Pulse repetition-frequency multiplication in a coupled cavity passively mode-locked semiconductor lasers

Using a delay differential equation model with two time delays, we investigate the dynamics of a semiconductor laser with an active cavity coupled to an external passive cavity. Our numerical simulations indicate that when the coupling between the two cavities is strong enough and the round-trip time of the active cavity is an integer multiple of the round-trip time of the external passive cavity, a harmonic mode-locking regime can develop in the laser with the pulse repetition period close to the passive cavity round-trip time. We also demonstrate that the output field intensity sensitively depends on the relative position of the frequency combs of the two cavities giving rise to a resonant behavior. The period and width of the resonances depend on the ratio of the round-trip times to the coupling between the two cavities. We show that the coupled cavity system under consideration can demonstrate a bistability between different regimes of generation.