Optimized Cryogenic FBG Sensitivity Coefficient Calibration for High-Precision Thermal Expansion Measurements

Under conventional temperature regimes, optical fiber sensors (OFS) have been extensively utilized for real-time monitoring of strain and temperature responses in transportation, energy, and civil engineering structures. However, in low-temperature environments, OFS applications confront challenges such as poor adhesion, diminished sensitivity, weak signal transmission, and complex compensation requirements. Therefore, it is crucial to conduct in-depth research on the high-precision calibration characteristics of strain and temperature in cryogenic conditions. In this study, we propose an enhanced calibration technique for fiber Bragg grating (FBG) strain/temperature sensitivity, covering a low-temperature range of 77–296 K. Employing a custom-designed low-temperature testing apparatus, we calibrated the thermo-optic coefficients, temperature, and strain sensitivity coefficients. Subsequently, this improved method was applied to measure the coefficient of thermal expansion of various materials. The results confirm the accuracy of the proposed method and present the strain/temperature measurement properties of FBG. This research provides guidance for the precise use of FBG in low-temperature environments.