Temperature sensing characteristics of metal coated FBG during dynamic cooling process

Temperature sensitivity of bare Fiber Bragg Grating (FBG) is quite low when used as sensor in the cryogenic environments, which is due to the decrease of thermal expansion coefficient and thermo-optic coefficient of silica at cryogenic temperatures. Therefore, metals with high thermal expansion coefficients were used to protect bare FBGs and increase their cryogenic temperature sensitivities. Specifically, FBGs were firstly coated with a thin conductive nickel layer by chemical plating and then electroplated with nickel, copper, zinc and tin. Sensing characteristics of the bare FBG and metal coated FBGs during dynamic cooling process were systematically studied assisted by a temperature controlled stage with the resolution of 0.1 °C. Results show that: (i) FBGs are well protected with the metal coatings; (ii) during dynamic cooling process, the curves of wavelength shift of the bare FBG and metal coated FBGs are both nonlinear; (iii) temperature sensitivities of Ni-coated FBG, Cu-coated FBG, Zn-coated FBG and Sn-coated FBG in the cryogenic environments are about 1.5 times, 2 times, 2.5 times and 3 times of that of the bare FBG, respectively; (iv) temperature sensitivities of FBGs decrease slightly with the increase of the cooling rates.