Numerical study of femtosecond pulse generation using quadratic polarization switching mode-locking

Summary form only given. Until now quadratic polarization switching mode-locking (QPS-ML) has not been applied to broad bandwidth laser. This is the reason why we have carried numerical investigation on the QPS-ML capabilities for the generation of femtosecond pulses. We have developed a numerical model that simulates the behaviour of an intracavity doubling crystal acting like a nonlinear phase-plate with an intensity dependent retardation. A rectilinear polarizer was introduced in the cavity resulting in a saturable absorber effect suitable for mode-locking. Starting from a free-running CW regime, we show that the parametric interaction between the different longitudinal modes of the laser cavity can result in a steady state where modal phases are locked at each other. Dispersion effects that are of tremendous importance in the femtosecond regime were introduced. Phase and group velocity mismatches between fundamental and second harmonic pulses were taken into account, as well as group velocity dispersions. Then, assuming perfect compensation of second order overall dispersions for each cavity round trip, we calculated the self-consistent pulse temporal profile that builds up in the resonator. Various parametric configurations were studied for QPS: perfectly phase-matched type II SHG crystal, slightly phase-mismatched type II and type I SHG crystals. The influence of various parameters such as doubling crystal length, laser power level, nonlinear losses on the pulse duration and profile were determined.