Generation of pulses with variable multi-GHz repetition rate in a SOA-based fiber laser with gain instability

We demonstrate what we believe to be a novel approach for tuning of the pulse repetition rate in a semiconductor optical amplifier (SOA)-based fiber laser by exploiting gain instability induced by self-sustained cross-gain modulation (XGM). By adjusting the relative delay in the negative feedback loop, stable high-contrast pulse trains are generated with repetition rates tunable from 1.85 GHz to 6.2 GHz and beyond, without the need for active modulation or saturable absorption. Experimental results agree well with theory, confirming the impact of feedback loop length difference on pulse dynamics. This simple, compact, and energy-efficient design shows promise for tunable high-repetition-rate pulsed sources in telecommunications, metrology, and sensing.