This review focuses on silica-based optical fibers, guiding light through the total internal reflection mechanism, that are currently used in telecommunications and sensor networks. Section I briefly introduces the fiber and fiber sensor technologies. Section II presents their various applications in radiation environments. In section III, the radiation effects on optical fibers are explained, first at a microscopic scale, with the description of point defects generated by irradiation. Then, the macroscopic consequences of the appearance of these point defects are discussed, in particular the phenomena of radiation-induced attenuation (RIA), radiation-induced emission (RIE) and radiation-induced refractive index change (RIRIC). RIA impacts nearly all applications under irradiation. We then detail the various parameters, intrinsic or extrinsic to the optical fibers that influence their amplitudes and kinetics. This improved knowledge paved the way for the radiation hardening of optical fibers through various approaches, either at material, component or system levels, that are described in section IV. Recent advances are presented for passive and for active rare-earth doped optical fibers and systems. Then, the use of fiber optic sensors is introduced, starting in section V with the latest advances in radiation-hardened point or distributed sensors. Then, the potential of this technology for radiation detection, beam instrumentation and dosimetry applications is assessed in section VI. Finally, the future prospects and the main challenges to be met in order to accelerate the implementation of this technology in the harshest environments are discussed.

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

This paper presents the state of the art distributed sensing systems, based on optical fibres, developed and qualified for the French Cigéo project, the underground repository for high level and intermediate level long-lived radioactive wastes. Four main parameters, namely strain, temperature, radiation and hydrogen concentration are currently investigated by optical fibre sensors, as well as the tolerances of selected technologies to the unique constraints of the Cigéo’s severe environment. Using fluorine-doped silica optical fibre surrounded by a carbon layer and polyimide coating, it is possible to exploit its Raman, Brillouin and Rayleigh scattering signatures to achieve the distributed sensing of the temperature and the strain inside the repository cells of radioactive wastes. Regarding the dose measurement, promising solutions are proposed based on Radiation Induced Attenuation (RIA) responses of sensitive fibres such as the P-doped ones. While for hydrogen measurements, the potential of specialty optical fibres with Pd particles embedded in their silica matrix is currently studied for this gas monitoring through its impact on the fibre Brillouin signature evolution.

We report here the response of a commercial ultra-low loss (ULL) single-mode (SM) pure silica core (PSC) fiber, the Vascade EX1000 fiber from Corning, associated with 0.16 dB/km losses at 1.55 µm to 40 keV X-rays at room temperature. Today, among all fiber types, the PSC or F-doped ones have been demonstrated to be the most tolerant to the radiation induced attenuation (RIA) phenomenon and are usually used to design radiation-hardened data links or fiber-based point or distributed sensors. The here investigated ULL-PSC showed, instead, surprisingly high RIA levels of ~3000 dB/km at 1310 nm and ~2000 dB/km at 1550 nm at a limited dose of 2 kGy(SiO₂), exceeding the RIA measured in the P-doped SM fibers used for dosimetry for doses of ~500 Gy. Moreover, its RIA increased as a function of the dose with a saturation tendency at larger doses and quickly recovered after irradiation. Our study on the silica structure suggests that the very specific manufacturing process of the ULL-PSC fibers applied to reduce their intrinsic attenuation makes them highly vulnerable to radiations even at low doses. From the application point of view, this fiber cannot be used for data transfer or sensing in harsh environments, except as a very efficient radiation detector or beam monitor.

We investigated the influence of modifying the voltage of an X-ray tube with a tungsten anode between 30 kV and 225 kV, and therefore its photon energy spectrum (up to 225 keV), on the Total Ionizing Dose deposited in a single-mode, phosphorus-doped optical fiber, already identified as a promising dosimeter. Simulation data, obtained using a toolchain combining SpekPy and Geant4 software, are compared to experimental results obtained on this radiosensitive optical fiber and demonstrate an increase of the deposited dose with operating voltage, at a factor of 4.5 between 30 kV and 225 kV, while keeping the same operating current of 20 mA. Analysis of simulation results shows that dose deposition in such optical fibers is mainly caused by the low-energy part of the spectrum, with 90% of the deposited energy originating from photons with an energy below 30 keV. Comparison between simulation and various experimental measurements indicates that phosphosilicate fibers are adapted for performing X-ray dosimetry at different voltages.

We investigated the nature, optical properties, and decay kinetics of point defects causing large transient attenuation increase observed in silica-based optical fibers exposed to short duration and high-dose rate x-ray pulses. The transient radiation-induced attenuation (RIA) spectra of pure-silica-core (PSC), Ge-doped, F-doped, and Ge + F-doped optical fibers (OFs) were acquired after the ionizing pulse in the spectral range of [∼0.8–∼3.2] eV (∼1500–∼380 nm), from a few ms to several minutes after the pulse, at both room temperature (RT) and liquid nitrogen temperature (LNT). Comparing the fiber behavior at both temperatures better highlights the thermally unstable point defects contribution to the RIA. The transient RIA origin and decay kinetics are discussed on the basis of already-known defects absorbing in the investigated spectral range. These measurements reveal the importance of intrinsic metastable defects such as self-trapped holes (STHs), not only for PSC and F-doped fibers but also for germanosilicate optical fibers as clearly evidenced by our LNT measurements. Furthermore, our results show that fluorine co-doping seems to decrease the RIA related to the strain-assisted STHs absorption bands in both types of optical fibers. Regarding Ge-doped glasses, besides a description of the defects responsible of the RIA, highlighting the STHs' role in their transient response, we provide a clear correlation between the GeX and GeY centers’ kinetics. In conclusion, the presented results improve our understanding of the transient RIA origin in the ultraviolet and visible domains. The lack of knowledge about the defects causing the RIA in the near-infrared domain will require future studies.