Numerical Insights into the Pulse Instability in a GHz Repetition-Rate Thulium-doped Fiber Laser

We report on the pulse instability in GHz repetition-rate thulium-doped fiber laser from the numerical perspective, hinging on a combined physical model that incorporates rate equations of rare-earth ion energy levels and nonlinear Schrodinger equation. In terms of this more explicit model, we predict two pulsating regimes, i.e., gain- and soliton-induced pulsations. Further, the study also reveals a novel type of pulse instability in a continuous wave mode locking (CWML) regime, in which the pulse-intensity fluctuates as well as a notable pedestal of radio-frequency spectrum arises. The proposed modeling can be universally applied, and the numerical results are able to offer reliable guidance to design high repetition rate mode-locked fiber lasers.