In this review paper, we present radiation effects on silica-based optical fibers. We first describe the mechanisms inducing microscopic and macroscopic changes under irradiation: radiation-induced ...attenuation, radiation-induced emission and compaction. We then discuss the influence of various parameters related to the optical fiber, to the harsh environments and to the fiber-based applications on the amplitudes and kinetics of these changes. Then, we focus on advances obtained over the last years. We summarize the main results regarding the fiber vulnerability and hardening to radiative constraints associated with several facilities such as Megajoule class lasers, ITER, LHC, nuclear power plants or with space applications. Based on the experience gained during these projects, we suggest some of the challenges that will have to be overcome in the near future to allow a deeper integration of fibers and fiber-based sensors in radiative environments.
We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the ...harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0-to-5 m) and temperature (20-to-100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants.
The radiation-induced attenuation (RIA) levels and kinetics depend on several parameters, including silica-based optical fiber composition, operation wavelengths, and preirradiation treatments. We ...studied in this work how the <inline-formula> <tex-math notation="LaTeX">{X} </tex-math></inline-formula>-ray RIA in the visible and near-infrared domains, up to a total ionizing dose (TID) of 100 kGy(SiO2), of a standard telecom Ge-doped fiber, changes when this optical fiber is subjected to different preirradiation treatments such as an H2-loading, with or without an additional thermal treatment at 50 °C. The performed systematic study, at room temperature (RT), allows to better characterize the influence of the presence of either molecular H2 or bonded atomic hydrogen on the generation and bleaching mechanisms of radiation-induced point defects. Our results show that if the hydrogen can passivate point defects at lower doses (< a few kGy), it globally negatively impacts the visible and near-IR RIA at doses larger than a few kGy. This hardening solution can then not be employed for this class of optical fibers (OFs).
We report pump-probe transient absorption measurements addressing the photocycle of the Germanium lone pair center (GLPC) point defect with an unprecedented time resolution. The GLPC is a model point ...defect with a simple and well-understood electronic structure, highly relevant for several applications. Therefore, a full explanation of its photocycle is fundamental to understand the relaxation mechanisms of such molecular-like systems in solid state. The experiment, carried out exciting the sample resonantly with the ultraviolet (UV) GLPC absorption band peaked at 5.1 eV, gave us the possibility to follow the defect excitation-relaxation dynamics from the femto-picosecond to the nanosecond timescale in the UV-visible range. Moreover, the transient absorption signal was studied as a function of the excitation photon energy and comparative experiments were conducted on highly- and weakly-germanium doped silica glasses. The results offer a comprehensive picture of the relaxation dynamics of GLPC and allow observing the interplay between electronic transitions localized on the defect and those related to bandgap transitions, providing a clear evidence that the role of dopant high concentration is not negligible in the earliest dynamics.
Fiber Bragg gratings (FBGs)-based temperature sensors present numerous advantages such as small packaging, fast acquisition rate, and accuracy for structural health-monitoring applications in nuclear ...environments. Among the various classes of FBGs, Type I FBGs are inscribed with a UV continuous or pulsed laser on a photosensitive fiber or with femtosecond laser even in nonphotosensitive fibers. These gratings, however, cannot survive at temperatures exceeding 400 °C. Regenerated FBG (RFBG) gratings, instead, are derivative from type I grating: when a high thermal treatment (> 650 °C) is applied after the inscription on a prehydrogenated fiber, a new grating (RFBG) appears with different thermal properties. It withstands temperatures as high as 1000 °C, opening the way to new application fields such as the temperature monitoring of the nuclear reactor cores. This work investigates the radiation response of RFBGs originated from type I seed FBGs inscribed with an argon laser (244 nm) in two different fibers (SMF-28e fiber or in a B/Ge co-doped fiber), loaded with either H 2 or D 2 before the inscription to enhance the fiber photosensitivity. Regeneration was achieved at 650 °C or 900 °C depending on the fiber type. After this, the RFBGs were irradiated under 40-keV X-rays at two different dose rates 1 Gy(SiO 2 )/s or 10 Gy/s and at two temperatures of irradiation-25 °C or 250 °C. At room-temperature (RT) irradiation, a Bragg Wavelength Shift (BWS) of about 35 pm was observed for the SMF-28e and more than 130 pm for the B/Ge fiber at 400-kGy dose, and the combined temperature and radiation constraints reveal that the RFBGs are radiation-tolerant. Indeed, no BWS is observed anymore, at the highest temperature of irradiation.
Ge-doped optical fibers (OFs) are considered radiation-tolerant waveguides. Even if their transmission in the infrared (IR) domain is degraded under irradiation, for most of the applications, their ...radiation-induced attenuation (RIA) remains acceptable. Space harsh environment is characterized by low dose and dose-rate constraints meaning that germanosilicate fibers are often employed for data links. However, the temperature can largely vary in space and is known to impact the RIA levels and kinetics. We studied here systematically the combined temperature and radiation effects induced on the transmission, at the telecom wavelengths, of a Ge-doped fiber, between −80 °C and 80 °C up to a total ionizing dose (TID) of 10 kGy(SiO 2 )-1 Mrad. Our measurements highlight larger RIA levels at low temperatures than at room temperature (RT). At our highest TID, it increases by a factor of ~40 and ~20, respectively, at 1310 and 1550 nm, when the irradiation is performed at −80 °C instead of RT. A model is reported to study the activation energy of the radiation-induced point defects responsible for the IR-RIA. This simple model could help in predicting the fiber vulnerability for various mission profiles.
We demonstrate a real-time high-spatial-resolution (3 cm) ionizing dose 2-D mapping sensor achieved by analyzing a phosphorus-doped silica-based optical fiber with a Rayleigh-optical frequency domain ...reflectometry (R-OFDR) interrogator. The dose is monitored through the radiation-induced attenuation (RIA) around 1550 nm measured on the OFDR traces. This study aims at studying the sensor performances and assessing its dose mapping capabilities. The RIA spectrum from the P-doped fiber is first characterized and the fiber radiation sensitivity is calibrated by evaluating the dose rate. Then, the dose-rate dependence (from 10 mGy/s up to 4 Gy/s) of the RIA levels and kinetics has been investigated up to a cumulated dose of 2 kGy (SiO<inline-formula> <tex-math notation="LaTeX">_{{2}}\text {)} </tex-math></inline-formula>. The achievable spatial RIA resolution with the R-OFDR is equal to 3 cm for precise distributed measurements. Finally, a 2-D square mapping of our X-ray beam has been performed during irradiation with a 10-s acquisition period. These results highlight the significance of distributed and active R-OFDR dosimetry for doses up to 2 kGy, as well as beam shape sensing with high spatial resolution.
Radiation-induced attenuation (RIA) based dosimetry exploiting phosphosilicate optical fiber has been established as a reliable and precise dosimetry technique, up to at least 500 Gy(SiO 2 ), in ...particular in the infrared and visible domains. For lower doses, the larger radiation sensitivity coefficients of those fibers in the visible domain (ξ > 100 dB·km −1 ·Gy −1 ) than in the IR domain (ξ ∼ 4 dB·km −1 ·Gy −1 ) appear as more appealing. However, in radiation-rich locations the devices are hardly to manipulate and have to be replaced after a certain dose when the dynamic range of the interrogator is attained or when doses exceed 500 Gy. To overcome this issue, we investigate the feasibility to reset and re-use P-doped multimode fiber dosimeters operating in the visible domain through photobleaching. To achieve this purpose, we injected post-irradiation five different 4 mW lasers 1550 nm, 1060 nm, 640 nm, 514 nm and 403 nm in an irradiated (5 Gy; 10 mGy/s) P-doped multimode OF to evaluate their potential to bleach the visible RIA. With the most efficient laser (403 nm), at least 80% of the visible range RIA is bleached in 2 hours. Successive re-irradiations of the same sample present the same RIA kinetics, confirming that the fiber calibration coefficients remain unaffected by the photobleaching process. Those results open the way to more advanced fiber dosimeter architectures with longer lifetime and higher achievable dose ranges.
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