Invited - Experimental characterization of optical feedback and current modulation effects on the spatial coherence of a semiconductor laser

Semiconductor lasers are very sensitive to optical feedback. Here we study experimentally the effect of optical feedback on the spatial coherence of a diode laser using the speckle technique. Speckle is a noisy structure arising from the interference of coherent waves as they propagate through a diffusive medium. Using a multimode fibre as the diffusive medium, we observe that, during the laser turn-on, without feedback, the speckle contrast increases gradually (revealing a gradual increase in spatial coherence), but, with sufficiently strong feedback, the speckle contrast increases sharply (revealing an abrupt increase in spatial coherence). For pump currents above the threshold, high-contrast regions alternate with low-contrast regions. We also observe that, under appropriate current modulation, high-contrast regions are suppressed. Our findings may find application in laser-based illumination systems, because optical feedback can be used in combination with current modulation to reduce speckles over a wide range of pump currents.