Mechanism And Influence Of Crystal Thermal Effect In Optical Parametric Process

Optical parametric oscillation technology provides a effective way to realize highperformance,high-power,high-beam-quality,high-stability and wide spectrum range continuous tinning laser output.Nonlinear gain medium is the core device for optical parametric process.MgO: PPLN crystal is one of the most important materials in the field of optical parametric oscillator because of its excellent optical properties and excellent spectral characteristic.In recent decades,the output power of the mid-infrared optical parametric oscillator has been refreshed all the time,and the maximum output power has reached nearly 100 watts.However,with the increasing of pump power,the thermal effect of the nonlinear crystal becomes more and more obvious,which seriously affect the performance of optical parametric oscillator.In this paper,the influence of crystal thermal effect on optical parametric process is studied,which provides some guidance for the realization of high-power and high-beam-quality mid-infrared laser output.The main research contents are as follows:1,The development of high-power optical parametric oscillator is introduced.With the increasing pump power,intra-cavity signal light power and idler light power become higher,which leads to negligible temperature gradient in crystal.Due to the strong dispersion characteristics of MgO: PPLN,and optical parametric process is very sensitive to the refractive index of parametric light,the effect of crystal heat accumulation on optical parametric process can not be neglected.In high power pump experiment,the beam quality of mid-infrared output laser has been significantly degraded.2,Based on fundamental theory of second-order nonlinearity,the nonlinear wave equation is deduced by simplification and approximation,and then three wave mixing coupled wave equations are deduced.The QPM quasi-phase matching method is briefly introduced.Compared with the traditional phase matching method,its nonlinear transformation efficiency can be improved remarkably.OPO gain characteristics and SRO pump threshold and conversion efficiency are analyzed under small signal approximation condition.3,Combined with MATLAB numerical analysis software and COMSOL finite element analysis software,SRO working model and crystal hear transfer model are established.In low pump power,special signal light spectrum constitution is analyzed.In high pump power,crystal temperature gradient distribution is analyzed,which found that idler light increasing power is the main reason for nearly straight upward temperature rise in crystal.Introduced the temperature into SRO working model,its found that temperature rise can not only broaden parametric light spectrum,but also make its central wavelength shift and effect parametric gain.In the end,crystal heat accumulation leads to wavefront distortion in parametric light.Due to the strong dispersion characteristics of MgO:PPLN,several degrees temperature rise will cause a big OPD,then lead to beam quality deterioration.The research provides some guidance for MgO:PPLN based high-power and highbeam-quality optical parametric oscillator.