Optical feedback phenomena in semiconductor lasers

Optical feedback phenomena in semiconductor lasers attracted considerable interest during the last decade. It has been recognized quite early, that the linewidth can be reduced considerably with optical feedback and external cavity lasers are routinely used if linewidths down to a few kHz are required. However, optical feedback phenomena have been found to be responsible also for mode hopping, for strong excess noise in the coherence collapse regime and other regimes have been identified with stable microwave oscillations