A fiber Bragg grating has been inscribed in a 100-µm diameter sapphire optical fiber with the phase mask technique and a fs-laser emitting at 800 nm. The grating was placed inside a sealed alumina capillary to protect the fiber from the environment. Then the fiber was set inside an oven and cycled up 7 times to a maximum temperature of 1500°C during 2 h. We observed that after two cycles, the grating is stabilized and no more hysteresis on the Bragg wavelength is observed. However, the temperature uncertainty is as high as 15°C and is principally due to modal interference. Then the grating is submitted to a 3-day annealing and two annealing successive 4-day annealing – for a total of eleven days – at a temperature of 1500°C. During this treatment, the grating amplitude remained constant and the Bragg wavelength showed no significant drift. As a conclusion, the packaged grating did not exhibit any erasure during these annealing experiments and perform reliable temperature measurement up to 1500°C.
Fiber Bragg Gratings (FBGs) are used to measure shock velocity, detonation velocity, shock wave profile or pressure profile in inert and energetic materials. Such thin probe, with a diameter below 150 μm, can be inserted directly into materials without disturbing the physical phenomena. Chirped FBGs are used to track the shock wave in the grating using wavelengths. The velocity (few km/s) and shock wave profile measurements are realized by recording the CFBG’s reflected spectral width. Pressure measurements at few GPa levels use dynamic spectrometers, two approaches are compared: parallel acquisition using an Arrayed-Waveguide-Grating and time-multiplexing by wavelength-to-time conversion using dispersion.
High pressure shock profiles are monitored using a long Fiber Bragg Grating (FBG). Such thin probe, with a diameter of
typically 150 μm, can be inserted directly into targets for shock plate experiments. The shocked FBG’s portion is
stressed under compression, which increases its optical group index and shortens its grating period. Placed along the 2D
symmetrical axis of the cylindrical target, the second effect is stronger and the reflected spectrum shifts towards the
shorter wavelengths. The dynamic evolution of FBG spectra is recorded with a customized Arrayed Waveguide Grating
(AWG) spectrometer covering the C+L band. The AWG provides 40 channels of 200-GHz spacing with a special flattop
design. The output channels are fiber-connected to photoreceivers (bandwidth: DC - 400 MHz or 10 kHz – 2 GHz).
The experimental setup was a symmetric impact, completed in a 110-mm diameter single-stage gas gun with Aluminum
(6061T6) impactors and targets. The FBG’s central wavelength was 1605 nm to cover the pressure range of 0 – 8 GPa.
The FBG was 50-mm long as well as the target’s thickness. The 20-mm thick impactor maintains a shock within the
target over a distance of 30 mm. For the impact at 522 m/s, the sustained pressure of 3.6 GPa, which resulted in a Bragg
shift of (26.2 ± 1.5) nm, is measured and retrieved with respectively thin-film gauges and the hydrodynamic code
Ouranos. The shock sensitivity of the FBG is about 7 nm/GPa, but it decreases with the pressure level. The overall
spectra evolution is in good agreement with the numerical simulations.
Dynamic measurements of detonation velocity profiles are performed using long Chirped Fiber Bragg Gratings (CFBGs). Such thin probes, with a diameter of typically 150 μm, are inserted directly into a high explosive sample or simply positioned laterally. During the detonation, the width of the reflected optical spectrum is continuously reduced by the propagation of the wave-front, which physically shortens the CFBG. The reflected optical intensity delivers a ramp down signal type, which is directly related to the detonation velocity profile. Experimental detonation velocity measurements were performed on the side of three different high explosives (TNT, B2238 and V401) in a bare cylindrical stick configuration (diameter: 2 inches, height: 10 inches). The detonation velocity range covered was 6800 to 9000 m/s. The extraction of the detonation velocity profiles requires a careful calibration of the system and of the CFBG used. A calibration procedure was developed, with the support of optical simulations, to cancel out the optical spectrum distortions from the different optical components and to determine the wavelength-position transfer function of the CFBG in a reproducible way. The 40-mm long CFBGs were positioned within the second half of the three high explosive cylinders. The excellent linearity of the computed position-time diagram confirms that the detonation was established for the three high explosives. The fitted slopes of the position-time diagram give detonation velocity values which are in very good agreement with the classical measurements obtained from discrete electrical shorting pins.
Safety must always prevail in Nuclear Power Plants (NPPs), as shown at Fukushima-Daiichi. So, innovations are clearly needed to strengthen instrumentations, which went inoperative during this nuclear accident as a consequence of power supply losses. Possible improvements concern materials and structures, which may be remotely monitored thanks to Optical Fiber Sensors (OFS). We detail topics involving OFS helpful for monitoring, in nominal conditions as well as during a severe accident. They include distributed sensing (Rayleigh, Raman, Brillouin) for both temperature sensing and structure monitoring as well as H2 concentration and ionizing radiation monitoring. For future plants, Fiber Bragg Grating (FBG) sensors are considered up to high temperature for sodium-cooled fast reactor monitoring. These applications can benefit from fiber advantages: sensor multiplexing, multi-km range, no risk-to-people, no common failure mode with other technologies, remote sensing, and the ability to operate in case of power supply lost in the NPP.
Following the pioneering work of the Lawrence Livermore National Laboratory, Chirped Fiber Bragg Gratings are investigated as in situ, real-time, wavelength-position discriminators for measuring detonation speeds inside explosives.
Corrosion of reinforced bar (rebar) in concrete structures represents a major issue in civil engineering works, being its detection and evolution a challenge for the applied research. In this work, we present a new methodology to corrosion detection in reinforced concrete structures, by combining Fiber Bragg Grating (FBG) sensors with the electrochemical and physical properties of rebar in a simplified assembly. Tests in electrolytic solutions and concrete were performed for pitting and general corrosion. The proposed Structural Health Monitoring (SHM) methodology constitutes a direct corrosion measurement potentially useful to implement or improve Condition-Based Maintenance (CBM) program for civil engineering concrete structures.
Safety while tunneling is one of the main challenges for underground constructions, avoiding
confinement losses, which remain an important risk for public works, leading to additional delays and high
insurance costs. In such applications, usual surface instrumentations cannot be set up because of high building
density in many overcrowded cities. Tunnelling deals with the challenge of requiring ground surface
undisturbed. One original concept proposed in the framework of the European Tunconstruct project, consists
in very early settlement detection close to the tunnel vault and before any detectable effect on the surface. The
adopted solution is to set-up a sensing element inserted into a directional drilling excavated above the
foreseen tunnel. The methodology is based on the well known Brillouin Optical Time Domain Reflectometry
(B-OTDR) in singlemode optical fibres and a special cable design dedicated to bending measurement. Two
cables, based on different industrial manufacturing processes, have been developed taking into account the
strain sensitivity required, the flexibility and the robustness for borehole installation, a low power attenuation
and storage on a drum. Industrial prototypes have been manufactured and validated with tests in open air
where settlement profiles geometry can be accurately controlled. Demonstration on job site took place on The
Greater Cairo Metro Line 3 (CML3) at the beginning of 2009.
A multi-channel fibre optic OSL dosimeter system is developed by the CEA LIST for quality control in cancer radiation
therapy, in the framework of the European Integrated Project MAESTRO.
A wavelength tunable erbium-doped fiber ring laser dedicated to high-speed interrogation of Fiber Bragg Grating (FBG) sensors has been realised and characterized. A standard FBG is used as the output mirror whereas an intra cavity spectral filter formed by a π-phase-shifted FBG is implemented to narrow the output laser spectrum down to 0.5 pm. The emitting wavelength is tuned at 500 Hz over 3.6 nm by simultaneously straining both the standard and π-phase-shifted FBG with a piezoelectric actuator. An all-fiber Michelson interferometer is used to accurately determine the spectral scanning range. Bragg wavelength increments equal to 1.3 pm are resolved combining both wavelength referencing with an absorption gas cell and amplitude normalisation versus the laser output power. This instrumentation is designed to be embedded in a train cabin for making on-board strain and temperature measurements on pantographs with FBG sensors.
This paper describes the laboratory tests and field trial on the Saint-Jean bridge (Bordeaux, France) of both FBG extensometers (long and short-gage) and associated instrumentation, in the context of its industrial transfer.
Most civil engineering structures have been built in the 50's and 60's and reach similar level of degradation accelerated by loading conditions and corrosion. In Europe, National Authorities and the European Commission promote Health Monitoring concepts, instrumentation of existing structures and help in the design of new durable structures of higher performance. In this context, the CEA-List has achieved a non-exclusive industrial transfer of its Bragg grating sensing technology for civil engineering applications to Hydrolog (French SME), supported by the European Community and the french ministry of Industry. In order to check the reliability and user-friendliness of this instrumentation, eleven spectrally-multiplexed Bragg grating-based extensometers, four FBG temperature sensors and an acquisition unit have been installed into the Saint-Jean bridge in Bordeaux, France with the help of the Infrastructure Regional Direction (DRE-Aquitaine) and the Bordeaux Authority (Communaute Urbaine de Bordeaux). A standardized loading of the bridge has been performed on October 29, 1001, with the purpose of correlating its mechanical reaction to loading conditions. Moreover, the equipment has been operating for one year to take into account the winter-summer cycle.
A new kind of instrumentation based on Optical Fiber Bragg grating sensors is proposed for the main relevant monitoring needs in the electric power industry. The challenging metrological properties of these components are presented and their good resistance to (gamma) -ray irradiations experimentally proved.
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