Passively mode-locked optically pumped vertical external cavity surface-emitting lasers (OP-VECSELs) have unique properties, such as excellent beam quality, high-average output power and high repetition rate. Many applications, including frequency comb and supercontinuum, require pulses in the femtosecond regime. A numerical model in the femtosecond regime is essential to understand the pulse formation mechanism. In this paper, we present a numerical model of passively mode-locked VECSELs in the femtosecond regime. We analyze the influence of gain parameters, such as small-signal gain, saturation energy, and the gain bandwidth on the pulse characteristics. The results of the simulation can provide a direction for designing the gain chip.
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