Abrupt Transition to Coherent Emission in a Semiconductor Laser with Optical Feedback

Semiconductor lasers are very sensitive to optical feedback. Optical feedback is well known to reduce the laser threshold, but the characteristics on the transition from low-coherence radiation (dominated by spontaneous emission) below threshold to high-coherence radiation (dominated by stimulated emission) above threshold have not yet been investigated. Here, we study experimentally how the coherence of the light emerges during the turn-on of a semiconductor laser when it is and not optically fedback by a mirror. With a speckle technique, we show that the transition into coherence varies from smooth to abrupt as the amount of light fed back to the laser increases.