Cascaded Nonlinear Effects In Periodically Poled Lithium Niobate Crystal

Recently, with the development of pulse lasers, nonlinear optical frequency conversion of broadband lasers has been attracted much attention. However, broadband pulse is difference from long pulse and continuous wave. Its high incident intensity would inevitably cause high-order nonlinearity, which has little effect on the frequency conversion of long pulse and continuous wave. The spectrum of broadband pulse is very wide, not only the central wavelength but also other wavelength components should be considered. Therefore,the frequency conversion of broadband pulse is very complicated.This dissertation focuses on the research of Cascaded Quadratic Nonlinearity( CQN)process and Cascaded Third Harmonic Generation( THG). The main content is described as follows:1. Theoretical and experimental studies of CQN process. The characteristics of spectrum broadening, spectral shift and energy transfer were studied. Coupled equations of CQN process was deduced and numerically solved using Split-step Fourier Method. The contribution of incident intensity, phase mismatch, group velocity mismatch, group velocity dispersion and crystal length were analyzed. Using the periodically poles lithium niobate( PPLN), multi-peak spectra( 300 nm) with the typical SPM-like characteristics were obtained experimentally by changing the incident intensity and wavelength. Spectral red shifts( 50 nm) and blue shifts( 60 nm) were observed by changing the incident FF wavelength.2. Theoretical and experimental studis of cascaded THG. The physical model of the cascaded THG was established and the explicit formula solutions were derived. The effects of parameters, such as the walk off length, incident pulse width, phase mismatch, incident intensity and the propagation distance were analyzed. The threshold value of the incident pulse width was about 20 fs when the walk-off effect was considered. The obtained results showed that the CE of cascaded THG could be adjusted by phase mismatch, i.e. △k SHG and△k THG. The walk-off effect could be eliminated to some extent by optimizing pump intensity.Using the multiple-grating periodically poled Mg O-doped lithium niobate( Mg O: PPLN),pumped by 100 fs optical parametric amplifier pulses, the maximum CE was about 10.8%.With the approximately phase matching condition, the temperature tuning bandwidth was about 120 ℃, and the THG spectral width was about 9 nm.