Effect of crystals’ surface shape distortion on conversion efficiency of third harmonic generation
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摘要: 在惯性约束聚变（ICF）领域，为了满足物理实验需求，需要采用短波长的紫外激光打靶。目前，国内外高功率固体激光装置普遍采用谐波转换的方式来获得三倍频紫外激光。高效的频率转换必须满足相位匹配条件，而晶体的反射面形畸变过大对准直精度和倍频效率均匀性都会产生不利影响。通过实验验证了晶体准直光斑的质量退化主要来源于晶体因夹持和重力导致的反射面形畸变，证明了改进晶体的夹持方式可以有效改善晶体面形，提高准直精度和准直光斑质量，并显著提升三倍频转换效率。Abstract: In the field of inertial confinement fusion (ICF), UV laser can effectively improve the beam-target coupling efficiency and suppress the hydrodynamic instability of laser plasma interaction. Using large aperture KDP crystal or KD*P crystal to triple the frequency of laser created by high power solid-state laser facility (wavelength is 1 μm) is an effective way to obtain high power UV laser output. At present, most high power laser facilities take method of I/II type third harmonic generation. The phase matching condition is required in the third harmonic generation, and the conversion efficiency is easily influenced by the detuned angle of crystals. The quality of the return light spot of crystals’ surface can reflect the state of wavefront distortion, and then evaluate the degree of angle-detuning at the crystal. The factors which create detuned angle are analyzed and judged, within which the surface shape distortion of crystals (PV is more than 30 μm) caused by clamping and gravity is the main part. According to improving the surface shape of crystals (PV is less than 10 μm) in the experiment, the quality of the return light spot of crystals has been significantly improved, and then the conversion efficiency of third harmonic generation has an obvious increase.
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图 2 三倍频效率与晶体失谐角的关系
Figure 2. Relationship between conversion efficiency of third harmonic generation and detuned angle of crystals
图 3 失谐角与面形畸变及波前畸变的关系
Figure 3. Relationship between detuned angle and surface shape distortion，between detuned angle and wavefront distortion
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