The detection and quantification of the presence of certain chemical species is of central importance regarding permanent structural health monitoring of key industrial fields and civil infrastructures such as oil extraction boreholes or radioactive waste repositories, where H2 is released. With this work we propose and test a competitive technique able to measure the concentration of hydrogen and deuterium thanks to their diffusion into the silica glass of a standard optical fiber, already employed for the distributed monitoring of large infrastructures. The proposed technique, based on Chirped-Pulse Phasesensitive Reflectometry (CP-φOTDR), could represent a novel solution for this problem, thanks to its ability to provide dynamical measurements of refractive index change, with great linearity and sensitivities of 10-8 refractive index units, featuring spatial resolutions of a few meters and kilometric sensing ranges.
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 µε uncertainty (versus 17 με), Brillouin-based methods are more reliable than Rayleigh-based on cross-correlation for strain difference over 500 με; while with the same parameters Rayleigh-based systems are able to provide a higher effective spatial resolution.
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.
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 < 0.3 nm; side lobe suppress ratio – SLSR >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.
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.
We study the effect of hydrogen gas diffusion in silica optical fibers on Brillouin and Rayleigh scatterings. By modeling hydrogen diffusion kinetics as a function of temperature and pressure and by measuring simultaneously Rayleigh and Brillouin scattering on G652 single-mode fiber samples during H2 desorption (previously exposed to 175 bars H2 at 80°C), we have demonstrated experimentally that acoustic velocity increases linearly with H2 concentration with a ratio of about (4.8 m/s) / (%mol H2).
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 γ-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.
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.
In this paper, we report the fabrication and characterization of a new concept of optical fibers whose cladding is composed of palladium particles embedded into the silica glass cladding. Since conventional fiber processes are not suitable for such realizations, we developed an original process based on powder technology to prepare our specific preforms. Step, graded index and photonic crystal optical fibers with original shapes were realized. The use of high purity powders as raw materials combined to a specific preforms heat treatment allowed the fabrication of resistant and long length metal-cladding optical fibers. Microstructured Pd-SiO2 composite cladding optical fibers with single-mode behavior and optical losses lower than 2 dB/m at 1530 nm were characterized. Hydrogen-induced attenuation sensitivity of these fibers at the 1245 nm wavelength was demonstrated after long H2 exposure. Dehydrogenation kinetics calculations and experiments were studied.
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.
The use of electromagnetic sensors such as Time Domain Reflectometry (TDR) probes has gained increasing importance for water content measurements since several years, for long term monitoring of structures, among which radioactive waste repositories. TDR probes are basically sensitive to electromagnetic properties of the host material, clayrock in our case Prior to perform in-situ experiments with TDR probes, it is mandatory to have an accurate knowledge of the electromagnetic properties of clayrock as a function of their water content. We developed a new laboratory dielectric measurement device, consisting of a coaxial transmission line, enabling characterization of intact clayrock permittivity over the 50 MHz - 1 GHz frequency range. The study has shown a large variation of complex permittivity with (i) water content, the parameter of interest and (ii) frequency. The frequency dependence is induced by different relaxation processes. In a second step, these data are introduced in an original semi analytical model (Distributed Points Sources Method) in order to obtain a reliable modeling of the TDR probe. Taking into account some experimental aspects of the TDR probe, we propose to introduce a in this paper the effect of an air gap between the TDR antennas and the surrounding media. The effect of this influent parameter is evaluated owing to our DPSM modeling, and some solutions are proposed to overcome the problem.
KEYWORDS: Optical fibers, Sensors, Scattering, Environmental sensing, Signal attenuation, Gamma radiation, Single mode fibers, Environmental monitoring, Fiber optics sensors, Signal to noise ratio
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.
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.
Usually, strain and temperature coefficients of Brillouin frequency shift (BFS) of optical fibers result from experimental
measurements. For the first time, theoretical strain dependence of BFS is analyzed depending on fiber properties. Based
on a FEM-2D modeling, the strain coefficient Cε is determined considering geometrical profile, doping composition and
drawing conditions. Theoretical results showed great accordance with measurements for different types of single-mode
fibers. The Cε coefficient is evaluated with a relative uncertainty better than 8%. Significant Cε magnitude variation has
been observed depending on doping profile. This paves the way towards major improvement of strain optical fiber
sensors.
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.
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.
A 1.48-micrometer unstable-cavity laser is coupled into a single-mode fiber using three microlenses. Reproducible coupling of very high power is demonstrated with different types of lenses (plano-convex or bi-convex, with different apertures). Over 550 mW in single-mode fiber were reproducibly reached; to our knowledge, it is the highest power coupled into a single-mode fiber from a single semiconductor laser at this wavelength. Tolerance measurements on all of the coupling elements of a three-lens system are reported for the first time; an unexpected very large tolerance on the axial displacement of the second lens was measured. Results and interpretation with the aid of Gaussian and aberration simulations are also presented. Finally, we report on the first realization of a module with 46% chip to fiber coupling efficiency.
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