The dual-tripler scheme for third harmonic generation (THG) is studied by the coupled equations under the small signal and high power condition (1.5 GW/cm2). The design has been proved in the experiment. It found that this design can increase the third harmonic conversion efficiency of broad bandwidth, but the distance between two triplers has an important impact on the third harmonic conversion efficiency in the experiment. 19.75% THG efficiency with chirp pulse of 3.5 nm bandwidth at average power of 1.5 GW/cm2 was achieved at 25 mm optimum distance between two triples in the experiment. It will be useful to solving the problem of the large bandwidth of the THG in the high power laser system.

A theoretical calculation is presented for third-harmonic generation in KDP crystal for type Ⅱ- type Ⅱ polarization-mismatch scheme by using the four-stepped Runge-Kutta method. For the “Shenguang Ⅱ” frequency tripling system, the doubling angle θd and the tripling angle θt are located at optimized situation, by changing detuned values of the polarization angle θp, the corresponding changes of pulse shape properties was gotten. The results show that when the polarization angle θp is located at optimized situation, the full width at half maximum (FWHM) τ is minimum and the pulse shape rises fastest. Increasing the θp from optimized situation causes the τ widens faster and the pulse shape rises slower than decreasing the θp from optimized situation. In the experiment, by changing the polarization angle θp, the pulse shapes consistency of the “Shenguang Ⅱ” eight frequency tripling beams has been accomplished. This work offers a method for adjusting crystals and actualizing power balance.

使用四阶龙格-库塔算法数值求解Ⅱ类-Ⅱ类偏振失配方案的三倍频稳态耦合波方程组。针对“神光Ⅱ”激光驱动器的三倍频系统，在二倍频匹配角θd和三倍频匹配角θt均处于最佳匹配情况下，以1°为单位改变偏振分配角θp的偏离量，得到对应的三倍频波形特性的变化。研究表明，偏振分配角θp处于最佳理论匹配位时，对应的三倍频波形半峰全宽最窄，且波形上升沿抬起也最快; θp从最佳匹配位增加比从最佳匹配位减小对应的半峰全宽变宽更快，对应的上升沿抬起更慢。根据所得理论结果，在实验中调节偏振分配角θp,达到了改变三倍频输出波形特性，进而实现八路三倍频波形一致的目的。该研究结果对于更快地实现激光驱动器装置的功率平衡有一定的应用价值。

从耦合波方程出发,分别在小信号、高功率密度(1.5 GW/cm2)条件下研究了晶体串接三次谐波(THG)转换方案,并利用实验验证了该方案。实验观察到这种方案可以有效提高宽带激光三次谐波转换效率,但混频晶体之间的距离对宽带三次谐波转换效率影响明显。在1.5 GW/cm2平均功率密度下,实验中最佳距离25 mm处,对带宽3.5 nm的啁啾脉冲取得了19.75%的三倍频效率,这对于高功率激光宽带三次谐波转换的解决很有意义。

从耦合波方程出发,分别在小信号、高功率(1.5 GW/cm2)条件下研究KDP晶体串接三次谐波转换。当两块混频晶体的长度选择为8 mm和6 mm,晶体分别偏离原混频匹配角0.35 mrad和-0.25 mrad时可以有0.3 nm的谐波转换带宽,同时系统的三次谐波转换效率与两块混频晶体之间的距离有密切关系,当两块晶体之间的距离使从第一块混频晶体出射的光波之间的相位差改变π时,会使第一块混频晶体产生的三次谐波大部分回流到基频和倍频光,从而使转换效率大幅度下降,最合适的距离应当使光波之间的相位差改变为2π。

针对重要的χ(2):χ(2)级联非线性相移,在理论上论证了可以用光谱分辨的双光束耦合方法来测量χ(2):χ(2)级联非线性相移,在实验上研究了飞秒时域级联非线性相移随波矢失配量的变化规律。这一新颖的测量方法灵敏度高、实际使用简单,应用低能量的飞秒激光振荡器,就能方便地测量微弱χ(2)晶体的级联非线性相移。

基于三次谐波转换过程中位相扰动的传输变换,理论上研究了非线性晶体表面周期性纹波对三倍频激光束聚焦特性的影响,建立了多级衍射模型.该理论模型能解析地分析多级衍射峰的强度分布及角度分布,可为高功率激光驱动器三倍频器的设计提供理论依据.

采用光谱分辨的双光束耦合方法,精确测量了KDP,KTP,PPLN,BBO,LBO及LiIO3等常用χ(2)非线性光学晶体的非线性折射率。实验测量工作使用800 nm工作波长的克尔透镜锁模钛宝石激光振荡器。

首次在理论上研究了位相畸变激光束的谐波转换。从波动方程出发,应用空间频谱方法处理位相畸变光束的非线性耦合问题,建立了适用于位相畸变且空间光强不均匀光束的谐波转换的理论模型。

针对高斯型随机位相畸变，应用夫琅和费(Fraunhofer)衍射积分研究了畸变光束的聚焦特性和光束质量M2因子，给出了它们随位相畸变特征量变化的近似分析和数值计算结果，可为高功率激光器的设计提供理论依据。