KDP晶体激光损伤机理研究

大口径KDP晶体是唯一可用作激光约束核聚变(ICF)中Pockels盒和倍频器件的晶体材料,但是低的抗激光损伤阈值使其应用受到了限制.本文从电子-空穴对的产生及稳定机制、光伤实体的本质等方面总结了多年来人们对KDP晶体激光损伤机理的研究进展,尤其从多光子电离、碰撞电离、激光加热三个方面定性阐述了电子-空穴对的产生机制, 而电子-空位对的稳定机制是探讨光损伤的关键步骤.另外从晶体生长过程及后处理两个方面初步讨论了提高光伤阈值和光学均匀性的途径.     

硼酸对KDP晶体光学特性的影响

生长了不同硼酸掺杂浓度的KDP晶体,用超显微镜了晶体内部的散射颗粒,检测了晶体的光损耗和透过率特性。实验结果表明,较高浓度的硼酸对KDP晶体的光学均匀特性有较大影响,并使晶体具有显著的旋光性和波片效应。     

双掺杂TGS晶体——LUTGS

分别在58～54℃和45～38℃温区生长了完整的LUTGS单晶.揭示了两种晶体的生长习性.通过对两种晶体电滞回线、介电、热释电性能的测量,发现两温区生长的LUTGS单晶的热释电材料优值M(P/ε)无大差异,但都比纯TGS有显著提高.     

Phase matching characteristics of deuterated ammonium dihydrogen phosphate crystals

Refractive indices for crystals ammonium dihydrogen phosphate(ADP), 30% deuterated ADP(DADP), 50%DADP, and 70% DADP are measured from 253 to 1529 nm with 5 × 10-6accuracy. Numerical fits to modified double-pole Sellmeier equation are made. Second-harmonic generation, third-harmonic generation phase matching(PM) angles, and noncritical PM(NCPM) wavelengths are calculated using the Sellmeier parameters. The deuterated crystals show smaller PM angles than pure crystal. Fourth-harmonic generation process can be realized by DADP in smaller deuterium content than deuterated potassium dihydrogen phosphate(DKDP).The measured NCPM wavelengths are consistent with the calculated value. PM characteristics are compared between DADP and DKDP.     

Growth Characteristics and Mechanism of Surface and Bulk Damage in KDP and DKDP Crystals

By testing the number increase and size growth of surface and bulk laser induced damage in KDP and DKDP crystals, we observe different growth characteristics of surface and bulk damage under multiple 355nm laser irradiations. The size of the surface damage grows exponentially, but that of the bulk damage does not grow. In contrast, the bulk damage number increases, but that of surface damage does not increase significantly. We attribute the differences to the different formation of the damage initiators and the different damage testing volumes.     

不同方法DKDP晶体生长和损伤阈值

生长方法是影响DKDP晶体生长和光损伤阈值的一个重要因素。分别采用传统降温法和点籽晶快速生长法,利用同种原料从氘化程度为85%的溶液生长了DKDP晶体,并选取部分Ⅱ类3倍频晶片进行3倍频光损伤阈值和透过性能测试。实验表明,不同生长方法对DKDP晶体的损伤阈值以及DKDP晶体的生长速度的影响效果正好相反,即由于生长溶液过饱和度的差别,点籽晶快速生长法晶体生长速度为传统方法的10倍,但晶体损伤阈值下降了50%,且紫外波段透过性能下降明显。     

A Thermal Approach to Model Laser Damage in KDP and DKDP Crystals

A thermal model is considered in order to better understand the mechanism of laser induced damage in KDP and DKDP crystals. We demonstrate that the expressions of pinpoint density and damage probability, predicted by the thermal model, are consistent with the experimental data. We also discuss the effect of particle interaction on the thermal model.     

Cascaded third-harmonic-generation converter based on a single ADP crystal

Based on a specially designed optical structure, an efficient cascaded third-harmonic-generation(THG) output of a 1064-nm, pico-seconds pulse laser is successively realized by using an NH_4H_2PO_4(ADP) crystal that acts as the secondharmonic-generation component and sum-frequency-generation component. The maximum THG output is 1.61 mJ, and the highest conversion efficiency from 1064 nm to 355 nm reaches 35%, which are obviously superior to the results obtained using a KDP crystal under the same circumstance. The further phase-matching analysis indicates that this THG configuration of ADP crystal can be applied to various fundamental wavelengths in a range of 1 μm–1.1 μm. Compared with the previously reported KDP THG converter, which is based on a similar principle, the present ADP THG converter is favorable for large-energy, high-efficiency operation because of the larger effective nonlinear optical coefficient deffand higher laser damage threshold.     

Na+对KDP晶体生长的影响

本文利用传统的降温法和"点籽晶"快速生长法在不同Na+掺杂浓度的溶液中生长KDP晶体,定量研究了Na+对KDP晶体生长的影响.实验发现:Na+的存在降低了溶液的稳定性,致使KDP晶体柱面容易扩展.Na+的存在对KDP晶体的光学性能基本没有影响.     

不同波长三倍频DKDP晶体的激光损伤

采用传统降温法,利用高纯原料从氘化程度为80%的溶液生长了四方相磷酸二氘钾(DKDP)晶体,并按Ⅱ类三倍频方式切割晶体。三倍频用DKDP晶体的最大问题在于其抗光伤阈值低于KDP晶体,严重限制了激光输出的能量密度和晶体使用寿命。考察了不同波长下三倍频DKDP晶体的损伤阈值,以及激光退火效应。实验表明,激光退火对于DKDP晶体的损伤阈值有显著的提升作用,基频、倍频、三倍频的提升效果分别达到1.4,1.9,2.7倍,是改善DKDP晶体抗光伤能力的有效途径。