechanism pically a more c when omplex t compar ued nab to ilit the y mechan simpleis curr m when com ent or thermal pared tuning to the si of mpl theeDML current or therma [7]. l tuning of the DML [7]. Figure 6. Representation of a generic statistical distribution of lasers over their initial wavelength () in Figure 6. Representation of a generic statistical distribution of lasers over their initial wavelength (λ) a set of three wafer runs. Dierent wafers may result in dierent central s. (A): distribution of initial in a set of three wafer runs. Different wafers may result in different central λs. (A): distribution of of wafer #2; (B): example of an initial s distribution for three dierent wafers. initial λ of wafer #2; (B): example of an initial λs distribution for three different wafers. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 11 of 19 Appl. Sci. 2020, 10, x 11 of 19 4.3. Integrated Photonics Packaging 4.3. Integrated Photonics Packaging Photonic packaging technology has achieved a higher signiﬁcance under optical communication Photonic packaging technology has achieved a higher significance under optical communication systems’ recent developments [82] by covering the optical and electronic connections in/out of the systems’ recent developments [82] by covering the optical and electronic connections in/out of the PIC. Packaging process developments are of great importance for the next generation of optical PIC. Packaging process developments are of great importance for the next generation of optical components [6]. Being one of the most complex segments of the integrated component viability, components [6]. Being one of the most complex segments of the integrated component viability, it it can pose several challenges, such as limitations in terms of volume, cost, RF performance, power can pose several challenges, such as limitations in terms of volume, cost, RF performance, power dissipation, and its high-volume manufacturing capability [83], which can highly impact the cost of dissipation, and its high-volume manufacturing capability [83], which can highly impact the cost of the solution. the solution. Currently, packaging is seen as one of the most signiﬁcant bottlenecks in the development of Currently, packaging is seen as one of the most significant bottlenecks in the development of commercially relevant PIC devices [60,84]. The packaging design ﬂow is divided into three main commercially relevant PIC devices [60,84]. The packaging design flow is divided into three main areas: the optical design, the electrical design, and the thermal management of the module. A ﬁrst areas: the optical design, the electrical design, and the thermal management of the module. A first open-access PIC assembly and packaging pilot line of Europe’s leading industrial and research open-access PIC assembly and packaging pilot line of Europe’s leading industrial and research organization teams (i.e., PIXAPP) are dedicating eorts to the development of a photonic packaging organization teams (i.e., PIXAPP) are dedicating efforts to the development of a photonic packaging platform to gather state-of-the-art technologies and procedures [62]. This highly interdisciplinary platform to gather state-of-the-art technologies and procedures [62]. This highly interdisciplinary team provides single-point access to PIC assembly and packaging for users. The project key objectives team provides single-point access to PIC assembly and packaging for users. The project key objectives include: (i) custom solutions through standard packaging technologies; (ii) training and education as a include: (i) custom solutions through standard packaging technologies; (ii) training and education as future workforce; and (iii) link the PIC ecosystem through the development of packaging standards a future workforce; and (iii) link the PIC ecosystem through the development of packaging standards and roadmaps [17]. Considering the typical wavelength gain band of an active device, ranging from and roadmaps [17]. Considering the typical wavelength gain band of an active device, ranging from 20 nm to 30 nm for the O- to L-bands, and the technology wavelength map presented in Figure 7, 20 nm to 30 nm for the O- to L-bands, and the technology wavelength map presented in Figure 7, several approaches may be required to merge in a single device the upstream/downstream signals. several approaches may be required to merge in a single device the upstream/downstream signals. There are several network standards, like XGSPON, which require upstream/downstream wavelengths There are several network standards, like XGSPON, which require upstream/downstream spaced by more than 200 nm. A 200 nm spacing brings higher challenges and may require hybrid wavelengths spaced by more than 200 nm. A 200 nm spacing brings higher challenges and may integration, which poses additional packaging challenges [7]. require hybrid integration, which poses additional packaging challenges [7]. Figure 7. Diagram describing transmitter and receiver technologies with potential use in access Figure 7. Diagram describing transmitter and receiver technologies with potential use in access networks. Typical gain and lasing media proﬁles are presented, with 20 nm to 60 nm bandwidths for networks. Typical gain and lasing media profiles are presented, with 20 nm to 60 nm bandwidths for common current materials, PINs with dierent doping materials, and APDs. (A): transmitter/(pre-) common current materials, PINs with different doping materials, and APDs. (A): transmitter/(pre-) ampliﬁcation side; (B): receiver side. amplification side; (B): receiver side. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 12 of 19 In the same technological space, NGPON2 requires only 60 nm between the upstream and downstream, thus a fully monolithic approach may be achievable by changing slightly the doping or applying regrowth. Figure 7 provides a representation of the gain curve for a generic material doping, the abstraction of a laser for the transmitter side, and three types of available receiver techniques/devices. On the transmitter side tunability limitation of a laser is depicted and corroborated by the limited gain media in which the laser light is generated, see Figure 7A. This constraint results in 20 nm to 30 nm tunability range, considering laser operation with signiﬁcant output power. Simple tuning based on thermal eects usually results in an available tuning range of 3 nm to 4 nm. A combination of a tunable device with an external cavity supported by a convenient control mechanism results in a wider tuning range. On the receiver side several techniques are described, such as a positive-intrinsic-negative photodiode (PIN), APD, and coherent [7,8], see Figure 7B. The PIN is one of the cheapest and simplest detecting techniques presenting a wide light wavelength detection range. Its proﬁle can be changed by the type of materials and doping used. PINs can be easily integrated into e.g., InP PIC, which is already available for bandwidths exceeding >35 GHz and operational wavelength ranges covering the common ﬁber telecommunication bands [33,60,85]. APDs are tougher to integrate, however, from the system point of view, they result in improved sensitivity due to their intrinsic avalanche gain. The referred receiving technologies are wide wavelength band, however, with a coherent detection scheme tunable reception can be achieved. Depending on the arrangement, a dierent number of associated PINs (from dual-polarization dierential with 8 PINs to quasi-coherent single photodetector) simultaneous gain and ﬁltering (stemming from the local oscillator beating with the signal) can be implemented [7,77,86]. To achieve the optical interface between the PIC and the ﬁber, there are several challenges to be surpassed, such as the mode adaptation between the WG in the PIC and the cylinder-shaped ﬁber core [87]. Spot size convert (SSC) modules in the PIC, lensed ﬁbers connected through V-grooves, holders, and other techniques are used to achieve consistent solutions. The PIC packaging interfacing of the electrical connections between the PIC and the PCB is usually carried out by wire-bonding [34]. As bandwidth grows, the trend is to replace the bonding wires by ﬂip-chip solutions to reach higher thermal and electrical performance. Furthermore, packaging should be a gateway to solve power dissipation, which is expected to increase signiﬁcantly with density [7]. Packaging solutions based on holders with high thermal conductivity and inherent thermal monitoring is an attractive solution to overcome this constraint. 4.4. Control Complexity, Power Dissipation and Form Factor The dierent PON standards and technologies have their own requirements in terms of complexity control, especially for tunable devices. When wavelength control is not essential, simpler actuation systems with lower power are required [88]. Nonetheless, once the wavelength spectrum becomes more crowded, the options have to be restricted and the requirements for more complex control increase [7]. A summary of the technology and expected power for its optoelectronic interfaces versus the complexity and requirements for a certain interface is depicted in Figure 8. A general sweep, from low data rate with large wavelength range and short-reach to higher data rate with tighter wavelength range and longer reaches, are also presented in the Figure 8. Additionally, the technologies with an approximate relative power requirement classiﬁcation (from low to high power) are also identiﬁed. The x-axis represents the optical requirements, ranging from low data rate and short-reach to long-reach DWDM and high data rate. The red arrows describe the complexity steps from the system requirements. Appl. Sci. 2020, 10, 4024 13 of 19 Appl. Sci. 2020, 10, x 13 of 19 Figure 8. Diagram of requirements in terms of control, complexity, and power for different Figure 8. Diagram of requirements in terms of control, complexity, and power for dierent technologies. technologies. Potential form factor solutions able to cope with the required power dissipation are also presented in thePotenti far right al f oform f Figuraector 8. Technology solutions a evolution ble to cope wi poses incr th the requi easing challenges red power dissi in the contr patol ion a complexity re also and presented in t required power he far right of Fig . As the demanded ure 8. Tec power hnology ev grows,o alchange ution poses in the in form creasing ch factor is allenges needed. in the control Form factors have complexity and two major characteristics: required power. As minimum the dem volume anded and pmaximum ower grow dissipated s, a change power in th[e form 89]. factor is needed. An appr Form oximate factors have two major chart of the average char pricing acteri for stics: minimum GPON, XGSPON, volume and a NGPON2 nd maximum dissip evolution versus ated power [89]. time is presented in Figure 9. GPON faced a strong price drop with its increased manufacturability, An approximate chart of the average pricing for GPON, XGSPON, and NGPON2 evolution pushed by the worldwide broad adoption of the technology by the operators. XGSPON is following versus time is presented in Figure 9. GPON faced a strong price drop with its increased the same steps, which for the user can be the motto to have 10 Gbit solutions sooner. Technically, manufacturability, pushed by the worldwide broad adoption of the technology by the operators. XGSPON generically adopted the physical layer characteristics of IEEE 802.3av 10GEPON systems, XGSPON is following the same steps, which for the user can be the motto to have 10 Gbit solutions e.g., diering in the burst mode timing requirements [70,78]. Moreover, to jump-start the market, sooner. Technically, XGSPON generically adopted the physical layer characteristics of IEEE 802.3av initial implementations of XGSPON systems relaxed the timing requirements to fully adopt 10GEPON 10GEPON systems, e.g., differing in the burst mode timing requirements [70,78]. Moreover, to jump- standards [70]. With this rapid price erosion model, the proﬁt margins in the supply chain depreciate start the market, initial implementations of XGSPON systems relaxed the timing requirements to very fast. As NGPON2 is based on the limited tunability of DWDM lasers and receivers, it poses fully adopt 10GEPON standards [70]. With this rapid price erosion model, the profit margins in the some challenges for its adoption by manufacturers, vendors, and operators. This fact has delayed its supply chain depreciate very fast. As NGPON2 is based on the limited tunability of DWDM lasers deployment process. NGPON2 technology diers from the traditional model since it goes out of the and receivers, it poses some challenges for its adoption by manufacturers, vendors, and operators. O-band, making the laser control and manufacturability more stringent. Thus, it results in a potentially This fact has delayed its deployment process. NGPON2 technology differs from the traditional model slower cost reduction and more sustainability from the supply chain point of view [7]. since it goes out of the O-band, making the laser control and manufacturability more stringent. Thus, it results in a potentially slower cost reduction and more sustainability from the supply chain point of view [7]. Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 14 of 19 Appl. Sci. 2020, 10, x 14 of 19 Figure 9. Representation of average price evolution of PON technology for GPON, XGSPON, Figure 9. Representation of average price evolution of PON technology for GPON, XGSPON, and and NGPON2 bulk type BOSA (not PIC based). Pricing was empirically collected along the years from NGPON2 bulk type BOSA (not PIC based). Pricing was empirically collected along the years from several providers and averaged in the graph. several providers and averaged in the graph. The indicative pricing evolution of the traditional bulk type BOSA for the three technologies is The indicative pricing evolution of the traditional bulk type BOSA for the three technologies is depicted in Figure 9. Early, broad and late adoption notes refer to perception and predictions estimated depicted in Figure 9. Early, broad and late adoption notes refer to perception and predictions by the authors given the present standing of the technology. Pricing is indicative and may not represent estimated by the authors given the present standing of the technology. Pricing is indicative and may all realities/contexts, and is based on, per vendor and per order, 100 pieces at the early adoption stage, not represent all realities/contexts, and is based on, per vendor and per order, 100 pieces at the early 100k pieces at broad adoption, and 1M pieces at late adoption stages, see Figure 9. adoption stage, 100k pieces at broad adoption, and 1M pieces at late adoption stages, see Figure 9. A reduction in physical volume, number of sub-assemblies, and calibration steps are expected A reduction in physical volume, number of sub-assemblies, and calibration steps are expected with the migration to PICs. These steps may greatly reduce the price of the subassemblies based on PICs with the migration to PICs. These steps may greatly reduce the price of the subassemblies based on when compared with traditional bulk BOSA [7,8]. As identiﬁed above, by having upstream/downstream PICs when compared with traditional bulk BOSA [7,8]. As identified above, by having on approximately the same wavelength band, NGPON2 can be a great applicant for monolithic PIC. upstream/downstream on approximately the same wavelength band, NGPON2 can be a great In such a case, ONU and OLT can coexist in the same PIC platform without further processing. applicant for monolithic PIC. In such a case, ONU and OLT can coexist in the same PIC platform XGSPON, besides being already stressed in price, may require O-band and L-band integration, which without further processing. XGSPON, besides being already stressed in price, may require O-band can entail extra processing and packaging steps, thus increasing the limitations of PIC viability in this and L-band integration, which can entail extra processing and packaging steps, thus increasing the wavelength range. DWDM is also included in the wavelength plan of NGPON2 and is already an limitations of PIC viability in this wavelength range. DWDM is also included in the wavelength plan ITU-T standard being used for implementing several telecommunication systems, which also poses a of NGPON2 and is already an ITU-T standard being used for implementing several good opportunity for the use of the PIC technology. telecommunication systems, which also poses a good opportunity for the use of the PIC technology. 5. Conclusions 5. Conclusions PICs are a promising technology with great potential in several ﬁelds including telecom, sensing, PICs are a promising technology with great potential in several fields including telecom, sensing, and bio-photonics. In this work, we have reviewed the PON technology standards and evaluated the and bio-photonics. In this work, we have reviewed the PON technology standards and evaluated the challenges and beneﬁts of meeting its requirements with the prospective usage of PIC. Depending challenges and benefits of meeting its requirements with the prospective usage of PIC. Depending on on the requirements, the type of materials, and components, a tailored approach has to be carefully the requirements, the type of materials, and components, a tailored approach has to be carefully speciﬁed. The dierent materials used in the available solutions for implementing PIC result in dierent specified. The different materials used in the available solutions for implementing PIC result in behaviors, steps, processes, and capabilities. The main constraint of PIC is still the limited gain band different behaviors, steps, processes, and capabilities. The main constraint of PIC is still the limited of the active devices which are in its core. This may imply advanced processing and/or packaging gain band of the active devices which are in its core. This may imply advanced processing and/or packaging techniques to be able to cope with the existing standards, e.g., regrowth and hybrid Appl. Sci. 2020, 10, x; doi: www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4024 15 of 19 techniques to be able to cope with the existing standards, e.g., regrowth and hybrid packaging. After carefully choosing the standard to be followed and the packaging approach, its control complexity also has to be cautiously considered to guarantee that the solution is feasible and adapted. Speciﬁcally, for PON the most stringent requirements are the cost and robustness of the proposed solution. We have discussed several of the challenges to be considered when equating the introduction of PIC in the next-generation PONs, like productization, wideband wavelength range (O, C, and L-bands), laser control, and tunability mechanisms. PON standards have dierent ﬂavors that result in quite dierent requirements, especially regarding the wavelength ranges to be covered by the two trac directions, i.e., the upstream and downstream. For instance, GPON and XGSPON have speciﬁcations of >200 nm spacing between upstream and downstream, while NGPON2 requires around 60 nm for the same parameter. Due to technological inherent limitations of some technologies, hybrid integration may be required, e.g., XGSPON and GPON. Smaller bands will ease this process and therefore potentiate monolithic PIC implementation, simplifying packaging and deployment, e.g., NGPON2. PICs have typically two major interfaces, electrical and optical, each with dierent requirements, techniques, and available materials to be used. Packaging and technical considerations such as size, power, RF compliance, and sealing are relevant. In a nutshell, the manufacturability is anticipated to be one of the most critical steps for the technology to be successful, especially in PON. In our vision, the use of PICs in PON and other subsystems is very close to being an eective reality with prospective advantages. Author Contributions: This is a review paper, each author contributed to all chapters. C.P. and A.T. prepared the manuscript, i.e., writing, review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the European Regional Development Fund (FEDER), through the Competitiveness and Internationalization Operational Programme (COMPETE 2020) of the Portugal 2020 framework [Project Virtual Fiber Box with Nr. 033910 (POCI-01-0247-FEDER-033910)]; plugPON (POCI-01-0247-FEDER-047221); FEDER through the Regional Operational Programme of Centre (CENTRO 2020) of the Portugal 2020 framework [Project HeatIT with Nr. 017942 (CENTRO-01-0247-FEDER-017942)]; and IT (UID/EEA/50008/2019). Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Teixeira, A.; Shahpari, A.; Ferreira, R.; Guiomar, F.P.; Reis, J.D. Coherent Access. In Proceedings of the OFC 2016—Optical Fiber Communication Conference, Anaheim, CA, USA, 20–24 March 2016; p. M3C.5. 2. Shahpari, A.; Ferreira, R.M.; Luis, R.S.; Vujicic, Z.; Guiomar, F.P.; Reis, J.D.; Teixeira, A.L. Coherent Access: A Review. J. Lightwave Technol. 2017, 35, 1050–1058. [CrossRef] 3. Micolta, J.C.V. 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Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)

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Photonic Integrated Circuits for NGPON2 ONU Transceivers (Invited)

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