Study on the high-power, high-repetition-rate and multi-wavelength femtosecond laser system
高功率高重复频率多波长飞秒激光系统的研究

High-power and high-repetition-rate multi-wavelength femtosecond pulses are obtained by the nonlinear frequency up-conversion of large mode area photonic crystal fiber femtosecond system. Effects of focal length on the transverse mode of second harmonic are theoretically analyzed and experimentally demonstrated, showing that the longer the focal length, the better the mode quality. Under the condition of fundamental average power of 21.8 W, pulse duration of 110 fs and repetition rate of 50 MHz, generation of the second, third and fourth harmonics are achieved at the wavelengths of 520, 347, and 261 nm, with the maximum average power of 10.5, 4.7, and 2.14 W, respectively. The conversion efficiencies of frequency doubling and frequency tripling are 48.2% and 21.6%, and the conversion efficiency of frequency quadrupling from the second harmonic is 20.4%. Pulse duration of the fourth harmonic pulse is 408 fs, as measured by the cross-correlation technique.