Research On Third Harmonic Generation And Pulse Self-compression With All-solid-state Femtosecond Laser

The emergence of ultrafast lasers provides strong and powerful tool for scientific research.Ultraviolet,which combines the characters of short wavelength,high photon energy as well as high time resolution,has important application value in the fields of physical chemistry,biomedicine,and ultra-precision micro-machining.Most of waveband of ultrafast lasers is concentrate on mid-infrared and infraed.Frequency conversion is a commonly methods to generate ultraviolet.However,group velocity mismatch and the third-order nonlinear effects limit the conversion efficiency.Therefore,it is of great significance to carry out this research.In this dissertations,a series of work focused on the femtosecond laser third harmonic generation was carried out.Meanwhile,in order to explore a simple pulse compression scheme,the pulse self-compression due to cascaded second-order nonlinearity was studied theoretically and experimentally.The main research works in this dissertation are summarized as follows.1.Based on self-developed second harmonic generation program,the numerical simulation of ultrafast laser frequency conversion was realized.The coupled wave equations of the frequency doubling process was carried out numerically using the split-step Fourier transform method.The laser parameters and the crystal parameters of the experiment were used.Further,the main factors in frequency conversion,for example group velocity mismatch and phase mismatch,were considered in the simulastion.Using this program,results that agree with the experiment were obtained.2.Based on different femtosecond lasers and nonlinear crystals,an experiment system of third harmonic generation was designed and built to achieve high-efficiency infraredultraviolet light conversion.First,a commercial laser with center wavelength at 1030 nm,pulse duration of 105 fs and the power into the device of 5 W was used as the pump.Using a 1.7-mm long Li B3O5 crystal,a maximum frequency doubling power of more than 3 W was obtained.Through the delay line system,the fundamental laser and second harmonic laser are used to obtain third harmonics laser.A 343-nm ultraviolet pulse with the output power above 1 W and the spectral full width half maximum of 2.9 nm was obtained.Further,based on the above experiment setup,ultraviole pulses were generated by pumping with home-made femtosecond all-solid-state laser at 1050 nm with 98-fs pulse duration.With a longer ?-Ba B2O4 crystal,350-nm ultraviolet pulses with output power of 1.48 W were obtained,corresponding to a conversion efficiency of 20%.3.Experimental research of pulse self-compression was achieved using a Li B3O5 crystal.The experimental setup was similar to that of the previous frequency doubling.The maximum second harmonic generation efficiency was achieved at first,and the phase mismatch was realized by rotating crystal.The spectral full width half maximum and pulse width of the remaining fundamental pulse after Li B3O5 has been recorded.As a preliminary result,the 105-fs laser pulse was compressed to 78 fs.