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Mesures réparties par réflectométrie fréquentielle sur fibres optiques

Ekaterina Boldyreva 1, 2
1 LCFO - Laboratoire Capteurs Fibres Optiques
DM2I - Département Métrologie Instrumentation & Information : DRT/LIST/DM2I
2 LAAS-OSE - Équipe Optoélectronique pour les Systèmes Embarqués
LAAS - Laboratoire d'analyse et d'architecture des systèmes
Abstract : The present PhD thesis have been carried out at the Laboratoire de Mesures Optiques (Optical Measurement Laboratory) based at CEA Saclay Nuclear Research Center, France. It concerns theoretical analysis and experimental realizations of a “distributed Optical Fiber Sensor” based on Optical Frequency Domain Reflectometry using the Rayleigh backscattering phenomena. As the OFS community know, such technology has been developed by the US company Luna Innovations Inc., which keeps carefully secret any details of both the technology involved and the signal processing of such instruments, named OBR (Optical Backscattered Reflectometer). Advantageously, the present Rayleigh-OFDR method provides a very high spatial resolution (in comparison with other methods based on reflectometry, i.e. OTDR, DTS Raman, Brillouin BOTDR and BOTDA…) and so remains extremely sought for many applications, especially for Structural Health Monitoring. In the present PhD thesis the theoretical aspects of the Rayleigh OFDR method has been presented. As a reminder, in order to measure a physical parameter profile along the fiber, it is required to determine two subjects simultaneously: the distance at which the physical parameter is applied (i.e. the sensor address) to the fiber under test, and the amplitude of this parameter (i.e. “the measurement” itself). A relation between the influence of any environment parameter (temperature, strain, etc.) and the fiber core refractive index changes has to be used. These relations have been shown, as far as the hypothesis linked to the theoretical aspects. Two different methods for such measurement have been proposed: first of all the “correlation method” (used by Luna) and second “a tensor-based method”, fully innovative as developed during the present PhD thesis. The correlation method may be considered as more “universal” as based on the measurement of spectral shifts caused by the physical parameter to measure. But, in the same time, we consider it is less accurate, as a systematic error may somehow fail the spectral shift calculation algorithm. The tensor method do not suffer of such systematic error, but it remains only applicable to strain sensing, as based on the elasto-optic effect. Signal processing procedure has been elaborated and tested for both methods, and of course discussed in the manuscript. In the same time, performances of the existing Luna OBR 4600 reflectometer has been tested at Lab. and are reported for both temperature and strain measurements. An optical mock-up for Rayleigh OFDR method has also been developed. A software, dedicated to signal processing has been fully developed either. This program has been tested for simulated sensing signals and for raw data delivered by the Luna OBR 4600 reflectometer. Results of such tests could be considered as satisfactory, even some questions to be solved remains. Finally, some practical examples of Rayleigh OFDR industrial applications have been shown and detailed. Moreover, in such context of real applications on field, an innovative Rayleigh OFDR system devoted to liquid sodium leakage detection around secondary cooling circuitry of generation IV nuclear reactors has been developed and tested on a nuclear installation. An industrial prototype of such measurement system has been elaborated. Finally, way of progress are analyzed and discussed for future R&D activities.
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Ekaterina Boldyreva. Mesures réparties par réflectométrie fréquentielle sur fibres optiques. Optique [physics.optics]. INP Toulouse, 2016. Français. ⟨NNT : 2016INPT0080⟩. ⟨tel-02274547⟩

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