Optimization of surface bonding methods for fiber Bragg grating sensors at cryogenic temperatures

•Optimized surface bonding configuration for FBG sensors in extreme environment.•Excellent signal is observed in FemtoSecond Grating with polyimide coating at cryogenic temperatures.•Bonding FBG using cryogenic bonding agent Stycast and vacuum grease Apiezon N have been investigated. Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements.