Abstract

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.

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