摘要
锥形光纤是光波导理论和技术领域的一个很重要的研究课题，由锥形光纤构成的单模光纤器件包括有光纤耦合器、光纤波分复用器、光纤准直器、光纤扩束器、光纤滤波器和由锥形光纤构成的光纤传感器件包括有耦合式传感器、双锥形弯曲传感器和渐逝波传感器及其它熔锥型光纤器件已广泛应用于光纤通信和光纤传感等领域。
根据国内外有关熔融拉锥耦合理论的研究概况和现状，本论文首先从熔锥形光纤的动态形状曲线、空气层对熔锥形光纤传输特性的影响和熔锥形光纤波导耦合三个方面入手，以解决目前不能严格而完整地表示熔锥形光纤的形状曲线函数和熔锥形光纤耦合的分段处理等问题．然后，将所获得的结果具体应用于三种光纤无源器件的研制开发上。最后，对熔锥耦合式光纤渐逝波传感器进行了理论和实验研究。
本论文共分十章。第一章概述了光纤熔锥型耦合理论的发展状况。第二章根据熔锥型光纤的形成机理，导出了数理微分方程，利用动态熔融区长度的概念，结合初始、边界和锥区体积守恒条件，由行波法首次获得完整而精确地描述光纤熔融拉锥动态形状曲线解析函数，这种解析解对于任意参数的灯头，能方便、精确和完整地描述熔融光纤在不同拉锥过程中锥体动态形状的变化，并以大量光纤熔融拉锥实验进行了验证。应该指出：只要已知初始熔融区长度，获得的解能预知任意拉伸长度下的拉锥锥形曲线。从而，完整地解决了以往在光纤熔锥耦合的理论分析计算上难以严格导出拉锥动态形状曲线函数的困难。第三章将等效阶跃光纤方法推广到任意折射率分布的多包层结构光纤。多包层结构光纤的折射率分布一旦确定，利用这一方法能精确地计算得到其等效阶跃光纤的归一化折射率差，从而可方便地预知该多包层光纤的各传输特性参数。同时，将该方法应用于锥形光纤，以考虑空气层对锥形光纤波导光传输特性及其耦合效应的影响。第四章首先建立任意横截面介质光波导间的模耦合方程及其耦合系数的计算公式，并就相同单模光纤的情况，与R．Vanclooster等人的经典理论计算结果进行比较，验证了本论文的耦合理论。同时，结合第二，三章的理论结果，通过对光纤熔融拉锥时耦合输出功率随拉伸长度变化的动态耦合过程的精确描述及其与实验结果的比对，验证本论文的系列理论。第五章至第八章分别分析研究四种光纤无源器件，也即三纤双环熔锥互耦全光纤滤波器、单模光纤超平坦宽带熔锥型耦合器、开环三纤耦合光纤光开关以及基于2×2熔锥型单模光纤耦合器和磁流体薄膜的光纤光开关。第九章在分析研究光纤渐逝波及其耦合传感原理的基础上，对熔锥耦合式高灵敏度光纤渐逝波温度传感器系统进行较深入的理论和实验研究。最后，第十章对整篇论文进行了总结。
The fused tapering optical fibers are the significant research project both in the theory and in the technology of the optical waveguide. Many optical fiber devices can be developed from the fused tapering optical fibers such as couplers, wavelength division multiplexers (WDMs), coaxial couplers, beam expanders, wavelength filters and so on. In addition, many types of optical fiber sensors such as coupler sensors, biconically tapered bend sensors, evanescent-wave sensors can also be made from the fused tapering optical fibers devices. And in such sensors the transmission through the tapered coupling zone depends on the index refraction of the external medium. In all, the fused tapering optical fiber devices have been extensively used in optical fiber communications and optical fiber sensors.
Based on the current status of the coupling theory of the fused tapering optical fibers, at first, this dissertation mainly deals with three aspects including: (1) the dynamic-shape-curve function for the fused tapering optical fibers; (2) the air-layer influence for the transmission characteristics of the fused tapering optical fibers; (3) the waveguide-coupling of the fused tapering optical fibers, in order to obtain the accurate and integrative expression of the shape-curve and to solve the problem of the sectional treatment of the coupling for the fused tapering optical fibers. Then it makes introduction of three novel optical devices which have been developed with the obtained theoretical results. At last, in this dissertation fused tapering fiber coupler evanescent-wave sensors are investigated both in theory and in experiment.
The dissertation is arranged in ten chapters. In chapter 1, there are brief introductions of basic configurations and characteristics, development trends, and application fields of the tapered fibers. In chapter 2, a physics-mathematics equation of dynamic-shape curves just for the fused tapering optical fibers is deduced. The closed-form solution of the equation can be acquired by use of its initial, boundary, volume-conserved conditions and the traveling-wave method. The shape curves in different tapering processes can be described by the solution conveniently, accurately and integratively. A large quantity of experimental data has been provided and shown to have a coincidence with the theoretical results properly. Especially, when the initial length of the fused zone is known, the solution can predict the whole tapering shape curve with an arbitrary elongating length. Thereby, we solved the long before troubling problem in that how to deduce the function of the dynamic tapering-shape-curve strictly and integratively in the theoretical calculation of the coupling for the fused tapering optical fibers. In chapter 3, an equivalent step-index-fiber method (ESFM) has been extended to multicladding fibers for any refractive index profile. Upon defining the refractive index profile for the multicladding fiber, the equivalent normalized index difference for the equivalent step-index-fiber can be exactly derived by the method so that varieties of transmission parameters for the fiber would be predicted conveniently. Then the extended method has been used to analyze the transmission and coupling characteristics of the fused tapering optical fiber. In chapter 4, firstly, coupling-mode equations for the medium optical waveguide with the any cross-section-shape are established and the coupling coefficients are derived. Then, under the condition of the same single-mode optical fibers, the coupling theory has been confirmed by comparing with the results of R. Vanclooster's classic coupling theory. Meanwhile, combining with the theoretical results in chapter 2 and 3, a series of the theories in this dissertation can be confirmed by describing accurately the dynamic coupling processes for the output coupling powers with the elongating length and comparing with the experimental results. In chapter 5, 6, 7 and 8, four novel optical fiber devices are presented and analyzed theoretically and experimentally using the above theory of the fused tapering fibers, which include all-fiber filter with three fiber-two ring mutually fused biconical couplings, super-flat-wideband single-mode optical fiber couplers ranged from 1250nm to 1650nm, fiber-optic switch using a three fiber-one open loop coupler, and a fiber-optic switch using 2×2 fiber fused coupler and magnetic fluid film. In chapter 9, based on analysis of evanescent wave transmission and coupling sensor on single-mode optical fibers, an optical fiber evanescent wave temperature sensor has been investigated comprehensively in theory and experiment. At last, in chapter 10, the conclusion for the whole dissertation is made.
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