Three-dimensional (3D) nonlinear photonic crystals have received intensive interest as an ideal platform to study nonlinear wave interactions and explore their applications. Periodic fork-shaped gratings are extremely important in this context because they are capable of generating second-harmonic vortex beams from a fundamental Gaussian wave, which has versatile applications in optical trapping and materials engineering. However, previous studies mainly focused on the normal incidence of the fundamental Gaussian beam, resulting in symmetric emissions of the second-harmonic vortices. Here we present an experimental study on second-harmonic vortex generation in periodic fork-shaped gratings at oblique incidence, in comparison with the case of normal incidence. More quasi-phase-matching resonant wavelengths have been observed at oblique incidence, and the second-harmonic emissions become asymmetric against the incident beam. These results agree well with theoretic explanations. The oblique incidence of the fundamental wave is also used for the generation of second-harmonic Bessel beams with uniform azimuthal intensity distributions. Our study is important for a deeper understanding of nonlinear interactions in a 3D periodic medium. It also paves the way toward achieving high-quality structured beams at new frequencies, which is important for manipulation of the orbital angular momentum of light.

Fast and stable phase control is essential for many applications in optics. Here, we propose an all-fiber all-optical phase modulation scheme based on a Fabry–Perot interferometer (FPI) and an Er/Yb co-doped fiber (EYDF). By using the EYDF as an F-P cavity via rational design, a phase shift with a modulation sensitivity of 0.0312π/mW is introduced to the modulator. The phase shifts in the EYDF consist of a thermal phase shift and a nonlinear phase shift with a ratio of 19:1, and the corresponding temporal responses of the modulation are 204 ms and 2.5 ms, respectively. In addition, the compact FPI is encapsulated to provide excellent stability for the modulator.

Here we report on a simple-to-implement and cost-effective approach for laser pulse contrast enhancement, based on the ${\chi}^{(3)}$ nonlinear self-focusing effect. An intentionally induced and gently controlled self-focusing in a thin glass transforms the time-dependent intensity into variation in beam divergence. Followed by a spatial discriminating filter, only the strongly focused fraction traverses the setup, at the expense of efficiency. A numerical model, accounting for the pulse and material parameters via a Gaussian ABCD matrix, provides an estimate for the instantaneous beam waist and transmission efficiency, which enables us to evaluate the resulting contrast enhancement. The estimated contrast enhancement spans between 0.5 and 2.5 orders of magnitude, in conjunction with approximately 25%–90% estimated efficiency, depending on the pulse parameters. In a preliminary experiment we demonstrated the effect with 10s-μJ sub GW regime with approximately 40 $\%$ efficiency and a contrast improvement of more than or equal to 20 dB.

基于光子晶体非线性效应和线性干涉效应设计了全光异或、非和与逻辑门。应用反演定理拆分较复杂逻辑表达式，通过级联组合设计了全光或非门和四输入与门逻辑器件。本文利用时域有限差分法进行仿真模拟计算，对非线性环形腔的耦合特性进行了分析，然后在信号波长为1.47 μm条件下设计了上述逻辑器件，且通过可扩展输入端可设计出更多输入的器件。分析了信号功率对四输入与逻辑器件逻辑功能的影响。结果表明信号光源功率在1.1 W/μm²到3.4 W/μm²之间时，输出端的逻辑对比度均大于10 dB。所设计器件响应时间最短仅1.6 ps，占用面积小，易于扩展与集成，在光处理系统和集成光路中有较大应用前景。

为解决相位生成载波-反正切解调算法(PGC-Atan)的非线性失真问题，搭建了基于改进型PGC-Atan算法的非本征型法珀传感器(EFPI)解调系统。首先，理论分析了载波相位调制深度(C)偏离最优值、伴生调幅、载波相位延迟等非线性因素对经典PGC-Atan算法中参与反正切运算的正弦与余弦两路信号的影响。然后，针对外调制或伴生调幅较小的情况，提出了一种基于系数补偿的改进型PGC-Atan算法(PGC-CC-Atan)。该算法通过构造与C值和载波相位延迟有关的系数，消除反正切运算中的非线性参数。针对内调制情况，提出了一种基于椭圆拟合的改进型PGC-Atan算法(PGC-EF-Atan)。该算法通过基于分块矩阵的最小二乘法拟合椭圆并提取3个椭圆参数，进而将受非线性因素影响的正弦与余弦两路信号校正为正交信号。最后，通过仿真验证了改进型算法的正确性，并采用高调制特性的垂直腔面发射激光器(VCSEL)和常规腔长的EFPI等搭建PGC解调系统，对比经典PGC-Atan算法与两种改进型算法的解调性能，证实了改进型算法非线性失真抑制的有效性。实验结果表明：一定C值范围内，两种改进型算法可在非线性因素影响下有效解调。PGC-EF-Atan算法相较于PGC-CC-Atan算法，解调信纳比提升了11.602 dB，总谐波失真降低了10.951%。两种改进型算法中，PGC-EF-Atan算法对非线性失真的抑制效果更好，且解调线性度良好，准确度高。

针对索驱悬吊式重力卸荷系统被动缓冲器带来的欠驱动问题，提出了基于气动人工肌肉（Pneumatic Artificial Muscle， PAM）主动式缓冲器控制方法。首先，分析用于地面微/低重力模拟的悬吊式重力卸荷系统，为了克服PAM高度非线性特性，提出基于块结构的非线性神经网络建模方法，其次分析吊索与航天器相互作用过程中挠性引起的力扰动，最后采用非线性模型预测跟踪控制。相比传统PID控制方法，该方法具有参数调节简单，实时跟随性能好，以及对卸荷系统目标惯性参数的摄动具有控制性能不变性等优势。实验结果表明，设置不同扰动情况跟随力误差都能保证在3%以内，实验证明了基于PAM主动缓冲器的可行性，所提出的控制方法能够在挠性不确定性的情况下实现力跟随控制。

提出了基于光强非线性响应的光致热弹光谱光强修正方法，实现了激光光强的准确修正。控制DFB激光器工作于波长调制模式，设置激光调制频率为16 369.75 Hz，通过光纤放大器增强其出射光强非线性项的幅值，光束经多次反射池后汇聚于石英音叉根部激发光热信号，采用数字锁相放大器解调得到对应的谐波信号，通过多项式拟合谐波信号背景，反演得到与浓度及光强分别对应的谐波信号。实验结果表明，当光强从22.03 mW变化至3.16 mW，谐波信号背景幅值与光强具有良好的线性关系，线性相关系数大于0.998，归一化后的谐波信号幅值波动小于0.37%。针对甲烷测量，系统在较大的浓度范围内具有良好的线性响应，谐波信号信噪比表明系统的最低检测限达0.22×10^-6。该研究为光致热弹光谱的光强修正提供了一种新的方法，可有效提高LITES系统在长期测量应用中的稳定性。