摘要
微型化与集成化是近年来先进光子技术和器件的重要发展方向，微纳米工艺技术是实现光子器件微型化与集成化的基础。借助微纳米工艺技术，人们可以按照需求来设计制备具有优异性能的光纤微纳米结构和器件。同时，理论和实验的研究表明，随着功能结构尺度的减小，光纤器件的特性及性能等都表现出与传统宏观结构器件显著不同的特点。这些特异的物理性质具有广阔的实际应用和理论研究前景。本文以此为出发点，从理论和实验上分析了两种具有代表性的基于微纳米尺度结构的新型光纤模间干涉器件，包括其模式与耦合特性和相关微纳米工艺技术，论文详细研究和探讨了它们的工作原理、制备过程以及功能特性，同时通过与同类传统尺寸结构光纤器件的比较验证了基于微纳米技术的光纤模间干涉器件在性能上的优越性。本文的主要内容如下：
第一章介绍了两种颇具代表性的模间干涉型光纤器件及其相关微纳米加工工艺。一种代表性模间干涉器件是光纤光栅，通过光纤表面和表层微纳米薄膜涂层工艺，可实现具有大范围可调谐特性的光纤光栅器件；另一种代表性模间干涉器件是锥形光纤，通过微纳米光纤制备工艺，可以获得具有高灵敏度传感特性的锥形微纳米光纤器件。
第二章采用四层光纤模型理论分析了覆盖高折射率微纳米涂敷层的长周期光纤光栅的包层模特性，并通过耦合模理论对四层模型长周期光纤光栅谐振波长和频谱特性进行了分析；研究了谐振峰波长与涂敷层相关参数的关系，并分析了内包层厚度在提高长周期光纤光栅调谐范围方面的作用。
第三章从理论上深入分析了具有弯曲结构的锥形微纳米光纤中的模式耦合及干涉特性。建立了局部弯曲的锥形微纳米光纤的数学模型，为理论分析提供所需的数学依据；并利用Matlab仿真工具采用数值分析法对不同弯曲半径下锥形微纳光纤中的模式耦合进行了分析；详细阐述了基于局部弯曲的锥形微纳米光纤的模间干涉仪的工作原理，分析了弯曲锥形微纳米光纤的几何参数对输出干涉波形的影响，并给出了参数优化的原则。
第四章从实验上研究了覆盖微纳米液晶涂敷层的长周期光纤光栅的制备过程和大范围调谐特性。利用基于菲涅耳反射的折射率测量方法，对不同温度下的液晶折射率进行了测量和分析；采用简单的刷涂工艺在长周期光纤光栅表面制备了不同厚度的液晶涂敷层；通过实验研究了该光纤器件的热光特性和电光特性，并将仿真计算的结果与实验结果进行了比较。研究表明，覆盖约800nm厚度的液晶涂敷层的长周期光纤光栅在58°C-60°C的温度范围内会出现模式迁移现象，在此区域内，该光纤器件对温度具有非常高的响应灵敏度。文中利用这一特性，设计并实现了在特定温度下的长周期光纤光栅的大范围电光调谐，最大调谐范围达到约10nm。
第五章从实验上详细研究了基于局部弯曲的锥形微纳光纤模间干涉仪的制备工艺和传感特性。利用实验室自制的微纳光纤拉伸系统制备了具有不同锥长和束腰直径的微纳光纤，并将其弯曲成一近似对称的‘C’形弯曲结构形成模间干涉仪；详细研究了它们的温度特性以及折射率和微位移传感特性。理论和实验的研究表明，当弯曲的锥形光纤的束腰直径约为1.92μm时，该模间干涉仪的传输谱基本不受环境温度的影响。该光纤器件的环境折射率传感灵敏度和微位移传感灵敏度分别为658nm/RIU (refractive index unit)和102pm/μm。该器件可用于高灵敏度传感。
第六章对论文工作进行了总结和展望。
Miniaturizaion and integration are the current trends in photonictechnology. Micro/nanofabrication technology is the basis to manufacure theminiaturized or integrated optical devices. With the help ofmicro/nanofabrication technology, people can design and prepare variousoptical fiber micro/nano structures and devices. The optical devices exhibitssignificantly difference in characteristics, compared with the bulky countparts,as the functional structure size decreases to micro and/or nanometers. Thesespecial physical properties have wide theoretical research prospect andpractical application. In this dissertation, two kinds of novel fiber modalinterference devices with micro/nano structures are theoretically analized andexperimentally studied, including the mode coupling properties and relevantmicro/nanofabrication technologies. The operation principle, preparationprocess and functional properties of the proposed optical fiber devices arestudied in detail. By comparison with that of the traditional sized optical fiberdevices, the advantages in performance of the micro/nano structure basedones are verified. The main contents of this dissertation include:
Two kinds of typical optical fiber based modal interference devices andthe relevant micro/nanofabrication technology are proposed in the firstchapter. One typical optical fiber modal interference device is the opticalfiber grating. Adopting optic fiber surface micro/nano structure technology, a wide range tunable optical fiber grating based optical device is achieved. Theother typical device is the tapered optical fiber. Through the micro/nanooptical fiber fabrication technology, optical micro/nano fiber taper basedoptical devices is realized, which can be used for high resolution sensing.
In the second chapter, the properties of the cladding modes in longperiod fiber gratings coated with a high refractive index micro/nanometeroverlay are theoretically studied. The resonant wavelength and spectralcharacteristics of the four layer model long period grating are also analizedbased on the coupled-mode theory. Besides, the transmission spectra of longperiod grating with different overlay thickness and refractive indices arenumerically calculated. After investigating the relation of the resonantwavelength with the cladding thickness, we analyze the methods to improvethe tuning range of the long period fiber grating.
In the third chapter, the mode coupling and interference characteristicsin a locally-bent microfiber taper are theoretically analized. Themathematical model for locally curved micro/nano fiber taper is established.The working principle of its modal interference is described and the influenceof the fiber taper geometry on the interference fringes is discussed. Theoptimization principle for geometry parameters is presented.
In the forth chapter, a new structure LPFG coated with nanosizedhigh-refractive-index liquid crystal (LC) layer is experimentally realized. Therefractive indices of LC at different temperatures are measured. Using thesample brush coating technology, we have prepared different thickness LClayers on the surface of LPFGs. The cladding mode reorganization in highrefractive index coated LPFGs is theoretically analyzed and experimentallyobserved with the aim of exploring the sensitivity of the resonance wavelength to the change of the refractive index in a nanoscale overlay.Experimental results show that the transition between cladding modes andoverlay modes occurs when the refractive index (HRI) of the LC overlay ischanged from1.477to1.515by increasing its temperature from20°C to65°C. The spectral tuning ability of LPFGs coated with a HRI LC layer byelectro-optic modulation on a LC layer is also demonstrated, and themaximum tuning range can reach approximately10nm by choosing a highlysensitive operating point in the transition region, which is vertified with thetheoretical results in chapter2.
In the fifth chapter, the fabrication process and optic sensing propertiesof the locally bent microfiber taper based modal interferometer are presented.The microfiber taper fabricated by adiabatically stretching a heatedsingle-mode fiber is made in a C-shape bent to form a modal interferometer.The microfiber taper waist diameter can be optimized to minimize thespectral shift of the interferometer owing to the environmental temperaturechange. We show that the transmission spectrum of a microfiber taper withdiameter of about1.92μm has substantially small temperature dependence,which agrees well with the theoretical estimation. The proposed modalinterferometer has a high refractive index sensitivity of~658nm/RIU for RI=1.333-1.353and a high microdisplacement sensitivity of~102pm/μm. Theproposed device can be used for precision sensing applications.
Conclusions and expectation are made for the dissertation in the lastchapter.
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