Bend Limitation of a Polyimide-Coated Optical Fiber at Cryogenic Temperature of 77 K

A Raman-based distributed temperature sensor (RmDTS) system is a promising method for quench detection in high-temperature superconducting (HTS) magnets. To sense the temperature distribution of HTS magnets, optical fibers need to be wound with HTS wires synchronously. Accordingly, the bend limitation of the sensing optical fibers at 77 K is very important for an HTS coil temperature measurement. In this study, a segment with a 2.5-m-long polyimide-coated multimode optical fiber was bent as the curvature radii of 3, 5, 10, 15 and 25 mm, respectively, and applied to sense a room and a liquid nitrogen temperature. The experimental results of bending and without bending optical fiber show that when the curvature radius of the optical fiber is smaller than 10 mm, the temperature measurement is unreliable. The recommended minimal curvature radius of the used optical fiber for applying the RmDTS system to monitor the HTS coil temperature is 10 mm.