Mechanisms and Implementation of Thermo-Optical Annealing in Silica Fiber Sensors for Radiation-Induced Attenuation Mitigation

Abstract

In the context of quench detection systems for fusion superconducting magnets, temperature sensors based on optical fibers provide an effective solution for rapid, distributed measurement, with low sensitivity to electromagnetic interference. At the cryogenic temperatures and high radiation doses associated with this application, however, optical fibers undergo radiation-induced attenuation (RIA): light-absorbing point defects form within the silica glass structure, reducing the longevity and effectiveness of these sensors. In this work, we investigate the underlying microscopic defects and mechanisms of RIA and assess strategies for mitigation, namely, annealing via heat treatment (thermal annealing) and annealing via light propagation through the fiber (optical annealing, or “photobleaching”). We design a white light absorption spectroscopy setup with in-situ irradiation and optical annealing, working at liquid nitrogen temperature and different post-irradiation warm-up rates. For the pure silica core and F-doped cladding fibers studied, the RIA spectrum obtained is decomposed into known radiation-induced defect absorption bands, highlighting the key role of self-trapped holes in RIA at telecommunication wavelengths. Furthermore, absorption spectroscopy experiments are performed to show that thermal annealing at liquid nitrogen temperature is negligible, validating the transferability of the experimental results obtained at 77 K to 20 K applications. The decomposition of RIA into different defect contributions is supported by cold post-irradiation electron paramagnetic resonance (EPR) spectroscopy of fiber preform fragments, which reveals the presence of two types of paramagnetic centers: self-trapped holes and E'_gamma centers. The post-irradiation transient grating spectroscopy (TGS) technique is adapted to glass samples with continuous cooling at liquid nitrogen temperature and in-situ optical annealing. With this technique, we could observe the changes in thermal and acoustic properties resulting from the evolution of defect populations, with the potential to complement other experimental techniques to better understand RIA build-up and annealing kinetics. To improve the modeling of thermo-optical annealing, we propose future experiments including isothermal annealing tests and a larger exploration of optical annealing parameters. Our RIA build-up and annealing tests can help companies aiming to operate optical fibers under irradiation at cryogenic temperatures optimize their heat treatments to restore fiber transmission and the prevention of RIA during operation.