Femtosecond nonlinear polarization evolution based on cascade quadratic nonlinearities

We experimentally demonstrate that one can exploit nonlinear phase shifts p roduced in type I phase-mismatched second-harmonic generation to produce in tensity-dependent polarization evolution with 100-fs pulses. An amplitude m odulator based on nonlinear polarization rotation provides passive amplitud e-modulation depth of up to similar to 50%. Applications of the amplitude a nd phase modulations to mode locking of femtosecond bulk and fiber lasers a re promising and are discussed. (C) 2000 Optical Society of America.