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。

The dual-tripler scheme of KDP crystals for third harmonic generation (THG) is studied by the equations of three interacting waves under the small signal and the high power (1.5 GW/cm2) condition. The 8 mm type II KDP tripler with an detuning angle of 0.35 mrad whereby the addition of a 6 mm type II tripler with an detuning angle of -0.25 mrad can increase the bandwidth of third harmonic generation to 0.3 nm at 1.5 GW/cm2. Meanwhile, the conversion efficiency of the third harmonic generation is closely related to the distance between two triplers. If the relative phase among the three interacting waves changes π during the distance, the third harmonics from the first tripler can return to the second harmonics and the fundamental frequency and the conversion efficiency will drop by a wide margin. To ensure the optimum performance of the dual-tripler system, it is necessary to find the correct air gap during which the relative phase among the three interacting waves changes 2π.