Influence of external optical feedback on the alpha factor of semiconductor lasers

This Letter presents the results revealing the influence of external optical feedback (EOF) on the alpha factor, or the linewidth enhancement factor, of semiconductor lasers operating on single mode. First, a method is proposed for the measurement of the alpha over a wide range of optical feedback level, which provides an effective way for investigating the dependence of the alpha on laser operating conditions. Second, experimental investigation based on the proposed method is performed on a GaAlAs laser diode with a multiquantum well structure. It is discovered that the alpha value remains approximately constant with increasing injection current, but has a strong dependence on EOF.