Subharmonic synchronous mode-locking of a monolithic semiconductor laser operating at millimeter-wave frequencies

Optical pulse trains at millimeter-wave frequencies are generated by subharmonic synchronous mode-locking of a monolithic distributed Bragg reflector semiconductor laser, by which an initially passively mode-locked semiconductor laser is stabilized by injecting optical pulses at subharmonic frequencies of its resonant frequency. The stabilized pulse trains are characterized in terms of phase noise, timing jitter, locking range and modulation depth under various injection conditions including injected signal power levels, pulsewidths, and subharmonic numbers. It is also shown that subharmonic synchronous mode-locking can provide pulse trains with a very low level of phase noise (⩽-86 dBc/Hz @ 10 kHz offset), reasonably wide locking ranges (4-20 MHz) and low levels of amplitude modulation (96%-99%). Such pulse characteristics are compared with those achieved by the subharmonic hybrid mode-locking scheme, where stabilization is realized by injecting electrical signals at subharmonic frequencies of the laser´s resonant frequency. It is shown that subharmonic hybrid mode-locking is only effective at low subharmonic numbers (2-6), while subharmonic synchronous mode-locking can be realized with much larger subharmonic numbers. It is also revealed from the comparison that while subharmonic hybrid mode-locking scheme is simple and cost-effective approach for the generation of high-frequency signals from semiconductor lasers, subharmonic synchronous mode-locking scheme can offer pulse trains using very low-frequency driving electronics with superior performance such as larger locking ranges, and lower levels of phase noise and amplitude modulation