Sensing Of Surrounding Refractive Index Based On Polarization Characteristics Of Long-Period Fiber Gratings

Based on the three-layered medium model and full-vector coupled-mode equations,we propose two novel surrounding refractive index(SRI)sensing schemes based on the Stokes parameters characteristics of the output light from a thin-cladding UV-induced birefringent long-period fiber grating(LPFG).One is based on the core-only birefringent LPFG with multiple input wavelengths,the other is based on the core-only birefringent LPFG with single input wavelength.The work in the thesis is categorized as follows:(1)Conventional LPFG-based SRI sensors require a broadband light source to trace the resonant wavelength shift as a result of SRI variations.However,as for the SRI sensor proposed here,a tunable laser source is needed and the linear measurement scope is enlarged.In this thesis,we propose a novel SRI sensor based on the Stokes parameters characteristics of the output light from a thin-cladding UV-induced birefringent LPFG.To demonstrate the sensing principle,the LPFG with core-only photoinduced birefringence is analyzed using full-vector coupled-mode equations and the Stokes parameters.The effects of LPFG parameters and incident light parameters on the sensing properties are analyzed,respectively.The result indicates that when the completely-polarized input light is in the wavelength range of 1690nm-1710 nm,the LPFG-based sensor with a cladding radius of 20.75?m shows a good linearity performance over a much broader SRI measurement range from 1 to 1.46.The sensor sensitivity is 0.02588/RIU(RIU is refractive index unit).The novel SRI measurement approach has the potential to be applied to the humidity and vapor sensing in the SRI range of 1 to 1.33.(2)The LPFG sensors based on a tunable laser source and conventional LPFG sensing normally make SRI measurement in the range of 1.32 to 1.46 with a small range of linear sensitivity.To achieve a broader linear measurement,a method for SRI sensing with a single wavelength,which is based on the core-only photoinduced birefringence of LPFG,is proposed and studied using the full-vector coupled-mode theory.With a completely-polarized light near the resonant wavelength of the LPFG being launched into the fiber,the spherical distance change yielded by the output light polarization state variation on the Poincare sphere are investigated as the SRI varies.The analysis indicates that the SRI sensor with a cladding radius of 20.75?m shows a good linearity performance and high sensitivity of 0.356/RIU over the SRI measurement range from 1 to 1.30.Unlike the conventional LPFG sensors,the proposed LPFG sensor may be applied to the humidity and vapor sensing.The analysis provides the theoretical basis for the design of other types of birefringent-LPFG sensors.