Abstract
In the process of power scaling large-area quantum cascade lasers (QCLs), challenges such as degradation of beam quality and emission of multilobed far-field modes are frequently encountered. These issues become particularly pronounced with an increase in ridge width, resulting in multimode problems. To tackle this, an innovative multi ridge waveguide structure based on the principle of supersymmetry (SUSY) was proposed. This structure comprises a wider main waveguide in the center and two narrower auxiliary waveguides on either side. The high-order modes of the main waveguide are coupled with the modes of the auxiliary waveguides through mode-matching design, and the optical loss of the auxiliary waveguides suppresses these modes, thereby achieving fundamental mode lasing of the wider main waveguide. This paper employs the finite difference eigenmode (FDE) method to perform detailed structural modeling and simulation optimization of the 4.6 μm wavelength quantum cascade laser, successfully achieving a single transverse mode QCL with a ridge width of 10 μm. In comparison to the traditional single-mode QCL(with a ridge width of about 5 μm), the MRW structure has the potential to increase the gain area of the laser by 100%. This offers a novel design concept and methodology for enhancing the single-mode luminous power of mid-infrared quantum cascade lasers, which is of considerable significance.
High-power quantum cascade lasers are widely employed in fields such as directional infrared countermeasures, long-distance detection of hazardous chemicals, infrared laser guidance, and long-distance free-space optical communication
In recent years, a new parity-time (PT) symmetric design method has been established, which utilizes the concept of selective breaking of parity-time (PT) symmetr
Based on the idea of supersymmetry and PT symmetry, this paper adds additional optical structure to traditional ridge waveguide laser to suppress the higher-order modes while increasing the ridge width. The design principle is as follows. The core idea of the unbroken SUSY optical architecture is to construct a superpaired system of a known optical syste
The cross-section of the MRW structure proposed in this study is shown in

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Fig. 1 Schematic structure of the MRW device structure and principle:(a) Cross-section of the MRW; (b) Design principle of mode coupling
图1 MRW器件结构和原理示意图:(a)器件横截面; (b)主波导与左、右辅助波导的模式耦合设计原则
This study uses the MODE solver in Lumerical to perform mode analysis on the device under study, and by setting the imaginary part ni of the complex refractive index to impose additional losses. According to (α is positive for gain, α is negative for loss), nr is calculated based on the material composition. To enhance mode selection, it is assumed that a uniform loss of 20 c
A simulation was conducted to determine the width of each waveguide based on the relationship between the ridge width and the propagation constant of a single ridge waveguide, and the results are shown in

Fig. 2 The propagation constant real part of TM modes in QCL as a function of ridge width
图2 QCL中TM模的传播常数实部和脊宽的关系
ΔLoss1 is the mode loss difference between the fundamental mode with the lowest loss and the high-order mode with the second lowest loss in the existing modes of the main waveguide. The larger this value, the stronger the mode recognition ability and the better the selectivity for the fundamental mode. ΔLoss2 is the mode loss difference between TM1 of the main waveguide and TM0 of the left auxiliary waveguide. It represents the coupling efficiency of the first-order mode of the main waveguide and the fundamental mode of the left auxiliary waveguide. The smaller this value, the greater the mode overlap and the higher the coupling efficiency. Similarly, ΔLoss3 is the mode loss difference between TM2 of the main waveguide and TM1 of the right auxiliary waveguide. It represents the coupling efficiency of the second-order mode of the main waveguide and the first-order mode of the right auxiliary waveguide. At the same time, considering that when the trench depth is shallow, the laser may exhibit the characteristics of a super wide single ridge laser (more than 20 μm), a reasonable trench depth needs to be considered. According to the design principle in

Fig. 3 The number of modes in the laser at different trench etching depthes(The geometric parameters are WM = 10 μm, WL = 4.9 μm, WR = 6.5 μm, respectively.)
图3 不同沟槽刻蚀深度下激光器的模式数,对应的结构参数分别为WM =10 μm,WL= 4.9 μm,WR = 6.5 μm
To achieve fundamental mode lasing, the selection principle of MRW structural parameters is to ensure that the fundamental mode selectivity in the main waveguide is the strongest while the coupling efficiencies of high-order modes are the highest, that is, the fundamental mode is constrained in the main waveguide, and the energy of high-order modes is mainly concentrated in the lossy auxiliary waveguide.

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Fig. 4 Effect of the ridge width on the coupling results:(a) Loss margin of modes as a function of the left ridge width; (b) Loss margin of modes as a function of the left ridge width
图4 脊宽对耦合结果的影响:(a)左侧波导脊宽对MRW结构模式损耗差值的影响; (b)右侧波导脊宽对MRW结构模式损耗差值的影响

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Fig. 5 Effect of different parameters on loss margin of modes in MRW structure:(a)Loss margin of modes as a function of trench etch depth; (b) Loss margin of modes as a function of etch side depth; (c) Loss margin of modes as a function of trench etch depth
图5 不同参数对MRW结构模式损耗差值的影响:(a)沟槽深度对MRW结构模式损耗差值的影响;(b)侧壁深度对MRW结构模式损耗差值的影响;(c)沟槽宽度对MRW结构模式损耗差值的影响
After two modes are coupled, they split into pairs of supermodes, resulting in a significant change in the distribution of field strength (energy) compared to the uncoupled modes. Using the optimized structural parameters mentioned earlier, the energy distribution of the modes before and after coupling is calculated.

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Fig. 6 Field strength distribution of the fundamental modes in the laser:(a) Field strength component of the fundamental mode supported by the 10 μm independent single ridge laser; (b) Field strength component of the fundamental mode supported by the 6.1 μm independent single ridge laser; (c) Field strength component of the fundamental mode in the main waveguide of MRW laser; (d) Field strength component of the fundamental mode in right auxiliary waveguide of MRW laser
图6 激光器中基模的场强分布:(a)10 μm独立单脊激光器基模的场强分布; (b)6.1 μm独立单脊激光器基模的场强分布; (c)MRW激光器主波导中基模的场强分布; (d) MRW激光器右辅助波导中基模的场强分布

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Fig. 7 Field strength distribution of high-order mode in the laser:(a) Field strength component of 1st order mode supported by the 10 μm independent single ridge laser; (b) Field strength component of the fundamental mode supported by the 4.4 μm independent single ridge laser; (c) Field strength component of the first order supermode in main waveguide of MRW laser; (d) Field strength component of first order supermode in the left auxiliary waveguide of MRW laser
图7 激光器中高阶模式的场强分布:(a)10 μm独立单脊激光器一阶模的场强分布; (b)4.4 μm独立单脊激光器基模的场强分布; (c)MRW激光器中主波导一阶超模的场强分布; (d)MRW激光器中左辅助波导一阶超模的场强分布缺图
Fig. 8 Field strength distribution of high-order mode in the laser:(a) Field strength component of 2nd order mode supported by the 10 μm independent single ridge laser; (b) Field strength component of 1st order mode supported by the 6.1 μm independent single ridge laser; (c) Field strength component of the second order supermode in the main waveguide of MRW laser; (d) Field strength component of second order supermode in the right auxiliary waveguide of MRW laser
图8 激光器中高阶模式的场强分布:(a)10 μm独立单脊激光器二阶模的场强分布; (b)6.1 μm独立单脊激光器一阶模的场强分布; (c)MRW激光器中主波导二阶超模的场强分布; (d)MRW激光器中右辅助波导二阶超模的场强分布
Figure 8(a) shows the uncoupled mode TM2 of the 10 μm single ridge laser. As per the design principle illustrated in

Fig. 9 Mode discrimination of MRW-QCL(red circles) and conventional QCL(black triangles)
图9 MRW-QCL(红心圆)与传统QCL(黑三角)的模式区分能力
In this study, a new wide-ridge MRW MWIR-QCL waveguide structure was designed based on the unbroken SUSY principle. The structure comprises a primary waveguide situated in the center, accompanied by a pair of lossy auxiliary waveguides on either side, and the sidewall etching depth in this structure is about 0.7 μm deeper than the trench depth. By coupling the high-order modes of the main waveguide with the modes of the auxiliary waveguide, the lasing thresholds of the main waveguide high-order modes were raised, achieving the fundamental mode lasing of the device. Compared with array structures or taper waveguide structures, this structure is relatively simple and straightforward to fabricate. Additionally, compared with the conventional 5 μm wide single ridge waveguide MWIR-QCL, this design realizes a single-mode waveguide structure with a 10 μm ridge width, thereby increasing the active area by 100%, which is of great significance for achieving high power and single transverse mode output of QCL. In future research, we will further consider thermal effects and heat dissipation structure design, study the impact of trench filling on heat dissipation, and strive to achieve wide ridge QCL single-mode output of high-power continuous mode.
References
Quach P, Liu S F, Jollivet A, et al. A GaN/AlN quantum cascade detector with a broad response from the Mid-infrared (4.1 μm) to the Visible (550 nm) Spectral Range[J]. Applied Physics Letters, 2020, 116 (17) :171102. 10.1063/5.0003615 [Baidu Scholar]
Fei T, Zhai S T, Zhang J C, et al. High power λ ~ 8.5 μm quantum cascade Laser grown by MOCVD operating continuous-wave up to 408 K[J]. Journal of Semiconductors, 2021, 42 (11) :112301. 10.1088/1674-4926/42/11/112301 [Baidu Scholar]
Wang Z, Beck M, Wang R, et al. Monolithic integration of mid-infrared quantum cascade lasers and frequency combs with passive waveguides[J]ACS Photonics, 2022, 9(2):426-431. 10.1021/acsphotonics.1c01767 [Baidu Scholar]
Liu C W, Zhai S Q, Zhang J C, et al. Free-space communication based on quantum cascade laser[J]. Journal of Semiconductors, 2015, 36(9):85-88. 10.1088/1674-4926/36/9/094009 [Baidu Scholar]
Figueiredo P, Suttinger M, Go R, et al. Progress in high-power continuous-wave quantum cascade lasers[J]. Applied Optics, 2017, 56(31):H15. 10.1364/ao.56.000h15 [Baidu Scholar]
Liu Y H, Zhang J C, Yan F L, et al. Coupled ridge waveguide distributed feedback quantum cascade laser arrays[J]. Applied Physics Letters, 2015, 106(14):553. 10.1063/1.4917294 [Baidu Scholar]
Heydari D, Bai Y, Bandyopadhyay N, et al. High brightness angled cavity quantum cascade lasers[J]. Applied Physics Letters, 2015, 106(9):941. 10.1063/1.4914477 [Baidu Scholar]
Yu S, Fang Z, Wang Z Z, et al.On-chip single-mode thin-film Lithium Niobate fabry-perot resonator laser based on sagnac loop reflectors[J].Optics Letters, 2023, 48(10):2660-2663. 10.1364/ol.484387 [Baidu Scholar]
Praveena S, Senthilnathan K. A review: rise of pt-symmetry for laser applications[J]. Optik, 2023, 289(17): 171260. 10.1016/j.ijleo.2023.171260 [Baidu Scholar]
Song A Y, Kalapala A R K, Gibson R, et al. Controllable finite ultra-narrow quality-factor peak in a perturbed dirac-cone band structure of a photonic crystal slab[J]. Applied Physics Letters, 2021, 119(03):03115. 10.1063/5.0056243 [Baidu Scholar]
Fu Ting, Wang Yu-Fei, Wang Xue-You, et al. Microstructure lasers based on parity-time symmetry and supersymmetry[J]. Chinese Journal of Lasers(傅廷, 王宇飞, 王学友等.基于PT对称和超对称的微结构激光器.中国激光), 2021, 48(12):1201005. [Baidu Scholar]
Hokmabadi M P, Nye N S, El-Ganainy R, et al. Supersymmetric laser arrays[J]. Science, 2019, 363(6427):623-626. 10.1126/science.aav5103 [Baidu Scholar]
Qiao X, Midya B, Gao Z, et al. Higher-dimensional supersymmetric microlaser arrays[J]. Science, 2021, 372(6540):403-408. 10.1126/science.abg3904 [Baidu Scholar]
Zhao X L, Zeng S, Sweatt L, et al. High-power single-mode triple-ridge waveguide semiconductor laser based on supersymmetry[J]. AIP Advances, 2021, 11(9):095216. 10.1063/5.0060287 [Baidu Scholar]
Hayenga W E, Garcia-Gracia H, Cristobal E S, et al. Electrically pumped microring parity-time-symmetric lasers[J].Proceedings of the IEEE, 2020, 108 (5) :827-836. 10.1109/jproc.2019.2935901 [Baidu Scholar]
Abbas M, Ziauddin, Zhang Y C, et al. Phase dependent parity time symmetry in a quantum dot nanostructure[J]. Optics & Laser Technology, 2023, 162 (10):109259. 10.1016/j.optlastec.2023.109259 [Baidu Scholar]
Fu T, Wang Y F,Wang X Y,et al. Mode Control of quasi-PT symmetry in laterally multi-mode double ridge semiconductor laser[J]. Chinese Physics Letters, 2020, 37(4):044207. 10.1088/0256-307x/37/4/044207 [Baidu Scholar]
Guo A, Salamo G J, Duchesne D, et al. Observation of PT-symmetry breaking in complex optical potentials[J]. Physical Review Letters, 2009, 103(9):093902. 10.1103/physrevlett.103.093902 [Baidu Scholar]
Gu Z Y, Zhang N, Lyu Q, et al. Experimental demonstration of PT-symmetric stripe lasers[J]. Laser & Photonics Reviews, 2016, 10(4):588-594. 10.1002/lpor.201500114 [Baidu Scholar]
Chen Y T, Wang J W, Chen W J, et al. Reciprocal waveguide coupled mode theory[J], Acta Physica Sinc, 2020, 69 (15): 154206. 10.7498/aps.69.20200194 [Baidu Scholar]
Guan Y J, Lu X Y, Cheng F M, et al. Continuous-wave Distributed Bragg reflector quantum cascade lasers with fine single-mode tuning up to 102°c at λ∼8.4μm[J], Optics Communications, 2023, 528 (12): 128994. 10.1016/j.optcom.2022.128994 [Baidu Scholar]
Yang R K, Zhang D L, Zheng X T, et al. Optics and thermal co-optimal design of high-power quantum cascade lasers[J]. Laser & Infrared(杨若珂, 张东亮, 郑显通等.大功率量子级联激光器的光学与热学协同优化设计.激光与红外), 2023, 53 (9) :1350-1359. [Baidu Scholar]