Degenerate and nondegenerate lateral-mode patterns in quantum cascade lasers

We experimentally investigate near-field lateral-mode patterns of 10-25-μm -wide quantum cascade wide-ridge lasers with few lateral modes using a 1-GHz-resolution spatially resolving midinfrared spectrometer. The results supported by simple box-model theory demonstrate that, depending on the laser geometry, one can either obtain lateral-mode frequency degeneracy or find the high-order lateral modes to be blueshifted or redshifted from the fundamental. We show that, with proper laser geometry, all longitudinal and lateral modes can be frequency nondegenerate and, therefore, be easily suited for spatiospectral mode combining to increase brightness.