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.

Fiber Bragg grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 10 ¹⁸ -10 ¹⁹ n/cm ² , the radiation-induced changes in the material density and refractive index may drastically bias the measurements. This paper evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 1019 n/cm2 (E 1 MeV). We studied the responses of FBGs manufactured by three different laboratories in the same singlemode optical fiber but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWSs) vary from a few picometers (equivalent temperature error <;1 °C) to nearly 1 nm (~100 °C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare fiber samples through other experimental techniques. Our results show that by using specific hardening techniques, the FBGbased temperature measurements in a nuclear research reactor experiment may become feasible.

Fiber Bragg gratings (FBGs) are one of the most used optical fiber sensors and they have recently drawn the attention of several research groups for their potential applications in harsh radiation environments. Up to now, their performances have been mainly evaluated under ionizing radiations, such as X-rays or $\gamma $ -rays. We compare here the effects of different irradiation types, including X-rays, protons, and electrons, on type I FBGs written by UV laser exposure in Ge-, P/Ce-, and B/Ge-doped single-mode optical fibers. Different irradiation conditions were used according to the sources; 6-MeV electron irradiations were performed at a dose rate of 120 Gy(SiO ₂ )/s up to an accumulated dose of 500 kGy; whereas for the 63-MeV protons, the estimated equivalent dose rate was of 0.75 Gy(SiO ₂ )/s up to a total dose of 7 kGy. In order to compare their effects with those induced by X-rays, two irradiations with 45-keV photons were performed with different dose rates [0.75 and 60 Gy/s (SiO ₂ )] up to a total dose of 10 and 500 kGy, respectively. We demonstrated that X-rays and protons induce comparable effects at doses of about 10 kGy, whereas the behavior under electron beam appears to be strongly dependent on the fiber composition. For example, the grating in the B/Ge co-doped fiber is the most sensitive to electrons and the most resistant to X-rays; whereas the FBG inscribed in the H ₂ -loaded P/Ce co-doped fiber has exactly the opposite behavior.

Fiber Bragg Gratings (FBGs) present strong advantages for temperature or strain sensing in harsh radiation environments even if their properties are affected by radiations. The amplitudes and kinetics of these radiation induced changes depend on numerous parameters, intrinsic or extrinsic to the FBGs themselves. In this paper, we characterized 40 keV X-ray radiation effects on type I FBGs inscribed in pre-hydrogenated SMF-28 from Corning through an ultraviolet laser exposure at 244 nm (cw). We performed a systematic study of the influence of several FBG manufacturing parameters on their radiation response up to 100 kGy(SiO$_2$) highlighting Radiation-Induced Bragg Wavelength Shifts (RI-BWS) up to 130 pm (~13°C error for temperature measurements) but no decrease of those FBG reflectivity. Among the investigated parameters are the duration and temperature (100°C or 300°C) of the thermal treatments applied post-inscription to stabilize the FBG and to complete the H$_2$ out-gassing. For such type of FBG, the device has to be recoated after inscription, we then characterize the impact of this manufacturing step on the FBG response, showing that its recoating with NAO-81 acrylate slightly degrades its radiation resistance. In addition to this study, the influence of two other parameters have also been characterized: RI-BWS increases with the dose rate in the range 1 Gy/s to 50 Gy/s and a pre-irradiation at 1.5 MGy does not stabilize type I FBG response to a second irradiation.

The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two different organic–inorganic sol–gels were selected to fabricate the photonic device: TiO₂–SiO₂ and ZrO₂–SiO_2, acting as the waveguide core and the cladding, respectively. A systematic study was performed to determine the manufacturing parameters controlling their properties. The results show that large surfaces can be functionalized via sol–gel methods using the direct laser-writing approach. The structures are characterized in terms of refractive index, and the guiding properties were investigated through simulations and experiments, indicating an excellent behavior regarding the light guidance in a straight waveguide or in the 2D matrix structure grid. Additionally, preliminary tests show that the presence of impact can be easily detected after damage through the induced optical losses on large surfaces. This proof of concept sensor is a promising tool for structural health monitoring. To achieve the ultimate goal, the integration of this photonic sensor will be later performed on aircraft wings.