Effect of Bonding Material and Fixing Techniques on the Calibration of Copper–Indium Bimetallic-Coated FBG-Based Cryogenic Temperature Sensors for Aerospace Applications

To enable fiber Bragg gratings (FBGs) to operate effectively as cryogenic temperature sensors, metals with high thermal expansion coefficients should be deposited onto the surface of an optical fiber. In this work, targeting aerospace applications, a copper–indium bimetallic-coated FBG designed for operation at around 850 nm was manufactured using sputtering and electrodeposition techniques. Furthermore, the influence of bonding materials, specifically Kapton tape and Apiezon N grease, as well as fixation methods, on the performance of the coated FBG was investigated during cryogenic temperature calibration. The results demonstrate that with Kapton tape bonding, the Bragg wavelength shift (BWS) was approximately 1.6 times greater than that obtained with Apiezon N grease at 77 K during sensor calibrations. However, in order to have efficient thermal coupling between the sample holder and the metal-coated sensor in harsh environments such as cryogenic and vacuum environments, the optimal choice of bonding material as well as its architecture is mandatory to achieve high sensor sensitivity. Therefore, this study gives insights into the fact that metal-coated FBGs’ calibration accuracy and real-time stability during sensor deployments can be affected by the fixing techniques and the bonding materials used.