FEMTOSECOND PULSE COMPRESSION WITH NONLINEAR CASCADING PHASE-SHIFT EFFECTS

我们证明了超高强度激光脉冲的I型二次谐波产生过程中的初始相位失配可以用来补偿三阶非线性，不仅在时间剖面上，而且在基频和二次谐波脉冲的空间分布上。在对三阶非线性进行补偿的过程中，二次谐波脉冲的空间分布表现出与输入基频脉冲的良好再现。我们证明了在基脉冲中引入频率啁啾可以有效地抑制超高强度激光脉冲I型二次谐波产生中的三次非线性效应。与传统的二次谐波产生方案相比，该方案具有良好的二次谐波脉冲时空分布，能量转换效率提高了1.23倍。本文对I型二次谐波源级联效应的理论分析和数值计算进行了如下两种情况的描述：(1)忽略群速失配、群速色散、衍射、漂移和吸收的I型二次谐波源；(2)皮秒激光脉冲的I型二次谐波产生涉及群速度失配、群速度色散和漂移。此外，还给出了用KDP晶体产生I型二次谐波的数值结果。为了提高从泵浦到种子的能量转换效率，我们提出了一种简单的OPCPA方案，通过在长泵浦脉冲和短种子脉冲之间引入时间延迟，使种子脉冲与泵浦脉冲的所有部分相互作用，同时保证增益。我们将给出所提出的OPCPA方案的数值结果，并证明泵能量可以有效地传递到种子脉冲和惰轮脉冲中。

We demonstrated that an initial phase mismatch in type I second-harmonic generation processes with ultrahigh intensity laser pulses can be used to compensate for third-order nonlinearity not only in temporal profiles but also in spatial distributions of fundamental and second-harmonic pulses. The spatial profile of the second-harmonic pulse shows a good reappearance of the input fundamental pulse throughout the compensation for the third-order nonlinearity.We demonstrated that a frequency chirp initially introduced to fundamental pulse can be used to effectively suppress cubic nonlinear effect in a type I second-harmonic generation with ultra-high intensity laser pulses. The energy conversion efficiency can be increased by a factor of 1.23 with good temporal and spatial profiles of second-harmonic pulse in comparison with conventional schemes of second-harmonic generation.Theoretical analysis and numerical calculation for cascading effects in type I second-harmonic generation are described for two cases as the following: (1) a type I second-harmonic generation provided that group-velocity mismatch, group-velocity dispersion, diffraction, walk-off and absorption can be neglected; (2) a type I second-harmonic generation with picosecond laser pulses involved the group-velocity mismatch, group-velocity dispersion and walk-off. In addition, numerical results for a type I second-harmonic generation with a KDP crystal are also given.We propose a simple OPCPA scheme to increase the energy conversion efficiency from the pump to seed by introducing a time delay between the long pump and short seed pulses so that the seed pulse interacts with all portions of the pump pulse, and the gain is guaranteed simultaneously. We will present numerical results for the proposed OPCPA scheme and demonstrate that the pump energy can be efficiently transferred into the seed and idler pulses.