Microwave Frequency Comb Generation by Gain-Switching Versus Relaxation Oscillations

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current J of the laser leading to microwave combs in two distinct regimes. For J close to the threshold current Jth, the pulse repetition rate f(rep) is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching and leads to comb spacings in the tens of MHz range. In contrast, for J similar or equal to 2 Jth, the repetition rate f(rep) is observed to be related to the relaxation-oscillation frequency f(RO). The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy.