Development Of Local-structure-manipulated In-line Fiber-optic Interferometers And Their Sensing Applications

A good variety of in-line fiber-optic interferometers have been developed by exploiting the interference effect in optical fibers manipulated by the local fiber structures, which would be one important approach to realize different fiber-optic functional devices. This thesis aims to achieve highly sensitive in-line fiber-optic interferometers with smart structure, compact size, stable performances, low cost, and ease of deployment, and through taking the advantages presented by different interferometer structures, research work has been performed on the design, fabrication, and sensing performance characterization of the proposed fiber interferometers. The main content and research outcome of this thesis include the following aspects:Firstly, the important role of the fiber interferometerstructure on its performances has been emphasized through a brief introduction of the historical trail in the research progress of fiber-optic interferometers and related interferometer classification standard. A discussion about different techniques to control the fiber local structures provides a basis for the realization of fiber local structure control in this thesis. Based on an introduction of the mode properties, the conception of effective refractive index, and related simulation methods, the mode coupling characteristics of the optical waveguide around the fiber local structure with abrupt transition have been described, which provides a theoretical basis for the manipulation of light beam propagation to form different fiber-optic interferometers. And following a summary of the operation principles for different in-line fiber-optic interferometers, the spatial frequency analysis technique is introduced to analyze the optical path difference for the proposed fiber interferometers, which provides a basic method for the understanding and analysis of the formation mechanism and sensing characteristics of the local-structure-manipulated in-line fiber interferometers.Secondly,an up-fusion-bi-taper-pair (UFBTP)-based highly sensitive refractive index (RI) and bending sensor has been proposed based on the fundamental principle of the in-line fiber mode Mach-Zehnder interferometer (IL-FMMZI).Through fabricating the fiber Bragg grating (FBG) in the interference segment of the UFBTP, simultaneous measurement of index-temperature and bending-temperature have been achieved and the temperature cross sensitivity has been resolved effectively. In order to further improve the index sensitivity of the UFBTP, flame-scanning-tapering in combination with the HF-acid etching method have been employed to enhance the evanescent field intensity of the UFBTP. Within a refractive index range of1.33～1.41, the proposed UFBTP experimentally shows a sensitivity up to-48nm/RIU, and its bending sensitivity reaches-19nm/m-1for particular curvature range. For dual-parameter measurement, the index and temperature sensitivity reach-197nm/RIU and63.7pm/℃,respectively.Thirdly, the abrupt fusion between the microfiber (MF) and single-mode fiber (SMF) has been proposed and achieved, and the thinnest diameter of the MF is16μm. An in-line extrinsic fiber Fabry-Perot interferometer (IL-EFFPI) and an in-line intrinsic fiber Fabry-Perot interferometer (IL-IFFPI) are constructed based on the mutation splicing between MF and SMF, and their spectral as well as sensing characteristics have been investigated. Related studies indicate that the strain response of the open cavity IL-EFFPI based on the MF splicing is dependent on the diameter and length of the MF, while its index response is independent on the MF parameters. For the IL-EFFPI with a cavity length of～21μm and MF diameter of44μm, the strain sensitivity reaches167.4nm/N and the index sensitivity is about1331nm/RIU around the index of1.33. The structural parameters of the IL-IFFPI based on MF splicing could be conveniently adjusted, which is particularly suitable for the environmental parameter monitoring of confined space. For the IL-IFFPI with a cavity length of～136μm, the temperature sensitivity reaches15.3pm/℃with good linearity for a temperature range of25℃-1000℃. Finally, two types of in-line fiber MZI, namely the in-line fiber multimode MZI(IL-FMMZI) and an in-line fiber cavity MZI (IL-FCMZI), have been constructed by using the abrupt splicing between the MF and SMF, and their index sensing as well as temperature sensing characteristics have been investigated. For the IL-FMMZI with a cavity length of～10mm and MF diameter of～47μm, the index sensitivity is up to 436.58nm/R IU for the index range of1.33～1.38. For the IL-FCMZI with a cavity length of-688μm, its index sensitivity reaches-126056nm/RIU for the index range of1.4444～1.4462. Its temperature sensitivity is-31pm/℃with good linearity. Our proposed index sensor with ultrahigh sensitivity is particularly suitable for non-invasive biochemical sensing applications.