Reducing Radiation Effects on Fiber Optic Quench Detection Sensors With Optical Annealing

Optical fiber sensors have many advantages for instrumentation in superconducting magnets, including a small footprint, immunity to electromagnetic interference, fully dielectric construction, and fast response times. However, using these sensors in magnets for fusion devices presents unique challenges from the combination of low operating temperatures and high radiation doses. In particular, Radiation Induced Attenuation (RIA) can cause severe signal degradation. One way to reduce RIA is optical annealing, in which high-intensity light propagating along the fiber is used to temporarily remove the defects responsible for RIA. Here we present measurements of RIA in fluorine-doped single mode fibers subjected to up to 1.2 MGy of gamma irradiation while maintained at 77 K. Optical annealing with 200 mW of 970 nm light increased the survivable dose by a factor of 2000 relative to a minimally annealed fiber, but the effect was significantly reduced when the sample length was extended from 45 to 205 m.