40 GHz mode-beating with 8Hz linewidth and 64fs timing jitter from a synchronized mode-locked quantum-dash laser diode

Linewidth narrowing of the radio frequency beat-tones and the optical-modes is experimentally investigated in a ˜40GHz quantum-dash mode-locked laser diode subject to optical injection of 10GHz pulses. In comparison to the 75kHz linewidth exhibited by the beat-tones in passive mode-locking conditions, a remarkable reduction to less than 8Hz is achieved when the laser is under optical injection. From this beat-tone signal, an integrated root-mean-square timing jitter of 64fs is calculated. In addition, a quadratic profile of the optical linewidth with the wavelength is observed in active locking, reaching a minimum of 1.7MHz for the longitudinal modes around 1530nm and progressively increasing to 37.4MHz for modes at 1525nm.