Analysis of factors influencing self-Q-switching of erbium-doped fiber

Abstract

Self-Q-switched erbium-doped fiber (EDF) lasers combine a high damage threshold, a compact, miniaturizable structure, and the ability to generate stable Q-switched pulses. However, in communication systems, the self-Q-switching behavior of gain fibers can cause optical damage. To investigate these issues, we investigated high-gain erbium-doped fiber used as a saturable absorber (EDF-SA). Experiments show that, within a specific length range, increasing the EDF-SA length significantly enhances Q-switching performance: It increases the maximum single-pulse energy and reduces pulse width—improvements that are important for high-performance all-fiber Q-switched lasers. We attribute this improvement to the greater pump-light absorption efficiency of longer absorbers. Further experiments show that, for a fixed EDF length, peak power and single-pulse energy increase with pump power up to a point and then decline. This reversal results from amplified spontaneous emission at high pump powers, which depletes stored energy before Q-switching occurs. Moreover, inserting a non-Q-switched EDF segment effectively suppresses self-Q-switching in high-gain EDFs, providing a practical means to protect communication devices from optical damage. We attribute the ability of EDFs to produce Q-switched pulses primarily to ion-pair interactions and fast carrier recovery dynamics rather than to the saturable-absorption curve alone. Finally, based on measured data, we hypothesize that the observed self-pulsing originates from relaxation oscillations, and it was observed that stronger reabsorption leads to a lower relaxation oscillation frequency, while higher pump power results in a higher relaxation oscillation frequency. These results deepen the understanding of the physical mechanisms underlying self-pulsing in gain fibers.