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Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal

Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate... applied sciences Article Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal Yang Yue *, Qiang Wang and Jon Anderson Juniper Networks, 1133 Innovation Way, Sunnyvale, CA 94089, USA; qiwang.thresh@gmail.com (Q.W.); jonanderson@juniper.net (J.A.) * Correspondence: yyue@juniper.net; Tel.: +1-408-745-2000 Received: 16 March 2019; Accepted: 13 June 2019; Published: 15 June 2019 Abstract: In this article, we review the latest progress on data center interconnect (DCI). We then discuss di erent perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption, following 400G pluggable optics in DCI applications. Next, we experimentally investigate the capacity-reach matrix for high-baud-rate and high-order quadrature amplitude modulation (QAM) single-carrier signals in the unamplified single-mode optical fiber (SMF) link. We show that the 64 GBd 16-QAM, and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond of unamplified fiber link. Keywords: 400 Gigabit Ethernet; coherent communications; data center interconnect; fiber optics links and subsystems; optical communications; QSFP-DD transceiver 1. Introduction To meet the ever-growing demands on the internet bandwidth, the Ethernet speed has evolved at an astonishing pace, from 10 Megabit Ethernet to 100 Gigabit Ethernet (100 GbE) within the past 30 years [1]. In 2017, 400GbE standard was ratified by the IEEE P802.3bs Task Force [2]. This built the foundation for industrial deployment of 400GbE in the global network. It is anticipated that 400GbE will be rapidly adopted over the next few years [3]. Recently, 400GbE pluggable optics is under development to satisfy the demand on bandwidth-consuming applications, including data-center-based cloud services [4–6]. For intra data center interconnect (DCI) application, optical direct detection with four-level pulse amplitude modulation (PAM-4) has been widely adopted to enable the low cost 400GbE optical transceivers, which can carry up to 400 Gb/s Ethernet data over either parallel fibers or multiple wavelengths. Direct detection PAM-4 optics has also been standardized by Multi-Source Agreement (MSA) groups to achieve link distance from 500 m to 10 km (e.g., 400G-FR8, 400G-DR4, 400G-LR4, etc.) [7]. For inter DCI application, pluggable 400G digital coherent optics (DCO) with single-carrier dual-polarization quadrature amplitude modulation (DP-QAM) signal is being proposed to achieve 40 km and beyond fiber link [8,9]. PAM-4 signal with direct detection only uses the intensity of the light to carry information. Instead, DP-QAM signal with coherent detection technology can utilize a few degrees of freedom for photons, including polarization, intensity, and phase. Consequently, coherent technology can transmit a single-carrier 400 Gb/s signal with significantly improved spectral eciency. Furthermore, its receiver sensitivity can be significantly improved over the one of intensity modulation with direct detection (IM/DD) systems. In this article, we first introduce the application scenarios of pluggable optics in DCI applications. Next, we discuss di erent perspectives on the 400G pluggable module, including form factor, Appl. Sci. 2019, 9, 2455; doi:10.3390/app9122455 www.mdpi.com/journal/applsci Appl. Sci. 2019, x, x 2 of 10 In this article, we first introduce the application scenarios of pluggable optics in DCI applications. Appl. Sci. 2019, 9, 2455 2 of 9 Next, we discuss different perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption. Then, we experimentally investigate the fiber transmission performance of 16-QAM and 64-QAM single-carrier architecture, digital signal processing (DSP), and module power consumption. Then, we experimentally signals at various baud rates, aiming at 400 Gb/s DCI applications with unamplified link. Here, we investigate the fiber transmission performance of 16-QAM and 64-QAM single-carrier signals at various −2 consider 25% overhead (OH) soft-decision forward error correction (SD-FEC), which gives a 4.2 × 10 baud rates, aiming at 400 Gb/s DCI applications with unamplified link. Here, we consider 25% overhead bit error ratio (BER) threshold [10]. For 64 GBd 16-QAM signals with 12 dB and 18 dB link margins, (OH) soft-decision forward error correction (SD-FEC), which gives a 4.2  10 bit error ratio (BER) it is possible to transmit 400 Gb/s net data rate over 40 km and 80 km unamplified fiber link, threshold [10]. For 64 GBd 16-QAM signals with 12 dB and 18 dB link margins, it is possible to transmit respectively. With 18 dB link margin, 64 GBd 64-QAM signal can produce greater than 600 Gb/s net 400 Gb/s net data rate over 40 km and 80 km unamplified fiber link, respectively. With 18 dB link margin, data rate for an unamplified link as long as 40 km. We further study the impact of transmitter (Tx) 64 GBd 64-QAM signal can produce greater than 600 Gb/s net data rate for an unamplified link as long side power on the transmission performance and obtain a high-baud-rate QAM signal transmission as 40 km. We further study the impact of transmitter (Tx) side power on the transmission performance matrix in unamplified link. For various signals, extra 6-dB more power can extend the fiber and obtain a high-baud-rate QAM signal transmission matrix in unamplified link. For various signals, transmission reach to approximately 30 km. It is thus critically important to increase the Tx output extra 6-dB more power can extend the fiber transmission reach to approximately 30 km. It is thus power or improve the Rx sensitivity, under the available power and space constrains of the DCO critically important to increase the Tx output power or improve the Rx sensitivity, under the available pluggable module. power and space constrains of the DCO pluggable module. 2. 400G Data Center Interconnect Use Cases 2. 400G Data Center Interconnect Use Cases Data centers have been proliferated over the past few years, which are driven by more and more Data centers have been proliferated over the past few years, which are driven by more and more bandwidth-hungry applications, including enterprise digital transformation, cloud services, and the bandwidth-hungry applications, including enterprise digital transformation, cloud services, and the coming 5G applications. In such an era, enterprises, service providers, and cloud providers need a coming 5G applications. In such an era, enterprises, service providers, and cloud providers need a DCI network that can quickly adapt to the changes in business and operational demands. To meet DCI network that can quickly adapt to the changes in business and operational demands. To meet the the above requirements, data center networks are expanding in number and capacity, driving the above requirements, data center networks are expanding in number and capacity, driving the need need for higher-speed DCI solutions. Top cloud service providers, such as Amazon, Facebook, for higher-speed DCI solutions. Top cloud service providers, such as Amazon, Facebook, Google, Google, and Microsoft, have already adopted 100 Gb/s and will quickly transfer to 400 Gb/s and and Microsoft, have already adopted 100 Gb/s and will quickly transfer to 400 Gb/s and beyond. beyond. Figure 1 illustrates typical application scenarios of 400 Gb/s pluggable optics for DCI. As shown Figure 1 illustrates typical application scenarios of 400 Gb/s pluggable optics for DCI. As shown in Figure 1a, the reach of an unamplified link is power limited, and it mainly depends on the optical in Figure 1a, the reach of an unamplified link is power limited, and it mainly depends on the optical transmitter output power, the fiber link loss, and the optical receiver sensitivity. For the amplified transmitter output power, the fiber link loss, and the optical receiver sensitivity. For the amplified dense wavelength division multiplexing (DWDM) link displayed in Figure 1b, it is noise limited, dense wavelength division multiplexing (DWDM) link displayed in Figure 1b, it is noise limited, and and its reach depends on the available optical signal to noise ratio (OSNR) at the optical receiver. its reach depends on the available optical signal to noise ratio (OSNR) at the optical receiver. Figure 1. Typical application scenarios of 400 Gb/s pluggable optics for data center interconnect. (a) Figure 1. Typical application scenarios of 400 Gb/s pluggable optics for data center interconnect. Unamplified point-to-point applications for 40 km reach; (b) amplified dense wavelength division (a) Unamplified point-to-point applications for 40 km reach; (b) amplified dense wavelength division multiplexing multiplexing (DWDM) applications for (DWDM) applications for 80–1 80–120 20 km reach. km reach. 3. 400G Pluggable Module Form Factors and Power Requirement 3. 400G Pluggable Module Form Factors and Power Requirement Due to the emerging bandwidth-hungry applications, the photonics communications industry is Due to the emerging bandwidth-hungry applications, the photonics communications industry unceasingly driven towards higher-capacity, smaller-size, and lower-power solutions. Standardization is unceasingly driven towards higher-capacity, smaller-size, and lower-power solutions. organizations, including the Institute of Electrical and Electronics Engineers (IEEE) and the Optical Internetworking Forum (OIF), are leading the industrial transformation of photonics communications society from using discrete optical components to utilizing photonics integration enabled packet Appl. Sci. 2019, x, x 3 of 10 Appl. Sci. 2019, 9, 2455 3 of 9 Standardization organizations, including the Institute of Electrical and Electronics Engineers (IEEE) and the Optical Internetworking Forum (OIF), are leading the industrial transformation of photonics optics [11,12]. Lately, line and client-side industrial trials on interoperability have been demonstrated communications society from using discrete optical components to utilizing photonics integration successfully using 100 Gb/s CFP2 and 400 Gb/s CFP8 pluggable optical modules, respectively [13–17]. enabled packet optics [11,12]. Lately, line and client-side industrial trials on interoperability have Nowadays, the industry is marching towards 400 Gb/s pluggable optical modules with even more been demonstrated successfully using 100 Gb/s CFP2 and 400 Gb/s CFP8 pluggable optical modules, compact form factors. The first debuts of 400 Gb/s client and line-side pluggable QSFP modules are respectively [13–17]. Nowadays, the industry is marching towards 400 Gb/s pluggable optical planned for year 2019 and 2020. modules with even more compact form factors. The first debuts of 400 Gb/s client and line-side Based on the DSP’s physical location, there are two types of pluggable coherent modules: analog pluggable QSFP modules are planned for year 2019 and 2020. coherent Base optics d on the (ACO) DSP’s physic and DCO. al location For 400G , there pluggable are twcoher o type ent s of pl transceivers, uggable cohe the reindustry nt module has s: an reached alog ancoherent agreement opton ics ( implementing ACO) and DCO. For DCO, which 400G pl has uggthe able DSP coherent tran inside the sceivers, the industry has reached module. A few form factors have an agreement on implementing DCO, which has the DSP inside the module. A few form factors have emerged recently as the candidates for 400G DCO pluggable modules, such as CFP2, CFP8, OSFP, emerged recently as the candidates for 400G DCO pluggable modules, such as CFP2, CFP8, OSFP, and QSFP-DD [18–22], whose mechanical dimensions and power consumptions are shown in Figure 2. and QSFP-DD [18–22], whose mechanical dimensions and power consumptions are shown in Figure These transceivers are equipped with eight pairs of di erential signal pins, which can support 400G 2. These transceivers are equipped with eight pairs of differential signal pins, which can support 400G transmission speed using 50G PAM-4 electrical interface [23]. One can see that CFP2 and CFP8 have transmission speed using 50G PAM-4 electrical interface [23]. One can see that CFP2 and CFP8 have larger physical dimensions than those of OSFP and QSFP-DD, thus, they have more room for additional larger physical dimensions than those of OSFP and QSFP-DD, thus, they have more room for components and allow a larger power budget. Consequently, OSFP and QSFP-DD are targeted for additional components and allow a larger power budget. Consequently, OSFP and QSFP-DD are the DCI application, while CFP2 and CFP8 are planned for the long-haul applications. The line-card targeted for the DCI application, while CFP2 and CFP8 are planned for the long-haul applications. fitting into one rack unit (RU) space can accommodate 36 QSFP-DD pluggable modules. The total The line-card fitting into one rack unit (RU) space can accommodate 36 QSFP-DD pluggable modules. capacity of the line-card is 14.4 Tb/s. Currently, the industry is converging behind QSFP-DD due to The total capacity of the line-card is 14.4 Tb/s. Currently, the industry is converging behind QSFP- its high port density in the front panel, its backward compatibility with QSFP28/QSFP+, and thus a DD due to its high port density in the front panel, its backward compatibility with QSFP28/QSFP+, larger ecosystem. and thus a larger ecosystem. Figure 2. 400G pluggable coherent transceiver form factors. CFP: C form-factor, OSFP: octal small Figure 2. 400G pluggable coherent transceiver form factors. CFP: C form-factor, OSFP: octal small form form factor, QSFP-DD: quad small form factor double density. factor, QSFP-DD: quad small form factor double density. Figure 3 illustrates the typical architecture of a 400G pluggable coherent transceiver. There are Figure 3 illustrates the typical architecture of a 400G pluggable coherent transceiver. There three functional blocks inside the module, one DSP chip, one transmitter and receiver optical are three functional blocks inside the module, one DSP chip, one transmitter and receiver optical subassembly (TROSA), and one power/control unit. Besides the power and control signal, the host subassembly (TROSA), and one power/control unit. Besides the power and control signal, the host side also provides up to 400GbE client traffic to the pluggable coherent module through eight pairs side also provides up to 400GbE client trac to the pluggable coherent module through eight pairs of differential signal pins. 400 Gb/s single-carrier signal (>60 GBd 16-QAM) can thus be generated out of di erential signal pins. 400 Gb/s single-carrier signal (>60 GBd 16-QAM) can thus be generated of the Tx output. The other port of receiver (Rx) input captures the received signal, and coherent out of the Tx output. The other port of receiver (Rx) input captures the received signal, and coherent detection is performed by the TROSA and DSP. detection is performed by the TROSA and DSP. The pluggable coherent optics starts from 100 Gb/s CFP. During the past few years, its evolution of density has been quite amazing. Meanwhile, a lot of challenges to the whole industry have been triggered. Power consumption is always one of the key constrains for industrial applications. As shown in Figure 2, the requirement on power consumption for 400 Gb/s QSFP-DD DCO is less than 15 W. Over the years, the power consumption of the coherent DSP has reduced significantly [24–26]. Table 1 illustrates the evolution of the power consumption of coherent DSP. One can see that, with 40 nm complementary metal-oxide-semiconductor (CMOS) process, 100G coherent trac consumes approximately 50 W power. Nowadays, with the latest 7 nm CMOS technique, the industry can bring Appl. Sci. 2019, 9, 2455 4 of 9 the power consumption of DSP down to less than 2 W, which is larger than a 20 times reduction. This tremendous improvement can potentially fit the 400G-ZR QSFP-DD DCO under a 15 W power envelop. As shown in Table 2, DSP, TROSA, and the other components are allocated only 7 W, 6–7 W, and 1–2 W power budget, respectively. In such tight power and size constrains, the industry has adopted 10–12 dB link budget in the power limited unamplified case for up to 40 km reach. Appl. Sci. 2019, x, x 4 of 10 Figure 3. Typical architecture of 400G pluggable coherent transceiver. DSP: digital signal processing, Figure 3. Typical architecture of 400G pluggable coherent transceiver. DSP: digital signal processing, TROSA: transmitter and receiver optical subassembly, Tx: transmitter, Rx: receiver. TROSA: transmitter and receiver optical subassembly, Tx: transmitter, Rx: receiver. TableThe p 1. lugg Coher ablent e coherent digital op signal tics start prs ocessing from 100 G (DSP) b/s Cpower FP. During t consumption he past few evolution. years, its evol CMOS: ution of density has been quite amazing. Meanwhile, a lot of challenges to the whole industry have been complementary metal-oxide-semiconductor. triggered. Power consumption is always one of the key constrains for industrial applications. As DSP 2012 2015 2017 2019 shown in Figure 2, the requirement on power consumption for 400 Gb/s QSFP-DD DCO is less than 15 W. Over the years, the power consumption of the coherent DSP has reduced significantly [24–26]. CMOS node 40 nm 28 nm 16 nm 7 nm Table 1 illustrates the evolution of the power consumption of coherent DSP. One can see that, with Power/100G <50 W <20 W <7 W <2 W 40 nm complementary metal-oxide-semiconductor (CMOS) process, 100G coherent traffic consumes approximately 50 W power. Nowadays, with the latest 7 nm CMOS technique, the industry can bring Table 2. 400G-ZR QSFP-DD power consumption. TROSA: transmitter and receiver optical subassembly, the power consumption of DSP down to less than 2 W, which is larger than a 20 times reduction. This QSFP-DD: quad small form factor double density. tremendous improvement can potentially fit the 400G-ZR QSFP-DD DCO under a 15 W power envelop. As shown in Table 2, DSP, TROSA, and the other components are allocated only 7 W, 6–7 Module Power DSP TROSA Other W, and 1–2 W power budget, respectively. In such tight power and size constrains, the industry has QSFP-DD 7 W 6–7 W 1–2 W adopted 10–12 dB link budget in the power limited unamplified case for up to 40 km reach. 4. Experimental Table 1. Setup Coherent digital signal processing (DSP) power consumption evolution. CMOS: complementary metal-oxide-semiconductor. Figure 4 shows our experimental setup to investigate single-carrier high-baud-rate and high-order DSP 2012 2015 2017 2019 QAM signal transmission in unamplified fiber link. A 92 GS/s digital-to-analog converter (DAC) CMOS node 40 nm 28 nm 16 nm 7 nm with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of Power/100G <50 W <20 W <7 W <2 W dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter [27]. Table 2. 400G-ZR QSFP-DD power consumption. TROSA: transmitter and receiver optical The root-raise cosine filter with a roll-o factor of 0.2 was applied in the following experiment. We used subassembly, QSFP-DD: quad small form factor double density. two tunable narrow-linewidth (<100 kHz) lasers with 16 dBm maximal output power, and set them to Module Power DSP TROSA Other 1550.12 nm (i.e., 193.4 THz). One is with maximal power for the optical input to the DP-IQ modulator, QSFP-DD 7 W 6–7 W 1–2 W and the other is set to 12 dBm to act as the local oscillator (LO) for the coherent receiver. The laser to the modulator is separated into two orthogonal polarizations, and each polarization is modulated by the 4. Experimental Setup corresponding IQ signals. An automatic bias control (ABC) is used to control the bias voltage applied to Figure 4 shows our experimental setup to investigate single-carrier high-baud-rate and high- the DP-IQ modulator. At the Rx side, the optical signal is first de-modulated by using a 90 heterodyne order QAM signal transmission in unamplified fiber link. A 92 GS/s digital-to-analog converter coherent detector. Four linear trans-impedance amplifiers (TIAs) with di erential outputs are set to (DAC) with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of automatic gain control (AGC) mode. This minimizes the BER performance dependence on the LO dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and power, and four tributaries of analog electrical signals are generated. Then, an 8-bit analog-to-digital the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter converter (ADC) with 63 GHz analog bandwidth and 4.5 e ective number of bits (ENOB) samples the [27]. The root-raise cosine filter with a roll-off factor of 0.2 was applied in the following experiment. analog signals into discrete digital samples at 160 GS/s for post signal processing. In the DSP, chromatic We used two tunable narrow-linewidth (<100 kHz) lasers with 16 dBm maximal output power, and dispersion set them to (CD) 15is 50first .12 nm compensated. (i.e., 193.4 THz) Then . One is wi polarization th maximmode al power dispersion for the optica (PMD) l inpuis t to the DP- compensated IQ modulator, and the other is set to 12 dBm to act as the local oscillator (LO) for the coherent receiver. and polarization tributaries are de-multiplexed by a blind equalization using the radius directed The laser to the modulator is separated into two orthogonal polarizations, and each polarization is equalization (RDE). A 21-tap equalizer is implemented in the time domain with a Ts/2 spacing [28]. modulated by the corresponding IQ signals. An automatic bias control (ABC) is used to control the bias voltage applied to the DP-IQ modulator. At the Rx side, the optical signal is first de-modulated by using a 90° heterodyne coherent detector. Four linear trans-impedance amplifiers (TIAs) with differential outputs are set to automatic gain control (AGC) mode. This minimizes the BER Appl. Sci. 2019, x, x 5 of 10 performance dependence on the LO power, and four tributaries of analog electrical signals are generated. Then, an 8-bit analog-to-digital converter (ADC) with 63 GHz analog bandwidth and 4.5 effective number of bits (ENOB) samples the analog signals into discrete digital samples at 160 GS/s for post signal processing. In the DSP, chromatic dispersion (CD) is first compensated. Then Appl. Sci. 2019, 9, 2455 5 of 9 polarization mode dispersion (PMD) is compensated and polarization tributaries are de-multiplexed by a blind equalization using the radius directed equalization (RDE). A 21-tap equalizer is implemented in the time domain with a Ts/2 spacing [28]. Furthermore, the blind estimation Furthermore, the blind estimation algorithm is used to estimate and compensate the carrier frequency algorithm is used to estimate and compensate the carrier frequency offset between the Tx signal and o set between the Tx signal and the LO laser, followed by the carrier phase noise [29]. the LO laser, followed by the carrier phase noise [29]. Figure 4. Experimental setup. DAC: digital-to-analog converter, IQ: in-phase (I) and quadrature (Q), Figure 4. Experimental setup. DAC: digital-to-analog converter, IQ: in-phase (I) and quadrature (Q), VOA: variable optical attenuator, SMF: single-mode fiber, LO: local oscillator, ADC: analog-to-digital VOA: variable optical attenuator, SMF: single-mode fiber, LO: local oscillator, ADC: analog-to-digital converter, DSP: digital signal processing. converter, DSP: digital signal processing. In our experiment, we tried to emulate the power limited unamplified case, which is being In our experiment, we tried to emulate the power limited unamplified case, which is being adopted by the industry with 10–12 dB link margin. According to the receiver sensitivity of 64 GBd adopted by the industry with 10–12 dB link margin. According to the receiver sensitivity of 64 GBd 16-QAM signal, we adjusted the variable optical attenuator (VOA) to set the transmitter optical 16-QAM signal, we adjusted the variable optical attenuator (VOA) to set the transmitter optical output output power to a reference point PTx Ref, which is 12 dB greater than the receiver sensitivity at a BER power to a reference point P , which is 12 dB greater than the receiver sensitivity at a BER of −2 Tx Ref of 4.2 × 10 . In our experiment, we generated 16-QAM and 64-QAM with symbol rates from 45 to 86 4.2  10 . In our experiment, we generated 16-QAM and 64-QAM with symbol rates from 45 to GBd. By adjusting the VOA, we also increased the Tx optical power into the single-mode optical fiber 86 GBd. By adjusting the VOA, we also increased the Tx optical power into the single-mode optical (SMF) by 3 dB and 6 dB. Optical 16-QAM and 64-QAM signals at various baud rates were then fiber t (SMF) ransmit by ted 3 odB ver and an SMF 6 dB. link wit Optical h di 16-QAM stance from and 1064-QAM to 110 kmsignals . Different at com various binatbaud ions ofrates a few f wer ibee r then sections were used to achieve specific fiber length, and their corresponding losses are illustrated in transmitted over an SMF link with distance from 10 to 110 km. Di erent combinations of a few fiber Table 3. sections were used to achieve specific fiber length, and their corresponding losses are illustrated in Table 3. Table 3. Fiber loss characterization. Table 3. Fiber loss characterization. Length (km) 20 40 60 80 90 100 110 Loss (dB) 4.7 7.8 12.6 15.5 17.6 20.0 22.8 Length (km) 20 40 60 80 90 100 110 Loss (dB) 4.7 7.8 12.6 15.5 17.6 20.0 22.8 5. Fiber Transmission Performance Results and Discussion In this section, we first study the fiber transmission performance with high-baud-rate 16-QAM 5. Fiber Transmission Performance Results and Discussion signals in the unamplified link. Then, we extend our experiment to the high-baud-rate 64-QAM signals. Finally, we summarize a transmission matrix showing capacity and distance for 16-QAM and In this section, we first study the fiber transmission performance with high-baud-rate 16-QAM 64-QAM signals at various baud rates. signals in the unamplified link. Then, we extend our experiment to the high-baud-rate 64-QAM signals. Finally, we summarize a transmission matrix showing capacity and distance for 16-QAM and 64-QAM 5.1. High-Baud-Rate 16-QAM signals at various baud rates. For 400 Gb/s DCI applications, the industry is adopting DP-16QAM signals. With additional FEC overhead, the DP-16QAM signal can also fuel the 400 Gb/s metro, regional, and even long-haul 5.1. High-Baud-Rate 16-QAM DWDM applications. Figure 5a displays the fiber transmission performance of the 56 GBd 16-QAM For 400 Gb/s DCI applications, the industry is adopting DP-16QAM signals. With additional signal with different Tx side optical power. As the optical link is power-limited, the transmission FEC overhead, the DP-16QAM signal can also fuel the 400 Gb/s metro, regional, and even long-haul performance can be improved for longer transmission by increasing the output power of Tx. With 6- DWDM applications. Figure 5a displays the fiber transmission performance of the 56 GBd 16-QAM dB more optical power than PTx Ref, the 56 GBd 16-QAM signal can transmit over 100 km with a pre- −2 FEC BER of 2.5 × 10 . From the figure, one can see that, with 3-dB more optical power than PTx Ref, it signal with di erent Tx side optical power. As the optical link is power-limited, the transmission is sufficient to achieve 80 km error-free transmission after FEC. With different optical power of the performance can be improved for longer transmission by increasing the output power of Tx. With 6-dB more optical power than P , the 56 GBd 16-QAM signal can transmit over 100 km with a pre-FEC Tx Ref BER of 2.5  10 . From the figure, one can see that, with 3-dB more optical power than P , it is Tx Ref sucient to achieve 80 km error-free transmission after FEC. With di erent optical power of the Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Experimental result shows that a larger than 50 km unamplified optical communication link can be achieved using P . Tx Ref With 6-dB more optical power than P , the system can achieve larger than 90 km transmission Tx Ref in the unamplified link. Consequently, enhanced output power of Tx or improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. Appl. Sci. 2019, x, x 6 of 10 Appl. Sci. 2019, x, x 6 of 10 Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Experimental result shows that a larger than 50 km unamplified optical communication link can be Experimental result shows that a larger than 50 km unamplified optical communication link can be achieved using PTx Ref. With 6-dB more optical power than PTx Ref, the system can achieve larger than achieved using PTx Ref. With 6-dB more optical power than PTx Ref, the system can achieve larger than 90 km transmission in the unamplified link. Consequently, enhanced output power of Tx or Appl. Sci. 2019, 9, 2455 6 of 9 90 km transmission in the unamplified link. Consequently, enhanced output power of Tx or improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. (a) (b) (a) (b) Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point link Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point link for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. link for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. Furthermore, we compare 16-QAM signal transmission performance at various baud rates with Furthermore, we compare 16-QAM signal transmission performance at various baud rates with Furthermore, we compare 16-QAM signal transmission performance at various baud rates with PTx Ref and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted the P and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted PTx Ref and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted the Tx Ref 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with PTx Ref and 6-dB the 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with P and 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with PTx Ref and 6-dB Tx Ref more Tx side optical power, their reaches are around 40 km and 75 km, which are mainly limited by 6-dB momor re Tx side e Tx side optoptical ical power power , their , their reache reaches s are aro areuar nd 4 ound 0 km 40 and 7 km and 5 km 75 , wh km, ich ar which e mar ainl e mainly y limited limited by the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud rate the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud rate by the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud system. system. rate system. (a) (b) (a) (b) Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output power at the reference point. power at the reference point. power at the reference point. 5.2. High-Baud-Rate 64-QAM 5.2. High-Baud-Rate 64-QAM 5.2. High-Baud-Rate 64-QAM Compared with 16-QAM, the 64-QAM signal has 1.5× more spectral efficiency, and thus can Compared with 16-QAM, the 64-QAM signal has 1.5× more spectral efficiency, and thus can Compared with 16-QAM, the 64-QAM signal has 1.5 more spectral eciency, and thus can carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd and 64 GBd 64-QAM signals over different link distances. Compared with the 16-QAM, the 64-QAM and 64 GBd 64-QAM signals over different link distances. Compared with the 16-QAM, the 64-QAM and 64 GBd 64-QAM signals over di erent link distances. Compared with the 16-QAM, the 64-QAM signal has a smaller Euclidean distance between neighboring constellation points. It is thus much more susceptible to noise sources, such as laser phase noise and Rx electrical noise. In addition, compared with 16-QAM, the nonlinearity introduced by the transfer function of the IQ modulator has more impact on the 64-QAM signals. In our experiment, the modulation nonlinearity is mitigated by adjusting the electrical driving signal. As shown in Figure 7b, with 6-dB more optical power of the Tx side optical power than P , the 64 GBd 64-QAM signal can transmit up to 40 km in the unamplified Tx Ref link, which gives greater than 600 Gb/s net data rate. Appl. Sci. 2019, x, x 7 of 10 signal has a smaller Euclidean distance between neighboring constellation points. It is thus much more susceptible to noise sources, such as laser phase noise and Rx electrical noise. In addition, compared with 16-QAM, the nonlinearity introduced by the transfer function of the IQ modulator has more impact on the 64-QAM signals. In our experiment, the modulation nonlinearity is mitigated by adjusting the electrical driving signal. As shown in Figure 7b, with 6-dB more optical power of the Tx side optical power than PTx Ref, the 64 GBd 64-QAM signal can transmit up to 40 km in the Appl. Sci. 2019, 9, 2455 7 of 9 unamplified link, which gives greater than 600 Gb/s net data rate. (a) (b) Figure 7. BER as a function of transmission distance in unamplified point-to-point link for (a) 45 GBd Figure 7. BER as a function of transmission distance in unamplified point-to-point link for (a) 45 GBd 64-QAM signals; (b) 64 GBd 64-QAM signals. 64-QAM signals; (b) 64 GBd 64-QAM signals. 5.3. High-Baud-Rate QAM Transmission Matrix 5.3. High-Baud-Rate QAM Transmission Matrix After transmitting 16-QAM and 64-QAM signals with various baud rates and di erent Tx After transmitting 16-QAM and 64-QAM signals with various baud rates and different Tx side powers, we summarized the single-carrier signal transmission matrix, using the net data rate and side powers, we summarized the single-carrier signal transmission matrix, using the net data rate achievable unamplified link distance. As shown in Figure 8, the link distance data points are and achievable unamplified link distance. As shown in Figure 8, the link distance data points are −2 2 interpolated at 4.2 × 10 SD-FEC threshold. The net data rate calculates only the information bits, by interpolated at 4.2  10 SD-FEC threshold. The net data rate calculates only the information bits, excluding the 25% FEC OH. For each combination of baud rate and QAM order, we chose two points by excluding the 25% FEC OH. For each combination of baud rate and QAM order, we chose two of optical power at the Tx side, PTx Ref and 6-dB more optical power than PTx Ref. For the 64 GBd 16-QAM points of optical power at the Tx side, P and 6-dB more optical power than P . For the 64 GBd Tx Ref Tx Ref signals, these conditions provide 12 dB and 18 dB link margin, respectively. From Figure 8 one can see 16-QAM signals, these conditions provide 12 dB and 18 dB link margin, respectively. From Figure 8 that, for the same modulation format, lowering the baud rate gives an extended reach at the sacrifice of one can see that, for the same modulation format, lowering the baud rate gives an extended reach at the net data rate. Similarly, for the same baud rate, increasing the QAM order gives a higher net data the sacrifice of the net data rate. Similarly, for the same baud rate, increasing the QAM order gives a rate at the expense of the system’s reach. For PTx Ref Tx side optical power, the 45 GBd 16-QAM signal higher net data rate at the expense of the system’s reach. For P Tx side optical power, the 45 GBd Tx Ref can still achieve 80 km reach in the unamplified link, which provides 288 Gb/s net data rate. For the Tx 16-QAM signal can still achieve 80 km reach in the unamplified link, which provides 288 Gb/s net side with 6-dB more optical power than PTx Ref, the 64 GBd 16-QAM signal can achieve both 80 km data rate. For the Tx side with 6-dB more optical power than P , the 64 GBd 16-QAM signal Tx Ref transmission and larger than 400 Gb/s net data rate. In such a condition, the 64 GBd 64-QAM signal can can achieve both 80 km transmission and larger than 400 Gb/s net data rate. In such a condition, produce greater than 600 Gb/s net data rate for an unamplified link as long as 40 km. By comparing the the 64 GBd 64-QAM signal can produce greater than 600 Gb/s net data rate for an unamplified link two conditions of optical power at the Tx side, one can see that 6-dB more power can extend the fiber as long as 40 km. By comparing the two conditions of optical power at the Tx side, one can see that transmission reach to approximately 30 km. Consequently, it is critically important to increase the 6-dB more power can extend the fiber transmission reach to approximately 30 km. Consequently, it is output power at the Tx side or improve the sensitivity at the Rx side. Increasing the laser’s power, critically important to increase the output power at the Tx side or improve the sensitivity at the Rx reducing the modulation loss, using two separate lasers for Tx and LO are a few potential options. side. Increasing the laser ’s power, reducing the modulation loss, using two separate lasers for Tx and LO are a few potential options. Appl. Sci. 2019, x, x 8 of 10 Figure 8. Net data rate as a function of transmission distance for 16-QAM and 64-QAM signals at Figure 8. Net data rate as a function of transmission distance for 16-QAM and 64-QAM signals at various baud rates. various baud rates. 6. Conclusions In this article, we introduce 400GbE DCI using pluggable optics. Different aspects of the 400G pluggable module, including form factor, architecture, and power consumption, are discussed. Experimental investigation shows that the 64 GBd 16-QAM and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond unamplified fiber link. Capacity- reach matrix is finally generated with different baud rates, QAM order, and Tx side power in the unamplified fiber link. We find that increasing the Tx output power or improving the Rx sensitivity is the key for better system performance in the power-limited coherent fiber link. A 6-dB more transmitted power can extend the fiber transmission reach to approximately 30 km for both modulation formats investigated. It is thus important for the industry to develop accordingly, under the power and space constrains of the 400G DCO pluggable module. Author Contributions: Conceptualization, Y.Y. and Q.W.; methodology, Y.Y. and Q.W.; software, Y.Y. and Q.W.; validation, Y.Y. and Q.W.; formal analysis, Y.Y. and Q.W.; investigation, Y.Y. and Q.W.; resources, Y.Y., Q.W. and J.A.; data curation, Y.Y. and Q.W.; writing—original draft preparation, Y.Y. and Q.W.; writing— review and editing, Y.Y., Q.W. and J.A.; visualization, Y.Y. and Q.W.; supervision, J.A. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge Xuan He and Jeffery J. Maki for the fruitful discussion on the work. The authors also gratefully acknowledge vigorous encouragement and sturdy support on innovation from Domenico Di Mola at Juniper Networks. Conflicts of Interest: The authors declare no conflicts of interest. References 1. 40 Gb/s and 100 Gb/s Fiber Optic Task Force. Available online: http://www.ieee802.org/3/bm/ (accessed on 26 January 2019). 2. IEEE P802.3bs 400 Gb/s Ethernet Task Force. Available online: www.ieee802.org/3/bs/ (accessed on 1 March 2019). 3. Rokkas, T.; Neokosmidis, I.; Tomkos, I. Cost and Power Consumption Comparison of 400 Gbps Intra-Datacenter Transceiver Modules. In Proceedings of the 2018 International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, 1–5 July 2018. 4. Urata, R.; Liu, H.; Zhou, X.; Vahdat, A. Datacenter Interconnect and Networking: From Evolution to Holistic Revolution. In Proceedings of the 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 11–15 March 2018. 5. Baveja, P.P.; Li, M.; Wang, D.; Hsieh, C.; Zhang, H.; Ma, N.; Wang, Y.; Lii, J.; Liang, Y.; Wang, C.; Ho, I.L.; Zheng, J. 56 Gb/s PAM-4 Directly Modulated Laser for 200G/400G Data-Center Optical Appl. Sci. 2019, 9, 2455 8 of 9 6. Conclusions In this article, we introduce 400GbE DCI using pluggable optics. Di erent aspects of the 400G pluggable module, including form factor, architecture, and power consumption, are discussed. Experimental investigation shows that the 64 GBd 16-QAM and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond unamplified fiber link. Capacity-reach matrix is finally generated with di erent baud rates, QAM order, and Tx side power in the unamplified fiber link. We find that increasing the Tx output power or improving the Rx sensitivity is the key for better system performance in the power-limited coherent fiber link. A 6-dB more transmitted power can extend the fiber transmission reach to approximately 30 km for both modulation formats investigated. It is thus important for the industry to develop accordingly, under the power and space constrains of the 400G DCO pluggable module. Author Contributions: Conceptualization, Y.Y. and Q.W.; methodology, Y.Y. and Q.W.; software, Y.Y. and Q.W.; validation, Y.Y. and Q.W.; formal analysis, Y.Y. and Q.W.; investigation, Y.Y. and Q.W.; resources, Y.Y., Q.W. and J.A.; data curation, Y.Y. and Q.W.; writing—original draft preparation, Y.Y. and Q.W.; writing—review and editing, Y.Y., Q.W. and J.A.; visualization, Y.Y. and Q.W.; supervision, J.A. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge Xuan He and Je ery J. Maki for the fruitful discussion on the work. The authors also gratefully acknowledge vigorous encouragement and sturdy support on innovation from Domenico Di Mola at Juniper Networks. Conflicts of Interest: The authors declare no conflicts of interest. References 1. 40 Gb/s and 100 Gb/s Fiber Optic Task Force. Available online: http://www.ieee802.org/3/bm/ (accessed on 26 January 2019). 2. IEEE P802.3bs 400 Gb/s Ethernet Task Force. Available online: www.ieee802.org/3/bs/ (accessed on 1 March 2019). 3. Rokkas, T.; Neokosmidis, I.; Tomkos, I. Cost and Power Consumption Comparison of 400 Gbps Intra-Datacenter Transceiver Modules. In Proceedings of the 2018 International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, 1–5 July 2018. 4. Urata, R.; Liu, H.; Zhou, X.; Vahdat, A. Datacenter Interconnect and Networking: From Evolution to Holistic Revolution. In Proceedings of the 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 11–15 March 2018. 5. Baveja, P.P.; Li, M.; Wang, D.; Hsieh, C.; Zhang, H.; Ma, N.; Wang, Y.; Lii, J.; Liang, Y.; Wang, C.; et al. 56 Gb/s PAM-4 Directly Modulated Laser for 200G/400G Data-Center Optical Links. In Proceedings of the 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 11–15 March 2018. 6. Yue, Y.; Wang, Q.; Maki, J.J.; Anderson, J. Enabling Technologies in Packet Optics. In Proceedings of the 2018 Asia Communications and Photonics Conference (ACP), Hangzhou, China, 26–29 October 2018. 7. IEEE P802.3bs 200 Gb/s and 400 Gb/s Ethernet Task Force. Available online: www.ieee802.org/3/bs/ (accessed on 26 January 2019). 8. Sone, Y.; Nishizawa, H.; Yamamoto, S.; Fukutoku, M.; Yoshimatsu, T. Systems and technologies for high-speed inter-oce/datacenter interface. In Proceedings of the 2017 SPIE Photonics West, San Francisco, CA, USA, 28 January–2 February 2017. 9. OIF 400ZR. Available online: http://www.oiforum.com/technical-work/hot-topics/400zr-2/ (accessed on 20 May 2019). 10. Zhu, Y.; Li, A.; Peng, W.; Kan, C.; Li, Z.; Chowdhury, S.; Cui, Y.; Bai, Y. Spectrally-Ecient Single-Carrier 400G Transmission Enabled by Probabilistic Shaping. In Proceedings of the 2017 Optical Fiber Communications Conference and Exposition (OFC), Los Angeles, CA, USA, 19–23 March 2017. 11. Isono, H. Latest standardization trends for client and networking optical transceivers and its future directions. In Proceedings of the 2018 optoelectronics, photonic materials and devices conference (SPIE OPTO), San Francisco, CA, USA, 27 January–1 February 2018. 12. Cole, C. Beyond 100G client optics. IEEE Commun. Mag. 2012, 50, s58–s66. [CrossRef] Appl. Sci. 2019, 9, 2455 9 of 9 13. Garrafa, N.; Salome, O.; Mueller, T.; Carcelen, O.P.; Calabretta, G.; Carretero, N.; Galimberti, G.; Keck, S.; López, V.; van den Borne, D. Multi-vendor 100G DP-QPSK line-side interoperability field trial over 1030 km. In Proceedings of the 2017 Optical Fiber Communications Conference and Exposition (OFC), Los Angeles, CA, USA, 19–23 March 2017. 14. Nelson, L.E.; Zhang, G.; Padi, N.; Skolnick, C.; Benson, K.; Kaylor, T.; Iwamatsu, S.; Inderst, R.; Marques, F.; Fonseca, D.; et al. SDN-Controlled 400GbE end-to-end service using a CFP8 client over a deployed, commercial flexible ROADM system. In Proceedings of the 2017 Optical Fiber Communications Conference and Exposition (OFC), Los Angeles, CA, USA, 19–23 March 2017. 15. Birk, M.; Nelson, L.E.; Zhang, G.; Cole, C.; Yu, C.; Akashi, M.; Hiramoto, K.; Fu, X.; Brooks, P.; Schubert, A.; et al. First 400GBASE-LR8 interoperability using CFP8 modules. In Proceedings of the 2017 Optical Fiber Communications Conference and Exposition (OFC), Los Angeles, CA, USA, 19–23 March 2017. 16. Nelson, L.E. Advances in 400 Gigabit Ethernet Field Trials. In Proceedings of the 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 11–15 March 2018. 17. Yue, Y.; Wang, Q.; Yao, J.; O’Neil, J.; Pudvay, D.; Anderson, J. 400GbE Technology Demonstration Using CFP8 Pluggable Modules. Appl. Sci. 2018, 8, 2055. [CrossRef] 18. CFP-MSA. Available online: http://www.cfp-msa.org/ (accessed on 1 March 2019). 19. SFF Committee. Available online: http://www.s committee.com/ie/ (accessed on 1 March 2019). 20. OSFP. Available online: http://osfpmsa.org/index.html (accessed on 1 March 2019). 21. QSFP-DD. Available online: http://www.qsfp-dd.com/ (accessed on 1 March 2019). 22. OIF: Optical Internetworking Forum. Available online: http://www.oiforum.com/ (accessed on 1 March 2019). 23. Cole, C. Future datacenter interfaces based on existing and emerging optics technologies. In Proceedings of the 2013 IEEE Photonics Society Summer Topical Meeting Series, Waikoloa, HI, USA, 8–10 July 2013. 24. Geyer, J.C.; Rasmussen, C.; Shah, B.; Nielsen, T.; Givehchi, M. Power Ecient Coherent Transceivers. In Proceedings of the 2016 European Conference on Optical Communication (ECOC), Dusseldorf, Germany, 18–22 September 2016. 25. Frey, F.; Elschner, R.; Fischer, J.K. Estimation of Trends for Coherent DSP ASIC Power Dissipation for Di erent Bitrates and Transmission Reaches. In Proceedings of the ITG-Symposium Photonic Networks, Leipzig, Germany, 11–12 May 2017. 26. Zhang, H.; Zhu, B.; Park, S.; Doerr, C.; Aydinlik, M.; Geyer, J.; Pfau, T.; Pendock, G.; Aroca, R.; Liu, F.; et al. Real-time transmission of 16 Tb/s over 1020km using 200Gb/s CFP2-DCO. Opt. Express 2018, 26, 6943–6948. [CrossRef] [PubMed] 27. Wang, Q.; Yue, Y.; Anderson, J. Compensation of Limited Bandwidth and Nonlinearity for Coherent Transponder. Appl. Sci. 2019, 9, 1758. [CrossRef] 28. Yue, Y.; Wang, Q.; Anderson, J. Transmitter skew tolerance and spectral eciency tradeo in high baud-rate QAM optical communication systems. Opt. Express 2018, 26, 15045–15058. [CrossRef] [PubMed] 29. Faruk, M.; Savory, S. Digital Signal Processing for Coherent Transceivers Employing Multilevel Formats. J. Lightwave Technol. 2017, 35, 1125–1141. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal

Applied Sciences , Volume 9 (12) – Jun 15, 2019

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applied sciences Article Experimental Investigation of 400 Gb/s Data Center Interconnect Using Unamplified High-Baud-Rate and High-Order QAM Single-Carrier Signal Yang Yue *, Qiang Wang and Jon Anderson Juniper Networks, 1133 Innovation Way, Sunnyvale, CA 94089, USA; qiwang.thresh@gmail.com (Q.W.); jonanderson@juniper.net (J.A.) * Correspondence: yyue@juniper.net; Tel.: +1-408-745-2000 Received: 16 March 2019; Accepted: 13 June 2019; Published: 15 June 2019 Abstract: In this article, we review the latest progress on data center interconnect (DCI). We then discuss di erent perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption, following 400G pluggable optics in DCI applications. Next, we experimentally investigate the capacity-reach matrix for high-baud-rate and high-order quadrature amplitude modulation (QAM) single-carrier signals in the unamplified single-mode optical fiber (SMF) link. We show that the 64 GBd 16-QAM, and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond of unamplified fiber link. Keywords: 400 Gigabit Ethernet; coherent communications; data center interconnect; fiber optics links and subsystems; optical communications; QSFP-DD transceiver 1. Introduction To meet the ever-growing demands on the internet bandwidth, the Ethernet speed has evolved at an astonishing pace, from 10 Megabit Ethernet to 100 Gigabit Ethernet (100 GbE) within the past 30 years [1]. In 2017, 400GbE standard was ratified by the IEEE P802.3bs Task Force [2]. This built the foundation for industrial deployment of 400GbE in the global network. It is anticipated that 400GbE will be rapidly adopted over the next few years [3]. Recently, 400GbE pluggable optics is under development to satisfy the demand on bandwidth-consuming applications, including data-center-based cloud services [4–6]. For intra data center interconnect (DCI) application, optical direct detection with four-level pulse amplitude modulation (PAM-4) has been widely adopted to enable the low cost 400GbE optical transceivers, which can carry up to 400 Gb/s Ethernet data over either parallel fibers or multiple wavelengths. Direct detection PAM-4 optics has also been standardized by Multi-Source Agreement (MSA) groups to achieve link distance from 500 m to 10 km (e.g., 400G-FR8, 400G-DR4, 400G-LR4, etc.) [7]. For inter DCI application, pluggable 400G digital coherent optics (DCO) with single-carrier dual-polarization quadrature amplitude modulation (DP-QAM) signal is being proposed to achieve 40 km and beyond fiber link [8,9]. PAM-4 signal with direct detection only uses the intensity of the light to carry information. Instead, DP-QAM signal with coherent detection technology can utilize a few degrees of freedom for photons, including polarization, intensity, and phase. Consequently, coherent technology can transmit a single-carrier 400 Gb/s signal with significantly improved spectral eciency. Furthermore, its receiver sensitivity can be significantly improved over the one of intensity modulation with direct detection (IM/DD) systems. In this article, we first introduce the application scenarios of pluggable optics in DCI applications. Next, we discuss di erent perspectives on the 400G pluggable module, including form factor, Appl. Sci. 2019, 9, 2455; doi:10.3390/app9122455 www.mdpi.com/journal/applsci Appl. Sci. 2019, x, x 2 of 10 In this article, we first introduce the application scenarios of pluggable optics in DCI applications. Appl. Sci. 2019, 9, 2455 2 of 9 Next, we discuss different perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption. Then, we experimentally investigate the fiber transmission performance of 16-QAM and 64-QAM single-carrier architecture, digital signal processing (DSP), and module power consumption. Then, we experimentally signals at various baud rates, aiming at 400 Gb/s DCI applications with unamplified link. Here, we investigate the fiber transmission performance of 16-QAM and 64-QAM single-carrier signals at various −2 consider 25% overhead (OH) soft-decision forward error correction (SD-FEC), which gives a 4.2 × 10 baud rates, aiming at 400 Gb/s DCI applications with unamplified link. Here, we consider 25% overhead bit error ratio (BER) threshold [10]. For 64 GBd 16-QAM signals with 12 dB and 18 dB link margins, (OH) soft-decision forward error correction (SD-FEC), which gives a 4.2  10 bit error ratio (BER) it is possible to transmit 400 Gb/s net data rate over 40 km and 80 km unamplified fiber link, threshold [10]. For 64 GBd 16-QAM signals with 12 dB and 18 dB link margins, it is possible to transmit respectively. With 18 dB link margin, 64 GBd 64-QAM signal can produce greater than 600 Gb/s net 400 Gb/s net data rate over 40 km and 80 km unamplified fiber link, respectively. With 18 dB link margin, data rate for an unamplified link as long as 40 km. We further study the impact of transmitter (Tx) 64 GBd 64-QAM signal can produce greater than 600 Gb/s net data rate for an unamplified link as long side power on the transmission performance and obtain a high-baud-rate QAM signal transmission as 40 km. We further study the impact of transmitter (Tx) side power on the transmission performance matrix in unamplified link. For various signals, extra 6-dB more power can extend the fiber and obtain a high-baud-rate QAM signal transmission matrix in unamplified link. For various signals, transmission reach to approximately 30 km. It is thus critically important to increase the Tx output extra 6-dB more power can extend the fiber transmission reach to approximately 30 km. It is thus power or improve the Rx sensitivity, under the available power and space constrains of the DCO critically important to increase the Tx output power or improve the Rx sensitivity, under the available pluggable module. power and space constrains of the DCO pluggable module. 2. 400G Data Center Interconnect Use Cases 2. 400G Data Center Interconnect Use Cases Data centers have been proliferated over the past few years, which are driven by more and more Data centers have been proliferated over the past few years, which are driven by more and more bandwidth-hungry applications, including enterprise digital transformation, cloud services, and the bandwidth-hungry applications, including enterprise digital transformation, cloud services, and the coming 5G applications. In such an era, enterprises, service providers, and cloud providers need a coming 5G applications. In such an era, enterprises, service providers, and cloud providers need a DCI network that can quickly adapt to the changes in business and operational demands. To meet DCI network that can quickly adapt to the changes in business and operational demands. To meet the the above requirements, data center networks are expanding in number and capacity, driving the above requirements, data center networks are expanding in number and capacity, driving the need need for higher-speed DCI solutions. Top cloud service providers, such as Amazon, Facebook, for higher-speed DCI solutions. Top cloud service providers, such as Amazon, Facebook, Google, Google, and Microsoft, have already adopted 100 Gb/s and will quickly transfer to 400 Gb/s and and Microsoft, have already adopted 100 Gb/s and will quickly transfer to 400 Gb/s and beyond. beyond. Figure 1 illustrates typical application scenarios of 400 Gb/s pluggable optics for DCI. As shown Figure 1 illustrates typical application scenarios of 400 Gb/s pluggable optics for DCI. As shown in Figure 1a, the reach of an unamplified link is power limited, and it mainly depends on the optical in Figure 1a, the reach of an unamplified link is power limited, and it mainly depends on the optical transmitter output power, the fiber link loss, and the optical receiver sensitivity. For the amplified transmitter output power, the fiber link loss, and the optical receiver sensitivity. For the amplified dense wavelength division multiplexing (DWDM) link displayed in Figure 1b, it is noise limited, dense wavelength division multiplexing (DWDM) link displayed in Figure 1b, it is noise limited, and and its reach depends on the available optical signal to noise ratio (OSNR) at the optical receiver. its reach depends on the available optical signal to noise ratio (OSNR) at the optical receiver. Figure 1. Typical application scenarios of 400 Gb/s pluggable optics for data center interconnect. (a) Figure 1. Typical application scenarios of 400 Gb/s pluggable optics for data center interconnect. Unamplified point-to-point applications for 40 km reach; (b) amplified dense wavelength division (a) Unamplified point-to-point applications for 40 km reach; (b) amplified dense wavelength division multiplexing multiplexing (DWDM) applications for (DWDM) applications for 80–1 80–120 20 km reach. km reach. 3. 400G Pluggable Module Form Factors and Power Requirement 3. 400G Pluggable Module Form Factors and Power Requirement Due to the emerging bandwidth-hungry applications, the photonics communications industry is Due to the emerging bandwidth-hungry applications, the photonics communications industry unceasingly driven towards higher-capacity, smaller-size, and lower-power solutions. Standardization is unceasingly driven towards higher-capacity, smaller-size, and lower-power solutions. organizations, including the Institute of Electrical and Electronics Engineers (IEEE) and the Optical Internetworking Forum (OIF), are leading the industrial transformation of photonics communications society from using discrete optical components to utilizing photonics integration enabled packet Appl. Sci. 2019, x, x 3 of 10 Appl. Sci. 2019, 9, 2455 3 of 9 Standardization organizations, including the Institute of Electrical and Electronics Engineers (IEEE) and the Optical Internetworking Forum (OIF), are leading the industrial transformation of photonics optics [11,12]. Lately, line and client-side industrial trials on interoperability have been demonstrated communications society from using discrete optical components to utilizing photonics integration successfully using 100 Gb/s CFP2 and 400 Gb/s CFP8 pluggable optical modules, respectively [13–17]. enabled packet optics [11,12]. Lately, line and client-side industrial trials on interoperability have Nowadays, the industry is marching towards 400 Gb/s pluggable optical modules with even more been demonstrated successfully using 100 Gb/s CFP2 and 400 Gb/s CFP8 pluggable optical modules, compact form factors. The first debuts of 400 Gb/s client and line-side pluggable QSFP modules are respectively [13–17]. Nowadays, the industry is marching towards 400 Gb/s pluggable optical planned for year 2019 and 2020. modules with even more compact form factors. The first debuts of 400 Gb/s client and line-side Based on the DSP’s physical location, there are two types of pluggable coherent modules: analog pluggable QSFP modules are planned for year 2019 and 2020. coherent Base optics d on the (ACO) DSP’s physic and DCO. al location For 400G , there pluggable are twcoher o type ent s of pl transceivers, uggable cohe the reindustry nt module has s: an reached alog ancoherent agreement opton ics ( implementing ACO) and DCO. For DCO, which 400G pl has uggthe able DSP coherent tran inside the sceivers, the industry has reached module. A few form factors have an agreement on implementing DCO, which has the DSP inside the module. A few form factors have emerged recently as the candidates for 400G DCO pluggable modules, such as CFP2, CFP8, OSFP, emerged recently as the candidates for 400G DCO pluggable modules, such as CFP2, CFP8, OSFP, and QSFP-DD [18–22], whose mechanical dimensions and power consumptions are shown in Figure 2. and QSFP-DD [18–22], whose mechanical dimensions and power consumptions are shown in Figure These transceivers are equipped with eight pairs of di erential signal pins, which can support 400G 2. These transceivers are equipped with eight pairs of differential signal pins, which can support 400G transmission speed using 50G PAM-4 electrical interface [23]. One can see that CFP2 and CFP8 have transmission speed using 50G PAM-4 electrical interface [23]. One can see that CFP2 and CFP8 have larger physical dimensions than those of OSFP and QSFP-DD, thus, they have more room for additional larger physical dimensions than those of OSFP and QSFP-DD, thus, they have more room for components and allow a larger power budget. Consequently, OSFP and QSFP-DD are targeted for additional components and allow a larger power budget. Consequently, OSFP and QSFP-DD are the DCI application, while CFP2 and CFP8 are planned for the long-haul applications. The line-card targeted for the DCI application, while CFP2 and CFP8 are planned for the long-haul applications. fitting into one rack unit (RU) space can accommodate 36 QSFP-DD pluggable modules. The total The line-card fitting into one rack unit (RU) space can accommodate 36 QSFP-DD pluggable modules. capacity of the line-card is 14.4 Tb/s. Currently, the industry is converging behind QSFP-DD due to The total capacity of the line-card is 14.4 Tb/s. Currently, the industry is converging behind QSFP- its high port density in the front panel, its backward compatibility with QSFP28/QSFP+, and thus a DD due to its high port density in the front panel, its backward compatibility with QSFP28/QSFP+, larger ecosystem. and thus a larger ecosystem. Figure 2. 400G pluggable coherent transceiver form factors. CFP: C form-factor, OSFP: octal small Figure 2. 400G pluggable coherent transceiver form factors. CFP: C form-factor, OSFP: octal small form form factor, QSFP-DD: quad small form factor double density. factor, QSFP-DD: quad small form factor double density. Figure 3 illustrates the typical architecture of a 400G pluggable coherent transceiver. There are Figure 3 illustrates the typical architecture of a 400G pluggable coherent transceiver. There three functional blocks inside the module, one DSP chip, one transmitter and receiver optical are three functional blocks inside the module, one DSP chip, one transmitter and receiver optical subassembly (TROSA), and one power/control unit. Besides the power and control signal, the host subassembly (TROSA), and one power/control unit. Besides the power and control signal, the host side also provides up to 400GbE client traffic to the pluggable coherent module through eight pairs side also provides up to 400GbE client trac to the pluggable coherent module through eight pairs of differential signal pins. 400 Gb/s single-carrier signal (>60 GBd 16-QAM) can thus be generated out of di erential signal pins. 400 Gb/s single-carrier signal (>60 GBd 16-QAM) can thus be generated of the Tx output. The other port of receiver (Rx) input captures the received signal, and coherent out of the Tx output. The other port of receiver (Rx) input captures the received signal, and coherent detection is performed by the TROSA and DSP. detection is performed by the TROSA and DSP. The pluggable coherent optics starts from 100 Gb/s CFP. During the past few years, its evolution of density has been quite amazing. Meanwhile, a lot of challenges to the whole industry have been triggered. Power consumption is always one of the key constrains for industrial applications. As shown in Figure 2, the requirement on power consumption for 400 Gb/s QSFP-DD DCO is less than 15 W. Over the years, the power consumption of the coherent DSP has reduced significantly [24–26]. Table 1 illustrates the evolution of the power consumption of coherent DSP. One can see that, with 40 nm complementary metal-oxide-semiconductor (CMOS) process, 100G coherent trac consumes approximately 50 W power. Nowadays, with the latest 7 nm CMOS technique, the industry can bring Appl. Sci. 2019, 9, 2455 4 of 9 the power consumption of DSP down to less than 2 W, which is larger than a 20 times reduction. This tremendous improvement can potentially fit the 400G-ZR QSFP-DD DCO under a 15 W power envelop. As shown in Table 2, DSP, TROSA, and the other components are allocated only 7 W, 6–7 W, and 1–2 W power budget, respectively. In such tight power and size constrains, the industry has adopted 10–12 dB link budget in the power limited unamplified case for up to 40 km reach. Appl. Sci. 2019, x, x 4 of 10 Figure 3. Typical architecture of 400G pluggable coherent transceiver. DSP: digital signal processing, Figure 3. Typical architecture of 400G pluggable coherent transceiver. DSP: digital signal processing, TROSA: transmitter and receiver optical subassembly, Tx: transmitter, Rx: receiver. TROSA: transmitter and receiver optical subassembly, Tx: transmitter, Rx: receiver. TableThe p 1. lugg Coher ablent e coherent digital op signal tics start prs ocessing from 100 G (DSP) b/s Cpower FP. During t consumption he past few evolution. years, its evol CMOS: ution of density has been quite amazing. Meanwhile, a lot of challenges to the whole industry have been complementary metal-oxide-semiconductor. triggered. Power consumption is always one of the key constrains for industrial applications. As DSP 2012 2015 2017 2019 shown in Figure 2, the requirement on power consumption for 400 Gb/s QSFP-DD DCO is less than 15 W. Over the years, the power consumption of the coherent DSP has reduced significantly [24–26]. CMOS node 40 nm 28 nm 16 nm 7 nm Table 1 illustrates the evolution of the power consumption of coherent DSP. One can see that, with Power/100G <50 W <20 W <7 W <2 W 40 nm complementary metal-oxide-semiconductor (CMOS) process, 100G coherent traffic consumes approximately 50 W power. Nowadays, with the latest 7 nm CMOS technique, the industry can bring Table 2. 400G-ZR QSFP-DD power consumption. TROSA: transmitter and receiver optical subassembly, the power consumption of DSP down to less than 2 W, which is larger than a 20 times reduction. This QSFP-DD: quad small form factor double density. tremendous improvement can potentially fit the 400G-ZR QSFP-DD DCO under a 15 W power envelop. As shown in Table 2, DSP, TROSA, and the other components are allocated only 7 W, 6–7 Module Power DSP TROSA Other W, and 1–2 W power budget, respectively. In such tight power and size constrains, the industry has QSFP-DD 7 W 6–7 W 1–2 W adopted 10–12 dB link budget in the power limited unamplified case for up to 40 km reach. 4. Experimental Table 1. Setup Coherent digital signal processing (DSP) power consumption evolution. CMOS: complementary metal-oxide-semiconductor. Figure 4 shows our experimental setup to investigate single-carrier high-baud-rate and high-order DSP 2012 2015 2017 2019 QAM signal transmission in unamplified fiber link. A 92 GS/s digital-to-analog converter (DAC) CMOS node 40 nm 28 nm 16 nm 7 nm with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of Power/100G <50 W <20 W <7 W <2 W dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter [27]. Table 2. 400G-ZR QSFP-DD power consumption. TROSA: transmitter and receiver optical The root-raise cosine filter with a roll-o factor of 0.2 was applied in the following experiment. We used subassembly, QSFP-DD: quad small form factor double density. two tunable narrow-linewidth (<100 kHz) lasers with 16 dBm maximal output power, and set them to Module Power DSP TROSA Other 1550.12 nm (i.e., 193.4 THz). One is with maximal power for the optical input to the DP-IQ modulator, QSFP-DD 7 W 6–7 W 1–2 W and the other is set to 12 dBm to act as the local oscillator (LO) for the coherent receiver. The laser to the modulator is separated into two orthogonal polarizations, and each polarization is modulated by the 4. Experimental Setup corresponding IQ signals. An automatic bias control (ABC) is used to control the bias voltage applied to Figure 4 shows our experimental setup to investigate single-carrier high-baud-rate and high- the DP-IQ modulator. At the Rx side, the optical signal is first de-modulated by using a 90 heterodyne order QAM signal transmission in unamplified fiber link. A 92 GS/s digital-to-analog converter coherent detector. Four linear trans-impedance amplifiers (TIAs) with di erential outputs are set to (DAC) with 32 GHz analog bandwidth is used to generate electrical signals for the four tributaries of automatic gain control (AGC) mode. This minimizes the BER performance dependence on the LO dual-polarization (DP) in-phase and quadrature (IQ) modulator. The trace loss between the DAC and power, and four tributaries of analog electrical signals are generated. Then, an 8-bit analog-to-digital the DP-IQ modulator is compensated in the digital domain by a finite impulse response (FIR) filter converter (ADC) with 63 GHz analog bandwidth and 4.5 e ective number of bits (ENOB) samples the [27]. The root-raise cosine filter with a roll-off factor of 0.2 was applied in the following experiment. analog signals into discrete digital samples at 160 GS/s for post signal processing. In the DSP, chromatic We used two tunable narrow-linewidth (<100 kHz) lasers with 16 dBm maximal output power, and dispersion set them to (CD) 15is 50first .12 nm compensated. (i.e., 193.4 THz) Then . One is wi polarization th maximmode al power dispersion for the optica (PMD) l inpuis t to the DP- compensated IQ modulator, and the other is set to 12 dBm to act as the local oscillator (LO) for the coherent receiver. and polarization tributaries are de-multiplexed by a blind equalization using the radius directed The laser to the modulator is separated into two orthogonal polarizations, and each polarization is equalization (RDE). A 21-tap equalizer is implemented in the time domain with a Ts/2 spacing [28]. modulated by the corresponding IQ signals. An automatic bias control (ABC) is used to control the bias voltage applied to the DP-IQ modulator. At the Rx side, the optical signal is first de-modulated by using a 90° heterodyne coherent detector. Four linear trans-impedance amplifiers (TIAs) with differential outputs are set to automatic gain control (AGC) mode. This minimizes the BER Appl. Sci. 2019, x, x 5 of 10 performance dependence on the LO power, and four tributaries of analog electrical signals are generated. Then, an 8-bit analog-to-digital converter (ADC) with 63 GHz analog bandwidth and 4.5 effective number of bits (ENOB) samples the analog signals into discrete digital samples at 160 GS/s for post signal processing. In the DSP, chromatic dispersion (CD) is first compensated. Then Appl. Sci. 2019, 9, 2455 5 of 9 polarization mode dispersion (PMD) is compensated and polarization tributaries are de-multiplexed by a blind equalization using the radius directed equalization (RDE). A 21-tap equalizer is implemented in the time domain with a Ts/2 spacing [28]. Furthermore, the blind estimation Furthermore, the blind estimation algorithm is used to estimate and compensate the carrier frequency algorithm is used to estimate and compensate the carrier frequency offset between the Tx signal and o set between the Tx signal and the LO laser, followed by the carrier phase noise [29]. the LO laser, followed by the carrier phase noise [29]. Figure 4. Experimental setup. DAC: digital-to-analog converter, IQ: in-phase (I) and quadrature (Q), Figure 4. Experimental setup. DAC: digital-to-analog converter, IQ: in-phase (I) and quadrature (Q), VOA: variable optical attenuator, SMF: single-mode fiber, LO: local oscillator, ADC: analog-to-digital VOA: variable optical attenuator, SMF: single-mode fiber, LO: local oscillator, ADC: analog-to-digital converter, DSP: digital signal processing. converter, DSP: digital signal processing. In our experiment, we tried to emulate the power limited unamplified case, which is being In our experiment, we tried to emulate the power limited unamplified case, which is being adopted by the industry with 10–12 dB link margin. According to the receiver sensitivity of 64 GBd adopted by the industry with 10–12 dB link margin. According to the receiver sensitivity of 64 GBd 16-QAM signal, we adjusted the variable optical attenuator (VOA) to set the transmitter optical 16-QAM signal, we adjusted the variable optical attenuator (VOA) to set the transmitter optical output output power to a reference point PTx Ref, which is 12 dB greater than the receiver sensitivity at a BER power to a reference point P , which is 12 dB greater than the receiver sensitivity at a BER of −2 Tx Ref of 4.2 × 10 . In our experiment, we generated 16-QAM and 64-QAM with symbol rates from 45 to 86 4.2  10 . In our experiment, we generated 16-QAM and 64-QAM with symbol rates from 45 to GBd. By adjusting the VOA, we also increased the Tx optical power into the single-mode optical fiber 86 GBd. By adjusting the VOA, we also increased the Tx optical power into the single-mode optical (SMF) by 3 dB and 6 dB. Optical 16-QAM and 64-QAM signals at various baud rates were then fiber t (SMF) ransmit by ted 3 odB ver and an SMF 6 dB. link wit Optical h di 16-QAM stance from and 1064-QAM to 110 kmsignals . Different at com various binatbaud ions ofrates a few f wer ibee r then sections were used to achieve specific fiber length, and their corresponding losses are illustrated in transmitted over an SMF link with distance from 10 to 110 km. Di erent combinations of a few fiber Table 3. sections were used to achieve specific fiber length, and their corresponding losses are illustrated in Table 3. Table 3. Fiber loss characterization. Table 3. Fiber loss characterization. Length (km) 20 40 60 80 90 100 110 Loss (dB) 4.7 7.8 12.6 15.5 17.6 20.0 22.8 Length (km) 20 40 60 80 90 100 110 Loss (dB) 4.7 7.8 12.6 15.5 17.6 20.0 22.8 5. Fiber Transmission Performance Results and Discussion In this section, we first study the fiber transmission performance with high-baud-rate 16-QAM 5. Fiber Transmission Performance Results and Discussion signals in the unamplified link. Then, we extend our experiment to the high-baud-rate 64-QAM signals. Finally, we summarize a transmission matrix showing capacity and distance for 16-QAM and In this section, we first study the fiber transmission performance with high-baud-rate 16-QAM 64-QAM signals at various baud rates. signals in the unamplified link. Then, we extend our experiment to the high-baud-rate 64-QAM signals. Finally, we summarize a transmission matrix showing capacity and distance for 16-QAM and 64-QAM 5.1. High-Baud-Rate 16-QAM signals at various baud rates. For 400 Gb/s DCI applications, the industry is adopting DP-16QAM signals. With additional FEC overhead, the DP-16QAM signal can also fuel the 400 Gb/s metro, regional, and even long-haul 5.1. High-Baud-Rate 16-QAM DWDM applications. Figure 5a displays the fiber transmission performance of the 56 GBd 16-QAM For 400 Gb/s DCI applications, the industry is adopting DP-16QAM signals. With additional signal with different Tx side optical power. As the optical link is power-limited, the transmission FEC overhead, the DP-16QAM signal can also fuel the 400 Gb/s metro, regional, and even long-haul performance can be improved for longer transmission by increasing the output power of Tx. With 6- DWDM applications. Figure 5a displays the fiber transmission performance of the 56 GBd 16-QAM dB more optical power than PTx Ref, the 56 GBd 16-QAM signal can transmit over 100 km with a pre- −2 FEC BER of 2.5 × 10 . From the figure, one can see that, with 3-dB more optical power than PTx Ref, it signal with di erent Tx side optical power. As the optical link is power-limited, the transmission is sufficient to achieve 80 km error-free transmission after FEC. With different optical power of the performance can be improved for longer transmission by increasing the output power of Tx. With 6-dB more optical power than P , the 56 GBd 16-QAM signal can transmit over 100 km with a pre-FEC Tx Ref BER of 2.5  10 . From the figure, one can see that, with 3-dB more optical power than P , it is Tx Ref sucient to achieve 80 km error-free transmission after FEC. With di erent optical power of the Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Experimental result shows that a larger than 50 km unamplified optical communication link can be achieved using P . Tx Ref With 6-dB more optical power than P , the system can achieve larger than 90 km transmission Tx Ref in the unamplified link. Consequently, enhanced output power of Tx or improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. Appl. Sci. 2019, x, x 6 of 10 Appl. Sci. 2019, x, x 6 of 10 Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Tx side, the fiber transmission performance of the 16-QAM signals at 64 GBd is shown in Figure 5b. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Using 25% OH SD-FEC, the 64 GBd 16-QAM signal can still carry over 400 Gb/s net data rate. Experimental result shows that a larger than 50 km unamplified optical communication link can be Experimental result shows that a larger than 50 km unamplified optical communication link can be achieved using PTx Ref. With 6-dB more optical power than PTx Ref, the system can achieve larger than achieved using PTx Ref. With 6-dB more optical power than PTx Ref, the system can achieve larger than 90 km transmission in the unamplified link. Consequently, enhanced output power of Tx or Appl. Sci. 2019, 9, 2455 6 of 9 90 km transmission in the unamplified link. Consequently, enhanced output power of Tx or improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. improved sensitivity of Rx would be beneficial for 80 km extended long reach (ZR) application [9]. (a) (b) (a) (b) Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point link Figure 5. Bit error ratio (BER) as a function of transmission distance in unamplified point-to-point link for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. link for (a) 56 GBd 16-QAM signals; (b) 64 GBd 16-QAM signals. Furthermore, we compare 16-QAM signal transmission performance at various baud rates with Furthermore, we compare 16-QAM signal transmission performance at various baud rates with Furthermore, we compare 16-QAM signal transmission performance at various baud rates with PTx Ref and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted the P and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted PTx Ref and 6-dB more Tx side optical power, respectively. As shown in Figure 6a,b, we transmitted the Tx Ref 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with PTx Ref and 6-dB the 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with P and 16-QAM signal with up to 86 GBd symbol rate. For the 86 GBd 16-QAM signals with PTx Ref and 6-dB Tx Ref more Tx side optical power, their reaches are around 40 km and 75 km, which are mainly limited by 6-dB momor re Tx side e Tx side optoptical ical power power , their , their reache reaches s are aro areuar nd 4 ound 0 km 40 and 7 km and 5 km 75 , wh km, ich ar which e mar ainl e mainly y limited limited by the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud rate the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud rate by the increased receiver sensitivity (i.e., more receiving optical power is required) for higher baud system. system. rate system. (a) (b) (a) (b) Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM Figure 6. BER as a function of transmission distance in unamplified point-to-point link for 16-QAM signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output signals with (a) 6-dB more transmitter output power than the reference point; (b) transmitter output power at the reference point. power at the reference point. power at the reference point. 5.2. High-Baud-Rate 64-QAM 5.2. High-Baud-Rate 64-QAM 5.2. High-Baud-Rate 64-QAM Compared with 16-QAM, the 64-QAM signal has 1.5× more spectral efficiency, and thus can Compared with 16-QAM, the 64-QAM signal has 1.5× more spectral efficiency, and thus can Compared with 16-QAM, the 64-QAM signal has 1.5 more spectral eciency, and thus can carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd carry more information bits. Figure 7a,b illustrates the fiber transmission performance of the 45 GBd and 64 GBd 64-QAM signals over different link distances. Compared with the 16-QAM, the 64-QAM and 64 GBd 64-QAM signals over different link distances. Compared with the 16-QAM, the 64-QAM and 64 GBd 64-QAM signals over di erent link distances. Compared with the 16-QAM, the 64-QAM signal has a smaller Euclidean distance between neighboring constellation points. It is thus much more susceptible to noise sources, such as laser phase noise and Rx electrical noise. In addition, compared with 16-QAM, the nonlinearity introduced by the transfer function of the IQ modulator has more impact on the 64-QAM signals. In our experiment, the modulation nonlinearity is mitigated by adjusting the electrical driving signal. As shown in Figure 7b, with 6-dB more optical power of the Tx side optical power than P , the 64 GBd 64-QAM signal can transmit up to 40 km in the unamplified Tx Ref link, which gives greater than 600 Gb/s net data rate. Appl. Sci. 2019, x, x 7 of 10 signal has a smaller Euclidean distance between neighboring constellation points. It is thus much more susceptible to noise sources, such as laser phase noise and Rx electrical noise. In addition, compared with 16-QAM, the nonlinearity introduced by the transfer function of the IQ modulator has more impact on the 64-QAM signals. In our experiment, the modulation nonlinearity is mitigated by adjusting the electrical driving signal. As shown in Figure 7b, with 6-dB more optical power of the Tx side optical power than PTx Ref, the 64 GBd 64-QAM signal can transmit up to 40 km in the Appl. Sci. 2019, 9, 2455 7 of 9 unamplified link, which gives greater than 600 Gb/s net data rate. (a) (b) Figure 7. BER as a function of transmission distance in unamplified point-to-point link for (a) 45 GBd Figure 7. BER as a function of transmission distance in unamplified point-to-point link for (a) 45 GBd 64-QAM signals; (b) 64 GBd 64-QAM signals. 64-QAM signals; (b) 64 GBd 64-QAM signals. 5.3. High-Baud-Rate QAM Transmission Matrix 5.3. High-Baud-Rate QAM Transmission Matrix After transmitting 16-QAM and 64-QAM signals with various baud rates and di erent Tx After transmitting 16-QAM and 64-QAM signals with various baud rates and different Tx side powers, we summarized the single-carrier signal transmission matrix, using the net data rate and side powers, we summarized the single-carrier signal transmission matrix, using the net data rate achievable unamplified link distance. As shown in Figure 8, the link distance data points are and achievable unamplified link distance. As shown in Figure 8, the link distance data points are −2 2 interpolated at 4.2 × 10 SD-FEC threshold. The net data rate calculates only the information bits, by interpolated at 4.2  10 SD-FEC threshold. The net data rate calculates only the information bits, excluding the 25% FEC OH. For each combination of baud rate and QAM order, we chose two points by excluding the 25% FEC OH. For each combination of baud rate and QAM order, we chose two of optical power at the Tx side, PTx Ref and 6-dB more optical power than PTx Ref. For the 64 GBd 16-QAM points of optical power at the Tx side, P and 6-dB more optical power than P . For the 64 GBd Tx Ref Tx Ref signals, these conditions provide 12 dB and 18 dB link margin, respectively. From Figure 8 one can see 16-QAM signals, these conditions provide 12 dB and 18 dB link margin, respectively. From Figure 8 that, for the same modulation format, lowering the baud rate gives an extended reach at the sacrifice of one can see that, for the same modulation format, lowering the baud rate gives an extended reach at the net data rate. Similarly, for the same baud rate, increasing the QAM order gives a higher net data the sacrifice of the net data rate. Similarly, for the same baud rate, increasing the QAM order gives a rate at the expense of the system’s reach. For PTx Ref Tx side optical power, the 45 GBd 16-QAM signal higher net data rate at the expense of the system’s reach. For P Tx side optical power, the 45 GBd Tx Ref can still achieve 80 km reach in the unamplified link, which provides 288 Gb/s net data rate. For the Tx 16-QAM signal can still achieve 80 km reach in the unamplified link, which provides 288 Gb/s net side with 6-dB more optical power than PTx Ref, the 64 GBd 16-QAM signal can achieve both 80 km data rate. For the Tx side with 6-dB more optical power than P , the 64 GBd 16-QAM signal Tx Ref transmission and larger than 400 Gb/s net data rate. In such a condition, the 64 GBd 64-QAM signal can can achieve both 80 km transmission and larger than 400 Gb/s net data rate. In such a condition, produce greater than 600 Gb/s net data rate for an unamplified link as long as 40 km. By comparing the the 64 GBd 64-QAM signal can produce greater than 600 Gb/s net data rate for an unamplified link two conditions of optical power at the Tx side, one can see that 6-dB more power can extend the fiber as long as 40 km. By comparing the two conditions of optical power at the Tx side, one can see that transmission reach to approximately 30 km. Consequently, it is critically important to increase the 6-dB more power can extend the fiber transmission reach to approximately 30 km. Consequently, it is output power at the Tx side or improve the sensitivity at the Rx side. Increasing the laser’s power, critically important to increase the output power at the Tx side or improve the sensitivity at the Rx reducing the modulation loss, using two separate lasers for Tx and LO are a few potential options. side. Increasing the laser ’s power, reducing the modulation loss, using two separate lasers for Tx and LO are a few potential options. Appl. Sci. 2019, x, x 8 of 10 Figure 8. Net data rate as a function of transmission distance for 16-QAM and 64-QAM signals at Figure 8. Net data rate as a function of transmission distance for 16-QAM and 64-QAM signals at various baud rates. various baud rates. 6. Conclusions In this article, we introduce 400GbE DCI using pluggable optics. Different aspects of the 400G pluggable module, including form factor, architecture, and power consumption, are discussed. Experimental investigation shows that the 64 GBd 16-QAM and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond unamplified fiber link. Capacity- reach matrix is finally generated with different baud rates, QAM order, and Tx side power in the unamplified fiber link. We find that increasing the Tx output power or improving the Rx sensitivity is the key for better system performance in the power-limited coherent fiber link. A 6-dB more transmitted power can extend the fiber transmission reach to approximately 30 km for both modulation formats investigated. It is thus important for the industry to develop accordingly, under the power and space constrains of the 400G DCO pluggable module. Author Contributions: Conceptualization, Y.Y. and Q.W.; methodology, Y.Y. and Q.W.; software, Y.Y. and Q.W.; validation, Y.Y. and Q.W.; formal analysis, Y.Y. and Q.W.; investigation, Y.Y. and Q.W.; resources, Y.Y., Q.W. and J.A.; data curation, Y.Y. and Q.W.; writing—original draft preparation, Y.Y. and Q.W.; writing— review and editing, Y.Y., Q.W. and J.A.; visualization, Y.Y. and Q.W.; supervision, J.A. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge Xuan He and Jeffery J. Maki for the fruitful discussion on the work. The authors also gratefully acknowledge vigorous encouragement and sturdy support on innovation from Domenico Di Mola at Juniper Networks. Conflicts of Interest: The authors declare no conflicts of interest. References 1. 40 Gb/s and 100 Gb/s Fiber Optic Task Force. Available online: http://www.ieee802.org/3/bm/ (accessed on 26 January 2019). 2. IEEE P802.3bs 400 Gb/s Ethernet Task Force. Available online: www.ieee802.org/3/bs/ (accessed on 1 March 2019). 3. Rokkas, T.; Neokosmidis, I.; Tomkos, I. Cost and Power Consumption Comparison of 400 Gbps Intra-Datacenter Transceiver Modules. In Proceedings of the 2018 International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, 1–5 July 2018. 4. Urata, R.; Liu, H.; Zhou, X.; Vahdat, A. Datacenter Interconnect and Networking: From Evolution to Holistic Revolution. In Proceedings of the 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 11–15 March 2018. 5. Baveja, P.P.; Li, M.; Wang, D.; Hsieh, C.; Zhang, H.; Ma, N.; Wang, Y.; Lii, J.; Liang, Y.; Wang, C.; Ho, I.L.; Zheng, J. 56 Gb/s PAM-4 Directly Modulated Laser for 200G/400G Data-Center Optical Appl. Sci. 2019, 9, 2455 8 of 9 6. Conclusions In this article, we introduce 400GbE DCI using pluggable optics. Di erent aspects of the 400G pluggable module, including form factor, architecture, and power consumption, are discussed. Experimental investigation shows that the 64 GBd 16-QAM and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond unamplified fiber link. Capacity-reach matrix is finally generated with di erent baud rates, QAM order, and Tx side power in the unamplified fiber link. We find that increasing the Tx output power or improving the Rx sensitivity is the key for better system performance in the power-limited coherent fiber link. A 6-dB more transmitted power can extend the fiber transmission reach to approximately 30 km for both modulation formats investigated. It is thus important for the industry to develop accordingly, under the power and space constrains of the 400G DCO pluggable module. Author Contributions: Conceptualization, Y.Y. and Q.W.; methodology, Y.Y. and Q.W.; software, Y.Y. and Q.W.; validation, Y.Y. and Q.W.; formal analysis, Y.Y. and Q.W.; investigation, Y.Y. and Q.W.; resources, Y.Y., Q.W. and J.A.; data curation, Y.Y. and Q.W.; writing—original draft preparation, Y.Y. and Q.W.; writing—review and editing, Y.Y., Q.W. and J.A.; visualization, Y.Y. and Q.W.; supervision, J.A. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge Xuan He and Je ery J. 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Applied SciencesMultidisciplinary Digital Publishing Institute

Published: Jun 15, 2019

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