Period-doubling and period-quadrupling for an actively mode-locked laser diode with extended cavity

Active mode-locking of a laser diode in an external cavity has led to the observation of new instabilities. We report period-doubling manifested by the alternation of weak and strong pulses in the mode-locked laser pulse train. We also report period-quadrupling where four successive pulses have different amplitudes. These results are attributed to the imperfect antireflection coating of the semiconductor chip, the latter constituting a regenerative amplifier with a time-dependent delay for amplification. A qualitative analysis is proposed and validated by numerical simulations from a diode laser rate equation model.