Feedback noise in single-mode semiconductor lasers

In nearly every application of semiconductor lasers some externally reflected light is coupled back into the laser with a certain time delay. The noise and coherence properties are very sensitive to optical feedback. Coupling to external cavity modes induces frequency shifts, line-narrowing and broadening, competition among different external cavity modes, or dynamical instabilities. Strongly dispersive gain makes instabilities efficiently generate phaser noise, leading to coherence collapse. We present an overview of these phenomena and their theoretical descriptions.