Generation of the second harmonic of ultrashort pulses as a result of multiple phase-matched interaction in a nonlinear crystal

A study was made of the interaction of ultrashort second harmonic pulses in a nonlinear crystal placed inside a resonator with a train of ultrashort pulses from a YAG:Nd laser with active mode locking. It was found that at low radiation intensities, insufficient for significant phase modulation in the course of frequency conversion, it was possible to generate strong ultrashort pulses of the second harmonic as a result of multiple interaction of the harmonic radiation with a train of ultrashort pump pulses in an external resonator containing a nonlinear crystal. The experimental setup made it possible also to obtain information on the correlation between the phases in the laser radiation spectrum over periods which were a multiple of the period of intermode resonator beats.