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

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.