2-D Grating-Coupled Surface Emission From Phase-Locked Mid-IR Quantum Cascade Laser Array

Surface emission from a resonant leaky-wave coupled phase-locked array of mid-infrared (IR) quantum cascade lasers (QCLs) employing a metal/semiconductor 2
^nd
-order distributed feedback (DFB) grating, placed in the array elements, is proposed and analyzed. A quasi-3D modal analysis is performed including lateral (i.e., array-mode) and longitudinal (i.e., DFB) directions to predict the threshold-current densities of the competing modes. The grating is found to improve the intermodal discrimination, ensuring single-spatial-mode operation to high surface-emitted output powers. A five-element, surface-emitting phase-locked QCL array with 2
^nd
-order Ag/semiconductor DFB gratings in the array-element regions was fabricated, and provides, at 4.6 μm wavelength, 1.22 W surface-emitted peak pulsed power. The device emits in a narrow two-dimensional beam, although it is not yet optimized for single-spatial-mode operation. Analysis indicates that further optimized devices employing DFB/DBR gratings placed in the array elements increases the array intermodal discrimination, and as a result creates a larger fabrication tolerance for the array interelement width. Specifically, grating designs which favor the anti-symmetric longitudinal mode are preferred to obtain higher discrimination against unwanted lateral array modes.