The paper contains experimental results of three-frequency parametric conversion in the field of femtosecond laser pulses in periodically poled lithium niobate crystal. The spectral distribution and the power of radiation at the periotically poled lithium niobate output pumped by Ti:sapphire laser were measured. Special attention is paid to realization of parametric amplification at low frequency pumping. In order to interpret the results obtained we used simultaneous phase matching condition of two coupled three-frequency processes.

Paper contains experimental results and theoretical estimations of quasi-phase matched second harmonic generation in PPLN, where the nonlinear grating vector is noncollinear to wave vectors of fundamental radiation and its second harmonic.