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。