Dynamics of optical backward-injection-induced gain-depletion modulation and mode locking in semiconductor optical amplifier fiber lasers.

The optical gain-depletion-induced mode-locking dynamics of a semiconductor optical-amplifier-based fiber ring laser (SOAFL) backward injected by a purely sinusoidally modulated or digitally encoded distributedfeedback laser diode are theoretically and experimentally demonstrated. The effect of gain depletion and waveform on the mode-locked pulse width, pulse shape, and power of the SOAFL are interpreted from theoretical simulations. The shortest pulse width of 12 ps can be generated from an optically sinusoidal-wave-modulated SOAFL. By backward injecting the SOAFL with a digitally encoded optical signal of adjustable duty cycle, one can observe the optimized gain depletion time of 400-600 ps required for mode locking the SOAFL.