Metal coating for enhancing the sensitivity of fibre Bragg grating sensors at cryogenictemperature

Abstract

Fibre Bragg grating (FBG) sensors that are immune to electromagnetic interference couldadvantageously perform cryogenic temperature monitoring in superconducting magneticfields, but their intrinsic temperature sensitivity is quite poor and must be enhanced. Infact, the low thermal expansion coefficient of silica limits the temperature sensitivity ofbare FBG sensors at cryogenic temperature.In this paper the possibility of improving the temperature sensitivity of FBGsensors by metal coating is investigated. Specifically, zinc and copper coatingdepositions are performed by the traditional electrowinning process, after aluminiumpre-coating of the sensor. Coated FBG sensors are inspected by both opticaland metallographic techniques. SEM metallographic investigations show that ahomogeneous deposit is obtained, with good metal adhesion to the FBG sensor.Optical testing shows that the optical properties of the coated FBG sensors areslightly affected: aluminium pre-coating produces appreciable modification ofthe diffraction spectrum in both peak width and peak shift, while zinc coatingproduces a moderate peak shift and copper coating just enlarges the peak width.Results presented in this paper show that both metals appreciably increase thetemperature sensitivity of the FBG sensors. Zinc coating provides the highest sensitivityand high-resolution temperature measurements are possible at temperatures as low as15 K.