Andrei Gusarov

Radiation Effects on Silica-Based Optical Fibers: Recent Advances and Future Challenges

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.

High total dose radiation effects on temperature sensing fiber Bragg gratings

Fiber Bragg gratings (FBGs) written in a 10 mol.% Ge-doped core silica fiber using a phase mask were exposed to /spl gamma/-radiation. The transmission and reflection spectra were recorded during irradiation up to doses in excess of 1 MGy. There was no detectable change of the Bragg peak amplitude and the grating temperature sensitivity. The radiation-induced shift of the Bragg wavelength saturated at a dose of 0.1 MGy at a level less than 25 pm, which could still be decreased by optimization of the grating parameters. Our results confirm that FBGs are good candidates for sensing applications in radiation environments.

Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors

In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing ca- pabilities of fiber Bragg grating (FBG) temperature sensors. Our experi- ments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an air- cooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the differ- ence saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 10 19 neutron/cm 2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can sur- vive in such radiation conditions, but at 90°C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under well- specified conditions.

Proton- and Gamma-Induced Effects on Erbium-Doped Optical Fibers

We characterized the responses of three erbium-doped fibers with slightly different concentrations of rare-earth ions (240-290 ppm) and Al2O3 (7-10 wt.%) during proton and gamma-ray exposures. We have simultaneously measured the radiation-induced attenuation (RIA) around the Er3+ ion pumping wavelength (980 nm) and the associated changes of the Er3+ emission around 1530 nm. The three erbium-doped fibers show similar radiation responses. All fibers exhibit RIA levels between 9 times 10-3 and 1.7 times 10-2 dB m-1 Gy-1 at 980 nm and between 4 times 10-3 and 1.1 times 10-2 dB m-1 Gy-1 at 1530 nm. Protons and gamma-rays lead to similar radiation damages, with small differences between the protons of different energies (50 MeV and 105 MeV). Furthermore, we have performed online measurements of the spectral dependence of RIA from 600 to 1600 nm and offline measurements from 1200 to 2400 nm. The three fibers exhibit the same spectral response. Losses decrease monotonically from the visible to the infrared part of the spectrum. We have performed spectral decomposition of these RIA curves with the help of absorption bands previously associated with radiation-induced point defects. Our analysis shows that the main part of the RIA (600-1700 nm) in erbium-doped glass can be explained by the generation of Al-related point defects. The other defects related to the germanium and phosphorus doping of the silica seem to have a lower contribution to the induced losses. The Er3+ ion properties seem to be mainly unaffected by proton exposure, suggesting a solvation shell around the Er3+ ion formed by Al2O3 species.

Radiation Effects on Fiber Gratings

Fiber Bragg and long period gratings are photonic components that find numerous applications in telecommunication and sensing. In some cases, such as space, high-energy physics, and nuclear industry, those applications include the presence of ionizing radiation. It is therefore essential to evaluate their radiation response. In this paper, we review radiation effects on various types of fiber gratings.

Behavior of fibre Bragg gratings under high total dose gamma radiation

We report on the effect of MGy dose level /spl gamma/-irradiation on the parameters of fibre Bragg gratings intended for sensing applications. The /spl gamma/-radiation sensitivities of gratings written with near-UV 330 nm light in hydrogen loaded Ge-doped fibres and of a grating written in a N-doped fibre were found to be higher than that of gratings written in a 10 mol.% Ge-doped fibre without hydrogen loading. In the former cases, changes in the amplitude and the width of the Bragg peak were observed during /spl gamma/-irradiation while no change was observed in the latter case. For the grating written in the N-doped fibre, the radiation-induced shift of the Bragg peak did not saturate while for gratings written in hydrogen-loaded Ge-doped fibres it saturated to a higher level than for gratings written in unloaded Ge-doped fibre.

An Introduction to Radiation Effects on Optical Components and Fiber Optic Sensors

We review the effects of ionizing radiation on various types of optical components including optical fiber sensors and summarize some of their applications in particular environments where the presence of energetic radiation is a concern.

Stabilization of Fiber Bragg Gratings Against Gamma Radiation

The effect of gamma-radiation on the spectral characteristics of FBGs has been studied experimentally. The FBGs were fabricated in optical fibers with a GeO2 concentration in the core in a range from several to 99 mol.%. Various pre- and post-fabrication treatments were applied with the aim to improve the radiation tolerance of the FBGs. The best result was obtained for a Type IIa annealed grating written in SM310 fiber, where the Bragg peak shift saturated at a 12 pm level and an amplitude change of 0.04 dB with the maximal dose of about 50 kGy.

Effect of gamma–neutron nuclear reactor radiation on the properties of Bragg gratings written in photosensitive Ge-doped optical fiber

Abstract We have experimentally studied the effect of mixed gamma–neutron nuclear reactor radiation on the effective refractive index of Bragg gratings fabricated in Ge-doped photosensitive optical fiber. We compared the effect of radiation on gratings written with and without hydrogen sensitization and show that the use of hydrogen loading results in an increase of the sensitivity to reactor radiation. Neither grating annealing at 500 °C nor pre-irradiation of the fiber results in a significant increase of the radiation hardness of the Bragg gratings. The best radiation stability is observed for gratings fabricated without hydrogen loading.

Gamma radiation effects in Er-doped silica fibers

The radiation behavior of a commercially available Er-doped fiber is evaluated under varying gamma dose rates with in-situ spectral loss measurements. Complementary post-irradiation photoluminescence measurements allow us to better understand the radiation effects. Our results suggest that the microscopic environment of the Er/sup 3+/ ions is not much affected by the gamma irradiation, unlike the host matrix. We discuss the impact for potential applications of these commercially available fibers in radiation environments.