Fiber optic sensors for metrology, geophysics and strain measurements
Capteurs à fibres optiques pour la métrologie, la géophysique et les déformations mécaniques
Résumé
This HDR manuscript principally describes the research activities in which I have been involved since my appointment in August 2003 to my current laboratory, the Optoelectronics for Embedded Systems Group of the Laboratory for Analysis and Architecture Systems (LAAS-OSE). Before detailing these activities, I have also added a brief description of research work carried out during my PhD. This has been willingly added to this manuscript to demonstrate the evolution of my research activities over the past decade. Chapter 1 is a very brief introduction to my early career as a researcher in optics and fiber optics, basically describing work carried out on ruby crystal fiber-based sensors for harsh environments during my PhD at the University of Glasgow. Results show that c-axis ruby crystal fibers are uniquely sensitive to temperature effects while being insensitive to high levels of strains. Chapter 2 concerns an extrinsic-type fiber Fabry-Perot (EFFPI) interferometer which I initially developed during my post-doctoral fellowship at the Ecole des Mines de Nantes where I was responsible for setting up and leading the fiber optic sensing branch of the Instrumentation and Sensor Group. The proof-of-concept of the fiber interferometer was demonstrated before my departure to ENSEEIHT-INPT in Toulouse where I continue to work on its improvement at my current laboratory, LAAS-OSE, for metrology purposes. The initial polarization-based EFFPI which was developed earlier is thus described in the first half of Chapter 2 where a quadrature signal pair is obtained, hence the optical dual-cavity nature of the instrument. This is followed by its evolution into a modulation-based instrument, in the second half of the chapter, where, effectively, the introduction of a double-modulation scheme to the laser drive current enables a quadrature pair as well as the capability for detecting displacement amplitudes smaller than /4 to be achieved. This latter sensor has been conceived for applications in optical metrology and, more specifically, in geophysics under the ANR RISKNAT-sponsored LINES project. Three optical fiber-based geophysics instruments have been developed during the course of this project, namely, an EFFPI-based long baseline tiltmeter (or hydrostatic leveling system, HLS), an EFFPI-based borehole tiltmeter and an EFFPI-based seismometer. The modulation-based EFFPI is currently undergoing further development as a key component of the HLS for accelerator alignment at the CERN. A TRL 7 (technology readiness level) maturity state is envisaged at the outcome of this project. The final ambitions are to attain TRL 8 and TRL 9 before production and commercialization of the instrument for geophysics and industrial applications. Chapter 3 describes the second research activity which I lead at LAAS-OSE. This essentially involves the development of a novel technique for interrogating fiber Bragg grating-based (FBG) strain sensors based on optical feedback or self-mixing interferometry. The reflections off an FBG are retro-injected into the cavity of a laser diode, perturbing the internal fields. These result in a series of sawtooth fringes being detected by an internal photodiode and which are a function of the strength as well as frequency of the external strains applied on the FBG. This sensor has been demonstrated for dynamic strain measurements under a cantilever set-up. Further, a proof-of-concept is also experimentally demonstrated for extending the dynamic strain measurement range by 50% of the current limit via a low-frequency modulation scheme to the laser diode current. Chapter 4 is a summary of all the administrative tasks throughout my research career. It is organized around my supervision of PhD students including post-doctoral fellows, the various research projects that I have led and a selected list of my publications. Included is also a short discussion on the perspectives for leading further research as well as my services to my research community. I have also summarized my teaching duties and, to conclude this manuscript, my Curriculum Vitae is enclosed for perusal.
Ce manuscrit d'Habilitation décrit les principales activités de recherche que je mène au sein du groupe Optoélectronique pour les Systèmes Embarqués du LAAS depuis 2003. Dans un premier temps, les travaux porteront sur le développement des capteurs à fibres optiques à base de fibres cristallines de rubis pour des environnements hostiles à savoir température élevée et déformations mécaniques très importantes. Il est à noter que ces capteurs sont insensibles aux dernières perturbations, et par conséquence, ne mesurent que la grandeur physique ciblée. Une deuxième partie concerne le développement initial d'un interféromètre fibré de Fabry-Pérot extrinsèque (EFFPI). Ce premier dispositif est basé sur la décomposition du mode fondamental injecté dans l'interféromètre en deux signaux intrinsèques interférométriques déphasés en quadrature, d'où l'EFFPI à double-cavité optiques. L'EFFPI à double-cavité se montre cependant sensible aux effets de polarisation lors des perturbations induites (variations de température et vibration parasites). Pour éliminer ces difficultés, un EFFPI à modulation est développé. Ce travail est effectué dans le cadre d'un projet ANR qui a pour objectif de développer des nouveaux instruments pour les applications en géophysique. En effet, une double modulation du courant de la diode laser est appliquée, ce qui équivaut une modulation sur la longueur d'onde du laser. Ainsi, nous obtenons une condition de quadrature où l'amplitude du déplacement ainsi que sa direction peuvent être précisément déterminées sans aucune ambiguïté. Par ailleurs, grâce a cette double modulation, l'EFFPI est aussi capable de mesurer des très faibles déplacements, inférieurs à lambda/4 (< 327.50 nm pour lambda = 1310 nm). Trois instruments opto-géophysiques ont ainsi été développés à savoir un inclinomètre longue base à fibre optique, un inclinomètre de forage à fibre optique et un sismomètre à fibre optique. La troisième partie taitera le développement des capteurs à fibres optiques à réseaux de Bragg pour la mesure de déformations mécaniques. Pour ce travail, une nouvelle technique d'interrogation des fibres à réseaux de Bragg basée sur la rétro-injection optique (ou "self-mixing") est exploitée pour réaliser des capteurs de contraintes (déformations mécaniques) avec une bonne précision.
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