Frequency locking in a semiconductor laser with double incoherent optical feedback

Frequency locking (FL) in a semiconductor laser with double incoherent optical feedback is experimentally studied, and the characteristics of the locking states are analyzed through comparing the RF spectrum of double incoherent optical feedback and single incoherent optical feedback. Moreover, by varying the external loops length ratio, different orders of frequency locking states are observed. The influence of feedback strength and cavity length on locking performance is preliminarily investigated and the optimized FL state is obtained. This experimental investigation may offer a method to obtain frequency spacing tunable discrete broad-band signals.