A femtosecond optical parametric oscillator (OPO) pumped by the second harmonic of a Yb:KGW solid-state oscillator was investigated. A comparison between lithium triborate (LBO) and barium borate (BBO) crystals was carried out, with particular attention to the impact of phase mismatch on pulse characteristics. The phase mismatch induces cascaded nonlinearity, which acts as an effective nonlinear refractive index. Negative group delay dispersion of the resonator and the effective nonlinear refractive index allows soliton-like pulse formation with varying pulse durations and a bandwidth-limited time-bandwidth product. Deviation from the soliton equation was observed when intracavity power was varied. The measured pulse durations were lower than the classical soliton duration, indicating that the pulses were not classical solitons. A Z-scan experiment was used to determine nonlinear refractive indices under conditions identical to those in the oscillator cavity. Unexpectedly, the signs of nonlinear refraction as measured by the Z scan and indirectly observed in the OPO proved to be the opposite. This sign reversal could be interpreted in terms of the competing cascaded nonlinearities corresponding to self-phase modulation and cross-phase modulation between the signal and the pump pulses in the nonlinear OPO crystal.

中文翻译：

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大功率飞秒光学参量振荡器中的级联非线性

研究了由Yb：KGW固态振荡器的二次谐波激发的飞秒光学参量振荡器（OPO）。比较了三硼酸锂（LBO）和硼酸钡（BBO）晶体，特别注意相位失配对脉冲特性的影响。相位失配会引起级联的非线性，这是有效的非线性折射率。谐振器的负群延迟色散和有效的非线性折射率允许形成类似孤子的脉冲，并具有变化的脉冲持续时间和带宽受限的时间带宽积。当腔内功率变化时，观察到与孤子方程的偏差。测得的脉冲持续时间低于经典孤子持续时间，表明脉冲不是经典孤子。在与振荡器腔相同的条件下，使用Z扫描实验确定非线性折射率。出乎意料的是，通过Z扫描测量并在OPO中间接观察到的非线性折射迹象被证明是相反的。这种符号反转可以用竞争级联的非线性来解释，这些非线性对应于非线性OPO晶体中信号和泵浦脉冲之间的自相位调制和交叉相位调制。