Youcef Ouerdane

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.

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.

Interstitial O2 distribution in amorphous SiO2 nanoparticles determined by Raman and photoluminescence spectroscopy

The O2 content and emission properties in silica nanoparticles after thermal treatments in oxygen rich atmosphere have been investigated by Raman and photoluminescence measurements. The nanoparticles have different sizes with average diameter ranging from 7 up to 40 nm. It is found that O2 concentration in nanoparticles monotonically increases with nanoparticles size. This finding is independent on the measurement technique and evidences that oxygen molecules are not present in all the nanoparticles volume. This dependence is interpreted on the basis of a structural model for nanoparticles consisting of a core region able to host the oxygen molecules and a surface shell of fixed size and free from O2.

Transient Radiation Responses of Optical Fibers: Influence of MCVD Process Parameters

A dedicated set of fibers elaborated via the Modified Chemical Vapor Deposition (MCVD) technique is used to study the influence of composition and drawing parameters on their responses to an X-ray pulse representative of the radiation environments associated with Megajoule class lasers. These canonical fibers were designed to highlight the impact of these parameters on the amplitude and kinetics of the transient pulsed X-ray Radiation Induced Attenuation (RIA) at room temperature. From preforms differing by their core composition, three optical fibers were elaborated by varying the tension and speed during the drawing process. No or only slight RIA change results from the tested variations in drawing process parameters of Ge-doped, F-doped, and pure-silica-core fibers. This study reveals that the drawing process is not the main parameter to be optimized in order to enhance the radiation tolerance of MCVD specialty optical fibers for the LMJ harsh environment. From the hardness assurance point of view, a specialty fiber sufficiently tolerant to this environment should be robust against changes in the drawing process. The origins of the RIA observed in the different fibers are discussed on the basis of spectral decomposition of their measured RIA spectra, using sets of defects from the literature and related to the different core dopants. This analysis highlights the limits of the well-known defect set to reproduce the RIA above 1 μm for Ge-doped fibers whereas self-trapped holes and chlorine-related species seem responsible for the transient responses of pure-silica-core and F-doped fibers.

Optimization of the Design of High Power

In this paper, the optimization of the design of rare earth-doped cladding-pumped fiber amplifiers is investigated to improve their performance with respect to the constraints associated with space missions. This work is carried out by means of a computer code based on particle swarm optimization (PSO) and rate equation model. We consider a fiber that is radiation tolerant at the space dose levels, and we characterize the radiation response of the amplifier based on it. By simulations, we study how the design of the radiation-tolerant double-cladding Er3+/Yb3+-codoped fiber amplifiers (EYDFAs) can improve the global system response in space. The rate equations model includes the first and secondary energy transfer between Yb3+ and Er3+, the amplified spontaneous emission and the most relevant upconversion and cross relaxation mechanism among the Er3+ ions. The obtained results highlight that the developed PSO algorithm is an efficient and reliable tool to perform the recovering of the most relevant spectroscopic parameters and the optimum design of this kind of devices. These results demonstrated that the performance of high power optical amplifiers can be optimized through such a coupled approach, opening the way for the design of radiation-hardened devices for the most challenging future space missions.

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We studied the sensitivities at 1.55 /spl mu/m of three polarization-maintaining (PM) and three unstressed single-mode (SM) optical fibers under pulsed X-ray (/spl sim/1 MeV) and steady-state /spl gamma/-ray (/spl sim/1 MeV) radiation. Our results showed no differential radiation-induced attenuation (RIA) between the two orthogonal polarization axes of PM fibers for times ranging from 10/sup -6/ s to 100 s after ionization pulse (dose <1 kGy(SiO/sub 2/), dose rate >1MGy/s). The two inner cladding phosphorus-codoped PM fibers and their corresponding SM fibers exhibited the same RIA levels and kinetics after both irradiation types. Their radiation responses could be explained by their compositions and by the properties of the absorbing phosphorus-related defects generated in the light guiding region (core and inner cladding). The inner cladding fluorine-codoped PM fiber showed radiation-tolerant properties compared with its SM counterpart. We assumed, therefore, that the outer stress applying region dopants could have an impact on the fiber radiation sensitivity under both irradiation types. More particularly, we showed that the cladding phosphorus-codoping far from the light guiding region reduces the RIA related to the germanium defects in the fiber core. This effect could be due to the donor nature of the double phosphorus-oxygen bond and could be exploited to design radiation-hardened polarization-maintaining and single-mode optical fibers.

-Codoped Fiber Amplifiers for Space Missions by Means of Particle Swarm Approach

We investigated the impact of an oxygen preloading on pure-silica-core or fluorine-doped-core fiber responses to high irradiation doses (up to 1 MGy (SiO2)). Oxygen enrichment was achieved through a diffusion-based technique, and the long-term presence of O2 molecules was confirmed by micro-Raman experiments. Online radiation induced attenuation (RIA) experiments were carried out in both the pristine and the O2-loaded optical fibers to investigate the differences induced by this pretreatment in the UV and visible ranges. Contrary to results recently published on the positive impact of O2 on infrared RIA, our results reveal a RIA increase with O2 presence. Data are analyzed in order to better understand the microscopic processes involved during the irradiation and to evaluate possible hardening developments.

Pulsed X-ray and /spl gamma/ rays irradiation effects on polarization-maintaining optical fibers

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.

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In this paper, we study the radiation-induced point defects related to the phosphorus element that is commonly used to improve the optical properties of silica-based glasses but is responsible of a dramatic increase in their radiation sensitivity. To this aim, the influence of x-ray irradiation on prototype phosphorus-doped canonical fibers and their related preforms was investigated by in situ radiation induced attenuation (RIA), optical absorption, and electron spin resonance (ESR) spectroscopy. The RIA spectra in the (1.5–5 eV) range, can be explained by the presence of at least three absorption bands induced by radiation exposure. Additionally the X-dose dependence of such bands was studied. The main responsible defect for these absorption peaks was the phosphorus oxygen hole center (POHC) center, whose presence was also detected by ESR measurements both in irradiated fibers and preforms, together with the lineshape of the so called P2 defect. Correlations between the RIA bands and the ESR results all...

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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.