Phase-locking and delay-induced instabilities in a dual- frequency fiber laser

We show that the polarization modes of a dual-frequency fiber laser can be efficiently phase-locked using optical frequency-shifted feedback, leading to a stable optically-carried microwave clock signal. We also show that a delay-induced instability, characterized by periodic pulses in the envelope of the beat-note signal, takes place inside the phase-locking range. This observation agrees with the prediction of a delayed rate equation model.