The study on the gain of signal pulse in gain fibers and its application in simulation of mode-locked fiber lasers

The effect of the signal pulse on the population densities of upper-level particles of doped ions in gain fibers is studied. The population densities of upper-level particles decrease gradually after encountering the signal pulse in both the two-level system and the three-level system, and the decreasing degree is related to the power of the pulse. Since the power of the signal will change greatly in a short time in a pulsed laser, the time dependent rate equations are more suitable for describing the interaction between the pulse signal and doped particles than the steady state rate equations. Based on this, a simulation method of passively mode-locked fiber lasers, in which the gain of the signal is calculated by time dependent rate equations, is proposed. An erbium doped mode-locked fiber laser at 1564 nm is simulated as an example, and the pulse width and spectral width in the simulation results are close to the experimental results. By adding noise to the initial pulse, the stability of the model is verified, and the mode competition between different modes in the process of mode locking can be studied. This method can not only provide a reference for building mode-locked fiber lasers, but also provide a method for studying the mechanism of mode locking.