采用BBO晶体产生266 nm深紫外激光过程中的热特性研究

Thermal Characteristics of 266 nm DUV Laser Generated by BBO Crystal

基于近红外波段1064 nm激光的四倍频过程是产生深紫外激光的有效技术手段之一。然而,非线性晶体在产生深紫外激光的过程中,晶体内部严重的非线性吸收带来的热效应会破坏其相位匹配条件,进而制约深紫外激光功率的提升。为此,在利用偏硼酸钡(BBO)晶体作为1064 nm近红外光四倍频过程的非线性转化介质的基础上,通过对不同匹配温度下的晶体内部温升情况进行研究,发现提高晶体匹配温度可以有效减缓晶体内部由非线性吸收引起的热效应,改善深紫外激光功率输出。最终,在晶体匹配温度为180℃、绿光输入功率为16 W时,获得了重复频率为20 kHz、单脉冲宽度为4 ns、平均功率为2.56 W的266 nm深紫外激光。通过建立BBO晶体非线性吸收理论模型,进一步计算了不同匹配温度、不同紫外激光功率下的非线性吸收系数和归一化色心密度,结果显示,非线性吸收系数与紫外激光功率强度正相关,并且在紫外激光功率均为2.25 W时,相较于60℃,匹配温度为180℃的晶体其非线性吸收系数减小了近63%,色心密度也降低了57%,该研究结果对实现高功率的266 nm深紫外激光的输出具有重要意义。

Objective All-solid-state high-power deep ultraviolet (DUV) 266 nm lasers are widely employed in various fields,including Raman spectroscopy,photo-biological imaging,integrated circuit etching,and precision micromachining,owing to their compact structure,high single-photon energy,and good long-term stability.Thus far,the fourth harmonic generation (FHG) of a 1064 nm laser based on nonlinear crystals,including cesium lithium borate (CLBO) and barium metaborate (BBO),is the most widely adopted method for generating a DUV 266 nm laser.However,its significant nonlinear absorption results in a large temperature gradient inside the crystal,which prevents the complete phase matching of the entire crystal.Consequently,the output power of the DUV 266 nm laser saturates easily in experiments.Although many methods for alleviating nonlinear absorption and eliminating power saturation have been devised,one must comprehensively investigate the thermal characteristics of BBO crystals when they are used to generate highpower DUV 266 nm lasers.Therefore,in this study,the effects of the phase-matching temperature of BBO crystal on heat generation inside the crystal and the output power of a 266 nm laser are first investigated during FHG.Experimental data show that an increase in the phase-matching temperature can effectively alleviate the thermal effect caused by nonlinear absorption inside the crystal,thus improving the output power of the 266 nm laser.In particular,when the phase-matching temperature of the BBO crystal increases from 60℃to 180℃,the maximum output power of the 266 nm laser increases from 2.25 W to 2.56 W.Second,a theoretical model of nonlinear absorption is established to explain the experimental results,and the nonlinear absorption coefficient (βNLA) and normalized color center density at different phase-matching temperatures that affect heat generation in the crystal are calculated using the experimental data.This study provides an instructive reference for generating high-power 266 nm DUV lasers.Methods To achieve a 266 nm DUV laser,an all-solid-state cavity-dumping Nd∶YVO4 1064 nm laser with an average output power,a repetition rate,and a pulse width of 26 W,20 kHz,and 5 ns,respectively,was designed and then fabricated.Subsequently,a nonlinear LBO crystal with non-critical phase matching was employed to convert the fundamental-wave 1064 nm laser into a second-harmonic 532 nm laser,which yielded an average power of 16 W.Next,the heat generated inside the crystal during FHG was measured based on the temperature offsetΔT of the crystal heater when the optimal output power of the 266 nm DUV laser was achieved under different input powers of a 532 nm laser.Subsequently,the heat generated inside the crystal and the output power of the 266 nm laser were compared when the phase-matching temperature of the BBO crystal was 60℃,120℃,and180℃.Simultaneously,a theoretical model of nonlinear absorption was established by analyzing the mechanism of heat generation inside the crystal during FHG,and a two-dimensional heat-conduction equation was obtained.Finally,based on experimental data,the βNLA and normalized color center density at different phase-matching temperatures,which affected heat generation inside the crystal,were numerically simulated using finite-element analysis software.Results and Discussions When the phase-matching temperature of the BBO crystal is 60℃,as the input power of the 532 nm laser is increased gradually from 0 W to 4,8,12,and 16 W to achieve the optimal output power for the 266 nm laser,the temperature of the crystal heater decreases continuously from 60℃to 59℃,53℃,45℃,and 36℃,respectively[Fig.2(a)].In particular,when the 532 nm laser with a maximum power of 16 W is injected,theΔT of the heater is 24℃,at which time the maximum output power of the 266 nm laser is 2.25 W.However,when the phase-matching temperature is increased gradually to 120℃and 180℃,ΔT decreases gradually from 24℃to 22℃[Fig.2(b)]and then to 20℃[Fig.2(c)],whereas the maximum output power of the 266 nm laser increases from 2.25 W to 2.39 W and 2.56 W.The experimental results show that an increase in the phase-matching temperature of the BBO crystal can effectively alleviate the thermal effect inside the crystal and improve the output power of the 266 nm DUV laser.Using the nonlinear absorption theory model and solving the heat-conduction equation of the BBO crystal,when the phasematching temperature of the crystal is 60℃and as the power of the 266 nm laser increases gradually from 0.32 W to 1.24,2.09,and2.25 W,the fitted β_(NLA) increases from 0 to 0.079,0.128,and 0.189 cm/GW,respectively.However,it decreases as the phasematching temperature of the crystal increases.In particular,when the phase-matching temperature is 180℃and the power of the 266nm laser is 2.25 W,βNLA is only 0.07 cm/GW[Fig.4(a)].Meanwhile,compared with the case of 60℃,the color center density inside the crystal with phase-matching temperatures of 120℃and 180℃decreases by approximately 35%and 57%,respectively[Fig.4(b)].Conclusions An experimental setup for the generation of a 266 nm DUV laser based on the FHG of a 1064 nm laser is constructed to investigate the effects of the phase-matching temperature of BBO crystal on heat generation inside the crystal and the output power of a 266 nm laser.Results show a significant amount of heat generated inside the BBO crystal during FHG.However,increasing the phase-matching temperature of the BBO crystal can effectively alleviate the thermal effect of the crystal and improve the output power of the 266 nm DUV laser.Finally,when the phase-matching temperature of the BBO crystal is increased to 180℃,a 266 nm DUV pulse laser with a maximum power of 2.56 W,a repetition frequency of 20 kHz,and a pulse width of 4 ns is obtained.Based on a nonlinear absorption theoretical model,the root cause of heat generation inside the crystal during FHG is explained,and the βNLA and normalized color center density at different phase-matching temperatures that affect the heat generation of the crystal are calculated.The results show that when the power of the 266 nm laser is 2.25 W,the βNLA of the crystal with a phase-matching temperature of180℃is reduced by approximately 63%and the color center density is reduced by 57%compared with the results based on 60℃.Increasing the phase-matching temperature of the BBO crystal is shown to reduce the heat generated inside the crystal during FHG via a reduction in the βNLA and color center density of the crystal,thus improving the output power of the 266 nm DUV laser,which supports the experimental results of this study.