Mode selection and stability of a semiconductor laser with weak optical feedback

The authors present measurements that show that a semiconductor laser with weak optical feedback oscillates in the external-cavity mode with the narrowest linewidth rather than the mode with minimum threshold gain. A frequency fluctuation model that explains this narrowest linewidth operation is proposed. This model also correctly predicts mode-hopping frequencies. It is shown that the frequency span of the external-cavity modes determines the critical feedback level that causes coherence collapse