Effect of current modulation on the coherence of a semiconductor laser with optical feedback

The use of laser light for illumination produces speckles that degrade image quality, and different strategies have been developed to mitigate speckle. Optical feedback can reduce the amount of speckle, since it can decrease the coherence of the laser by inducing multi-mode, broad-band emission. However, the effect of optical feedback depends on the laser current and feedback parameters. When the current increases, regions of low speckle contrast often alternate with regions of high contrast, where the laser emission is single-mode. Here, we experimentally study the combined effect of optical feedback and direct modulation of the laser current, to determine whether modulation can be used to further reduce the speckle contrast. We find that current modulation does not further reduce the contrast, but under appropriate modulation parameters, the regions of high speckle contrast are fully suppressed and the contrast remains low in all the range of pump currents studied. The modulation frequencies that eliminate the high-contrast regions depend on the modulation amplitude. Our findings are relevant to laser-based illumination systems, because optical feedback can be used in combination with current modulation to reduce speckle over a wide range of pump currents.