Semiconductor lasers subject to polarisation-rotated optical feedback

The theoretically predicted effect that polarisation-rotated optical feedback should have on the dynamics of semiconductor lasers has not been observed in the majority of the published experimental studies. This paper offers an explanation for the observed differences, based on experimental and theoretical investigation of three different optical feedback configurations. One of the configurations contains a quarter-wave plate within the external cavity formed by the laser diode and the external mirror. This commonly used scheme admits multiple reflections for high-external reflectivities, which in turn ensures that both incoherent and coherent feedback are present in the reflected beam. When these higher-order reflections are accounted for, good agreement between the numerical and the experimental results is obtained. Hence, the discrepancies between the earlier theoretical and the experimental measurements occurred because the first theoretical models neglected the weak coherent part of the reflected beam.