Optical Fiber Bragg Grating Response Function and its Temperature Dependence for a Polymer Coating of Temperature Dependent Thermal Expansion Coefficient

In-fiber Bragg gratings are highly sensitive to thermal and strain fields and have been widely used as temperature and strain sensors. Effects of the strain-optic and thermo-optic elements of optical fiber Bragg grating (OFBG) response function are often mixed, inter-competing and complicated to isolate. Moreover, both these elements are influenced by the thermal expansion coefficients, \textit{$\alpha $}, of the fiber core, cladding and the protective coating composite. Polymer protective coatings with large \textit{$\alpha $} have been reported to improve temperature sensitivity ($\sim $ 0.1 nm/\r{ }C) of the response function. However, the results are complicated and less reliable due to non-linearity of \textit{$\alpha $}(${\rm T})$ when using wider temperature range (such as 20 K -- 400 K). This paper reports determination of the OFBG response function for a polymer coating of a large but linear \textit{$\alpha $}(${\rm T})$. The results have been compared with responses from OFBGs coated with materials of non-linear \textit{$\alpha $}(${\rm T})$.