Research on polygonal microcavity 9 μm quantum cascade lasers with waveguide output from a master oscillator power amplifier

A polygonal microcavity 9 μm quantum cascade laser (QCL) incorporating an output waveguide and a master oscillator–power amplifier (MOPA) configuration is proposed and numerically investigated to overcome the inherent trade-off between directional emission and beam quality in long-wavelength infrared (LWIR) microcavity lasers. The active region of a 9 µm InP-based QCL is designed using Nextnano to achieve wavelength-matched optical gain, while the coupling characteristics between the polygonal microcavity and the MOPA waveguide are systematically optimized using Ansys Lumerical. Finite-difference time-domain simulations are performed to examine the influence of key structural parameters, including the flare angle of the power amplifier, the length of the master oscillator section, and the amplifier cavity length, on the output intensity, slow-axis divergence, and diffraction behavior. The results indicate that an appropriately flared amplifier enables effective transverse mode reshaping, leading to significant suppression of higher-order diffraction and pronounced reduction of slow-axis divergence while preserving the resonant properties of the microcavity. The master oscillator length is found to mainly affect the phase distribution at the output facet, with a limited impact on the divergence angle. An optimal amplifier length exists that simultaneously enhances beam quality and output performance. These findings provide a practical design strategy for realizing a high-power, high-beam-quality, and integrated long-wavelength infrared microcavity QCL.