Far-field characteristics of mid-infrared angled-grating distributed feedback lasers

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Abstract
The far-field emission characteristics of mid-infrared angled-grating distributed-feedback (α-DFB) lasers with W active regions are calculated using a self-consistent beam-propagation formalism that is more general than previous analyses. The theoretical projections are compared with the results of an experimental study of optically pumped α-DFB devices. Near-diffraction-limited beam quality is obtained both theoretically and experimentally for pump stripes ⩽50 μm wide. While simulations employing the theoretical linewidth enhancement factor of 1.7 for the homogeneously-broadened W-laser gain spectrum predict that the good beam quality should be retained for stripes as wide as ≈200 μm, the data indicate a much more rapid degradation. That finding can be reproduced only by assuming that inhomogeneous broadening increases the structure’s linewidth enhancement factor to ≈5. The experiments and theory also yield a steering of the output beam to off-normal angles as large as 6° when temperature tuning shifts the gain peak away from the grating resonance.