Article

Frequency scanned phase sensitive optical time-domain reflectometry interrogation in multimode optical fibers

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Abstract

Standard multimode optical fibers normally support transmission over some 100 modes. Large differences in the propagation constant and the spatial distribution of distinct modes degrade the performance of phase-sensitive optical time-domain reflectometry measurements. In this work, we present a new realization of a coherent time-domain interrogation technique using single-mode operation in multimode fibers. We demonstrate effectively distributed strain sensing on three different multimode optical fibers. Up to 4 km of multimode fiber has been correctly interrogated, featuring a spatial resolution of 20 cm.

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... Ces différences entre les constantes de propagation est par exemple problématique pour interroger les fibres multimodes à l'aide de méthodes OTDR (Optical Time-Domain Reflectometry), où la connaissance de la vitesse de propagation du signal est cruciale pour remonter à la position d'un défaut. Néanmoins, éliminer les modes d'ordre élevé à l'aide d'un filtre adapté [113] permet de réaliser de telles mesures. Dans un modèle à rayons, le diamètre de coeur des fibres multimodes implique également des variations de l'angle d'incidence sur le dioptre à l'extrémité de la fibre. ...
Thesis
URL : http://www.theses.fr/2021NANT4049 - FR : La mesure de l'indice optique d’un milieu est un moyen d'obtenir des informations comme les éléments chimiques qui le composent, sa température ou sa pression. Le capteur dit de Fresnel, basé sur la mesure de la puissance réfléchie à l’extrémité d’une fibre optique monomode, est un moyen simple de réaliser des mesures d’indice optique, et est adapté à des mesures in-situ de réticulation ou de teneur en eau dans des structures composites. Néanmoins, ce capteur ponctuel ne permet de faire des mesures que sur une petite zone à l'extrémité de la fibre, le rendant très sensible aux perturbations locales. Cette thèse étudie la possibilité d’utiliser des structures de fibres spéciales en tant que capteur de Fresnel, d’un point de vue théorique et expérimental, afin d’élargir le champ de mesures possibles. Les fibres optiques biréfringentes sont d’abord considérées, avec l'objectif de réaliser des mesures d'anisotropie optique. Les fibres multimodes sont ensuite étudiées afin d'augmenter sensiblement le volume de mesure grâce à leur diamètre de coeur important, et de valider l’emploi de fibres plastiques à bas coût en tant que capteur de Fresnel. Enfin, l'utilisation de la spectroscopie sur un capteur de Fresnel est envisagée, avec l'objectif d'utiliser les possibles bandes d'absorption dans un milieu pour obtenir des informations supplémentaires sur ce dernier, ou bien de prendre en compte la diffusion de la lumière par des particules situées à l'extrémité de la fibre.
... The derived model is valid for any system for which the peak value of a resonance is evaluated through quadratic least-square fitting. In the case of coherent Rayleigh-based DOFS, for instance in direct-detection frequency-scanned φ-OTDR systems, the most widely and commonly-used method to estimate the relative value of the FS between reference and measurement signals is cross-correlation [18][19][20]. Cross-correlation is a standard method utilised for delay estimation in sonar and radar systems [21][22][23], and is also adopted in other coherent Rayleigh-based DOFS [24]. The presence of unavoidable additive noise in the traces being correlated fundamentally limits the performance of the cross-correlation estimator and leads to uncertainty in the estimated FS. Besides, other experimental parameters, such as spatial resolution, can also influence the accuracy of estimation. ...
... Recently, multimode fiber has been reported to be used in φ-OTDR systems [19][20][21][22]. A typical multimode fiber can usually support hundreds of spatial modes, which cannot be easily and separately demodulated with standard photodetectors in conventional φ-OTDR systems. ...
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