Fabrication-Tolerant-Design for Single-Lobe, Surface-Emitting Quantum Cascade Lasers

Grating-coupled, surface-emitting (GCSE) quantum-cascade lasers (QCLs) are attractive sources for realizing watt-range continuous-wave (CW) output powers in the mid-infrared spectral region. Linear GCSE QCLs generally operate in the antisymmetric longitudinal mode [1] , which produces a double-lobed beam pattern. The antisymmetric mode can be suppressed by using a metal/semiconductor, 2 nd- order distributed-feedback (DFB) grating [2] , allowing symmetric longitudinal-mode operation and a single-lobe beam pattern. However, such designs require rather tight tolerances in grating duty cycle which cause fabrication challenges. Nevertheless, we have previously reported experimental results from GCSE QCLs using this approach - a single-lobe, diffraction-limited beam with a peak pulsed output power of ~ 0.4 W from wide ridges (~ 21 μm) [3] . Alternatively, GCSE devices designed to operate in an antisymmetric mode can exhibit a single-lobe beam by using a central π-phaseshift in the gratings [4] . Here we show that 4.6 μm-emitting GCSE QCLs with a central π-phaseshift can be designed to solely operate in an antisymmetric mode for high CW output power, single-lobed beam operation, while maintaining strong intermodal discrimination over a wide range in grating duty cycle.