Single-mode giga-hertz optical pulses generated by a self-modulation of the relaxation oscillation frequency in a semiconductor laser with subharmonic polarization-rotated optical feedback

In this research, single-mode 3.76 GHz optical pulses were generated in a Fabry-Perot type single-mode semiconductor laser with polarization-rotated optical feedback (PROF) at a round-trip feedback distance of 50.8 cm, corresponding to a feedback frequency of 0.59 GHz. Experimental results and numerical simulations revealed that the pulse generation mechanism involved a self-modulation of the laser's relaxation oscillation frequency so that the oscillation frequency approximated to an integer multiple of the PROF round-trip feedback frequency. This effect is very different from similar experiments reported by many researchers before, in which the laser's output was amplitude modulated by the feedback frequency and an ultra-short feedback distance was required to generate giga-hertz optical pulses. Investigations about some characteristics of the self-modulation mechanism will be reported in the paper.