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1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0)

1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode... Hindawi Advances in OptoElectronics Volume 2019, Article ID 2719808, 7 pages https://doi.org/10.1155/2019/2719808 Research Article 1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0) Henning Fouckhardt , Ann-Kathrin Kleinschmidt, Johannes Strassner , and Christoph Doering Integrated Optoelectronics and Microoptics Research Group, Physics Department, Technische Universitat ¨ Kaiserslautern (TUK), P.O. Box 3049, D-67653 Kaiserslautern, Germany Correspondence should be addressed to Henning Fouckhardt; fouckhar@physik.uni-kl.de Received 11 March 2019; Accepted 16 April 2019; Published 17 June 2019 Academic Editor: Vasily Spirin Copyright © 2019 Henning Fouckhardt et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Previously in this journal we have reported on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with integrated twice-retracted 4f set-up and film-waveguide lens as the Fourier-transform element. Now we choose and report on a simpler approach for BAL-TMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a transverse constriction. eTh absolute value of the radius R of curvature of both mirror-facets convex in one dimension (1D) is R = L =2f with focal length f. eTh round trip length 2 L =4f again makes up for a Fourier-optical 4f set-up and the constriction resulting in a resonator-internal beam waist stands for a Fourier-optical low-pass spatial frequency filter. Good TMS#0 is achieved, as long as the constriction is tight enough, but filamentation is not completely suppressed. 1. Introduction external cavity is employed [7–14]. In all these instances eventually low-pass spatial frequency filtering is performed. Broad area (semiconductor diode) lasers (BALs) are intended Since feedback from an external cavity may also cause self- to emit high optical output powers (where “high” is relative pulsation due to destabilization of the emission process [15– and depending on the material system). As compared to 19], the transverse mode selection set-up might also be conventional narrow stripe lasers, the higher power is dis- integrated into the laser resonator [20, 21], a concept which we tributed over a larger transverse cross-section, thus avoiding presented earlier. Moreover, approaches with tapered lasers or catastrophic optical mirror damage (COMD). Typical BALs amplifiers or similar devices are known [22–25]. have emitter widths of around 100𝜇 m. Previously in this journal we have also reported on a The drawback is the distribution of the high output power concept for TMS#0, which has employed a twice-retracted over a large number of transverse modes (in cases without integrated 4f set-up with an actual length of 1f forming countermeasures) limiting the portion of the light power in the laser resonator [26]. One facet has incorporated the the fundamental transverse mode (mode #0), which ought to spatial frequency filter, while the other one has housed a be maximized for the sake of good light focusability. film-waveguide lens as the 1D Fourier-transform element. uTh stechniques havetobeused to support, prefer, or Experimental results have shown good TMS#0. The best select the fundamental transverse mode (transverse mode one-dimensional beam quality parameter measured has been selection TMS#0) by suppression of higher order modes 𝑀 =1.47. 1D already upon build-up of the laser oscillation. A technological disadvantage of the latter approach has In many cases reported in the literature, either a BAL been the sophisticated preparation of the film-waveguide lens facet, the transverse eeff ctive refractive index distribution, with a necessary dry-etch depth precision better than (i.e., below) 20 nm. Here we propose a simpler resonator design. or the pump current distribution is modified [1–8]. Or an 2 Advances in OptoElectronics 2. Concept and Laser Design 100 m 100 m In this contribution, we propose and report on the realization 32 m of a confocal BAL resonator with (in top-view) a bow- tie-shaped beam constriction of minimal width a defining the smallest transverse beam width half-way between the 1000 m cylindrical facets with Fresnel reflection. These mirror-facets are both convex in 1D (viewed from outside the resonator), Figure 1: Light microscope image (top-view) of a confocal BAL giving a stable resonator. resonator with a bow-tie-shaped laser ridge and, in this example, a Typically confocal resonators are not employed for semi- 32𝜇 m wide constriction in the middle plane. eTh resonator length and the radii of curvature are 1 mm. conductor lasers. An early contribution with a so-called confocal resonator is given in [27]. But one of the mirror- facets had been convex, while the other one had been concave f = R/2 = L/2 f = R/2 = L/2 or plane, yielding an unstable resonator. In our case, only mirror-facets, which are convex in 1D (see above) and of equal absolute value for the radius of curvature, are employed. A confocal resonator is den fi ed by the following equation: 𝑅=𝐿=2,𝑓 (1) Figure 2: Principal sketch of the confocal resonator with constric- where Ristheabsolutevalueoftheradiusofcurvature ofboth tion. eTh transverse beam width bonthe facetsisconsiderablylarger facets, L the resonator length, 2L the round trip length, and f than the opening width a of the low-pass spatial frequency filter in the common (absolute value of the) focal length of the curved the middle plane. mirror-facets. Both 1D curved mirror-facets perform a 1D spatial Fourier-transform, each from their respective front to their GaAsSb quantum dot (QD) layers in-between 50 nm wide back focal plane. Since the resonator length is 2f,these focal GaAs barriers [28]. The lasers emit at wavelengths of ca. 930 planes coincide with the plane in the longitudinal middle of nm. the resonator, called the “middle plane” from now on. It has to be stressed here that our confocal BAL approach Rays with low propagation angles with respect to the is not restricted to this material system and laser design, but it optical axis account for low spatial frequencies and thus for rather represents a general concept. Even unipolar quantum the fundamental transverse mode (#0). They are Fresnel- cascade lasers for the THz emission range might profit from reflected back into the resonator at the cylindrical mirror- it. facets with a reflectivity of about 31%. Rays with larger For comparison, we prepared several lasers from the same propagation angles, which correlate with larger spatial fre- batch/wafer: with no constriction or constrictions of 64, 48, quencies and higher transverse modes, are blocked by the 32, and 16𝜇 m as the smallest transverse width, respectively. transverse constriction, the latter thus acting as a 1D low-pass In all cases, the bow-ties were 100𝜇 mwide at the outer edges spatial frequency filter, intended to support the fundamental (see Figure 1 again). The device with a 16 𝜇 m wide constriction transverse mode. did not oscillate/lase. Figure 1 contains a light microscope image of one of our confocal BAL resonators in top-view with a bow-tie-shaped, dry-etched laser ridge and a constriction with a width of 3. Experimental Results and Discussion (in this case) 32𝜇 m in the middle plane [28]. The absolute value of the radius of curvature is R = 1 mm for both convex Figure 3 gives the laser characteristics for the confocal BAL, facets, identical to the resonator length L.Tothe best of our e.g., with the 64 𝜇 m wide constriction (low-pass spatial knowledge, the spatial resolution of the etch process does not frequency lfi ter) to verify that the laser devices are of very aec ff t the symmetry of the bow-tie shape. good quality, even at continuous wave and room temperature The advantage of using a confocal resonator for Fourier- operation. The differential quantum efficiency is 31.5% here. optical spatial frequency lfi tering is its ease of design and For later comparison, Figure 4 reveals the results in technological preparation as well as the fact that a relatively terms of the near- and far-field intensity distributions for the wide opening angle of the mirror-facets can be used even confocal BAL without any constriction. The device operating for a tide constriction a (beam width on facet b> a). That temperature has been around 90 K. is, the transverse beam width b onthefacetsisconsiderably And Figure 5 gives false-color plots (intensity coded as larger than the width a of the constriction (spatial frequency colors, black/blue for low intensities, white/red/yellow for filter) in the middle plane. The latter aspect is schematically large intensities) of the near-eld fi transverse intensity distri- illustrated in Figure 2. butions for confocal BALs of the same batch with dieff rent The layer sequence of our lasers is based on the AlGaAsSb smallest widths of the constriction. Actually the uppermost material system on GaAs substrates. The devices are pn plot is from the confocal BAL without constriction, and the junctions, i.e., laser diodes, edge-emitting with a 450 nm thick other ones stemfromthe devices with 64, 48, and 32𝜇 m active region consisting of eight Stransky-Krastanov-grown wide (smallest) constriction, respectively. (Please remember: beam width on facet b>a constriction width a Advances in OptoElectronics 3 GaAsSb(QD)/GaAs BAL with 64 mm wide constriction, continuous wave at 18 C, emission at 930 nm wavelength, slope above threshold 0.42 W/A, differential quantum efficiency 31.5% 0 0.6 1.2 pump current [A] Figure 3: Laser characteristics of a confocal BAL with 64 𝜇 m wide constriction just to show that the lasers are of very good quality. The laser threshold is at 1.2 A, and the output power per facet shows values around 60 mW for a pump current of about 16% above threshold. without constriction near-field intensity distribution 100 m −60 −50 −40 −30 −20 −10 0 10 20 30 40 50 60 distance from optical axis [m] far-field intensity distribution −40 −30 −20 −10 0 10 20 30 40 far-field angle [ ] Figure 4: Near- and far-field intensity distributions for the confocal BAL without constriction, for comparison. eTh laser has been operated around 90 K in continuous-wave emission. output power per facet [mW] normalized intensity normalized intensity 4 Advances in OptoElectronics without constriction 64 m (a) (b) 48 m 32 m (c) (d) Figure 5: False-color plots of the near-field transverse intensity distributions for confocal BALs of the same batch with different widths of the constriction in the middle plane (i.e., no constriction, 64, 48, and 32𝜇 m wide, respectively). In all cases, the pump current has been around 16% above laser threshold and the lasers have been operated around 90 K in continuous-wave emission. the device with a 16𝜇 m narrow constriction did not lase!) In As can be seen from the gur fi e captions, the devices have all cases the pump current has been around 16% above laser been operated (continuous wave) both at around 90 K and at threshold. Again the device operating temperature has been room-temperature (18 C). In both cases, we did not observe around 90 K. a significant increase in device instability upon a temperature In the sequence of results in Figure 5, an increasingly change by a couple of ten degrees Celsius. stronger confinement of the transverse intensity distribution The (even in the cases with constriction) still strong is obvious, resulting in a near-el fi d intensity distribution filamentation is also an unexpected result, since the bow-tie- similar to that of the desired fundamental transverse mode shape of the confocal resonator should have restricted the for a constriction with a width of 32𝜇 m the middle plane. possible longitudinal paths for gain lfi aments geometrically. But la fi mentation is not completely suppressed. On the other hand, light scattering from the roughness of To make a closer inspection possible, Figure 6 (top and the etched transverse bow-tie edges might cause an addi- middle row) shows both the near- and the far-field intensity tional coupling of the gain la fi ments. u Th s an attempt to distribution for the same operating conditions for the confo- improve the TMS#0 (i.e., to reduce the filamentation) further cal BAL with 32𝜇 m wide constriction in the middle plane. should go for a reduction of the mentioned roughness, Gaussian tfi s (red lines) are added as guides to the eye. which has been on the order of 100 to 500 nm (root- Obviously the fundamental transverse mode is sup- mean-square nominally) so far due to the roughness of ported, but filamentation is not totally suppressed. the structures on the lithographic mask as well as the Both the near- and the far-field intensity distributions— roughness induced by the reactive ion etching process measured with the help of objective lenses—show an intensity itself. offset. As illustrated in the bottom part of Figure 6 by a side- In thecasewiththe 32 𝜇 m wide constriction in the view sketch and a top-view scanning electron micrograph middle plane, the intensity distributions in Figures 5(d) and (SEM), this is due to the fact that the mirror-facets have 6 with a single-lobed far-field and a full far-field angle of been dry-etched, resulting in some distance (ca. 36 𝜇 m 5.1 (disregarding la fi mentation for a moment) allow for the long) between the bow-tie edge and the device/crystal edge. extraction of a 1D beam quality parameter of𝑀 =1.71. 1𝐷 Within this distance, the substrate has been laid bare upon etching, giving a plateau with a roughened surface. Part of the emitted light is diffusely reflected or rather scattered off 4. Conclusions the plateau. The scattered intensity portion accounts for the offset. A concept for fundamental spatial transverse mode selection At pump currents, more than 20% above threshold (TMS#0) of edge-emitting broad area (semiconductor diode) considerable TMS#0 is not observed, a problem which our lasers (BALs) is presented, which employs a 1D confocal approach has in common with most TMS concepts. resonator with a constriction in the middle plane, i.e., the Comparing devices (from the same wafer) we have not plane half-way between the equally strongly curved convex found any significant deviations in differential quantum mirror-facets. This plane serves both as the front and the back efficiency within our device and measurement tolerances focal plane of the curved facets and, thus, also as the Fourier- for dieff rent smallest widths of the constriction (except for transform plane in the sense of a Fourier-optical 4f set-up. the fact that the device with 16 𝜇 m constriction did not A transverse constriction in this plane is employed as a low- lase/oscillate at all). But the devices without any constriction pass spatial frequency filter in order to select the fundamental had a worse quantum efficiency, worse by up to a factor of 10. transverse mode (TMS#0). This is an unexpected result, since the TMS#0 via a confocal Several lasers have been prepared from the same batch, resonator with constriction should increase the fraction of differing from one another in the smallest width of the the total power in the fundamental transverse mode, but not transverse constriction. The lasers are of very good quality, necessarily the overall power. Further investigations have to revealed by a differential quantum efficiency of around be pursued on this issue. 30%. Advances in OptoElectronics 5 with 32 m wide constriction near-field intensity distribution 22.5 m 100 m −60 −50 −40 −30 −20 −10 0 10 20 30 40 50 60 distance from optical axis [m] far-field intensity distribution 5.1 −30 −20 −10 0 10 20 30 far-field angle [ ] facet active zone emission layer sequence 36 m scattering from plateau substrate - side view, only one facet shown - - SEM, top-view - Figure 6: Top and middle row: near- and far-field intensity distributions for the confocal BAL with a 32 𝜇 m wide constriction in the middle plane. Gaussian ts fi (red lines) are added for both intensity distributions as guides to the eye. eTh laser has been operated around 90 K in continuous-wave emission. Bottom: side-view sketch and top-view scanning electron micrograph (SEM) of one of the dry-etched mirror- facets. eTh radiation is partially diffusely reflected or scattered off the somewhat rough substrate plateau, which has resulted from the dry-etch process to define the mirror-facets. Transverse mode selection (TMS#0) is indeed achieved Data Availability via the confocal resonator design, that is for pump currents Most of the experimental data used to support the findings of not larger than 20% above threshold. Filamentation is not this study are included within the article. Partially previously completely suppressed. The best TMS#0 is achieved in case of the 32 𝜇 mwide reported studies and experimental data were used to support this study. These prior studies and datasets are cited at constriction. Here the measured one-dimensional (1D) beam quality parameter is𝑀 =1.71. relevant places within the text as references. Further or more 1D normalized intensity normalized intensity contact pad bow-tie edge plateau 6 Advances in OptoElectronics detailed data used to support the findings of this study are [11] S. Wolff and H. Fouckhardt, “Intracavity stabilization of broad area lasers by structured delayed optical feedback,” Optics available from the corresponding author upon request. Express, vol. 7, no. 6, pp. 222–227, 2000. [12] V. Raab and R. Menzel, “External resonator design for high- Conflicts of Interest power laser diodes that yields 400 mW of TEM power,” Optics Express, vol.27, no. 3,pp.167–169,2002. The authors declare that there are no conflicts of interest [13] S. K. Mandre, I. Fischer, and W. Els¨asser, “Control of the regarding the publication of this paper, neither concerning spatiotemporal emission of a broad-area semiconductor laser the funding nor for any other reason. by spatially filtered feedback,” Optics Express,vol.28, no. 13, pp. 1135–1137, 2003. [14] S. Wolff,A.Rodionov,V. E. Sherstobitov, and H. Fouckhardt, Acknowledgments “Fourier-optical transverse mode selection in external-cavity broad-area lasers: experimental and numerical results,” IEEE This research has been funded by the German Research Foun- Journal of Quantum Electronics,vol.39,no.3,pp.448–458,2003. dation (Deutsche Forschungsgemeinschaft, DFG) under con- [15] T. Heil, I. Fischer, and W. Elsaßer ¨ , “Coexistence of low- tracts FO157/44 and FO157/46. Technological assistance by frequency uc fl tuations and stable emission on a single high-gain the Nano Structuring Center (NSC) of the Technische Uni- mode in semiconductor lasers with external optical feedback,” versita¨t Kaiserslautern (TUK) is gratefully acknowledged. Physical Review A: Atomic, Molecular and Optical Physics,vol. 58,no.4,pp.R2672–R2675,1998. References [16] F. Rogister, P. Megr ´ et, O. Deparis, and M. Blondel, “Coexis- tence of in-phase and out-of-phase dynamics in a multimode [1] K.Shigihara,Y. 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Zeimer, mode selector,” Applied Physics Letters,vol.96, no. 18, Article and G. Erbert, “High beam quality in broad area lasers via ID 181104, 2010. suppression of lateral carrier accumulation,” IEEE Photonics [21] D. Hoffmann, K. Huthmacher, C. Doering, and H. Fouckhardt, Technology Letters, vol.27,no.17, pp. 1809–1812,2015. “Broad area lasers with folded-resonator geometry for inte- [6] J.Rong, E. Xing,Y. Zhanget al., “Low lateral divergence2 grated transverse mode selection,” in Proceedings of the Novel 𝜇 m InGaSb/AlGaAsSb broad-area quantum well lasers,” Optics In-Plane Semiconductor Lasers X,A.A.Belyaninand P. M. Express, vol.24,no. 7,pp. 7246–7252, 2016. Smowton, Eds., vol. 7953, SPIE Photonics West, San Francisco, [7] C. Zink, M. Niebuhr, A. Jechow, A. Heuer, and R. Menzel, Calif, USA, 2011. “Broad area diode laser with on-chip transverse Bragg grating [22] K.-H. Hasler, B. Sumpf, P. 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1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0)

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Copyright © 2019 Henning Fouckhardt et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Advances in OptoElectronics Volume 2019, Article ID 2719808, 7 pages https://doi.org/10.1155/2019/2719808 Research Article 1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0) Henning Fouckhardt , Ann-Kathrin Kleinschmidt, Johannes Strassner , and Christoph Doering Integrated Optoelectronics and Microoptics Research Group, Physics Department, Technische Universitat ¨ Kaiserslautern (TUK), P.O. Box 3049, D-67653 Kaiserslautern, Germany Correspondence should be addressed to Henning Fouckhardt; fouckhar@physik.uni-kl.de Received 11 March 2019; Accepted 16 April 2019; Published 17 June 2019 Academic Editor: Vasily Spirin Copyright © 2019 Henning Fouckhardt et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Previously in this journal we have reported on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with integrated twice-retracted 4f set-up and film-waveguide lens as the Fourier-transform element. Now we choose and report on a simpler approach for BAL-TMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a transverse constriction. eTh absolute value of the radius R of curvature of both mirror-facets convex in one dimension (1D) is R = L =2f with focal length f. eTh round trip length 2 L =4f again makes up for a Fourier-optical 4f set-up and the constriction resulting in a resonator-internal beam waist stands for a Fourier-optical low-pass spatial frequency filter. Good TMS#0 is achieved, as long as the constriction is tight enough, but filamentation is not completely suppressed. 1. Introduction external cavity is employed [7–14]. In all these instances eventually low-pass spatial frequency filtering is performed. Broad area (semiconductor diode) lasers (BALs) are intended Since feedback from an external cavity may also cause self- to emit high optical output powers (where “high” is relative pulsation due to destabilization of the emission process [15– and depending on the material system). As compared to 19], the transverse mode selection set-up might also be conventional narrow stripe lasers, the higher power is dis- integrated into the laser resonator [20, 21], a concept which we tributed over a larger transverse cross-section, thus avoiding presented earlier. Moreover, approaches with tapered lasers or catastrophic optical mirror damage (COMD). Typical BALs amplifiers or similar devices are known [22–25]. have emitter widths of around 100𝜇 m. Previously in this journal we have also reported on a The drawback is the distribution of the high output power concept for TMS#0, which has employed a twice-retracted over a large number of transverse modes (in cases without integrated 4f set-up with an actual length of 1f forming countermeasures) limiting the portion of the light power in the laser resonator [26]. One facet has incorporated the the fundamental transverse mode (mode #0), which ought to spatial frequency filter, while the other one has housed a be maximized for the sake of good light focusability. film-waveguide lens as the 1D Fourier-transform element. uTh stechniques havetobeused to support, prefer, or Experimental results have shown good TMS#0. The best select the fundamental transverse mode (transverse mode one-dimensional beam quality parameter measured has been selection TMS#0) by suppression of higher order modes 𝑀 =1.47. 1D already upon build-up of the laser oscillation. A technological disadvantage of the latter approach has In many cases reported in the literature, either a BAL been the sophisticated preparation of the film-waveguide lens facet, the transverse eeff ctive refractive index distribution, with a necessary dry-etch depth precision better than (i.e., below) 20 nm. Here we propose a simpler resonator design. or the pump current distribution is modified [1–8]. Or an 2 Advances in OptoElectronics 2. Concept and Laser Design 100 m 100 m In this contribution, we propose and report on the realization 32 m of a confocal BAL resonator with (in top-view) a bow- tie-shaped beam constriction of minimal width a defining the smallest transverse beam width half-way between the 1000 m cylindrical facets with Fresnel reflection. These mirror-facets are both convex in 1D (viewed from outside the resonator), Figure 1: Light microscope image (top-view) of a confocal BAL giving a stable resonator. resonator with a bow-tie-shaped laser ridge and, in this example, a Typically confocal resonators are not employed for semi- 32𝜇 m wide constriction in the middle plane. eTh resonator length and the radii of curvature are 1 mm. conductor lasers. An early contribution with a so-called confocal resonator is given in [27]. But one of the mirror- facets had been convex, while the other one had been concave f = R/2 = L/2 f = R/2 = L/2 or plane, yielding an unstable resonator. In our case, only mirror-facets, which are convex in 1D (see above) and of equal absolute value for the radius of curvature, are employed. A confocal resonator is den fi ed by the following equation: 𝑅=𝐿=2,𝑓 (1) Figure 2: Principal sketch of the confocal resonator with constric- where Ristheabsolutevalueoftheradiusofcurvature ofboth tion. eTh transverse beam width bonthe facetsisconsiderablylarger facets, L the resonator length, 2L the round trip length, and f than the opening width a of the low-pass spatial frequency filter in the common (absolute value of the) focal length of the curved the middle plane. mirror-facets. Both 1D curved mirror-facets perform a 1D spatial Fourier-transform, each from their respective front to their GaAsSb quantum dot (QD) layers in-between 50 nm wide back focal plane. Since the resonator length is 2f,these focal GaAs barriers [28]. The lasers emit at wavelengths of ca. 930 planes coincide with the plane in the longitudinal middle of nm. the resonator, called the “middle plane” from now on. It has to be stressed here that our confocal BAL approach Rays with low propagation angles with respect to the is not restricted to this material system and laser design, but it optical axis account for low spatial frequencies and thus for rather represents a general concept. Even unipolar quantum the fundamental transverse mode (#0). They are Fresnel- cascade lasers for the THz emission range might profit from reflected back into the resonator at the cylindrical mirror- it. facets with a reflectivity of about 31%. Rays with larger For comparison, we prepared several lasers from the same propagation angles, which correlate with larger spatial fre- batch/wafer: with no constriction or constrictions of 64, 48, quencies and higher transverse modes, are blocked by the 32, and 16𝜇 m as the smallest transverse width, respectively. transverse constriction, the latter thus acting as a 1D low-pass In all cases, the bow-ties were 100𝜇 mwide at the outer edges spatial frequency filter, intended to support the fundamental (see Figure 1 again). The device with a 16 𝜇 m wide constriction transverse mode. did not oscillate/lase. Figure 1 contains a light microscope image of one of our confocal BAL resonators in top-view with a bow-tie-shaped, dry-etched laser ridge and a constriction with a width of 3. Experimental Results and Discussion (in this case) 32𝜇 m in the middle plane [28]. The absolute value of the radius of curvature is R = 1 mm for both convex Figure 3 gives the laser characteristics for the confocal BAL, facets, identical to the resonator length L.Tothe best of our e.g., with the 64 𝜇 m wide constriction (low-pass spatial knowledge, the spatial resolution of the etch process does not frequency lfi ter) to verify that the laser devices are of very aec ff t the symmetry of the bow-tie shape. good quality, even at continuous wave and room temperature The advantage of using a confocal resonator for Fourier- operation. The differential quantum efficiency is 31.5% here. optical spatial frequency lfi tering is its ease of design and For later comparison, Figure 4 reveals the results in technological preparation as well as the fact that a relatively terms of the near- and far-field intensity distributions for the wide opening angle of the mirror-facets can be used even confocal BAL without any constriction. The device operating for a tide constriction a (beam width on facet b> a). That temperature has been around 90 K. is, the transverse beam width b onthefacetsisconsiderably And Figure 5 gives false-color plots (intensity coded as larger than the width a of the constriction (spatial frequency colors, black/blue for low intensities, white/red/yellow for filter) in the middle plane. The latter aspect is schematically large intensities) of the near-eld fi transverse intensity distri- illustrated in Figure 2. butions for confocal BALs of the same batch with dieff rent The layer sequence of our lasers is based on the AlGaAsSb smallest widths of the constriction. Actually the uppermost material system on GaAs substrates. The devices are pn plot is from the confocal BAL without constriction, and the junctions, i.e., laser diodes, edge-emitting with a 450 nm thick other ones stemfromthe devices with 64, 48, and 32𝜇 m active region consisting of eight Stransky-Krastanov-grown wide (smallest) constriction, respectively. (Please remember: beam width on facet b>a constriction width a Advances in OptoElectronics 3 GaAsSb(QD)/GaAs BAL with 64 mm wide constriction, continuous wave at 18 C, emission at 930 nm wavelength, slope above threshold 0.42 W/A, differential quantum efficiency 31.5% 0 0.6 1.2 pump current [A] Figure 3: Laser characteristics of a confocal BAL with 64 𝜇 m wide constriction just to show that the lasers are of very good quality. The laser threshold is at 1.2 A, and the output power per facet shows values around 60 mW for a pump current of about 16% above threshold. without constriction near-field intensity distribution 100 m −60 −50 −40 −30 −20 −10 0 10 20 30 40 50 60 distance from optical axis [m] far-field intensity distribution −40 −30 −20 −10 0 10 20 30 40 far-field angle [ ] Figure 4: Near- and far-field intensity distributions for the confocal BAL without constriction, for comparison. eTh laser has been operated around 90 K in continuous-wave emission. output power per facet [mW] normalized intensity normalized intensity 4 Advances in OptoElectronics without constriction 64 m (a) (b) 48 m 32 m (c) (d) Figure 5: False-color plots of the near-field transverse intensity distributions for confocal BALs of the same batch with different widths of the constriction in the middle plane (i.e., no constriction, 64, 48, and 32𝜇 m wide, respectively). In all cases, the pump current has been around 16% above laser threshold and the lasers have been operated around 90 K in continuous-wave emission. the device with a 16𝜇 m narrow constriction did not lase!) In As can be seen from the gur fi e captions, the devices have all cases the pump current has been around 16% above laser been operated (continuous wave) both at around 90 K and at threshold. Again the device operating temperature has been room-temperature (18 C). In both cases, we did not observe around 90 K. a significant increase in device instability upon a temperature In the sequence of results in Figure 5, an increasingly change by a couple of ten degrees Celsius. stronger confinement of the transverse intensity distribution The (even in the cases with constriction) still strong is obvious, resulting in a near-el fi d intensity distribution filamentation is also an unexpected result, since the bow-tie- similar to that of the desired fundamental transverse mode shape of the confocal resonator should have restricted the for a constriction with a width of 32𝜇 m the middle plane. possible longitudinal paths for gain lfi aments geometrically. But la fi mentation is not completely suppressed. On the other hand, light scattering from the roughness of To make a closer inspection possible, Figure 6 (top and the etched transverse bow-tie edges might cause an addi- middle row) shows both the near- and the far-field intensity tional coupling of the gain la fi ments. u Th s an attempt to distribution for the same operating conditions for the confo- improve the TMS#0 (i.e., to reduce the filamentation) further cal BAL with 32𝜇 m wide constriction in the middle plane. should go for a reduction of the mentioned roughness, Gaussian tfi s (red lines) are added as guides to the eye. which has been on the order of 100 to 500 nm (root- Obviously the fundamental transverse mode is sup- mean-square nominally) so far due to the roughness of ported, but filamentation is not totally suppressed. the structures on the lithographic mask as well as the Both the near- and the far-field intensity distributions— roughness induced by the reactive ion etching process measured with the help of objective lenses—show an intensity itself. offset. As illustrated in the bottom part of Figure 6 by a side- In thecasewiththe 32 𝜇 m wide constriction in the view sketch and a top-view scanning electron micrograph middle plane, the intensity distributions in Figures 5(d) and (SEM), this is due to the fact that the mirror-facets have 6 with a single-lobed far-field and a full far-field angle of been dry-etched, resulting in some distance (ca. 36 𝜇 m 5.1 (disregarding la fi mentation for a moment) allow for the long) between the bow-tie edge and the device/crystal edge. extraction of a 1D beam quality parameter of𝑀 =1.71. 1𝐷 Within this distance, the substrate has been laid bare upon etching, giving a plateau with a roughened surface. Part of the emitted light is diffusely reflected or rather scattered off 4. Conclusions the plateau. The scattered intensity portion accounts for the offset. A concept for fundamental spatial transverse mode selection At pump currents, more than 20% above threshold (TMS#0) of edge-emitting broad area (semiconductor diode) considerable TMS#0 is not observed, a problem which our lasers (BALs) is presented, which employs a 1D confocal approach has in common with most TMS concepts. resonator with a constriction in the middle plane, i.e., the Comparing devices (from the same wafer) we have not plane half-way between the equally strongly curved convex found any significant deviations in differential quantum mirror-facets. This plane serves both as the front and the back efficiency within our device and measurement tolerances focal plane of the curved facets and, thus, also as the Fourier- for dieff rent smallest widths of the constriction (except for transform plane in the sense of a Fourier-optical 4f set-up. the fact that the device with 16 𝜇 m constriction did not A transverse constriction in this plane is employed as a low- lase/oscillate at all). But the devices without any constriction pass spatial frequency filter in order to select the fundamental had a worse quantum efficiency, worse by up to a factor of 10. transverse mode (TMS#0). This is an unexpected result, since the TMS#0 via a confocal Several lasers have been prepared from the same batch, resonator with constriction should increase the fraction of differing from one another in the smallest width of the the total power in the fundamental transverse mode, but not transverse constriction. The lasers are of very good quality, necessarily the overall power. Further investigations have to revealed by a differential quantum efficiency of around be pursued on this issue. 30%. Advances in OptoElectronics 5 with 32 m wide constriction near-field intensity distribution 22.5 m 100 m −60 −50 −40 −30 −20 −10 0 10 20 30 40 50 60 distance from optical axis [m] far-field intensity distribution 5.1 −30 −20 −10 0 10 20 30 far-field angle [ ] facet active zone emission layer sequence 36 m scattering from plateau substrate - side view, only one facet shown - - SEM, top-view - Figure 6: Top and middle row: near- and far-field intensity distributions for the confocal BAL with a 32 𝜇 m wide constriction in the middle plane. Gaussian ts fi (red lines) are added for both intensity distributions as guides to the eye. eTh laser has been operated around 90 K in continuous-wave emission. Bottom: side-view sketch and top-view scanning electron micrograph (SEM) of one of the dry-etched mirror- facets. eTh radiation is partially diffusely reflected or scattered off the somewhat rough substrate plateau, which has resulted from the dry-etch process to define the mirror-facets. Transverse mode selection (TMS#0) is indeed achieved Data Availability via the confocal resonator design, that is for pump currents Most of the experimental data used to support the findings of not larger than 20% above threshold. Filamentation is not this study are included within the article. Partially previously completely suppressed. The best TMS#0 is achieved in case of the 32 𝜇 mwide reported studies and experimental data were used to support this study. These prior studies and datasets are cited at constriction. Here the measured one-dimensional (1D) beam quality parameter is𝑀 =1.71. relevant places within the text as references. 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