Research On Propagation Characteristics Of Ultrashort Pulse In PPLT

Periodically poled ferroelectric crystal (PPFC) is a new kind of microstructurecrystal which develops rapidly since1990s with poling period of the order of a fewmicrometers. PPFC can modulate the relative phase of the interactive waves byperiodically reversing the sign of quadratic nonlinear susceptibility, and it is widelyused to generate tunable wavelength coherent source, from ultraviolet to THz. Thenonlinear phase shift due to quasi-phase matched cascading quadratic nonlinearitycan counteract the linear diffraction during light propagation in the material, thusspatial soliton can be produced. The second-order nonlinearity allows solitonexcitation at low power threshold, and this is a very important advantage forpractical applications. Having particle-like behavior in propagation and interactions,optical soliton offer great potential for performing ultrafast all-optical switching oreven optical computations. In this dissertation we use femtosecond laser pulses asinput source, the characteristics of the spatial soliton and frequency conversionwhen pulses propagation in periodically poled lithium tantalite (PPLT) has beeninvestigate.In chapter2the coupled wave equations that describe second order nonlinearityare derived on the basis of Maxwell’s equations. The nonlinear phase shift owing tophase mismatch and the excitation progress of cascading quadratic spatial solitonare also discussed theoretically. The normalized equations governing spatial solitonin the case of quasi-phase matching is deduced. The transverse profiles offundamental (FF) soliton and second harmonic (SH) soliton in the simplest schemeare simulated.In chapter3the experiment of spatial soliton is performed. The transverse profileand power distribution of FM and SH soliton are measured. The dependence ofsoliton evolution on input power and crystal temperature is illustrated. The thresholdto excitation soliton is detected. The effect of input beam profile on the generationof multisoliton is discussed.In chapter4, second harmonic generation in QPM PPLT is investigated. The thirdorder QPM poling period is calculated when fundamental wavelength is800nm andtemperature of the crystal is188℃. The tuning between temperature and conversionefficiency is analyzed theoretically and measured experimentally. The theory of Cerenkov emission in nonlinear optics is reviewed. Experiments on Cerenkovemission are conducted, and the Cerenkov angle is calculated.