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We analyzed the simultaneous response to radiation and temperature exposure of radiation-sensitive optical fibers doped with aluminum, phosphorus and/or Germanium in the framework of the CERTYF (Combined Effects of Radiations, Temperature and hYdrogen on silica-based optical Fibers) project, whose aim is to build a predictive model based on intrinsic and extrinsic optical parameters.

Raman distributed temperature sensing techniques (Raman-DTS) are currently meeting a growing interest from the industry as they are a promising and cheap alternative to classical temperature measurements which require the deployment of many sensors. The reliability of the DTS measurements, as well as the traceability to the temperature standards, must be ensured throughout the entire period of use (typically over a few tens of years for nuclear waste repositories and hydraulic structures). LNE, in association with Andra and EDF has developed facilities dedicated to the metrological characterization of Raman DTS devices. A first benchmark performed on five devices from different manufacturers has been performed following experimental procedures which enable a relevant comparison of these instruments. This paper defines the proposed metrological features to be evaluated for each Raman DTS system, and presents a ranking method enabling to provide useful pieces of information to the final users for the selection of the most appropriate device to the requirements of their applications.

Distributed optical-fiber sensors (OFSs) based on scattering phenomena are nowadays used for several applications such as temperature or strain monitoring. Whereas very promising results have already been reported for the applications of Rayleigh- and Raman-based OFSs in harsh environments, only few studies concern the Brillouin-based ones. Two phenomena influence the Brillouin-based OFS when the sensing fiber is exposed to radiations: the radiation-induced attenuation (RIA), which reduces the sensing range by decreasing the signal-to-noise ratio, and a frequency shift of the Brillouin peak, which entails an error on the temperature or strain measurements. We report here an in situ study on the effects of $\gamma $ -rays on Brillouin optical time-domain analysis (BOTDA) OFS up to the accumulated dose of 10 MGy(SiO2) and those of X-rays on Brillouin optical time-domain reflectometer sensor up to 1 MGy. We compare the vulnerability levels for fiber composition, fiber packaging, as well as the influence of a preirradiation on the sensing performances. Whereas the Ge-doped fibers present a too high RIA that limits drastically the sensing range at high doses, especially with the BOTDA technique; the F-doped ones appear suitable for integration in high dose levels environments. Indeed the error induced by the Brillouin frequency shift on temperature measurements is smaller than 5 °C after a 10-MGy dose.

Optical fiber sensors of hydrogen gas (H2) are conventionally based on the reaction of a sensitive material deposited on the surface of a fiber. Long-term applications of H2 monitoring require more robust configurations, less sensitive to the degradations of the sensitive layer. To overcome this issue, we develop disruptive polarisation-maintaining optical fibers composed of a sensitive material (Palladium, Pd) integrated into the silica cladding. We present the development of two Panda-type optical fibers with or without embedded Pd particles. These fibers have been fabricated for evaluating, through the measurement of the birefringence, the contribution of Pd particles on the detection of H2 gas. We have specially developed a gas chamber for measuring on-line the detection of H2 during its diffusion into the fiber. Dynamic comparisons between both fibers demonstrate the contribution of Pd particles resulting in a faster response time (of about 20 h for our experimental conditions). These results pave the way to the realization of robust optical fibers with enhanced sensitivity to H2 gas for developing sensing systems compatible with long-term hydrogen monitoring applications in extreme and harsh environments, such as radioactive waste repositories.

We report distributed temperature measurements based on Raman scattering performed during steady state $\gamma $ -ray irradiation at a dose rate of 1 kGy( ${\rm SiO}_{2}$ )/h and up to a total ionizing dose (TID) of $\sim 0.1\ \hbox{MGy}$ . We characterize on-line the evolution of the performances of a single-ended Raman distributed temperature sensor (RDTS) during the $\gamma $ -ray exposure of different classes of commercial multimode fibers (MMFs) acting as the sensing element. RDTS is influenced by the radiation-induced attenuation (RIA) phenomena leading to both large errors in the temperature measurements and a diminution of the useful sensing length. The amplitude of the radiation-induced temperature error strongly depends on the fiber choice and on the irradiation conditions. For the single-ended RDTS operation in the targeted Cigeo application the selection of a radiation tolerant sensing fiber will be mandatory, but not sufficient, to overcome the expected severe ambient conditions around radioactive wastes. For efficient temperature sensing up to an accumulated dose of 0.1 MGy, pre-irradiation of the selected radiation resistant (RR) fibers appears also necessary to improve the sensor performances.

[EN] We have designed and implemented a fiber optic shape sensor for high-energy ionizing environments based on multicore optical fibers. We inscribed two fiber Bragg gratings arrays in a seven-core optical fiber. One of the arrays has been inscribed in a hydrogen-loaded fiber and the other one in an unloaded fiber in order to have two samples with very different radiation sensitivity. The two samples were coiled in a metallic circular structure and were exposed to gamma radiation. We have analyzed the permanent radiation effects. The radiation-induced Bragg wavelength shift (RI-BWS) in the hydrogen-loaded fiber is near ten times higher than the one observed for the unloaded fiber, with a maximum wavelength shift of 415 pm. However, the use of the multiple cores permits to make these sensors immune to RI-BWS obtaining a similar curvature error in both samples of approximately 1 cm without modifying the composition of the fiber, pre-irradiation or thermal treatment., Ministerio de Economia y Competitividad (DIMENSION TEC2017 88029- R); Generalitat Valenciana (IDI/FEDER/2018, PROMETEO 2017/103); H2020 Marie Sklodowska-Curie Actions (MSCA-ITN-ETN-722509); Universitat Politecnica de Valencia (PAID-01-18); Ministerio de Ciencia, Innovacion y Universidades (IJCI-2017-32476).

This paper describes the qualification of sensors envisioned for Cigeo, the planned deep disposal center for French radioactive waste. More precisely, it describes an in situ demonstrator, similar with the system planned to monitor intermediate level long lived radioactive wastes cells. These cells would be a tunnel, located 490 m deep, with 9 m in diameter, with concrete liners realized in three steps. The demonstrator was instrumented in 2011 in the Andra's Underground Research Laboratory. The instruments provide useful information on sensors' adaptability to the environment; robustness and technology comparisons (accordance and discrepancies). As an example, accordance between localized sensors and distributed measurements for strains, by optical fiber sensors and vibrating wire extensometers and temperatures, by optical fiber sensors and platinum probe, was verified. After 3 years more than 95% of sensors still provide measurements and show no drift over time.

We evaluated comparatively the radiation sensitivity of two long period gratings (LPGs) manufactured in a SMF28™ optical fiber through the melting-drawing method based on CO2 laser assisted by a micro-flame, of a commercial fiber Bragg grating (FBG), and of a commercially available Draw Tower Grating (DTG), all exposed to gamma radiation. The tests were run on-line using the optical frequency domain reflectometry (OFDR) technique, in order to provide unequal sensitivity, the gratings being wavelength multiplexed. The dose rate dependence of the LPGs response was evaluated for dose rates of 0.37 kGy/h and 0.24 kGy/h, up to the total dose of (33.9 ± 0.8) kGy and (21.5 ± 0.5) kGy. The FBG and DTG proved to be radiation hardened having a radiation sensitivity of 12 pm/kGy, while the LPGs radiation sensitivity was as high as 1.2 nm/kGy under the same irradiation conditions. A recovery of the radiation induced wavelength shift for LPGs was noticed at room temperature, at a rate of 2.2 pm/h, over a 211 h period. The effect of LPG wavelength change after exposure to gamma-ray can be exploited in the development of radiation dosimeters, up to the dose of 10 kGy, when saturation appears. The temperature sensitivity of the LPG2 changed from 48 pm/°C before the irradiation, to 50 pm/ °C after the irradiation for a total dose of 21 kGy.

International audience; This paper presents the state of the art distributed sensing systems, based on optical fibres, developed and qualified for the French Cigéo project, the underground repository for high level and intermediate level long-lived radioactive wastes. Four main parameters, namely strain, temperature, radiation and hydrogen concentration are currently investigated by optical fibre sensors, as well as the tolerances of selected technologies to the unique constraints of the Cigéo’s severe environment. Using fluorine-doped silica optical fibre surrounded by a carbon layer and polyimide coating, it is possible to exploit its Raman, Brillouin and Rayleigh scattering signatures to achieve the distributed sensing of the temperature and the strain inside the repository cells of radioactive wastes. Regarding the dose measurement, promising solutions are proposed based on Radiation Induced Attenuation (RIA) responses of sensitive fibres such as the P-doped ones. While for hydrogen measurements, the potential of specialty optical fibres with Pd particles embedded in their silica matrix is currently studied for this gas monitoring through its impact on the fibre Brillouin signature evolution.

We present a distributed fiber optic sensing scheme to study 3D temperature fields inside concrete containers submitted to fire. High spatial resolution (3cm to 13cm), temperature uncertainty better than 3°C and rapid measurements (in the minute range) were required. We thus compared advantages and drawbacks of three sensing chains, based on Rayleigh, Brillouin and Raman scatterings; such distributed measurements were also compared with thermocouples (up to 250 measuring points). Practical field implementation limits are also underlined. Fire tests are a promising application for distributed optical fiber sensors.

Radiation effects on the performances of TW-COTDR (Tunable Wavelength — Coherent Optical Time Domain Reflectometry) optical fiber sensors based on Rayleigh scattering have been studied online for Ge-doped and F-doped fibers. We confirm that the Rayleigh response is not affected by low radiation (dose ∼55 kGy). However we observed using the TW-COTDR technology, a radiation induced frequency shift increasing at higher radiation (dose ∼ 10MGy): up to 96 GHz for Ge-doped fiber and up to 25.5 GHz for F-doped fiber.

This paper reports the outcomes of the tests that we conducted as online measurements for the evaluation of one optical fiber long-period grating produced by a fusion technique in a single-mode radiation-hardened optical fiber, and subjected to gamma irradiation. During the irradiation, the grating temperature was monitored. Before the irradiation, the temperature sensitivity of the grating was 27.7 $\hbox{pm}/^{\circ}\hbox{C}$ , while the value of this parameter postirradiation was found to be 29.3 $\hbox{pm}/^{\circ}\hbox{C}$ . The spectral characteristics of the grating were measured (i) in the laboratory with an ANDO AQ6317C optical spectrum analyzer and (ii) online, for the first time , with a LUNA OBR 4600 backscatter reflectometer, operating in the frequency acquisition mode. Such online measurement enables the study of recovery effects during the irradiation. The wavelength dip of the grating shifted under gamma irradiation, with 16 pm/kGy for the maximum total dose of 45 kGy. At room temperature, the recovery of the irradiation-induced shift of the wavelength dip was almost complete in about 120 h, at a rate of 6.7 pm/h. Postirradiation heating of the sensor produced the reversing of the recovery effect. The investigation indicated that, up to 45 kGy, the grating is more sensitive to radiation than other optical fiber sensors.

International audience; We investigated the evolution of the performances of Pulse Pre Pump-Brillouin Time Domain Analysis (PPP-BOTDA) and Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) fiber-based temperature and strain sensors when the sensing optical fiber is exposed to two γ-ray irradiation conditions: (i) at room temperature and a dose rate of 370 Gy(SiO2)/h up to a total ionizing dose (TID) of 56 kGy; (ii) at room temperature and a dose rate of 25 kGy(SiO2)/h up to a TID of 10 MGy. Two main different classes of single-mode optical fibers have been tested in situ, radiation-tolerant ones: fluorine-doped or nitrogen-doped core fibers, as well as Telecom-grade germanosilicate ones. Brillouin and Rayleigh Sensitivities of N-Doped fibers were not reported yet, and these characterizations pave the way for a novel and alternative sensing scheme. Moreover, in these harsh conditions, our results showed that the main parameter affecting the sensor sensitivity remains the Radiation Induced Attenuation (RIA) at its operation wavelength of 1550 nm. RIA limits the maximal sensing range but does not influence the measurement uncertainty. F-doped fiber is the most tolerant against RIA with induced losses below 8 dB/km after a 56 kGy accumulated dose whereas the excess losses of other fibers exceed 22 dB/km. Both Rayleigh and Brillouin signatures that are exploited by the PPP-BOTDA and the TW-COTDR remain unchanged (within our experimental uncertainties). The strain and temperature coefficients of the various fibers under test are not modified by radiations, at these dose/dose rate levels. Consequently, this enables the design of a robust strain and temperature sensing architecture for the monitoring of radioactive waste disposals

We report the evaluation of one long period grating (LPG) and one fiber Bragg grating (FBG) under gamma irradiation. The LPG was produced by the melting-drawing method based on CO2 laser assisted by a micro-flame and was engraved in a commercial single mode fiber SMF28 from Corning, grating length 25 mm, grating pitch of 720 μm. After the manufacturing of the grating, the fiber was re-coated with Acrylate and the grating was inserted into special ceramic case transparent to gamma radiation. The FBG is commercialized by Technica SA, and it is written in SMF-28 optical fiber (λ= 1546 nm; grating length of 12 mm; reflectivity > 80 %; bandwidth – BW @3 dB 15 dB; Acrylate recoating). By on-line monitoring of the LPG wavelength deep with an optical fiber interrogator during the irradiation exposure and pauses, both the irradiation induced shift (maximum 1.45 nm) and the recovery (in the range of 200 pm) phenomena were observed. Temperature sensitivity of the LPS was not affected by gamma irradiation.

We present a novel concept of quasi-distributed flexible optical fiber extensometers fully compensated from thermal variations. Developed for structural health monitoring applications, the sensors are composed of a combination of intrinsic Fabry-Perot cavities as long-base extensometers, and point-like Bragg gratings inserted along the same fiber used as temperature sensors. This configuration enables a high degree of multiplexing, thus quasi-distributed sensing, and very efficient temperature compensation. Both types of sensors are read by a fiber-optic low-coherence interferometer, used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. Finally, we present the experimental validation of the whole measurement system, suitable for concrete structures instrumentation, as it includes an original optical fiber sheath packaging the optical fiber as a flexible sensor.

We report on the design and experimental validation of a distributed Brillouin-based optical fiber sensor embedded into concrete structures for temperature and strain measurement. A composite-made wave-like coating designed by finite-element analysis ensures the sensor is transferring optimally temperature and strain fields from the concrete to the optical fiber, where Brillouin scattering takes place. During all experiments, sensors have been interrogated with a commercially available Brillouin optical time-domain reflectometer unit. First, temperature sensitivity of the Brillouin frequency shift were evaluated in PANDA and SMF28 optical fibers, before wrapping them into the specific sheath for embedment into a 3 m-long reinforced concrete beam. Temperature measurements during concrete beam casting agreed with reference measurements, and showed the significant sensor coating influence. A month later, strain measurements performed during a four-point bending experiment showed promising results: linearity and reliability of measurements were demonstrated, under tensile as well as compressive loadings.

We studied the potential of a fiber-based sensor using the Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) technique to monitor the distributed strain profile inside a tunnel shotcrete, with and without yieldable concrete wedges. Design of the measuring system and on-site test are reported. We demonstrated ability and performances of optical fiber sensing cable through several weeks monitoring in shotcrete. Results are shown to be comparable to those provided by the reference vibrating wire extensometers and distributed measurements will be of major interest for advanced understanding of performances and limitation of shotcrete mechanical behavior.

International audience; Brillouin optical time-domain analysis (BOTDA) sensors offer remarkable advantages for the surveillance of the planned French deep geological radioactive wastes repository, called Cigéo1,2. In this work we study the performances of Brillouin distributed sensors in harsh environment. We evaluate the radiation tolerance of different sensor classes and their responses evolution during γ-ray exposition with 1kGy/h dose rate (to reach ~0.2MGy) and after 1, 3, 6 and 10 MGy accumulated doses. Measurements on strained Ge-doped SMF are reported to highlight the variation on Brillouin scattering proprieties, both intrinsic frequency position of Brillouin shift and its dependence on temperature and strain

This paper reports on the design and realization of a new optical fibre sensor for continuous measurement of concrete strain over very long distances. We propose a composite-made wave-like sensor body that enables continuous bonding between optical fibre and concrete. Finite element analysis shows that the stiffness of the optical fibre can be adapted to that of the concrete, reducing the strain concentrations and the need for a theoretical calibration factor. Moreover, unlike the body of traditional I-shaped sensors, a wave-like sensor body enables a symmetrical response in tensile and compressive loadings whatever the contact conditions may be. We have realized optical fibre extensometers based on this technique and on a low-coherence interferometry method for the measurements. Despite its wave-shape, the proposed sensor body does not introduce any losses or strain that would result from fibre microbending. The process of realization is adapted to distributed sensors and sensors with a very long gauge length. Furthermore, several sensors were multiplexed, demonstrating the ability to perform quasi-distributed measurements. Once embedded into concrete cylinders, the optical fibre sensors were tested under compressive and tensile loading, and compared with traditional sensors. The allocation of one of the multiplexed sensors as a dummy gauge provides self-compensation for thermal effects and ambient fluctuations. As a result, and without any calibration factor, optical sensor measurements perfectly agreed with external linear variable differential transducer measurements.

We report on the design and test of a truly distributed hydrogen sensor based on Brillouin scattering into single-mode G652 optical fibers. The proposed system enables remote, robust, long-term, and truly distributed hydrogen sensing into long-range structures, such as radioactive waste repositories, with a spatial resolution in the order of 2 m over several hundred meters. For the first time, we demonstrate that Brillouin frequency shift depends on hydrogen concentration in the silica core with a rate of approximately 0.21 MHz/%H2 . Influence parameters are discussed. We showed that temperature influence may be corrected with a collocated carbon-coated fiber.

Raman distributed temperature sensor (RDTS) measurements were performed during γ-radiation on three different classes of standard multimode fibers (pure, Ge-doped and F-doped). The sensor response is affected by the radiation induced attenuation phenomena leading to errors in the temperature measurements. The amplitude of this error strongly depends on the fiber type and the irradiation conditions. These results are promising in view of the integration of these RDTS into the deep geological repository for radioactive waste.

International audience; The integration of Raman-distributed temperature fiber-based sensors (RDTS) into the envisioned French deep geological repository for nuclear wastes, called Cigéo requires evaluating how the performances of RDTS evolve in harsh environments, more precisely in presence of H2 or γ-rays. Both H2 and radiations are shown to affect the temperature measurements made with the single-ended RDTS technology. The amplitudes of the observed effects depend on the different classes of multimode fibers varying in terms of composition and coatings. By selecting the most tolerant fiber structure for the sensing, we could maintain the RDTS performances for such application. A hardening by system studies will be mandatory before integration of single-ended RDTS in Cigéo.

Raman Distributed Temperature Sensors (RDTSs) offer exceptional advantages to monitor the envisioned French deep geological repository for nuclear wastes, called Cigeo. Both $\gamma $ -ray and hydrogen release from nuclear wastes can strongly affect the temperature measurements made with RDTS. We present experimental studies on how the performances of RDTS evolve in harsh environments like those associated with $\gamma $ -rays or combined radiations and ${{\rm H}_2}$ release. The response of two standard and one radiation tolerant multimode fibers (MMFs) are investigated. In all fibers the differential induced attenuation between Stokes and anti-Stokes signal, ${({{\alpha _{\rm AS}} - {\alpha _{\rm S}}})}$ causes a temperature errors, up to $30^\circ {\rm C}$ with standard multimode fibers (100 m) irradiated at 10 MGy dose. This degradation mechanism that is more detrimental than the radiation induced attenuation (RIA) limiting only the sensing range. The attenuation in the [800-1600 nm] spectral range at room temperature is explored for the three fibers $\gamma $ -irradiated and/or hydrogen loaded to understand the origin of the differential RIA. We show that by adapting the characteristics of the used fiber for the sensing, we could limit its degradation but that additional hardening by system procedure is necessary to correct the T error in view of the integration of our RDTS technology in Cigeo. The current version of our correction technique allows today to limit the temperature error to $\sim 2^\circ {\rm C}$ for 10 MGy irradiated samples.

In order to meet application requirements, dependence of the Brillouin frequency shift to strain should be enhanced. We evaluated the influence of several optical fiber dopants on Brillouin scattering strain sensitivity. Based on a FEM-2D modeling, we developed a model for Brillouin gain spectrum and its strain sensitivity. Geometrical profile, doping composition and drawing conditions are taken into account. We show alumina is a very interesting dopant to enhance strain sensitivity in optical fibers. This result is experimentally validated: up to 0.0639MHz/μƐ is obtained with a 5.2mol% Al2O3 doped fiber. We expect to reach 0.1MHz/μƐ with 25mol% doping level.

International audience; Raman Distributed Temperature Sensors (RDTS) offer exceptional advantages for the monitoring of the envisioned French deep geological repository for nuclear wastes, called Cigéo. Here, we present experimental studies on how the performances of RDTS evolve in harsh environments like those associated with H2 or g-rays. Both of them are shown to strongly affect the temperature measurements made with RDTS. We showed that by adapting the characteristics of the used fiber for the sensing, we could limit its degradation but that additional hardening by system studies will have to be developed before integration of RDTS in Cigéo.

The paper reports results of the long distance (25 km range) distributed optical fiber sensing by means of Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR) method. The tests were designed to verify the accuracy and repeatability of the method in long distance measurements, as well as compatibility with various optical fiber types. Results demonstrate the capability of the method to detect strain or temperature changes over long distances. This proposed method is compared to Brillouin sensing techniques, into the same fibers. Unlike the Brillouin-based methods, measurement uncertainty does not increase with increasing distance. We demonstrated 0.16°C uncertainty at 21km.

International audience; The use of distributed strain and temperature in optical fiber sensors based on Brillouin scattering for the monitoring of nuclear waste repository requires investigation of their performance changes under irradiation. For this purpose, we irradiated various fiber types at high gamma doses which represented the harsh environment constraints associated with the considered application. Radiation leads to two phenomena impacting the Brillouin scattering: 1) decreasing in the fiber linear transmission through the radiation-induced attenuation (RIA) phenomenon which impacts distance range and 2) modifying the Brillouin scattering properties, both intrinsic frequency position of Brillouin loss and its dependence on strain and temperature. We then examined the dose dependence of these radiation-induced changes in the 1 to 10 MGy dose range, showing that the responses strongly depend on the fiber composition. We characterized the radiation effects on strain and temperature coefficients, dependencies of the Brillouin frequency, providing evidence for a strong robustness of these intrinsic properties against radiations. From our results, Fluorine-doped fibers appear to be very promising candidates for temperature and strain sensing through Brillouin-based sensors in high gamma-ray dose radiative environments.

Distributed optical fiber sensors are a clue technology for the monitoring of geological repository for long-lived nuclear wastes. Environment includes hydrogen whose influence on Rayleigh and Brillouin scatterings is experimentally evaluated. Induced propagation losses are quantified, as a function of wavelength and fiber dopants. 1310nm resulted the most favorable working wavelength. Distance range would anyway be reduced down to a hundred meter. We show hydrogen also induces Brillouin frequency shifts, up to 21MHz which corresponds to large temperature and strain measuring errors if not corrected. Fluorine fibers appeared more sensitive for short hydrogen exposure than other tested fibers.

Brillouin sensing is an interesting technology to monitor strain inside large civil engineering structures like nuclear waste repository. In radiative environments, performance degradation of Brillouin systems still needs to be investigated. In this paper, we fully characterized the Brillouin response of two different optical fibers, including the SMF28 fiber from Corning, under high gamma doses. Radiations increase the fiber linear attenuation and can also lead to a shift in the Brillouin frequency. We reduced these effects by designing a radiation resistant optical fiber. This fiber is well adapted for distributed sensing application using Brillouin scattering in MGy dose level environment. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We present a post mortem study of the influence of energetic radiation on optical fiber Brillouin sensors, both Brillouin spectrum and its temperature dependency in two different fibers, a photosensitive optical fiber and a SMF28. The target application is nuclear wastes repository monitoring where optical fiber Brillouin sensors might be exposed to energetic radiation. UV exposure induced optical losses, Brillouin frequency shifts up to 28MHz and even a variation of the temperature dependency. The photosensitive optical fiber resulted more sensitive than SMF28™.

International audience; A new approach is proposed to monitor in situ the influence of gamma radiations on Brillouin properties of optical fiber extensometers. Experimental results are illustrated with the characterization of two fibers samples up to total dose of about 600Gy. The Brillouin frequency shift remains unaffected at such radiations level, as well as the spectral Brillouin signature or its dependence with strain. Meanwhile, propagation losses increase under radiations with an amplitude related to fiber dopants. The target application is nuclear wastes repository monitoring where higher doses are expected. UV radiation preliminary tests show that compaction phenomenon may occur at such high doses, inducing Brillouin frequency shift up to 20 MHz.

International audience; A study on the influence of UV radiation on stimulated Brillouin scattering in photosensitive germanosilicate optical fibres is presented. The UV exposure of a highly germanium-doped fibre was performed at a power density of ~142 W/cm2. In this configuration, a refractive index change of about 2.5×10−3 was measured, partially due to the compaction phenomenon as well as a 20 MHz permanent change in the Brillouin frequency shift (at 1.55 µm).

Structural health monitoring is a key factor in life cycle management of infrastructures. Truly distributed fiber optic sensors are able to provide relevant information on large structures, such as nuclear power plants or nuclear waste disposal facilities. The sensing chain includes an optoelectronic unit and a sensing cable made of one or more optical fibers. A new instrument based on Optical Frequency Domain Reflectometry (OFDR), enables to perform temperature and strain measurements with a centimeter scale spatial resolution over hundred of meters and with a level of precision equal to 1 μ strain and 0.1 °C. Several sensing cables are designed with different materials targeting to last for decades, either embedded in the concrete or attached to the surface of the structure. They must ensure an optimal transfer of temperature and strain from the concrete matrix to the optical fiber. Based on the European guide FD CEN/TR 14748 “Non-destructive testing – Methodology for qualification of non-destructive tests”, a qualification method was developed. Tests were carried out using various sensing cables embedded in the volume or fixed to the surface of plain concrete specimens and representative-scale reinforced concrete structural elements. Measurements were performed with an OFDR instrument, while mechanical solicitations were imposed to the concrete element. Preliminary experiments seem very promising since measurements performed with distributed sensing systems are found comparable to values obtained with conventional sensors used in civil engineering and with the Strength of Materials Modelling. Moreover, the distributed sensing system makes it possible to detect and localize cracks appearing in concrete during the mechanical loading.

A soil-embedded optical fiber sensing cable is evaluated for an embedded cavity detection and sinkhole warning system in railway tunnels. Tests were performed on a decametric structure equipped with an embedded 110 m long fiber optic cable. Both Brillouin optical time-domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) sensing techniques were used for cable interrogation, yielding results that were in good qualitative agreement with finite-element calculations. Theoretical and experimental comparison enabled physical interpretation of the influence of ground properties, and the analysis of embedded cavity size and position. A 5 mm embedded cavity located 2 m away from the sensing cable was detected. The commercially available sensing cable remained intact after soil collapse. Specificities of each technique are analyzed in view of the application requirements. For tunnel monitoring, the OFDR technique was determined to be more viable than the B-OTDR due to higher spatial resolution, resulting in better detection and size determination of the embedded cavities. Conclusions of this investigation gave outlines for future field use of distributed strain-sensing methods under railways and more precisely enabled designing a warning system suited to the Ebersviller tunnel specificities.

Optical fiber sensor is evaluated for sinkhole detection under railway tunnels. Tests performed on a decametric structure equipped with embedded 110 m-cable interrogated by Brillouin-OTDR and OFDR showed great performances in accordance with finite-element calculations.

We present a novel concept of quasi-distributed optical fibre extensometers for embedment into concrete, with optimized, continuous transfer of the strain field to the fibre, and fully corrected from thermal variations. These sensors have been developed for structural health monitoring applications. They are composed of a combination of optical cavities and Bragg gratings cascaded along a single fibre. The cavities, which are the parts sensitive to the concrete strain, are formed by partially reflecting elements inserted into the fibre. Their length, which is also the measurement basis, can range from 10 cm to several metres. Several cavities can be cascaded along a single fibre, allowing quasi-distributed strains measurements. Bragg Gratings are inserted along the same fibre, close to the cavities, and are used to measure the temperature locally. Both types of sensors are read by a fibre optic low coherence interferometer featuring a temporal delay line. The interferometer is used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. In this paper we present the design of the sensor, and in particular the study of the composite packaging whose shape and mechanical properties have been optimized by finite elements modelling to minimize the intrusion effect and ensure a continuous transfer of the strain field when embedded into concrete. We develop the optical reading method, presenting the theory of the sensor interrogation, or how to get the strain and temperature information. We describe also the instrumentation. Finally we present some laboratory experiments that show very good agreement between standard sensors and OFS, and an example of implementation into a bridge near Angouleme, France.

This paper reports on the design and realization of a new optical fibre sensor body meant for embedding distributed optical fibre sensors into concrete. We have carried out a finite element analysis which shows that the proposed wave-like sensor enables continuous bonding between optical fibre and concrete, symmetrical response in traction and compression whatever the contact conditions may be. The stiffness of the optical fibre can so be adapted to the concrete, which reduces strain concentrations and need for a theoretical calibration factor. Experimental validations are carried out, showing that despite its wave-shape, the proposed sensor body does not introduce any losses or strain that would result from fibre macrobendings. The process of realization is very cheap, thus dedicated to distributed sensors. We have realized optical fibre extensometers based on this technique and on low-coherence interferometry method for the measurements, including an original approach of thermal effect compensation. Preliminary results are very encouraging.

Temperature impacts the sensing performances under radiation of distributed optical fiber sensors exploiting Stimulated Brillouin Scattering in Ge-doped SMFs: SNR is improved at 100°C while frequency shift is about 4 MHz at 1 MGy whatever the temperature.

Both Rayleigh and Brillouin scatterings measurements in optical fibers are able to provide distributed strain and temperature profiles. Acquisition parameters, measurement quality, impact of the environment over time, effective spatial resolution obtained on field are some of the driving specifications. We report here some studies on Brillouin and Rayleigh scattering techniques, related to their performances concerning measurement quality, distance range and spatial resolution. Even if Rayleigh-based measurements are more accurate with 6 µe uncertainty (versus 17 μe), Brillouin-based methods are more reliable than Rayleigh-based on cross-correlation for strain difference over 500 μe; while with the same parameters Rayleigh-based systems are able to provide a higher effective spatial resolution.