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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.

We demonstrate a new technique for the direct, real-time, femtosecond scale temporal measurement based on the conversion of temporal information to the spectral domain. The potential of the method has been experimentally investigated with an optical fiber spectral compressor device, first stretching and up-chirping the pulses in a prism dispersive delay line, and afterwards compensating the induced chirp by means of cross-phase modulation in a single-mode fiber. Spectro-temporal imaging (STI) reduces the problem of high-resolution temporal measurements to standard spectrometry.

An experimental and theoretical study on a new passive mode-locking technique based on a second order non-linear crystal cut for type II SHG is presented. The method exploits the cascading of sum and difference frequency generation to realize an intensity-dependent polarization rotation. A numerical model was developed to predict and optimize in terms of pulse duration, the performance of a laser passively mode-locked by quadratic polarization switching. The influence of the cavity design, the influence of the nonlinear crystal GVD and the benefit of GVD compensation were considered. Predictions are compared with experimental data measured with a diode-pumped Nd:YVO4 laser. Transform-limited pulses as short as 2.8 ps were obtained at 1064 nm with an average power of 670 mW.