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OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Transverse single-mode edge-emitting lasers based on coupled waveguides Nikita Yu. Gordeev,1,2,* Alexey S. Payusov,1,2 Yuri M. Shernyakov,1,2 Sergey A. Mintairov,1 Nikolay A. Kalyuzhnyy,1 Marina M. Kulagina,1,2 and Mikhail V. Maximov1,2 1 2
Ioffe Institute, 26 Polytekhnicheskaya, 194021 St. Petersburg, Russia
St. Petersburg Academic University—Nanotechnology Research and Education Centre, 8/3 Khlopina, 194021 St. Petersburg, Russia *Corresponding author: [email protected] Received February 19, 2015; revised April 11, 2015; accepted April 12, 2015; posted April 13, 2015 (Doc. ID 234979); published May 1, 2015 We report on the transverse single-mode emission from InGaAs/GaAs quantum well edge-emitting lasers with broadened waveguide. The lasers are based on coupled large optical cavity (CLOC) structures where high-order vertical modes of the broad active waveguide are suppressed due to their resonant tunneling into a coupled single-mode passive waveguide. The CLOC lasers have shown stable Gaussian-shaped vertical far-field profiles with a reduced divergence of ∼22° FWHM (full width at half-maximum) in CW (continuous-wave) operation. © 2015 Optical Society of America OCIS codes: (250.5960) Semiconductor lasers; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/OL.40.002150
Currently, high-power edge-emitting diode lasers are widely used as reliable, efficient, and compact light sources for solid-state laser systems, direct material processing, printing, and medicine. One of the principal problems limiting the maximal output power of semiconductor lasers is catastrophic optical mirror damage (COMD), which occurs when the power density per laser facet area exceeds the certain value (COMD level) characteristic for the given waveguide materials [1]. Thus, reducing the optical power density by expansion of the laser optical field in the vertical direction is considered an effective way to increase the maximum output power [2]. Another positive feature of the expanded optical field is the narrower beam vertical divergence, which simplifies coupling of the lasers with subsequent optical systems. Expansion of the optical field should ensure transverse fundamental mode lasing since highorder vertical modes deteriorate the vertical far-field patterns and worsen the laser beam quality. Therefore, the waveguides should be designed in such a way that either the waveguide thickness does not exceed the cutoff condition of high-order transverse modes, or the high-order transverse mode emission is suppressed due to gain or loss discriminations. The loss discrimination increases the losses for the high-order modes by increasing their overlap with the absorptive layers in the laser structure. The gain discrimination implies adjusting the position of the active layer in a way that only the fundamental mode has a high optical-confinement factor (OCF). The waveguide designs should also ensure that the laser modes have far-field patterns as close as possible to a Gaussian shape. Various approaches for expanding the vertical optical field have been suggested [2–5]. Lasers containing mode expansion layers [3], narrow asymmetric waveguides (NAW) [4], and ultra-broad waveguides based on one-dimensional longitudinal photonic band crystal (PBC) structures [2] tend to have a higher sensitivity of the mode profile to minor changes in the refractive indices due to compositional, temperature, and current variations. Lasers with super-large optical cavities (SLOC) [5] are more robust against refractive index variations, 0146-9592/15/092150-03$15.00/0
though they also require precise discrimination of highorder modes. In this Letter, we propose a novel, simple, and effective approach to suppress high-order mode lasing in broadened waveguides. The proposed laser waveguide structures for reduction of the vertical far-field divergence are based on coupled optical waveguides and thus named as Coupled Large Optical Cavity structures (CLOC) in this Letter. The concept utilizes the effect of codirectional coupling [6] between two parallel waveguides placed in the close proximity. In this case, two phasematched eigenmodes of the coupled waveguides form two composite modes. Each of the composite modes is equally distributed between the two waveguides. The effect can also be described in a way that the waveguide eigenmode tunnels into the coupled waveguide, and its initial intensity in the waveguide is reduced approximately by twofold. The eigenmodes are always phase matched if the two waveguides are identical. If two coupled waveguides are different, the phase-matching condition is satisfied when effective refractive indices of two uncoupled eigenmodes are equal. Mode effective refractive indices depend on the waveguide thickness and refractive indices of the core and cladding layers. The broader the waveguide, the higher the mode effective refractive index. Parameters of two coupled different waveguides can be chosen in such a way that one selected high-order mode of the first waveguide and a fundamental mode of the second waveguide have equal effective refractive indices. This selected high-order mode of the first waveguide would effectively tunnel into the second waveguide, while the fundamental mode of the first waveguide remains unperturbed. The waveguide of a CLOC laser consists of a singlemode narrow passive waveguide optically coupled to a broadened active multimode waveguide with an embedded active region. A selected high-order mode, which normally satisfies the laser threshold condition, has the effective refractive index equal or nearly equal to that of the narrow passive waveguide mode. Due to the coupling, this high-order mode and the mode of the passive © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
waveguide are transformed into two composite modes. Each of the composite modes has reduced intensity in the active waveguide and, therefore, a reduced optical confinement factor compared to that of the original high-order mode. Significant fractions of the composite modes propagate in the passive waveguide, which can be doped up to rather high levels. Therefore, these composite modes have additional optical losses resulted from free-carrier absorption in the highly doped layers. This high doping does not affect the fundamental mode propagating mainly in the undoped active waveguide. Because of the waveguide coupling, in CLOC lasers selected high-order modes are transformed into composite modes, which do not lase due to the reduced OCFs and increased optical losses. This process allows transverse single-mode lasing in CLOC broadened waveguides. The proposed approach can be easily combined with OCF engineering via adjusting the active region position within the active waveguide. To prove the concept, we have modeled CLOC and reference lasers using the FIMMWAVE waveguide mode solver from Photon Design. Both laser structures had identical 2.5-μm-thick undoped GaAs waveguides (see Fig. 1), InGaAs double quantum well active regions emitting at the wavelength λ ≈ 1.04 μm, 300-nm-thick p-GaAs (p ∼ 2 × 1019 cm−3 ) contact layers, and n-GaAs (n ∼ 2 × 1018 cm−3 ) substrates. The active waveguide thickness was chosen near the cut-off of the fourth-order transverse mode. The laser waveguides were sandwiched between 1-μm-thick Al0.15 Ga0.85 As claddings with the doping level of 2 × 1018 cm−3 . The CLOC structure had an extra 625-nm-thick n-GaAs (n ∼ 2 × 1018 cm−3 ) passive waveguide separated from the active waveguide with 250-nm-thick n-Al0.15 Ga0.85 As (n ∼ 2 × 1018 cm−3 ) layer.
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The thickness of this layer corresponded to the penetration depth [7] of the second-order eigenmode P λ∕2π· N 2 − n2c −1∕2 , where nc is the refractive index of the Al0.15 Ga0.85 As layer, and N is the effective refractive index of the second-order eigenmode of the active waveguide. The passive waveguide thickness was chosen in such a way that its eigenmode has the effective refractive index nearly equal to N. Since the active region is placed in the center of the active waveguide, the even modes have a node at the active region, and there is no necessity to provide their tunneling into the passive waveguide. The refractive indices of the lasers were calculated using the Afromowitz model [8]. In our simulation, we also took into account carrier-induced refractive index changes caused by doping [9]. The simulation results are presented in Fig. 1. All mode intensity profiles are normalized to have the same integrated intensity. Fundamental modes in both structures have identical profiles. The first-order mode in the CLOC laser has a small wing on the passive waveguide side. The major difference between two simulated lasers is absence of a second-order mode and presence of the composite mode in the CLOC structure. The latter has noticeably reduced intensity in the active waveguide. We have calculated optical confinement factors and free-carrier optical losses for the fundamental, first-order, secondorder, and composite modes (see Table 1). Free-electron and hole absorption cross-sections used in the loss calculations were taken from [10]. The second-order mode of the reference laser has a slightly smaller OCF than the fundamental mode, while the OCF of the CLOC laser composite mode is more than twofold smaller than that of the fundamental mode. The composite mode has freecarrier optical losses of 4.7 cm−1 , which is much higher than the second-order mode has. According to these figures, the CLOC laser should operate on a single transverse fundamental mode. From our calculations, the simulated CLOC laser has a reduced vertical beam divergence of 23° FWHM (full width at half maximum). To prove this single transverse mode emission, we have experimentally investigated CLOC and reference lasers with the layer designs identical to the simulated ones. Two laser wafers were grown by metal-organic chemical vapor deposition (MOCVD) on n-GaAs (100) substrates. Both wafers were processed into 50-μmwide shallow-mesa ridge lasers by etching through the p-contact and partly through the p-cladding layers. The samples were mounted p-side down on copper heatsinks. The laser parameters were measured in pulsed (500 ns, 4.7 kHz) and CW (continuous-wave) modes at room temperature. Both CLOC and reference lasers showed Table 1. Calculated Mode Parameters for the Reference and CLOC Lasers Reference Laser Mode
Fig. 1. Refractive index profiles (left axes) and simulated intensity distributions (right axes) of the fundamental (solid line), first-order (dotted line), second-order (dashed–dotted line), and one composite (dashed line) modes for (a) the reference and (b) CLOC lasers. All modes have the same integrated intensity.
Fundamental First-order Second-order Composite
CLOC Laser
Optical Optical OCF (%) Losses, cm−1 OCF (%) Losses, cm−1 0.93 0.1 0.0013 — 0.79 1.3 not exist
0.93 0.1 0.0032 — not exist 0.42 4.7
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Fig. 2. Light-current characteristic of 50-μm-wide and 1-mmlong CLOC (solid line) and reference lasers (dashed–dotted line) measured at room temperature. The insets show far-field patterns for both lasers measured at the drive currents of 0.4 A, 2 A, and 3.5 A. The divergence values are for the CLOC laser.
similar parameters, namely, threshold current densities of ∼330 A∕cm2 in 2-mm-long and ∼700 A∕cm2 in 1-mm-long cavities, internal quantum efficiency of 88%, and intrinsic losses of 1.3 cm−1 . Due to the expanded optical field, the OCF was reduced, which led to low modal gain and as a result, the threshold current density was increased [4]. CW light-current curves for CLOC and reference lasers are presented in Fig. 2. The maximum output power is limited by a thermal rollover. Vertical far-field patterns measured at the CW drive currents of 0.4 A, 2 A, and 3.5 A are presented in the insets. At all levels of excitation, the CLOC lasers showed stable single-mode emission with the lowest divergence of 21.4° FWHM. Under the same pumping conditions, the reference lasers had pronounced lasing on high-order modes. Far-field patterns measured in pulsed mode are shown in Fig. 3. The lasers were pumped up to 20 thresholds, limited by the power supply. The reference lasers emitted on high-order modes over the entire drive current range. At higher excitation levels, their far-field patterns became a superposition of fundamental and high-order
Fig. 3. Vertical far-field patterns for the (a) CLOC and (b) reference lasers measured in pulsed mode (500 ns, 4.7 kHz, 20°C). Threshold current is ∼0.37 A. The curves for the CLOC laser at the currents of 0.4 A and 4 A are vertically shifted for better clarity.
modes. In the far-field patterns of the reference lasers [Figs. 2 and 3(b)], one can see multiple narrow peaks at large angles. We attribute these peaks to the coupling of the high-order mode to the substrate and contact layer. This coupling may take place if the cladding layers are not thick enough to confine an optical mode within the core layer [11]. In pulsed regimes, the CLOC lasers again showed extremely stable single-transverse-mode lasing with the divergence not exceeding 21.6° FWHM. The far-field patterns are well fitted by a Gaussian function as shown in Fig. 3(a). CLOC lasers due to their simple design are expected to be robust against variations in thicknesses and compositions of the waveguide layers. Adjusted CLOC structures can be easily incorporated into edge-emitting lasers operating in other wavelength ranges. In addition, CLOC structures can have more advanced designs where two different selected high-order modes tunnel from an active broadened waveguide into two different passive single-mode waveguides placed by either side of it. This advanced CLOC design may allow further broadening of laser waveguides. In summary, we have proposed a simple design of broadened laser waveguides based on coupled large optical cavity structures. The concept allows effective suppressing of the high-order mode lasing and thus reducing the vertical far-field divergence. The concept has been tested for InGaAs/GaAs lasers emitting in the 1040-nm wavelength range. The lasers with the 2.5-μm-thick waveguide operated in CW and pulse regimes and showed stable single transverse mode emission with the Gaussian shaped far-field patterns. The divergence of the CLOC lasers was ∼22° FWHM. The obtained results demonstrate that CLOC laser structures can be considered as a costefficient solution for edge-emitting lasers with improved beam quality. The authors acknowledge the support from the Russian Foundation for Basic Research (project no. 13-02-12184). References 1. D. Botez, Appl. Phys. Lett. 74, 3102 (1999). 2. M. J. Miah, T. Kettler, V. P. Kalosha, K. Posilovic, D. H. Bimberg, J. Pohl, and M. Weyers, IEEE J. Sel. Top. Quantum Electron. 21, 4900206 (2015). 3. G. Lin, S.-T. Yen, C.-P. Lee, and D.-C. Liu, IEEE Photonics Technol. Lett. 8, 1588 (1996). 4. B. S. Ryvkin, E. A. Avrutin, and J. T. Kostamovaara, J. Appl. Phys. 114, 013104 (2013). 5. A. Pietrzak, P. Crump, H. Wenzel, G. Erbert, F. Bugge, and G. Trankle, IEEE J. Sel. Top. Quantum Electron. 17, 1715 (2011). 6. B. E. Little and W. P. Huang, Prog. Electromagn. Res. 10, 217 (1995). 7. C. R. Pollock and M. Lipson, Integrated Photonics (Kluwer Academic, 2003), p. 278. 8. M. A. Afromowitz, Solid State Commun. 15, 59 (1974). 9. B. R. Bennett, R. A. Soref, and J. A. Del Alamo, IEEE J. Quantum Electron. 26, 113 (1990). 10. K. H. Hasler, H. Wenzel, P. Crump, S. Knigge, A. Maasdorf, R. Platz, R. Staske, and G. Erbert, Semicond. Sci. Technol. 29, 045010 (2014). 11. H. Wenzel, IEEE J. Sel. Top. Quantum Electron. 19, 1 (2013).
OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
Transverse single-mode edge-emitting lasers based on coupled waveguides Nikita Yu. Gordeev,1,2,* Alexey S. Payusov,1,2 Yuri M. Shernyakov,1,2 Sergey A. Mintairov,1 Nikolay A. Kalyuzhnyy,1 Marina M. Kulagina,1,2 and Mikhail V. Maximov1,2 1 2
Ioffe Institute, 26 Polytekhnicheskaya, 194021 St. Petersburg, Russia
St. Petersburg Academic University—Nanotechnology Research and Education Centre, 8/3 Khlopina, 194021 St. Petersburg, Russia *Corresponding author: [email protected] Received February 19, 2015; revised April 11, 2015; accepted April 12, 2015; posted April 13, 2015 (Doc. ID 234979); published May 1, 2015 We report on the transverse single-mode emission from InGaAs/GaAs quantum well edge-emitting lasers with broadened waveguide. The lasers are based on coupled large optical cavity (CLOC) structures where high-order vertical modes of the broad active waveguide are suppressed due to their resonant tunneling into a coupled single-mode passive waveguide. The CLOC lasers have shown stable Gaussian-shaped vertical far-field profiles with a reduced divergence of ∼22° FWHM (full width at half-maximum) in CW (continuous-wave) operation. © 2015 Optical Society of America OCIS codes: (250.5960) Semiconductor lasers; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/OL.40.002150
Currently, high-power edge-emitting diode lasers are widely used as reliable, efficient, and compact light sources for solid-state laser systems, direct material processing, printing, and medicine. One of the principal problems limiting the maximal output power of semiconductor lasers is catastrophic optical mirror damage (COMD), which occurs when the power density per laser facet area exceeds the certain value (COMD level) characteristic for the given waveguide materials [1]. Thus, reducing the optical power density by expansion of the laser optical field in the vertical direction is considered an effective way to increase the maximum output power [2]. Another positive feature of the expanded optical field is the narrower beam vertical divergence, which simplifies coupling of the lasers with subsequent optical systems. Expansion of the optical field should ensure transverse fundamental mode lasing since highorder vertical modes deteriorate the vertical far-field patterns and worsen the laser beam quality. Therefore, the waveguides should be designed in such a way that either the waveguide thickness does not exceed the cutoff condition of high-order transverse modes, or the high-order transverse mode emission is suppressed due to gain or loss discriminations. The loss discrimination increases the losses for the high-order modes by increasing their overlap with the absorptive layers in the laser structure. The gain discrimination implies adjusting the position of the active layer in a way that only the fundamental mode has a high optical-confinement factor (OCF). The waveguide designs should also ensure that the laser modes have far-field patterns as close as possible to a Gaussian shape. Various approaches for expanding the vertical optical field have been suggested [2–5]. Lasers containing mode expansion layers [3], narrow asymmetric waveguides (NAW) [4], and ultra-broad waveguides based on one-dimensional longitudinal photonic band crystal (PBC) structures [2] tend to have a higher sensitivity of the mode profile to minor changes in the refractive indices due to compositional, temperature, and current variations. Lasers with super-large optical cavities (SLOC) [5] are more robust against refractive index variations, 0146-9592/15/092150-03$15.00/0
though they also require precise discrimination of highorder modes. In this Letter, we propose a novel, simple, and effective approach to suppress high-order mode lasing in broadened waveguides. The proposed laser waveguide structures for reduction of the vertical far-field divergence are based on coupled optical waveguides and thus named as Coupled Large Optical Cavity structures (CLOC) in this Letter. The concept utilizes the effect of codirectional coupling [6] between two parallel waveguides placed in the close proximity. In this case, two phasematched eigenmodes of the coupled waveguides form two composite modes. Each of the composite modes is equally distributed between the two waveguides. The effect can also be described in a way that the waveguide eigenmode tunnels into the coupled waveguide, and its initial intensity in the waveguide is reduced approximately by twofold. The eigenmodes are always phase matched if the two waveguides are identical. If two coupled waveguides are different, the phase-matching condition is satisfied when effective refractive indices of two uncoupled eigenmodes are equal. Mode effective refractive indices depend on the waveguide thickness and refractive indices of the core and cladding layers. The broader the waveguide, the higher the mode effective refractive index. Parameters of two coupled different waveguides can be chosen in such a way that one selected high-order mode of the first waveguide and a fundamental mode of the second waveguide have equal effective refractive indices. This selected high-order mode of the first waveguide would effectively tunnel into the second waveguide, while the fundamental mode of the first waveguide remains unperturbed. The waveguide of a CLOC laser consists of a singlemode narrow passive waveguide optically coupled to a broadened active multimode waveguide with an embedded active region. A selected high-order mode, which normally satisfies the laser threshold condition, has the effective refractive index equal or nearly equal to that of the narrow passive waveguide mode. Due to the coupling, this high-order mode and the mode of the passive © 2015 Optical Society of America
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
waveguide are transformed into two composite modes. Each of the composite modes has reduced intensity in the active waveguide and, therefore, a reduced optical confinement factor compared to that of the original high-order mode. Significant fractions of the composite modes propagate in the passive waveguide, which can be doped up to rather high levels. Therefore, these composite modes have additional optical losses resulted from free-carrier absorption in the highly doped layers. This high doping does not affect the fundamental mode propagating mainly in the undoped active waveguide. Because of the waveguide coupling, in CLOC lasers selected high-order modes are transformed into composite modes, which do not lase due to the reduced OCFs and increased optical losses. This process allows transverse single-mode lasing in CLOC broadened waveguides. The proposed approach can be easily combined with OCF engineering via adjusting the active region position within the active waveguide. To prove the concept, we have modeled CLOC and reference lasers using the FIMMWAVE waveguide mode solver from Photon Design. Both laser structures had identical 2.5-μm-thick undoped GaAs waveguides (see Fig. 1), InGaAs double quantum well active regions emitting at the wavelength λ ≈ 1.04 μm, 300-nm-thick p-GaAs (p ∼ 2 × 1019 cm−3 ) contact layers, and n-GaAs (n ∼ 2 × 1018 cm−3 ) substrates. The active waveguide thickness was chosen near the cut-off of the fourth-order transverse mode. The laser waveguides were sandwiched between 1-μm-thick Al0.15 Ga0.85 As claddings with the doping level of 2 × 1018 cm−3 . The CLOC structure had an extra 625-nm-thick n-GaAs (n ∼ 2 × 1018 cm−3 ) passive waveguide separated from the active waveguide with 250-nm-thick n-Al0.15 Ga0.85 As (n ∼ 2 × 1018 cm−3 ) layer.
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The thickness of this layer corresponded to the penetration depth [7] of the second-order eigenmode P λ∕2π· N 2 − n2c −1∕2 , where nc is the refractive index of the Al0.15 Ga0.85 As layer, and N is the effective refractive index of the second-order eigenmode of the active waveguide. The passive waveguide thickness was chosen in such a way that its eigenmode has the effective refractive index nearly equal to N. Since the active region is placed in the center of the active waveguide, the even modes have a node at the active region, and there is no necessity to provide their tunneling into the passive waveguide. The refractive indices of the lasers were calculated using the Afromowitz model [8]. In our simulation, we also took into account carrier-induced refractive index changes caused by doping [9]. The simulation results are presented in Fig. 1. All mode intensity profiles are normalized to have the same integrated intensity. Fundamental modes in both structures have identical profiles. The first-order mode in the CLOC laser has a small wing on the passive waveguide side. The major difference between two simulated lasers is absence of a second-order mode and presence of the composite mode in the CLOC structure. The latter has noticeably reduced intensity in the active waveguide. We have calculated optical confinement factors and free-carrier optical losses for the fundamental, first-order, secondorder, and composite modes (see Table 1). Free-electron and hole absorption cross-sections used in the loss calculations were taken from [10]. The second-order mode of the reference laser has a slightly smaller OCF than the fundamental mode, while the OCF of the CLOC laser composite mode is more than twofold smaller than that of the fundamental mode. The composite mode has freecarrier optical losses of 4.7 cm−1 , which is much higher than the second-order mode has. According to these figures, the CLOC laser should operate on a single transverse fundamental mode. From our calculations, the simulated CLOC laser has a reduced vertical beam divergence of 23° FWHM (full width at half maximum). To prove this single transverse mode emission, we have experimentally investigated CLOC and reference lasers with the layer designs identical to the simulated ones. Two laser wafers were grown by metal-organic chemical vapor deposition (MOCVD) on n-GaAs (100) substrates. Both wafers were processed into 50-μmwide shallow-mesa ridge lasers by etching through the p-contact and partly through the p-cladding layers. The samples were mounted p-side down on copper heatsinks. The laser parameters were measured in pulsed (500 ns, 4.7 kHz) and CW (continuous-wave) modes at room temperature. Both CLOC and reference lasers showed Table 1. Calculated Mode Parameters for the Reference and CLOC Lasers Reference Laser Mode
Fig. 1. Refractive index profiles (left axes) and simulated intensity distributions (right axes) of the fundamental (solid line), first-order (dotted line), second-order (dashed–dotted line), and one composite (dashed line) modes for (a) the reference and (b) CLOC lasers. All modes have the same integrated intensity.
Fundamental First-order Second-order Composite
CLOC Laser
Optical Optical OCF (%) Losses, cm−1 OCF (%) Losses, cm−1 0.93 0.1 0.0013 — 0.79 1.3 not exist
0.93 0.1 0.0032 — not exist 0.42 4.7
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Fig. 2. Light-current characteristic of 50-μm-wide and 1-mmlong CLOC (solid line) and reference lasers (dashed–dotted line) measured at room temperature. The insets show far-field patterns for both lasers measured at the drive currents of 0.4 A, 2 A, and 3.5 A. The divergence values are for the CLOC laser.
similar parameters, namely, threshold current densities of ∼330 A∕cm2 in 2-mm-long and ∼700 A∕cm2 in 1-mm-long cavities, internal quantum efficiency of 88%, and intrinsic losses of 1.3 cm−1 . Due to the expanded optical field, the OCF was reduced, which led to low modal gain and as a result, the threshold current density was increased [4]. CW light-current curves for CLOC and reference lasers are presented in Fig. 2. The maximum output power is limited by a thermal rollover. Vertical far-field patterns measured at the CW drive currents of 0.4 A, 2 A, and 3.5 A are presented in the insets. At all levels of excitation, the CLOC lasers showed stable single-mode emission with the lowest divergence of 21.4° FWHM. Under the same pumping conditions, the reference lasers had pronounced lasing on high-order modes. Far-field patterns measured in pulsed mode are shown in Fig. 3. The lasers were pumped up to 20 thresholds, limited by the power supply. The reference lasers emitted on high-order modes over the entire drive current range. At higher excitation levels, their far-field patterns became a superposition of fundamental and high-order
Fig. 3. Vertical far-field patterns for the (a) CLOC and (b) reference lasers measured in pulsed mode (500 ns, 4.7 kHz, 20°C). Threshold current is ∼0.37 A. The curves for the CLOC laser at the currents of 0.4 A and 4 A are vertically shifted for better clarity.
modes. In the far-field patterns of the reference lasers [Figs. 2 and 3(b)], one can see multiple narrow peaks at large angles. We attribute these peaks to the coupling of the high-order mode to the substrate and contact layer. This coupling may take place if the cladding layers are not thick enough to confine an optical mode within the core layer [11]. In pulsed regimes, the CLOC lasers again showed extremely stable single-transverse-mode lasing with the divergence not exceeding 21.6° FWHM. The far-field patterns are well fitted by a Gaussian function as shown in Fig. 3(a). CLOC lasers due to their simple design are expected to be robust against variations in thicknesses and compositions of the waveguide layers. Adjusted CLOC structures can be easily incorporated into edge-emitting lasers operating in other wavelength ranges. In addition, CLOC structures can have more advanced designs where two different selected high-order modes tunnel from an active broadened waveguide into two different passive single-mode waveguides placed by either side of it. This advanced CLOC design may allow further broadening of laser waveguides. In summary, we have proposed a simple design of broadened laser waveguides based on coupled large optical cavity structures. The concept allows effective suppressing of the high-order mode lasing and thus reducing the vertical far-field divergence. The concept has been tested for InGaAs/GaAs lasers emitting in the 1040-nm wavelength range. The lasers with the 2.5-μm-thick waveguide operated in CW and pulse regimes and showed stable single transverse mode emission with the Gaussian shaped far-field patterns. The divergence of the CLOC lasers was ∼22° FWHM. The obtained results demonstrate that CLOC laser structures can be considered as a costefficient solution for edge-emitting lasers with improved beam quality. The authors acknowledge the support from the Russian Foundation for Basic Research (project no. 13-02-12184). References 1. D. Botez, Appl. Phys. Lett. 74, 3102 (1999). 2. M. J. Miah, T. Kettler, V. P. Kalosha, K. Posilovic, D. H. Bimberg, J. Pohl, and M. Weyers, IEEE J. Sel. Top. Quantum Electron. 21, 4900206 (2015). 3. G. Lin, S.-T. Yen, C.-P. Lee, and D.-C. Liu, IEEE Photonics Technol. Lett. 8, 1588 (1996). 4. B. S. Ryvkin, E. A. Avrutin, and J. T. Kostamovaara, J. Appl. Phys. 114, 013104 (2013). 5. A. Pietrzak, P. Crump, H. Wenzel, G. Erbert, F. Bugge, and G. Trankle, IEEE J. Sel. Top. Quantum Electron. 17, 1715 (2011). 6. B. E. Little and W. P. Huang, Prog. Electromagn. Res. 10, 217 (1995). 7. C. R. Pollock and M. Lipson, Integrated Photonics (Kluwer Academic, 2003), p. 278. 8. M. A. Afromowitz, Solid State Commun. 15, 59 (1974). 9. B. R. Bennett, R. A. Soref, and J. A. Del Alamo, IEEE J. Quantum Electron. 26, 113 (1990). 10. K. H. Hasler, H. Wenzel, P. Crump, S. Knigge, A. Maasdorf, R. Platz, R. Staske, and G. Erbert, Semicond. Sci. Technol. 29, 045010 (2014). 11. H. Wenzel, IEEE J. Sel. Top. Quantum Electron. 19, 1 (2013).
