Investigation On The Thermal Effect In BBO Crystal By Low-power Pumped Frequency Doubling

When nonlinear crystals are used for optical frequency conversion, crystal thermal effects would result in a lowering of conversion efficiency. Parts of the light energy will be absorbed by a nonlinear crystal which causes the rising of temperature in the local region where the light passes. With fixed pumping power and external cooling conditions, a relatively stable temperature distribution would be formed in the crystal, and the rising of the local temperature will lead to an inhomogeneous change of the refractive indices, which then will ruin the phase matching condition, so further worsen the output beam quality and cause thermal lens effect to damage crystals by self-focusing in a worse case.Firstly, thesis briefly introduced the physical-chemical properties and optical properties of the BBO crystal (the fundamental wavelength of laser is 1064nm), especially the phase-matching of frequency doubling, walk-off effect and acceptable temperature. The influence of temperature on the phase matching could be reflected preliminarily by the acceptable temperature, so the acceptable temperature of the type-I SHG of BBO crystal was calculated when the aperture length is equal to crystal length, which provides a reference for a following study. Subsequently, according to the Maxwell's equations, the field coupling wave equations were rederived using Kleinman, slow-varying and paraxial approximation conditions which established the basis of a temperature-field coupling model.Secondly, according to heat transfer equation, the temperature field coupling equations were established in isotropic and anisotropic media respectively, and a numerical solution of the temperature field equations of isotropy, and solving method of anisotropy are presented. In the case of isotropy, the numerical Fourier transform method was detailed, and by some theoretical assumption an algorithm of Mathematica for the coupling equations was established. Finally, the frequency doubling process was simulated using the software of Mathematica, the amplitudes and conversion efficiencies of the fundamental and harmonic beams, and temperature distributions were obtained, together with the time evolution of temperature and light field. The influence of the pumping power, crystal length and the wavelength of incident laser on the conversion efficiency and on temperature field was analyzed. Through a compensation for the phase mismatch, it was found that the original conversion efficiency can be restored. The results finally showed that the temperature distribution wields a great influence on the conversion efficiency and beam quality in a complicated pattern. Then a compensation for the phase mismatch induced by temperature can provide an effective tool to rescue the reduction of conversion efficiency which thus should be considered in practice.