Phase Shifts in Gain-Switched Semiconductor Laser Subharmonic Pulse Trains

Gain switching of semiconductor lasers by strong sinusoidal current modulation can generate Period Two (P2) and Period Three (P3) sub-harmonic pulse trains. It is known that these pulse trains can be interrupted by bursting events, for the P2 pulse train predominantly consisting of short segments of relatively weak pulses at the modulation frequency (a P1-like state) before the system returns to the dominant P2 state. Similarly, for systems operating in the P3 state, intermittent switching yields combinations of P1- and P2-like pulses. To better understand these experimental observations, we perform numerical simulations using a standard model coupling the circulating optical field to the carriers of the gain medium and then compare the results with our experimental measurements on a single-mode distributed feedback laser. The simulations show that low optical power events typically precede the interruptions of the dominant sub-harmonic pulse trains. During these events, stochastic sources, such as spontaneous emission noise, which interact with the circulating optical field are found to dominate the deterministic changes to the calculated trajectory. With output in either P2 or P3 dominant pulse trains, we numerically and experimentally demonstrate that the induced phase fluctuations follow the statistics of classical telegraph noise for the P2 state, and a three-state variant for the P3 state.