Observation of multi-path interference in broad-area semiconductor lasers with optical feedback

Multi-path interference effects induced by optical feedback in broad-area semiconductor lasers is experimentally studied. An external mirror and an internal laser cavity form a closed composite optical feedback loop. For a very small tilt of the external mirror for the exit facet along the stripe width of the active layer, the light emitted from the laser undergoes multi-path reflections in the composite cavity, and we therefore observe multi-path interference effects of the laser oscillations for a small mirror tilt. The laser output power always shows the fundamental mode of the single feedback loop together with an oscillation of a certain higher multi-path loop. The laser oscillation and the beam profile are strongly dependent on the tilt. We observed up to a nine-fold multi-path interference in the experiment.