Fabrication And Sensing Characteristics Of Polymer Bragg Fiber Gratings

As a new type of sensor based on wavelength detection, the fiber Bragg grating has many advantages such as small size, light weight, anti-electromagnetic interference, and good repeatability, and is one of the mainstreams in current research of sensors. While most of fiber Bragg grating sensors are manufactured within quartz optical fiber, the polymer fiber grating has been attracting more and more attention since it was first reported in 1999. Compared with quartz fiber gratings, Polymer fiber gratings have more advantages such as flexibility, good biocompatibility, smaller Young's modulus, larger thermo-optic coefficient and thermal expansion coefficient; so they can provide more accurately sensing with higher sensitivity and wider response range. In this thesis, Fabricating and writing polymer fiber Bragg grating within PMMA doping BDK and its sensing properties are studied in depth both in theory and experiment including fabricating single-mode and multi-mode polymer fiber Bragg grating, the sensing properties of stress, strain and temperature of single-mode polymer fiber Bragg grating, and the differences of the sensing properties for different modes of multi-mode polymer fiber Bragg grating. And a number of innovative achievements are gained. The main contents are summarized as follows. The physical model of polymer fiber Bragg gratings is described, and the influences of each parameter of the polymer fiber Bragg grating on its spectral characteristics are calculated by using coupled-mode theory. Several common fiber grating etching technologies are introduced. The influence of mask template zero-level diffraction on the period of fiber Bragg gratings is analyzed, and the improved mask etching method used in the thesis is proposed. The single-mode and multi-mode polymer optical fiber with a core doped by the photosensitive material BDK using drawing perform method, within which the Bragg grating is written, and its spectrum is measured. By monitoring the dynamic spectrum of the polymer fiber Bragg grating, the photosensitivity of the PMMA polymer optical fiber doped by BDK in the core is studied, and the change of its core refractive index with exploring time is analyzed. The sensing properties of stress, strain and temperature of polymer fiber Bragg gratings are studied, and are compared with that of quartz fiber gratings. Different from quartz, polymer materials is a kind of macromolecular material, and their biggest feature is the mechanical characteristics of the viscoelastic, mainly in two aspects of creep and stress relaxation. With the standard solid model of linear three components for polymers, viscoelastic characteristics of the polymer materials are analyzed, of which the influences on the sensing properties of stress, strain are also analyzed. In the model the influence of creepage on stress characteristics of polymer fiber gratings and the influence of stress relaxation on strain characteristics of polymer fiber gratings are mainly considered, which is consistent with the experimental results. And the results show that the polymer fiber Bragg grating will result in an instantaneous wavelength shift when applied stress, and it will creep towards longer wavelength as time and will reach equilibrium at a certain time. In a small stress range, the change of the creep is linearly proportional to the applied stress. Stress relaxation of polymer materials will influence the sensing properties of polymer fiber Bragg grating and also result in the slow drift of the Bragg wavelength, but the drift is very small in small strain conditions, and the change of Bragg wavelength with strain has good linearity. Compared with quartz fiber gratings, polymer fiber gratings have extremely high stress and temperature sensitivity, which is about ten times of that of the quartz fiber grating, and the strain sensitivity of polymer fiber gratings is comparable to that of quartz fiber gratings.The coupling of different modes in multi-mode fibers is analyzed using coupled-mode theory, and the reflecting spectrum of the multi-mode polymer fiber Bragg grating is simulated. For different modes in multi-mode polymer fiber gratings, the response characteristic of their Bragg wavelength to strain and temperature is analyzed in theory and verified in experiment. And the results show that the coefficients of Bragg wavelength sensitivity to strain for these modes are almost identical. The coefficients of Bragg wavelength sensitivity to temperature for these modes are related to thermo-optic coefficients of the core and cladding materials and the larger the difference of the thermo-optic coefficients of the core and cladding materials, the larger is the difference the coefficients of Bragg wavelength sensitivity to temperature. For polymer optical fiber, because different kinds of polymer materials have large differences of the thermo-optic coefficients and they may be compatible, we can construct a kind of polymer optical fiber, of which the core and cladding materials have large difference of thermo-optic coefficients, so different modes have different sensitivity to temperature. Based on this characteristic, a sensing method is proposed, which separately realizes strain and temperature sensing by utilizing different modes of polymer fiber gratings and some calculation is simulated.