Cryogenic fiber optic temperature sensors based on fiber Bragg gratings

Results for several cryogenic fiber optic sensors based on fiber Bragg gratings (FBG) will be presented. Bragg gratings are periodic changes in the index of refraction in the fiber core that reflect light at a specific wavelength. The peak wavelength also depends on the strain in the FBG. A shift in the peak position can be used to measure strain or temperature. Temperature sensitivity is directly related to the thermal coefficient of expansion (TCE). Results of different high-TCE materials including Polypropylene (PP), PMMA (polymethyl methacrylate), and Teflon (PFA) will be presented. We will present results from LN2 thermal cycling, calibration from 10 K to 300 K. and dynamic fatigue tests at ambient and LN2. Calibration results demonstrated PP coated FBG with sensitivity greater than 40 pm/K at 77 K. Sensitivity at 20 K was approximately 7 pm/K. A wavelength resolution of 10 pm would translate to a temperature resolution of +/-0.5 K at 77 K and +/- 1.5 K at 20 K. The FBG sensors were tested for dynamic fatigue at room and at liquid nitrogen temperatures. The acrylate-coated FBG were also thermally shocked from ambient to LN2 40 times, and then examined under an SEM. No evidence of micro cracking was observed. Multiple FBG sensors in one fiber were made and tested.