Research On The Theory And Sensing Characteristics Of Long-period Fiber Gratings

Based on the two fiber analysis models, namely the two-layered medium model andthree-layered medium model, and the sensoring theory of long-period fiber gratings, thesurrounding refractive index, temperature and axial strain sensitivities of a long-period fibergrating at different clad radii are analyzed. Moreover, in analogy with the Mach-Zehnderinterferometer, a cascaded long-period photonic crystal fiber grating temperature sensingstructure is proposed. Based on the coupled-mode theory and the transfer matrix method, thesensoring characteristics of the structure are studied. The work detailed in the thesis iscategorized as follows:(1) For the purpose of analyzing accurately the sensoring characteristics of a long-periodfiber grating and providing guidance to the analysis and design of long-period fiber gratingbased sensors and solving their cross sensitivity problems, the contrast between the effectiverefractive indices of the core mode in an uniaxial-crystal fiber yielded by two fiber analysismodels is demonstrated and analyzed. On its basis, we calculate the surrounding refractiveindex, temperature and axial strain sensitivities of a long-period fiber grating. The resultshows that the core-mode effective refractive indices difference produced by the two fiberanalysis models is significant, especially in the case of thin cladding layer, and moreover, thesensitivities calculated from the two layer model and/or the materials’ specific opticalcoefficients deviate markedly from those calculated from the three layer model and thecorresponding modal effective optical coefficients. Therefore, the latter is critical to theaccurate modeling of long-period fiber grating based sensors. On its basis, we study thesurrounding refractive index, temperature and axial strain sensing characteristics oflong-period fiber gratings for different clad radii and cladding mode orders theoretically andexperimentally. The result shows that the change of signs of the waveguide dispersion factorsof various cladding mode leads to the same change of the surrounding refractive index,temperature and axial strain sensitivities at a certain clad radius. Moreover, for a givencladding mode order, the surrounding refractive index sensitivity increases and thetemperature sensitivity decreases with the increase of the clad radius, whereas the axial strainsensitivity increases for the low-order cladding mode and decreases for the high-ordercladding mode.(2) The bandwidth of the transmission dips of a single long-period grating is, in mostcircumstances, comparatively large, which limits the temperature sensitivity and themeasurement accuracy. To solve the above problem and realize the wavelength tunability, wepropose a cascaded long-period photonic crystal fiber grating temperature sensing structure inthe form of a Mach-Zehnder interferometer. Based on the coupled-mode theory and thetransfer matrix method, the temperature sensing characteristics of the structure are studied. The temperature sensor is constructed by cascading two identical long-period gratings on aphotonic crystal fiber filled with liquid ethanol. Through controlling the temperature ofethanol, we can change the effective indices of the coupled modes, hence the resonantwavelengths of the transmission spectrum. Therefore, we realize a wavelength tunabletemperature sensor. The resonant wavelengths corresponding to the three transmission dipsshift to the red linearly with the temperature increases. The bandwidths of the transmissiondips are significantly reduced in comparison with that of a single long-period grating, whichwill improve the measurement accuracy of the sensor.