【作者】 王东辉；

【导师】 苑立波；

【作者基本信息】 哈尔滨工程大学 ， 光学工程， 2024， 博士

【摘要】 涡旋光束,又称为轨道角动量光束,以其特殊的螺旋相位和环形光强分布在过去的几十年里吸引了众多研究者的关注,并被广泛应用于自由空间和光纤通信、量子信息、显微成像、光镊、光学测量和微加工等领域。当前基于手性光纤光栅的涡旋光束生成及检测是热点研究方向之一。然而,该方法生成的涡旋光束模场面积受限、高阶涡旋光束纯度较低、涡旋光束的检测仍依赖图像系统等因素限制了手性光纤光栅涡旋光束生成及检测器件的发展。本论文基于特种光纤的独特结构,探讨了光纤结构参数与涡旋场模场直径、涡旋阶数的关系,研究了复调制手性光纤光栅的单向耦合、模式滤波等特性。同时针对实际应用需求,提出和制备了多种涡旋光器件,为涡旋光束的生成、滤波、检测提供全光纤方案。具体工作内容如下:1.本文提出并制备了一种基于手性双包层光纤的大模场涡旋光束生成器。这种光纤的内包层能扩大OAM_1,1涡旋模式的模场面积,并隔离环境折射率的影响。通过分析纤芯归一化频率、内包层直径和数值孔径对OAM_1,1模式模场直径、耦合长度、抗折射率干扰能力的影响,给出了双包层光纤的最优设计方案。使用自制光纤四电级扭转系统加工了手性双包层光纤光栅,所生成的OAM_1,1模式等效模场直径达44μm,并且不受环境折射率和输入偏振态的影响。2.本文提出并制备了一种异型纤芯结构的鲁罗克斯三角芯光纤,该光纤的纤芯由三组弧边构成,因而含有独特的角向三阶调制。通过纤芯轮廓的傅里叶级数和耦合模分析,得出了激发三阶涡旋模式所需的相位匹配和轨道角动量匹配条件。该光栅生成的三阶涡旋光束转换效率达99.5%,模式纯度高于95%。通过调节扭转螺距和包层收缩率,该光栅还能生成混合多阶涡旋光束。此外,也测试了这种手性光纤光栅的一阶和三阶耦合峰的传感特性,结果表明高阶涡旋模式能显著提升灵敏度,并与现有其他增敏技术兼容。3.本文提出了一种复调制手性光纤光栅,其调制项由空间螺旋的折射率实部和虚部（损耗）调制构成,两者在光纤端面内的方位角差为n/2。正向和反向传播的光波在这种光栅中受到的两种调制顺序不同,使得正向传播基模衰减,反向传播的基模耦合至OAM_1,1模式。耦合模分析的结果表明,不同涡旋手性、不同传播方向的光波拥有不同的相位匹配和轨道角动量匹配条件。基于光束传播法的仿真模型验证了该光栅的单向耦合特性。利用不同涡旋模式的损耗及耦合差异,使用这种复调制手性光纤光栅可构建强度型涡旋模式过滤及探测器。综上所述,本论文基于手性特种光纤研究了生成涡旋光束的模场直径、模式阶数等性质,并得出了不同功能涡旋场生成器的设计方法。同时提出了复调制手性光纤光栅作为强度型涡旋模式过滤及探测器。这些全光纤器件提供了高效、稳定、经济的涡旋光束生成及检测方案,对光纤通信和光纤传感研究有着重要意义。更多还原

【Abstract】 Vortex beams,also known as orbital angular momentum beams,have attracted the attention of many researchers in the past decades because of their special helical phase and ring intensity distribution,and have been widely used in free-space and fiber optic communication,quantum information,microscopic imaging,optical tweezers,optical measurement,and micromachining.At present,vortex beam generation and detection based on chiral fiber grating is one of the hot research directions.However,the vortex beam generated by this method is limited in the mode field area,the purity of higher-order vortex beams is low,and the detection of vortex beams is still dependent on the image system,which limits the generation of chiral fiber grating vortex beams and the development of detection devices.Based on the unique structure of special optical fiber,the relationship between optical fiber structure parameters and vortex mode field diameter and vortex order is discussed in this thesis.At the same time,a variety of optical vortex devices are proposed and fabricated to provide an all-fiber scheme for the generation,filtering and detection of vortex beams.The specific work contents are as follows:1.A large mode area vortex beam generator based on chiral double-clad fiber is proposed and fabricated.The inner cladding of the fiber can enlarge the mode field area of the OAM_1,1 vortex mode and isolate the influence of the ambient refractive index.By analyzing the influence of core normalized frequency,inner cladding diameter and numerical aperture on OAM_1,1 mode field diameter,coupling length and anti-refractive index interference ability,the optimal design scheme of double-clad fiber is given.The generated OAM_1,1 mode equivalent field diameter is up to 44μm,which is not affected by the ambient refractive index and input polarization state.2.A Reuleaux triangle core fiber with a special core structure is proposed and fabricated.The core of the fiber is composed of three sets of curved edges,and therefore contains a unique angular third-order modulation.By the Fourier series of the core profile and coupled mode analysis,the matching conditions of phase and orbital angular momentum required to excite the third-order vortex mode are obtained.The conversion efficiency of the third-order vortex beam generated by the grating is 99.5%,and the mode purity is higher than95%.By adjusting the twisted pitch and cladding shrinkage,the grating can also generate a mixed multi-order vortex beam.In addition,the sensing characteristics of the first and third order coupling peaks of the chiral fiber grating are also tested.The results show that the high-order vortex mode can significantly improve the sensitivity and is compatible with other existing sensitization techniques.3.A complex modulated chiral fiber grating is proposed,whose modulation term consists of the helix real part and the imaginary part（loss）of the refractive index,and the azimuth difference between the two in the fiber end face is n/2.The forward and backward propagation light waves are subject to two different modulation sequences in such a grating,causing the forward propagation fundamental mode attenuation and the backward propagation fundamental mode to be coupled to the OAM_1,1mode.The results of coupled mode analysis show that light waves with different vortex chirality and propagation direction have different phase matching and orbital angular momentum matching conditions.The simulation model based on beam propagation method verifies the unidirectional coupling characteristics of the grating.By taking advantage of the attenuation and coupling differences of different vortex modes,the complex chiral fiber grating can be used to construct a power vortex mode filter and detector.In summary,this thesis studies the properties of vortex beam,such as mode field diameter and mode order based on chiral special fiber,and obtains the design method of vortex field generator with different functions.At the same time,a complex modulated chiral fiber grating is proposed as a power vortex mode filter and detector.These all-fiber devices provide efficient,stable,and economical vortex beam generation and detection schemes,which is of great significance for the research of optical fiber communication and optical fiber sensing.更多还原