New Theoretical Method For The Coupling System Of The Fused Fibers And Its Application To Optical Fiber Devices And Sensors

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.