兼容多波长及多脉宽输出的频率转换系统设计

通过对多种频率转换方案的深入对比分析,设计了一套兼容多波长及多脉宽高效输出的频率转换系统。采用"Ⅱ+Ⅱ"类双晶体级联的方式,可以实现"偏振失配"三倍频和Ⅱ类双晶体正交级联二倍频两种模式间的快速切换。通过数值模拟程序优化了晶体构型,结果表明,当两块晶体厚度均为10mm时,可以兼顾低功率长脉冲和高功率短脉冲对效率和稳定性的需求。     

基于焦斑空间频率全域优化的偏振匀滑设计

偏振匀滑是利用光束偏振特性降低焦斑对比度的一种技术,它最大可降低焦斑对比度为原来的1/√2. 焦斑的频谱分析显示,传统楔形晶体的偏振匀滑对焦斑对比度的改善只集中在某些特定的空间频率,本文因此提出了一种可全域降低焦斑空间频率的偏振匀滑方法,它采用单轴晶体对入射光的角度不同而产生的相移不同的方法,实现激光两种正交偏振态在靶点的分离. 理论分析和数值模拟表明,新方法可以实现焦斑空间频率中高频段的全频域降低,焦斑对比度也可同时达到1/√2的最大程度的改善. 分析了连续相位板作为新方法引入激光入射角分布不同的条件,确定了刻蚀连续相位板面形的晶体同时实现焦斑整形和偏振匀滑的边界条件. 关键词： 偏振匀滑 焦斑功率谱 响应函数 焦斑整形     

高斯脉冲非理想条件下的谐波转换规律

对于实际的大口径高功率激光器而言,I/II类角度失谐三倍频系统的谐波转换规律常常偏离理想情况。为了研究其实际的谐波转换规律,以便于指导实际情况的晶体调试,研究了各种非理想条件下,比如1ns高斯脉冲,不同波面半径和晶体损耗对三倍频谐波转换的影响。研究结果表明:高斯形脉冲的三倍频外效率绝对值比平顶脉冲降低了10%;为了追求最高三倍频外效率,由于受1ns高斯脉冲和三倍频晶体损耗的影响,二倍频晶体的最佳内失谐角是160μrad而非220μrad,二倍频内部效率最佳平均值并非为67%,而是在60%~63%左右为最佳,并且晶体损耗越高,二倍频内部效率的最佳平均值越低;功率密度越高,波面半径和发散角对谐波转换的影响越大。     

Demonstration of Color Separation with 2ω KDP Wedge in High Power Laser Facilities

A 2ω wedge design is proposed with KDP crystal to disperse the unconverted light away from the target in a high power laser facility for inertial confinement fusion. The ultraviolet B-integral problem is released, and about 1.2 times in color separation angle is achieved according to both theoretical and experimental investigations when compared with conventional 3ω wedge. The frequency conversion etgciency is unaffected when the wedge is along the non-sensitive axis of the tripler.     

由焦面附近光强分布实现波前重构

介绍了一种由光束在焦面附近几个面上的强度分布重构出光束近场波前的方法。把波前重构问题作为一个非线性优化问题,利用光束口径区域内的正交多项式模拟光束波前相位分布,通过优化多项式各项的系数,使对应的波前分布在焦面附近相应的几个面上的强度分布与已知强度分布的偏差最小,即认为由此时的多项式系数决定的近场波前代表了真实的光束波前。针对方光束口径,利用Legendre多项式模拟光束波前分布,利用共轭梯度算法对多项式系数进行了优化计算;优化过程分为几步进行以减少计算时间。重构得到的波前分布与参考波前分布符合得很好。     

Stimulated Brillouin Scattering Damage of Large-Aperture Fused Silica Grating

Laser induced damage experiment is carried out on a large aperture laser facility. Severe damage is observed on a large-aperture fused silica grating which presents dense craters on the front surface and six cracks alternatively located at the front and the rear surface. The bizarre fact about the damage on the grating is that, unlike other optics, the damage craters are almost on the front surface. According to observation, damage phenomenon is due to the stimulated Brillouin scattering （SBS） effect occurring in the grating, which includes the transverse SBS, the back SBS and the zigzag SBS.     

70mm×70mm光束取样光栅性能测试研究

介绍了一种用于惯性约束聚变研究的光束取样光栅的研制情况,光栅尺寸为70mm?0mm。利用星光II高功率固体激光装置实验平台对光栅的性能参数进行了实验测试,实验结果表明:光栅的三倍频激光取样效率为5.6?光束取样角为13.2。     

Effect of Air Gap on Dual-Tripler Broadband Third-Harmonic Generation

We experimentally study the effect of the air gap on conversion efficiency and the spectrum of generated third-harmonic pulses in the dual-tripler broadband third-harmonic generation scheme. The experimental results are in good agreement with predictions that the 4-cm air gap is equivalent to a full cycle of phase mismatch among the three interacting pulses （i.e. the fundamental, second-harmonic and third-harmonic pulse）. The experimental results also show that the spectrum of the third-harmonic pulse is sensitive to the air gap. We also point out that the air gap effect can be ignored when the dual-tripler system is located in 1000 Pa atmosphere. These results will guide the design of the broadband third-harmonic generation system in high power lasers.     

基频倍频光群速度对超短脉冲三倍频的影响分析

在超短脉冲的谐波转换过程中, 由于不同波包之间的群速度失配的存在, 非线性晶体的有效厚度、转换效率及带宽等将相互制约, 不能获得理想结果。基于耦合波方程, 在平面波和小信号近似条件下推导出了关于基频、倍频光群速度关系的解析表达式。通过对200 fs超短脉冲在II类KDP晶体中的混频过程的模拟分析, 发现在满足相位匹配的前提下, 三次谐波转换效率随着基频、倍频光群速度倒数的差值的平方呈现出指数衰减的关系, 与理论结果吻合。得出的关于基频光、倍频光群速度的表达式, 作为三倍频群速度匹配关系式的必要补充, 对寻找合适的色散晶体和选择有效的匹配措施, 实现宽带三次谐波转换具有理论指导意义。     

Online determination of the source of stray light in the SG-Ⅲ high-power laser facility

Stray light in high-power laser facilities not only decreases the gain capacity of the main amplifier, but can also cause permanent damage to optical components, greatly threatening the safety operation of the SG-III. This Letter reports a technique for quickly pinpointing the source of stray light in the main amplification optical path of the SG-III. The achieved results indicate that the accuracy of our method is up to 1 m in an optical path of100 m. The judging method is effective for examining and removing harmful stray light in the SG-III laser facility and it can be promoted in huge laser facilities of this kind.