【摘要】 为实现半导体激光器高功率和高光束质量的协同输出,提出了一种集成多模干涉(MMI)耦合器的脊波导带间级联激光器(ICL)结构,并对其模式调控特性进行了系统研究。通过设计脊宽分别为8、11和14μm的法布里-珀罗(FP)器件,以及对应的集成MMI结构的MMI-ICL器件,结合光场仿真与实验测试,分析了MMI结构对高阶模式的抑制效果。实验结果表明,MMI-ICL器件的输出光束由基模主导,光束质量较FP器件显著提升。其中,11μm脊宽的MMI-ICL器件的远场发散角为26°,接近1.1倍衍射极限,实现了近衍射极限的基横模输出;而14μm脊宽的MMI-ICL器件的输出光束虽然存在少量二阶模分量,但基模占比显著提升,发散角从FP器件的41°降低至18°。理论仿真结果进一步表明,MMI结构通过自映像效应选择性损耗奇数阶模式,同时增加偶数阶模式的耦合效率。在脊宽数倍于波长的器件中,MMI结构能够有效抑制高阶模式,从而优化光束质量。研究结果为设计高光束质量的宽脊半导体激光器提供了新的思路。更多还原

【Abstract】 Objective Mid-infrared（3-4 μm） lasers are critical for applications such as free-space communication,infrared countermeasures,and gas sensing,demanding concurrent high power and beam quality.Interband cascade lasers（ICLs） based on antimonide superlattices combine high gain with low power consumption,offering significant advantages in this spectral region.However,conventional ICLs suffer from low vertical thermal conductivity due to their 2000+ layer superlattice structure,limiting active stages（<10） and forcing narrow ridge widths（<6 μm） to maintain fundamental-mode operation.This restricts the output power to tens of milliwatts,insufficient for long-range applications.Widening the ridge to boost the power inevitably excites higher-order transverse modes,degrading the beam quality.This work addresses this trade-off by integrating multimode interference（MMI） couplers into ICLs.The MMI structure selectively suppresses odd-order modes via self-imaging effects,enabling wide-ridge designs without compromising beam quality,representing a crucial advancement for power-intensive mid-infrared applications.Methods We design and fabricate Fabry-Pérot（FP） and MMI-ICL devices with three ridge widths of 8,11,and 14 μm（Fig.1）.The symmetric 1×1 MMI structure with a width of 36 μm and length of 1356 μm features the input/output waveguides centering on the MMI region and connected via 34° tapers to minimize reflection losses.The MMI length(L_MMI) is calculated using the self-imaging principle,targeting the first-order position for TE0/TE2 modes enhancement and TE1 mode suppression.COMSOL Multiphysics simulations are performed to analyze the modal losses as a function of ridge width for TE0,TE1,and TE2 modes（Fig.2）.A 7-stage ICL epitaxial structure is grown on n-GaSb via molecular beam epitaxy（MBE）.Ridge waveguides and MMI couplers are patterned by contact lithography and wet etching with a H₃PO₄-based solution.The devices feature SiO₂ passivation,Ti/Pt/Au top contacts,and Ge/Au/Ni/Au back contacts.Cleaved 3-mm-long cavities are mounted on copper heat sinks.The beam profiles（Fig.4） are measured at 3X threshold current using a Pyrocam IV beam analyzer.Power-current-voltage curves（Fig.6） are recorded under continuous wave（CW） operation.Results and Discussions For FP lasers,increasing the ridge width from 8 μm to 14 μm results in a progressive degradation of beam quality as shown in Figs.4（a）-（c）:the 8-μm device exhibits a single-lobe TE0 profile,the 11-μm device shows a dual-lobe TE1-dominated pattern,and the 14-μm device displays a triple-lobe TE0/TE1/TE2 mixed mode.In contrast,the output remains a singlelobe TEO-dominant profile across all ridge widths [Figs.4（d）-（f）] for the MMI-ICL samples,demonstrating the MMI effectiveness in suppressing higher-order modes.In addition,the MMI-ICLs achieve near-diffraction-limited performance as shown in Fig.5.The MMI-ICL with a 11-μm ridge shows a divergence angle of 26°,corresponding to a beam quality factor（M²） of 1.23（1.1 × diffraction limit）,compared to 38° for the FP counterpart.For the 14-μm device,the MMI-ICL reduces divergence angle from 41° to 18°（M²=1.57）,achieving near-diffractionlimited performance.Weak side lobes observed in the 14-μm MMI-ICL [Fig.3（b）] are attributed to the minor TE2 contributions,consistent with the simulated 96% transmission of this mode through the MMI section.The MMI-ICLs demonstrate an enhanced optical power without increasing threshold performance（Fig.6）.The enlarged active area in the MMI section boosts the output power across all ridge widths compared to those of the FP lasers.This improvement highlights the MMI ability to address the core trade-off between beam quality and power in wide-ridge semiconductor lasers.Conclusions This work demonstrates a breakthrough in high-power and high-beam-quality ICLs via integrated MMI couplers.MMI-ICLs with 14-μm ridge widths maintain a single-lobe output,outperforming the conventional FP designs and existing sidewallgrating technologies.The 11-μm MMI-ICL achieves the M² factor of 1.23（26° divergence angle）,representing a significant advancement toward diffraction-limited performance.The MMI-ICL platform resolves the critical power-beam quality trade-off,enabling next-generation mid-infrared sources for long-range sensing and communication.Future work will optimize MMI designs and explore phase-locked arrays to further enhance output power while maintaining beam coherence.更多还原