Experimental analysis of harmonic and sub-harmonic synchronization of 40 GHz mode-locked quantum-dash lasers under optical injection

Synchronization of a 40 GHz quantum-dash mode-locked Fabry-Pérot laser diode with optically injected pulse streams is experimentally studied by analyzing the linewidth of the associated 40 GHz beat-tones. Injected signals consist of trains of 1.6 ps pulses at various repetition rates, spanning from 10 to 160 Gb/s of encoded or not with pseudo-random on-off key data sequence. Data injections at the fundamental bit-rate (40 Gb/s), harmonic at 80 and 160 Gb/s and sub-harmonic at 10 and 20 Gb/s are used to synchronize the mode-locked laser. The response is a stable 40 GHz clock pulse train featuring a width of 1.8 ps. Moreover, the 40 GHz beat-tones associated to the recovered clock pulses exhibit a full-width at half maximum smaller than 8 Hz regardless of the injected data rate. It is observed that frequency components at 10 and 20 GHz do not create any amplitude modulation on the recovered 40 GHz clock pulses by the injection of data signals at 10 and 20 Gb/s. Similarly, synchronization of the quantum-dash laser with pulse streams at 80 and 160 Gb/s is sustained in spite of negligible components at and below 40 GHz are injected in these two data rates.