Study On Fiber Grating-based Novel Sensing Technologies And High Temperature Devices

Fiber gratings, as key passive fiber components, have a wide range of applications in optical fiber communication, optical fiber sensing and other fiber-optic technologies. The emergence of fiber gratings marks a technical milestone in the fiber-optic field. This thesis mainly focuses on the novel fiber grating sensing technology and the fabrication technique of high temperature fiber gratings. Comprehensive studies on these topics have been conducted both theoretically and experimentally.To begin with, a brief overview of the historical background of fiber and the development of fiber grating technology are given. The applications of fiber gratings in communication and sensing are introduced, and the classification of fiber grating sensing and its market prospect are discussed. The recent research topics on fiber grating sensing technology are also summarized.Then, a brief discussion on the photosensitivity of fiber is given. The basic theoretical analysis tools of fiber gratings, including diffraction theory, coupled-mode theory and transfer matrix method, are introduced. The classification of fiber grating is summarized and several typical kinds of fiber gratings are briefly introduced along with their typical spectra. Brief introduction on the fabrication techniques for fiber gratings, the hydrogen-loading setup and the fabrication system used in this thesis, is also given.After that, novel flow speed/rate sensing technology based on laser-heated fiber Bragg gratings (LHFBGs) is studied and developed. LHFBGs are novel components and have some special properties, e.g. photothermal conversion, laser-control wavelength tuning, which enable many unique applications. Based on analyzing the thermal distribution of the LHFBG and the hot-wire anemometry, LHFBG-based all-fiber air speed sensor and microfluidic flow rate sensor are proposed and experimentally demonstrated. By employing an FBG interrogator, the wavelength response, sensitivity and response time of these sensors are investigated in detail under different conditions, and high-resolution, high-sensitive flow speed/rate sensing is achieved.Furthermore, a fiber-optic refractive-index sensor based on laser-heated fiber long-period gratings (LHFLPGs) with active temperature compensation is proposed and experimentally demonstrated. Experimental investigation of the temperature sensitivity, refractive-index sensitivity and the wavelength tuning feature of the LHFLPGs, is well conducted. In theory, the proposed sensor is proved to be feasible, and temperature-insensitive refractive-index sensing is achieved experimentally.Finally, high temperature regenerated fiber Bragg gratings (RFBGs) are systematically studied. A novel fabrication technique for high temperature RFBGs is presented-during the viscoelastic tuning of the RFBG under high temperature, it is possible to employ a particular temperature profile to customize the profile of the grating period, enabling permanently tuned RFBGs with different wavelength and bandwidth. Using this technique, a narrowband uniform grating produced by uniform phase mask is permanently tuned into a broadband chirped grating, with arbitrary bandwidth and central wavelength. By employing the viscoelastic tuning technique, FBG array with three gratings with the same wavelength, is also permanently tuned into another FBG array with three different wavelengths, which is applicable to quasi-distributed high temperature sensing.In conclusion, novel fiber grating sensing technology and the fabrication technique of high temperature fiber gratings are investigated both theoretically and experimentally in this thesis, which would contribute to the development of the fiber grating technology and its applications.