Spatial beam profiles from a laser-diode system with optical feedback: the importance of interference.

The output beam profile of a laser diode with weak-to-moderate levels of optical feedback is shown to arise from interference of the emitted and feedback fields. This has been determined from a series of experiments, that measure the output spatial beam profile as the optical feedback field into the laser diode is spatially manipulated. Tilting, focusing, and aperturing the feedback field led to output beam profiles readily interpreted as the interference between the emitted and the feedback fields, provided the output of the laser-diode system with optical feedback has sufficient temporal coherence. Observation of the interference pattern in the spatial beam profile, at an appropriate level of optical feedback, can be used to study the relative wave front of the optical feedback and emitted fields and to estimate coupling coefficients.