Coherence properties of a semiconductor laser with feedback from a distant reflector: experiment and theory

The visibility, i.e. the absolute value of the field-autocorrelation function, of a semiconductor laser subject to optical feedback has been measured. The feedback was provided by a mirror that was placed at a distance exceeding the coherence length of the emitted light. The amount of feedback varied between zero and the maximum feedback, for which the laser still operated in a single longitudinal mode of the semiconductor chip. The results are compared with numerical solutions of the Lang and Kobayashi equations. Over the whole range of feedback excellent agreement between experiment and theory was found. However. this agreement requires an unusually high value of the linewidth enhancement factor, i.e. a=10; the reason for this remains unclear. The self-sustained noise model, which is based on a statistical-analytical solution of the Lang and Kobayashi equations and which has been used in the past for analyzing feedback effects, is found to yield an inaccurate description of the experiments. Suggestions are given to explain this inadequacy