Feng Xi, Shi Jinqiao, Liu Pei, Zhang Zhaowei
Opt Express. 2020 May 11;28(10):14310-14318. doi: 10.1364/OE.391686.
We present difference frequency generators (DFGs) using cascaded PPLN crystals, each with a distinct poling-period, as the parametric gain medium. We show that the phase-matching bandwidth of cascaded crystals is the combination of that of each individual crystal. In the non-phase-matched section of cascaded crystals, there exists periodic backward and forward frequency-conversion processes. Nonetheless, we demonstrate that such a periodic back-conversion process would not compromise the parametric gain bandwidth of cascaded nonlinear crystals. By using two PPLN crystals with poling periods of 31 µm and 29 µm, we experimentally obtained mid-infrared light sources having instantaneous-bandwidth covering 2.8-3.9 µm, which was roughly twice as much as that from a system based on a single crystal. Moreover, our numerical results show that light sources with an instantaneous-bandwidth covering 2.5-5 µm could be obtained by cascading more crystals. This scheme represents a promising technical route to transform conventional DFGs into a device capable of generating spatially-coherent light emission with very broad instantaneous-bandwidth.
我们展示了使用级联周期极化铌酸锂（PPLN）晶体作为参量增益介质的差频发生器（DFG），每个晶体都有不同的极化周期。我们表明，级联晶体的相位匹配带宽是每个单独晶体的相位匹配带宽之和。在级联晶体的非相位匹配部分，存在周期性的正向和反向频率转换过程。尽管如此，我们证明这种周期性的反向转换过程不会损害级联非线性晶体的参量增益带宽。通过使用极化周期分别为31 µm和29 µm的两个PPLN晶体，我们通过实验获得了瞬时带宽覆盖2.8 - 3.9 µm的中红外光源，这大约是基于单个晶体的系统的两倍。此外，我们的数值结果表明，通过级联更多晶体可以获得瞬时带宽覆盖2.5 - 5 µm的光源。该方案代表了一条将传统DFG转变为能够产生具有非常宽瞬时带宽的空间相干光发射的器件的有前途的技术路线。
