螺旋少模光纤长周期光栅及其模分复用特性研究

Helical long-period grating (HLPG) is a novel fiber passive component can achieve the generation and conversion of the optical orbital angular momentum (OAM) modes, which could be useful for the development of the all fiber OAM mode-division-multiplexing of optical communications. We propose the fabrication of the HLPGs by femtosecond pulse laser and carbon dioxide laser. The helical refractive index modulation could be induced in different area of the twisted fiber by the focused laser. The transmission characteristics of the HLPGs can be tuned flexibly. The theoretical model and numerical simulation will be compared with the experimental results to reveal the physical mechanism of the laser induced refractive index modulation. The contribution of the laser-induced stress changes and glass structure changes to the refractive index modulation will be evaluated. We will explore the technique to change the refractive index distribution of the HLPGs using the focused laser. The mode coupling characteristics of the HLPGs will be studied using the couple mode theory. The effect of the helical index modulation on the generation and conversion of fiber OAM modes will be revealed. The transmission spectra, mode distribution and helical index modulation of the HLPGs will be investigated experimentally. Based on the theoretical and experimental results, the fabrication of the HLPGs can be improved so that the optical couplers and mode converters based on the HLPGs can be fabricated. The generation and mode conversion of high order fiber OAM modes can be realized based on the fabricated HLPGs. The all fiber OAM mode-division-multiplexing of optical communications based on the HLPG mode converters will be investigated experimentally, which could have promising applications in next generation optical communications.

螺旋光纤长周期光栅（HLPG）是实现光纤轨道角动量（OAM）模式产生与转换的新型全光纤无源器件，可望对全光纤OAM模分复用光通信技术的发展有重要促进作用。本项目提出利用飞秒脉冲激光与二氧化碳激光制备HLPG，采用聚焦激光在旋转光纤的不同区域诱导螺旋式折射率调制，实现HLPG传输特性的灵活调节。将结合理论建模与数值仿真，研究激光诱导光纤折射率调制的物理机理，对比研究激光诱导的应力变化与玻璃结构改变对光纤折射率调制的贡献，探索利用聚焦激光调节HLPG折射率分布的技术方法。将基于耦合模理论研究光栅模式耦合特性，揭示光栅螺旋式折射率调制对光纤OAM模式激发与转换的影响，对HLPG的光谱、模场分布和螺旋式折射率分布进行实验研究，并结合理论分析的结果改进光栅制备方法，从而制备具有不同模式耦合特性的HLPG耦合器，实现高阶光纤OAM模式耦合与转换，并实验研究其全光纤OAM模分复用光通信应用。

模分复用（MDM）光通信技术，利用少模光纤中的不同模式拓展光通信系统的信道容量，近年来受到普遍关注，有望成为下一代光通信技术的主要候选技术之一。螺旋光纤长周期光栅（HLPG）是实现光纤轨道角动量（OAM）模式产生与转换的新型全光纤无源器件，可望对光纤OAM模分复用光通信技术的发展有重要促进作用。项目利用二氧化碳激光与飞秒脉冲激光制备了一系列长周期光纤光栅（LPFG）与HLPG，结合理论建模与数值模拟，研究了激光诱导光纤折射率调制的物理机理。主要研究内容包括：少模光纤与保偏光纤中LPFG与HLPG的制备与传感特性研究、宽带少模光纤LPFG模式耦合器的制备方法研究、全光纤OAM模式激发研究、少模光纤LPFG与HLPG的模分复用特性研究。对于全光纤OAM模式的激发与转换，HLPG有突出的优点，可以通过HLPG直接激发全光纤OAM模式。基于HLPG技术的全光纤OAM模分复用光通信有望在下一代光通信技术方面有重要的应用前景。