Why do output pulses split in actively Q-switched fiber lasers?

Pulse evolutions of a typical actively Q-switched fiber laser are numerically investigated by using the traveling wave method. Unlike those conventional actively Q-switched solid-state lasers, the output pulses in a few actively Q-switched fiber lasers often tend to break into multiple peaks in Q-switched envelopes. In this paper, the multipeak phenomena of output pulses are analyzed, and the mechanism for these phenomena is detailed. It is shown that the first transient pulse in the cavity is initiated by a quick switching of the Q-switching element, which actually injects part of amplified spontaneous emission (ASE) into the cavity in the form of a pulse; and then the formation of multipeak structures is related to the complicated evolution of the switching-induced perturbation, and is determined by the rise time of switching and conditions of photon density and population inversion during the Q switching. It is also shown that under certain conditions of cavity, pump and switching, a split-pulse output can turn to a single-pulse output. The new understanding of laser dynamics and the precise explanation for the pulse splitting in actively Q-switched fiber lasers are given in this paper.

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