Multiplet cascade in a semiconductor laser with optoelectronic feedback

We report experimentally and theoretically a dynamical scenario involving the formation of pulse multiplet structures, i.e., configurations of two, three, and more pulses per round trip, in a semiconductor laser with positive optoelectronic feedback. The delayed feedback loop includes a cascade of two band-limited amplifiers with nonlinear saturation of sigmoid type. The multiplets appear subcritically in the vicinity of the lasing threshold, resembling gain-switched operation, and evolve with the injection current.