Strain response of FBG at liquid nitrogen temperature

The strain sensing properties based on fiber Bragg gratings (FBGs) at the liquid nitrogen temperature (77 K) were investigated. It was found that the single central peak in the reflection spectrum of FBGs at the ambient temperature is split into multiple peaks at the cryogenic temperature. This abnormality is attributed to the severely inhomogeneous thermo-elastic strain over the greeting of FBGs due to the mismatch of the coefficients of thermal expansion between the glass fiber core, epoxy adhesive and metal substrate, which transforms the original uniform grating into a chirped one. Some technologies were developed to eliminate the multi-peak phenomenon and, eventually, the strain responses of FBGs at the ambient and liquid nitrogen temperatures were measured. The experimental results show that the strain sensibility of FBGs is independent of temperature, which demonstrates that FBGs are excellent cryogenic strain sensors.