We numerically investigated temporal characteristics of in-band-pumped gain-switched thulim-doped fiber lasers (GSTDFL). Our theoretical model was based on rate equations and optical power propagation equations including both temporal and spatial variations. This model was validated by comparing the numerical results with results from previous experimental work. Due to the lack of a clear definition of the numerical threshold of gain-switched fiber lasers in the literature, we first defined it in our calculations as the status obtained when the variation of the widths of the generated pulses became nonlinear. Then, we used the model to explore the temporal characteristics of such lasers at and above the laser threshold. We found that the threshold pump energy would increase when the pump pulse width was longer than 500 ns and the pulse widths of the laser pulses at the threshold were kept the same at different pump widths. Above the threshold, by calculating the temporal shapes of the laser pulses under different pump conditions, we studied the origin of the trailing spike phenomenon in the in-band-pumped GSTDFLs and determined the parameter space of the pump conditions that could generate the pulses having a Gaussian-like temporal shape and free from the trailing spikes. We further investigated the variation of the laser pulse widths at different laser configurations and discussed their implications. We believe these numerical results can be referenced qualitatively by the design and optimization of the in-band-pumped GSTDFLs and other in-band-pumped gain-switched fiber lasers, such as those with and dopants.
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