手征光纤布拉格光栅轨道角动量调控机理及应用研究

The all-fiber orbital angular momentum modulators based on fiber gratings have the advantages of good stability, low insertion loss, low cost, and being fully compatible with the communication network, which suggest extremely high scientific value and application potential in the fiel of OAM generation and modulation. Among them, the chiral fiber grating with helical refractive index modulation which can directly excite OAM without auxiliary equipment of phase adjustment, is acknowledged as an ideal device for generating and regulating OAM in fiber. Confined to the current state of the art, the investigation on the regulation of OAM is mainly focused on chiral long-period fiber gratings, and the research on orbital angular momentum modulation based on chiral fiber Bragg grating (CFBG) has only existed in the limited theoretical design so far. This project addresses this issue by conducting the following studies: (1) establishing CFBG theoretical analysis models based on femtosecond laser-induced refractive index modulation; (2) exploring and optimizing CFBG preparation processes; (3) revealing the mechanism of generation and regulation of OAM modes in CFBG; (4) development of a novel reflective OAM modulator based on CFBG. Through the research of this project, CFBG's high-efficiency and high-quality preparation is realized, the interaction between CFBG and OAM is clearly defined, and the development of reflective high-order, circularly-polarized, tunable OAM modulators is realized. Through the research of this project, CFBG with high-efficiency and high-quality is achieved, the interaction between CFBG and OAM is clearly defined, and the reflective high-order, circularly-polarized, tunable OAM modulators is developed.

基于光纤光栅的全光纤轨道角动量（OAM）调制器，具有稳定性好、插损小、成本低、可与通信网络完全兼容等优点，在OAM产生及调制领域具有极高科学研究价值和应用潜力。其中，手征光纤光栅以其螺旋的折射率调制，可以不借助相位调节辅助设备而直接激发OAM，是实现OAM产生及调制的理想器件。限于当前技术水平，OAM的调控研究主要集中于手征长周期光纤光栅，而基于手征光纤布拉格光栅（CFBG）的OAM调制研究至今仅存在于有限的理论设计中。本项目针对此问题，进行以下研究：(1)建立基于飞秒激光诱导折射率调制的CFBG理论分析模型；(2)探索并优化CFBG制备工艺; (3)揭示CFBG中OAM产生及调控机理;(4)研制基于CFBG的性能新颖的反射型OAM调制器。通过本项目的研究，实现CFBG的高效率、高质量的制备，明晰CFBG与OAM相互作用的规律，实现反射型的高阶、圆偏振、可调谐的OAM调制器的研制。

光子轨道角动量（OAM），是独立于振幅、频率、偏振的光子准本征属性。全光纤型的OAM转换器，具有与光通信系统完全兼容的优势，并且，利用光纤自身的柔性可弯曲特性，可以更方便地应用于光纤涡旋光镊、光纤内窥超分辨成像及光纤高分辨率传感等领域，是当前热点研究问题。手性光纤光栅（CFG）可以高效率地实现光纤基模到高阶OAM模式的转换，已经成为激发光纤型OAM的重要器件。限制于制备技术，短周期的CFG，以及任意CFG的制备仍然是一项挑战。本项目发展飞秒激光加工技术，首先实现了CFG的飞秒激光制备技术的突破，在单模光纤上实现了±1阶OAM模式的直接激发，其次，在九模式光纤、环形芯光纤上，实现了更高阶OAM模式的激发。在研究CFG与OAM模式相互作用的基础上，利用正交FBG组合，获得了CFG的特性，实现了高阶OAM的直接激发。OAM模式的可调谐性，可以为其应用提供便利及应用范围，但是正交准CFG及CFG，在可调上缺乏灵活性。因此，本项目利用单个FBG，在四模光纤上，实现了±1阶模式之间、±２阶模式之间的模式可调，在九模光纤上，初步实现了０和２，１和３阶OAM模式的可调，为后续进一步研究模式可调的OAM转换器，垫定了基础。在研究CFG与OAM的相互作用机理研究过程，在六模光纤及环形芯光纤中，实现了±1阶、±２阶、±３阶的OAM模式的直接激发，明晰了光纤光栅相位匹配条件与角动量守恒相互作用过程，实现了OAM模式的偏振态与螺旋相位的独立调制，为任意偏振态的OAM模式的产生及应用提供了实验指导及实用器件。此外，通过改变光纤直径尺寸，可以调节光纤模式色散转换点，实现了基于双共振的超宽带OAM模式耦合器，通过扭CFG，实现了中心波长的可调。最后，本项目对CFG及OAM在传感、光镊及全息复用中的应用进行了探索，获得了高灵敏的温度、折射率传感器、可应用于任意复杂环境的光纤涡旋光镊。