Theoretical Study On Multi-process Quadratic Nonlinear Frequency Conversions Of Femtosecond Optical Pulses

In recent years, femtosecond optical frequency combs have had a revolutionary impact on many application fields. However, these fields have also presented more requirements about optical combs, especially on wavelength range. Frequency conversions based on second-order nonlinear effects such as optical parametric oscillation(OPO),second-harmonic generation(SHG) and sum-frequency generation(SFG), etc. are effective methods to broaden the optical comb frequencies and realize a coverage at all wavelengths. This dissertation mainly researches on the nonlinear frequency conversion processes for generation of optical frequency comb at particular deep ultraviolet wavelength based on a high power Yb fiber laser pumping source for applications of precision frequency comb spectroscopy.A nonlinear envelope equation(NEE) method was employed in this thesis for simulation of ultrashort optical pulses' evolution in quasi-phase-matching(QPM)nonlinear crystals with using split-step Fourier method and Runge-Kutta scheme in matlab programming. Superior to traditional coupled three/multiple wave equations, NEE presented results more accurate and truthful about the complicated nonlinear interactions in QPM crystals, especially for broadband and multi-process cases.In this thesis, four schemes were proposed for generating 234 nm deep UV comb under the pump of a high power femtosecond Yb fiber laser with 20 W average power,250 MHz repetition rate, 100 fs pulse duration and 1040 nm central wavelength. For each scheme, both traditionally designed cascaded periodically poled LiNbO3 and aperiodically poled LiNbO3 designed by simulated annealing algorithm were studied respectively combined with NEE analytic method. Comparative study results showed that the scheme,in which the 1040 nm pump pulses are firstly converted to 1404 nm in a parametric process,and then frequency tripled in a third-harmonic generation process to 468 nm in a multi-purpose QPM LiNbO3 crystal, and finally frequency doubled to 234 nm, has the highest efficiency.