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We present numerical simulations concerning mode locking of continuous-wave (CW) picosecond and femtosecond lasers based on quadratic polarization switching inside a frequency-doubling nonlinear (NL) crystal. Our model calculates the pulses steady-state temporal profile for various values of phase and group velocity mismatches between fundamental and second-harmonic pulses and group velocity dispersions in the NL crystal. It takes also into account on the characteristics of the laser medium and of the resonator assuming perfect second order dispersion compensation for one cavity round trip. We report on successful comparison between the numerical results and previously published experiments concerning a CW picosecond laser. Then we apply our numerical model to the case of a conventional Ti:sapphire laser predicting as short as 195 fs duration pulses using a type II BBO crystal of 400 μm length.