Research On Thermo-optic Coupling Properties Of Several Typical Double-frequency Infrared Nonlinear Crystals

Since nonlinear optical phenomena were found in the late 1960 s,the characteristics of infrared nonlinear crystal have been studied widely.The research on the optical frequency conversion of infrared nonlinear crystal attracted plenty attention.In this paper,the thermo-optic coupling characteristics of infrared nonlinear crystal during doubling frequency of 9.6 ?m CO2 laser were investigated.A thermo-optic coupling model of a themal anisotropic infrared nonlinear crystal was founded.Based on the model,thermo-optic coupling characteristics of GaSe,ZnGeP₂ and AgGaSe₂ crystals were studied by using numerical simulation.The efficiency of double-frequency and the evolution process of temperature distribution were simulated.The specific work is as follows:1.A thermo-optic coupling model of a themal anisotropic infrared nonlinear crystal was founded.Considering the effect of beam walking off and temperature anisotropy,the model was founded.In the model,the heat conduction equation was solved using semi-analytical method.Second harmonic generation coupling equation was solved using numerical Fourier transform method.The relation of optical field and teamperature field was studied by calculating anisotropic thermal conductivity using iterative approach.2.Numerical simulation of the thermally anisotropic GaSe crystal thermo-optic coupling performance for second harmonic generation was conducted.Beacause GaSe crystal thermal anisotropy is large,and thermal conductivity in the horizontal is small,the temperature gradient cannot be neglected.While the heat source in the longitudinal axis is uniform,and heat conduction is large,so the temperature distribution is relatively flat.Numerical simulation implied,under weak cooling conditions,the boundary temperature increases,more heat energy stored in the crystal.Under the strong cooling conditions,the heat energy resulting from boubling frequency can escape easily,the heat in crystal is relatively less,time taken by teamperature becoming stable is short.Under the two conditions,after stable time,the temperature distribution is approximately the same.Beacause light absorption coefficient of GaSe crystal in second harmonic generation is larger,incrasing temperature cause refractive index change,then the phase matching conditions is destroyed,and the phase mismatch is serious,second harmonic generation light greatly reduced.Under the condition of weak refrigeration,the conversion efficiency of the crystal will decrease with the increase of temperature rapidly.After the stable distribution is achieved,the conversion efficiency is almost zero.The double frequency and double frequency reversal process is obvious.It is not effective to attempt to restore the double frequency conversion efficiency by controling temperature,even under strong cooling conditions.3.Numerical simulation of thermo-optic coupling characteristics of ZnGeP₂ crystal during doubling frequency was conducted.ZnGeP₂ crystal thermal anisotropy is weak,and thermal conductivity is high,so the temperature gradient of crystal in the transverse direction is relatively small,when longitudinal heat source is uniform,temperature distribution is relatively flat.Under the condition of weak cooling,the time taken by temperature reaching a stable distribution is 230 times longer than that of the strong cooling condition.The maximum temperature rise is 34 times longer than that of the strong cooling condition.Under two conditions,the temperature distribution of the stable time was not significantly different.Under strong cooling condition,the temperature field of ZnGeP₂ crystal becomes stable,the phase mismatch is not obvious,the conversion efficiency is not changed,and the output harmonic beam quality is better.Under the weak cooling conditions,ZnGeP₂ crystal absorbed light mainly fundamental frequency light,second harmonic light reduced rapidly.The phase mismatch is obvious,cumulative phase mismatch caused the double-frequency inversion,and conversion efficiency is almost zero when the temperature reaches stability.4.Numerical simulation of the AgGaSe₂ crystal thermo-optic coupling characteristics in the second harmonic generation was conducted.Thermal conductivity of AgGaSe₂ crystalis small,and the absorption coefficient is relatively small,the thermal anisotropy is weak.Numerical simulation shows,under the weak cooling conditions,the transverse temperature gradient is relatively small,and the longitudinal temperature distribution is uniform.When power is constant,with the increase of the beam waist radius,the temperature of the distribution of the stable time is extended;stable temperature distribution is roughly the same time.When waist radius is invariable,with the increase of fundamental frequency light power,phase mismatch becomes more sensitive,double-frequency light reduced rapidly.When the temperature reaches the stable distribution,the cumulative phase mismatch causes the second harmonic generation and second harmonic generation reversal process in the crystal,and the conversion efficiency is reduced to zero.5.Comparative analysis of thermo-optic coupling in Mid-infrared GaSe,ZnGeP2,AgGaSe₂ was conducted.Under the condition of strong cooling,the phase mismatch of the ZnGeP₂ crystal is not obvious,the conversion process and temperature distribution is almost not affected,which makes it has the highest conversion efficiency.The absorption coefficient of GaSe crystal is relatively large,teamperature rise rapidly,and phase mismatch is obvious,so the double-frequency efficiency is lower.Under the weak cooling conditions,temperature distribution of three kinds of crystal changes rapidly with time.Temperature field distribution on the longitudinal axis direction is uniform.The conversion efficiency of AgGaSe₂ crystal is the lowest.The thermal anisotropy of GaSe crystal is stronger.The phase mismatch in AgGaSe₂ and GaSe is obvious,and the second harmonic generation and the second harmonic generation reversal process will appear in the crystal.