高功率激光终端KDP晶体非共线高效三倍频及远场色分离方案数值模拟分析

为满足高功率激光装置对终端光学系统的改进要求,控制3ω光路透射元件厚度以降低激光损伤风险,避免3ω非对称聚焦与色分离元件对靶场调靶产生不利影响,本文利用非共线相位匹配原理讨论了KDP晶体Ⅰ类和Ⅱ类两种和频产生351 nm(3ω)激光及其远场色分离过程.模拟结果表明,室温20?C环境中除目前常用的共线和频外,1053 nm(ω)与526.5 nm(2ω)激光可选择Ⅰ类或Ⅱ类两种非共线和频方式实现高效3ω激光输出并在激光远场实现色分离,且具有足够的高效转换失谐角容宽.计算表明,与Ⅰ类和频类似,Ⅱ类和频也存在一个非临界相位匹配过程,其匹配方向约为θ(3ω)=86.53?.可通过增加晶体厚度克服其有效非线性系数较低的缺点,实现3ω高效输出,失谐角容宽可达±20 mrad.为满足靶场需要,解决非共线角容宽苛刻带来的调节不便,并进一步使光路紧凑,将楔角为12?的熔石英楔板置于倍频晶体之后,ω与2ω激光在熔石英楔板后表面可产生约3.5 mrad分离角.经非共线和频,使用薄透镜即可实现聚焦及色分离.该方案完全满足终端光学系统的改进要求,可作为可靠的备选方案之一.

Numerical simulation analysis of high efficient SFG and color separation in far field in high power laser facility based on noncollinear phase matching by KDP crystal

Asymmetric property of wedge lens in 3ω optical path which is used as frequency separation, and focusing element is considered to be an unfavourable factor for target alignment in inertial confinement fusion (ICF). Furthermore, the thickness of wedge lens in 3ω optical path will lead to laser induced damage inevitably. For the purpose of scheme improvement of final optical assembly, types I and II noncollinear sum frequency generation in KDP crystal at room temperature are discussed based on nonlinear coupled wave theory. As illustrated by simulated result, in addition to type II collinear SFG used in ICF recently, 351 nm (3ω) waves can be generated by type I or II noncollinear SFG process. This method can realize color separations of ω, 2ω, 3ω in far field without asymmetric element such as wedge lens and posses adequate tolerance of matching angle corresponding to the high efficiency conversion. As calculated, for type I SFG, when the noncollinear angle α is in the interval from 0° to 19.99°, phase matching condition can be satisfied in KDP crystal. The noncritical phase matching angle θ ₃ is 90° and the corresponding noncollinear angle α is about 19.99°. The tolerance of mismatching angle is about ± 20 mrad. For type II SFG, the noncollinear angle interval that can satisfy phase matching process is about 0°-13.55°. Like type I SFG, there is also an noncritical solution in type II process whose matching angle is about θ (3ω) = 86.53°. Because of the smaller effective nonlinear coefficient in this case, high efficiency conversion needs about 5 cm thick SFG crystal under 1 GW/cm². Correspondingly, tolerance of mismatching angle is about ± 20 mrad. Because of the harsh tolerance of noncollinear angle between ω and 2ω and for the purpose of compactness of final optical assembly, another method of noncollinear SFG is proposed: a piece of silica wedge with 12° wedged angle is mounted behind the SHG crystal in order to produce a 3.5 mrad intersection angle between ω and 2ω, and after type II noncollinear SFG process, ω, 2ω, 3ω will be frequency separated in far field automatically by using thin lens. The tolerance of incident angle corresponding to high efficient conversion is about ± 1.0 mrad. This scheme can improve the the final optical assembly used recently.