A silicon-based digitally tunable positive/negative dispersion controller (DC) is proposed and realized for the first time using the cascaded bidirectional chirped multimode waveguide gratings (CMWGs), achieving positive and negative dispersion by switching the light propagation direction. A 1 × 2 Mach–Zehnder switch (MZS) and a 2 × 1 MZS are placed before and after to route the light path for realizing positive/negative switching. The device has Q stages of identical bidirectional CMWGs with a binary sequence. Thus the digital tuning is convenient and scalable, and the total dispersion accumulated by all the stages can be tuned digitally from - ( 2^Q - 1 ) D₀ to ( 2^Q - 1 ) D₀ with a step of D₀ by controlling the switching states of all 2 × 2 MZSs, where D₀ is the dispersion provided by a single bidirectional CMWG unit. Finally, a digitally tunable positive/negative DC with Q = 4 is designed and fabricated. These CMWGs are designed with a 4-mm-long grating section, enabling the dispersion D₀ of about 4.16 ps / nm in a 20-nm-wide bandwidth. The dispersion is tuned from -61.53 to 63.77 ps / nm by switching all MZSs appropriately, and the corresponding group delay is varied from -1021 to 1037 ps.

介绍了时空锁模的基本原理和时空锁模的理论模型−吸引子解剖。从空间结构和全光纤结构两方面介绍了近年来国内外在时空锁模光纤激光器方面的研究进展，包括激光腔型的改进、输出性能的提升和实时动力学的观测等。最后分析了目前时空锁模激光器的优势和不足，并对其发展方向进行了展望：时空锁模激光器在产生高功率超短脉冲方面有着巨大的优势，但输出光斑质量差在一定程度上限制了它的实际应用；利用时空自相似演化、波前整形等技术提升光束质量将是未来时空锁模激光器的发展方向。

Transmission matrix (TM) allows light control through complex media, such as multimode fibers (MMFs), gaining great attention in areas, such as biophotonics, over the past decade. Efforts have been taken to retrieve a complex-valued TM directly from intensity measurements with several representative phase-retrieval algorithms, which still see limitations of slow or suboptimum recovery, especially under noisy environments. Here, we propose a modified nonconvex optimization approach. Through numerical evaluations, it shows that the optimum focusing efficiency is approached with less running time or sampling ratio. The comparative tests under different signal-to-noise levels further indicate its improved robustness. Experimentally, the superior focusing performance of our algorithm is collectively validated by single- and multispot focusing; especially with a sampling ratio of 8, it achieves a 93.6% efficiency of the gold-standard holography method. Based on the recovered TM, image transmission through an MMF is realized with high fidelity. Due to parallel operation and GPU acceleration, our nonconvex approach retrieves a 8685 × 1024 TM (sampling ratio is 8) with 42.3 s on average on a regular computer. The proposed method provides optimum efficiency and fast execution for TM retrieval that avoids the need for an external reference beam, which will facilitate applications of deep-tissue optical imaging, manipulation, and treatment.

提出了一种基于锥形渐变耦合结构的可扩展多模弯曲波导。该器件利用模式等效折射率匹配原理，通过对称的锥形渐变耦合结构，实现高阶模式与基模的相互转化，完成多模弯曲传输功能。同时，结合时域有限差分方法和粒子群优化算法，优化锥形渐变耦合结构区域，提升器件性能。实验测试结果表明，在1 520~1 600 nm的波长范围内，当输入模式分别为TE₀、TE₁、TE₂、TE₃和TE₄时，该器件的插入损耗分别小于1.71 dB、3.04 dB、2.90 dB、3.16 dB和4.00 dB，对应的串扰分别小于-10.60 dB、-11.35 dB、-10.92 dB、-10.35 dB和-11.45 dB。