Fibre à réseau de Bragg

Une fibre à réseau de Bragg (FBG, fiber Bragg grating en anglais) est un type de réseau de Bragg inscrit dans un court segment de fibre optique, qui réfléchit des longueurs d'onde particulières de la lumière et transmet toutes les autres. Cette fonction est réalisée en créant une variation périodique de l'indice de réfraction du cœur de la fibre, qui génère un miroir diélectrique spécifique à une longueur d'onde donnée. Ces fibres à réseau de Bragg peuvent être utilisées comme filtres, afin de supprimer ou de ne conserver que certaines longueurs d'onde spécifiques, ainsi que comme capteurs pour certaines applications. vignette|400x400px|Figure 1 : Schéma d'une fibre à réseau de Bragg, avec son profil d'indice de réfraction et sa réponse spectrale en transmission et en réflexion. Théorie Réseau de Bragg Pour une fibre d'indice effectif de mode guidé n_\text{eff} dans laquelle est inscrite une variation d'indice de période spatiale \Lambda, la long

Strain Measurement Validation Of Embedded Fiber Bragg Gratings

Stefano Trono

This study investigates the influence of strain state distribution on the accuracy of embedded optical fiber Bragg gratings (FBGs) used as strain sensors. An optical fiber embedded parallel to adjacent structural fibers in a graphite epoxy quasi-isotropic (90/+/- 45/0)(S) lay-up is evaluated with mechanical loading parallel to the fiber optic direction. Finite element analysis (FEA) is used to evaluate the fiber optic sensors' responses both in the far field and near field regions of the mechanical grips. Comparison between experimental fiber optic strains, strain gauges, and FEA provides good correlation in the far field with differences of less than 1%. However, in the near field region, some discrepancies are found and attributed to birefringence arising from complex strain states.

2010

Fiber bragg grating characterization by optical low coherence reflectometry and sensing applications

Philippe Giaccari

This work connects of three domains of fiber optics : the fiber Bragg gratings (FBG), the optical low coherence reflectometry (OLCR) and the fiber optical sensors (using FBGs, the OLCR or a combination of both). Fiber Bragg gratings are fiber optic devices characterized by permanent and periodic changes of the fiber core refractive index, which translates into a narrowband spectral reflection. FBGs are widely used nowadays in the telecommunications field, for example as reflection filters or dispersion compensators. Moreover, their sensitivity to temperature and strain make them ideal for sensing applications, in particular due to their very small size, their immunity against electromagnetic fields and their multiplexing capabilities. The major challenge with FBGs is to locally characterize the grating properties, in particular the core refractive index distribution along the grating. These data allow correcting imperfections during the writing process or to determine the distributions in sensing applications. In this work, we have reconstructed the complex coupling coefficient distribution of the grating by combining the OLCR technique and a reconstruction technique called "layer-peeling". A novel design for the OLCR has been proposed and realized. This instrument measures precisely the amplitude and phase of the complex fiber Bragg grating impulse response with micrometer resolution and a noise level below –120 dB. Using the layer-peeling method, the FBG complex coupling coefficient can be retrieved with a 20 µm resolution and an error of less than 5% (this value is obtained by comparing the reconstructions from both sides of the grating). Many studies have been conducted on axial strains in various samples and various experimental conditions. The most promising result concerns the study of non-homogeneous strain fields with the reconstruction technique that combines the OLCR and the "layer-peeling". The study of transversal strain field has also been conducted with FBGs written in birefringent fibers. A non-linear behavior has been observed and explained with the rotation of the fiber eigen axis. An important sensitivity anisotropy for different angles has been observed, but not fully explained. The influence of humidity and temperature on a polyimide coated FBG was also investigated. The sensitivities were measured as a function of the coating thickness. From this analysis a novel concept for an intrinsic relative humidity sensor using polyimide-recoated fiber Bragg gratings has been proposed. Tests in a controlled environment indicate that the sensor has a linear, reversible and accurate response behavior between 10 and 90 %RH and between 13 and 60 °C. The last but not least, a new fiber optic sub-nanometric scale vibrometer based on the OLCR technique has been developed. This sensor allows for the control of a fiber SNOM (Scanning Nearfield Optical Microscopy) tip oscillations in the air and in water. A very good accuracy is achieved with a noise level around 1 pm. The compactness and the easiness to use (auto-calibration and stability) of this sensor open up new measurement fields for the SNOM technique as, for example, with biological samples in liquids.

EPFL2003

Fiber bragg grating characterization by optical low coherence reflectometry and sensing applications

Philippe Giaccari

EPFL2003

In situ simultaneous strain and temperature measurement of adaptive composite materials using a fiber Bragg grating based sensor

Strain Measurement Validation Of Embedded Fiber Bragg Gratings

Fiber bragg grating characterization by optical low coherence reflectometry and sensing applications

Strain Measurement Validation Of Embedded Fiber Bragg Gratings

In situ simultaneous strain and temperature measurement of adaptive composite materials using a fiber Bragg grating based sensor

Fiber bragg grating characterization by optical low coherence reflectometry and sensing applications