Radiation hardening techniques for rare-earth based optical fibers and amplifiers

Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers. We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by helping identifying the best amplifier configuration for operation in the radiative environment

Integration of optical fibers in radiative environments: Advantages and Limitations

International audienceOptical fibers are now considered for use as part of systems, like control-command, diagnostics, sensors that will have to operate in harsh environments associated with future nuclear power plants, high energy physics facilities, nuclear waste repository,... For these applications, optical fibers present a number of advantages compared to coaxial cables, due to their electromagnetic immunity. However, it has been shown that radiations affect the fiber properties mainly through three macroscopic degradation mechanisms [1]. First, radiations generate point defects in the silica-based matrix of the fibers resulting in an increase of its linear attenuation resulting in the degradation or loss of propagated signal (Radiation-Induced Attenuation, RIA). Second, some of these defects can also emit light that will add to the signal, degrades the signal to noise ratio and some times saturates the detectors (Radiation-Induced Emission). Finally, for some environments, radiations can change the glass structure by a compaction phenomenon, resulting in degradation of fiber guiding or sensing properties. In this paper, we will review the main limiting degradation phenomenon for the fiber integration in the different environments studied by our group: inertial and magnetic fusion, space, nuclear waste repository,... More particularly, the intrinsic and extrinsic fiber parameters that influence the amplitude of radiation-induced effects will be presented. A very important one is the composition of the glass used for the making of the fiber core and optical cladding [2]. Radiations response of different classes of optical fibers: pure-silica core, fluorine, germanium, phosphorus-doped optical fibers will be presented for various harsh environments associated with military, civil applications [1,3]. Spectral dependence of RIA and RIE in these fibers will be discussed as the response of the fiber mainly depends on the wavelength of interest for the application: eg the ultraviolet-visible part of the spectrum for diagnostics, the infrared for telecommunications or sensing. Furthermore, we will also present the influence of the manufacturing process on the fiber vulnerability. We will show that its influence strongly depends on the considered application and harsh environment. Finally, different techniques will be presented that have been shown to be able to reduce the fiber radiation sensitivity. Their interest and limitations of use will be discussed for various environment

VULNÉRABILITÉ ET DURCISSEMENT DES FIBRES OPTIQUES AUX NOUVEAUX ENVIRONNEMENTS RADIATIFS

National audienceCet article discute certaines des avancées récentes concernant l'étude de la vulnérabilité et du durcissement (amélioration de la résistance aux radiations) des fibres optiques ou systèmes fibrés aux nouveaux environnements radiatifs (LMJ, ITER, LHC, spatial, industrie nucléaire), les origines de la dégradation des fibres optiques ou capteurs à fibres optiques sous irradiation. Enfin, on présente une proposition des principaux challenges à étudier pour faciliter l'intégration de ces composants au sein des installations existantes ou dans les futurs grands instruments qui sont en cours de développement

Feasibility of Radiation Dosimetry with Phosphorus-doped Optical Fibers in the UV-visible range of wavelengths

International audienceIn this work, we investigated the feasibility of using phosphorus (P)-doped optical fibers to monitor the dose levels during various types of irradiations like protons, neutrons, X-ray or rays. This class of fibers was shown to be a good candidate for dosimetry mainly in the near-IR and the IR and for extremely low dose ranges. In this study, we choose to evaluate the spectral and time dependence of the radiation-induced attenuation (RIA) of these fibers in the ultraviolet (UV) and visible range of wavelengths. In this spectral domain, the RIA levels are considerably higher than in the IR and time-resolved distributed dosimetry with better spatial resolution may be achieved if the fibers present the requested response (linear with dose, dose-rate independent)

Micro-Raman investigation of X or gamma irradiated Ge doped fibers

Micro-Raman spectra have been recorded on Ge doped optical fibers before and after 10 keV-X or c-ray irradiation up to doses of 1 MGy (X-ray) or 7.8 MGy (c-ray). Our data provide evidence that, at such dose levels, the glass matrix is not modified in a detectable way. We observed that varying the Ge doping levels from 0 to about 11 wt.%, X or c radiation sensitivity of the overall matrix remains unchanged. Such results are observed for fibers obtained with drawing conditions within the usual range used for the fabrication of specialty fibers as radiation-tolerant waveguides. Our data support the potentiality of fiberbased sensors using glass properties, e.g. Raman scattering, for applications in harsh environments as those encountered in nuclear power plants

Origins of radiation-induced attenuation in pure-silica-core and Ge-doped optical fibers under pulsed x-ray irradiation

We investigated the nature, optical properties, and decay kinetics of point defects causing large transient attenuation increase observed in silica-based optical fibers exposed to short duration and high-dose rate x-ray pulses. The transient radiation-induced attenuation (RIA) spectra of pure-silica-core (PSC), Ge-doped, F-doped, and Ge + F-doped optical fibers (OFs) were acquired after the ionizing pulse in the spectral range of [∼0.8-∼3.2] eV (∼1500-∼380 nm), from a few ms to several minutes after the pulse, at both room temperature (RT) and liquid nitrogen temperature (LNT). Comparing the fiber behavior at both temperatures better highlights the thermally unstable point defects contribution to the RIA. The transient RIA origin and decay kinetics are discussed on the basis of already-known defects absorbing in the investigated spectral range. These measurements reveal the importance of intrinsic metastable defects such as self-trapped holes (STHs), not only for PSC and F-doped fibers but also for germanosilicate optical fibers as clearly evidenced by our LNT measurements. Furthermore, our results show that fluorine co-doping seems to decrease the RIA related to the strain-assisted STHs absorption bands in both types of optical fibers. Regarding Ge-doped glasses, besides a description of the defects responsible of the RIA, highlighting the STHs' role in their transient response, we provide a clear correlation between the GeX and GeY centers' kinetics. In conclusion, the presented results improve our understanding of the transient RIA origin in the ultraviolet and visible domains. The lack of knowledge about the defects causing the RIA in the near-infrared domain will require future studies

Raman investigation of the drawing effects on Ge-doped fibers

International audienceWe have investigated the Raman activity of various germanosilicate fibers and their associated preforms. Our data indicate an enhancement in small rings' (3-member rings) concentration in the silica-based matrix of the fibers during the drawing process. The generation of such rings appears compatible with an increase of the sample density and fictive temperature. The data regarding the drawing effects on the fiber stress appear less clear, and it is possible to suggest that in some cases the drawing could lower the tensile stress. Finally we have also provided evidence that changing the drawing conditions within the usual range of application leads to no significant changes of the matrix structure as can be observed using Raman spectroscopy

Feasibility of radiation dosimetry with phosphorus-doped optical fibers in the ultraviolet and visible domain

International audienceWe investigated the feasibility of using phosphorus-doped optical fibers to monitor the levels of deposited dose during an irradiation. For this, we characterized their spectral and time dependence of the steady state 10 keV X-ray radiation-induced attenuation in the ultraviolet and visible range of wavelengths (200 nm- 900 nm). Their radiation sensitivity is very high with losses exceeding 10 dB m−1 for doses larger than 10 Gy and wavelengths shorter than 550 nm. Our results reveal a sub linear dose dependence of the induced losses that also depends on the dose rate (1 Gy s−1-50 Gy s−1) between 350 nm and 900 nm. For this spectral domain, excess of attenuation is due to the phosphorus oxygen-hole centers. P2 defects are responsible for the induced losses around 300 nm that linearly increase with the dose at least until 1 kGy and without dose rate effect. We measured no noticeable influence of the temperature (5 °C-50 °C) on the radiation-induced attenuation in the studied spectral domain. Our study shows that dosimetry with phosphorus-doped fibers seems possible in the ultraviolet (around 300 nm) with a sensitivity enhanced by a factor N100 compared to the one observed in the infrared region (>900 nm)

Radiation-Induced Defects in Fluorine-doped Silica-Based Optical Fibers: Influence of the H2-Loading

International audienceWe investigated the effects of 10-keV X-ray radiation on the transmission properties of F-doped optical fibers in the 200-850 nm range of wavelengths (1.5 – 6 eV). The influence of a H2-loading of the fiber on its radiation sensitivity is also presented