Jocelyn Perisse

Combined High Dose and Temperature Radiation Effects on Multimode Silica-Based Optical Fibers

We investigate the response of Ge-doped, P-doped, pure-silica, or Fluorine-doped fibers to extreme environments combining doses up to MGy(SiO 2) level of 10 keV X-rays and temperatures between 25 °C and 300 °C. First, we evaluate their potential to serve either as parts of radiation tolerant optical or optoelectronic systems or at the opposite, for the most sensitive ones, as punctual or distributed dosimeters. Second, we improve our knowledge on combined ionizing radiations and temperature (R&T) effects on radiation-induced attenuation (RIA) by measuring the RIA spectra in the ultraviolet and visible domains varying the R&T conditions. Our results reveal the complex response of the tested fibers in such mixed environments. Increasing the temperature of irradiation increases or decreases the RIA values measured at 25 °C or sometimes has no impact at all. Furthermore, R&T effects are time dependent giving an impact of the temperature on RIA that evolves with the time of irradiation. The two observed transient and stationary regimes of temperature influence will make it very difficult to evaluate sensor vulnerability or the efficiency of hardening approaches without extensive test campaigns.

Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels

We report a method for fabricating fiber Bragg gratings (FBG) resistant to very severe environments mixing high radiation doses (up to 3 MGy) and high temperatures (up to 230°C). Such FBGs have been written in two types of radiation resistant optical fibers (pure-silica and fluorine-doped cores) by exposures to a 800 nm femtosecond IR laser at power exceeding 500 mW and then subjected to a thermal annealing treatment of 15 min at 750°C. Under radiation, our study reveals that the radiation induced Bragg wavelength shift (BWS) at a 3 MGy dose is strongly reduced compared to responses of FBGs written with nonoptimized conditions. The BWS remains lower than 10 pm for temperatures of irradiation ranging from 25°C to 230°C without noticeable decrease of the FBG peak amplitude. For an applicative point of view, this radiation induced BWS corresponds to an additional error on the temperature measurements lower than 1.5°C, opening the way to the development of radiation-tolerant multi-point temperature sensors for nuclear industry.

Influence of neutron and gamma-ray irradiations on rad-hard optical fiber

We investigated point defects induced in rad-hard Fluorine-doped optical fibers using both a mixed source of neutrons (fluences from 1015 to 1017 n/cm2) and γ-rays (doses from 0.02 to 2 MGy) and by a γ-ray source (dose up to 10 MGy). By combining several complementary spectroscopic techniques such as radiation-induced attenuation, confocal micro-luminescence, time-resolved photo-luminescence and electron paramagnetic resonance, we evidenced intrinsic and hydrogen-related defects. The comparison between the two irradiation sources highlights close similarities among the spectroscopic properties of the induced defects and the linear correlation of their concentration up to 1016 n/cm2. These results are interpreted on the basis of the generation processes of defects from precursors sites, that are common to both γ-rays and neutrons. In contrast, the highest neutron fluence (1017 n/cm2) causes peculiar effects, such as the growth of a photoluminescence and variations of the spectral and decay properties of the emission related with nonbridging oxygen hole centers, that are likely due to silica network modification.

Radiation Vulnerability of Fiber Bragg Gratings in Harsh Environments

The difficulties encountered in the implementation of a temperature or strain sensor based on fiber Bragg grating (FBG) in a harsh radiative environment are introduced. We present the choices made to select both a radiation-resistant fiber in terms of transmission and also the grating inscription conditions necessary to write radiation tolerant FBGs in such fibers with a femtosecond laser. The radiation response of these gratings was also studied under radiation at dose up to 1 MGy. The comparison between Ge-free and Ge-doped fibers was highlighted.

Radiation effects on optical frequency domain reflectometry fiber-based sensor

We investigate the radiation effects on germanosilicate optical fiber acting as the sensing element of optical frequency domain reflectometry devices. Thanks to a new setup permitting to control temperature during irradiation, we evaluate the changes induced by 10 keV x rays on their Rayleigh response up to 1 MGy in a temperature range from -40°C up to 75°C. Irradiation at fixed temperature points out that its measure is reliable during both irradiation and the recovery process. Mixed temperature and radiation measurements show that changing irradiation temperature leads to an error in distributed measurements that depends on the calibration procedure. These results demonstrate that Rayleigh-based optical fiber sensors are very promising for integration in harsh environments.

Radiation Hardened Optical Frequency Domain Reflectometry Distributed Temperature Fiber-Based Sensors

We study the performance of Optical Frequency Domain Reflectometry (OFDR) distributed temperature sensors using radiation resistant single-mode optical fibers. In situ experiments under 10 keV X-rays exposure up to 1 MGy( SiO2) were carried out with an original setup that allows to investigate combined temperature and radiation effects on the sensors within a temperature range from 30°C to 250°C. Obtained results demonstrate that optical fiber sensors based on Rayleigh technique are almost unaffected by radiation up to the explored doses. We show that a pre-thermal treatment stabilize the sensor performance increasing the accuracy on temperature measurement from ~ 5°C down to ~ 0.5°C by reducing the packaging-related errors (such as ones related to coating modification) that could be introduced during the measurement. These results are very promising for the future integration of Rayleigh based sensors in nuclear facilities.

Radiation-Hardened Fiber Bragg Grating Based Sensors for Harsh Environments

Fiber Bragg Grating (FBG) based sensors are nowadays used for several applications, but, even if they present advantages for their incorporation into radiation environments, commercial-off-the-shelf devices cannot still be used in harsh conditions. We recently reported a procedure for fabricating FBGs resistant to severe constraints combining both high radiation doses up to MGy levels and operation temperatures exceeding 200°C (RadHard FBGs). Following these results, the European project HOBAN was granted by Kic InnoEnergy with the aim of developing and marketing FBG-based temperature and strain monitoring systems suitable for harsh nuclear environments (350°C temperature and MGy dose levels), with their associated instrumentation devices. In this framework, we present an accurate study about the robustness of the radiation-response of these RadHard FBGs against the main grating inscription parameters. Up to the accumulated X-ray dose of 1 MGy(SiO2), no significant radiation induced Bragg wavelength shift is observed meaning that radiations induce errors below ± 0.4°C in the temperature estimation. Moreover, a study about the dose-rate dependence (1 to 50 Gy/s) of the gratings response is also reported and confirms the high radiation hardness of our RadHard FBGs at all dose rates.

Investigation of Coating Impact on OFDR Optical Remote Fiber-Based Sensors Performances for Their Integration in High Temperature and Radiation Environments

The response of optical frequency-domain reflectometry-based temperature sensors is here investigated in harsh environments (high temperature, high radiation dose) focusing the attention on the impact of the fiber coating on the sensor performances in such conditions. Our results demonstrate that the various coating types evolve differently under thermal treatment and/or radiations, resulting in a small (<;5%) change in the temperature coefficient of the sensor. The identified procedure, consisting of a prethermal treatment of the fiber at its maximum coating operating temperature, is here verified up to 150 °C for high-temperature acrylate and up to 300 °C for polyamide coating. This method allows a stabilization of the temperature coefficients. Finally, we show that radiation does not affect scattering phenomenon, temperature coefficients (CTs) remain identical within 1% fluctuations up to 10 MGy dose, and that permanent radiation-induced attenuation reached values stands for the development of high-spatial resolved distributed temperature for harsh environment associated with high temperature (up to 300 °C) and ionizing radiation up to the MGy dose level.

Origin of the visible absorption in radiation-resistant optical fibers

In this work we investigated the point defects at the origin of the degradation of radiation-tolerant optical fibers used in the visible part of the spectrum for plasma diagnostics in radiation environments. For this aim, the effects of γ-ray irradiation up to the dose of 10 MGy(SiO2) and post-irradiation thermal annealing at 550°C were studied for a Fluorine-doped fiber. An absorption peaking around 2 eV is mainly responsible for the measured radiation-induced losses, its origin being currently debated in the literature. On the basis of the unchanging shape of this band with the radiation dose, its correlation with the 1.9 eV photoluminescent band and the thermal treatment results we assign the asymmetric absorption around 2 eV to an unique defect, the NBOHC, instead of a set of various defects.

Optical Frequency Domain Reflectometer Distributed Sensing Using Microstructured Pure Silica Optical Fibers Under Radiations

We investigated the capability of micro-structured optical fibers to develop multi-functional, remotely-controlled, Optical Frequency Domain Reflectometry (OFDR) distributed fiber based sensors to monitor temperature in nuclear power plants or high energy physics facilities. As pure-silica-core fibers are amongst the most radiation resistant waveguides, we characterized the response of two fibers with the same microstructure, one possessing a core elaborated with F300 Heraeus rod representing the state-of-the art for such fiber technology and one innovative sample based on pure sol-gel silica. Our measurements reveal that the X-ray radiations do not affect the capacity of the OFDR sensing using these fibers to monitor the temperature up to 1 MGy dose whereas the sensing distance remains affected by RIA phenomena.