We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds −43 rad/MPa × m at 1.55 μm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.

Raman Distributed Temperature Sensors (RDTS) are attractive for the monitoring of large structures in nuclear power plants such as containment structures and coolant loop systems. We demonstrate the high radiation tolerance of a Raman distributed fiber optic temperature sensor, up to total gamma doses in excess of 300 kGy, using a double-ended configuration and commercially-available 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.

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 capabilities of fiber Bragg grating (FBG) temperature sensors. Our experiments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an aircooled 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 difference saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 1019 neutron/cm2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can survive 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.