1.2         Motivation and thesis outline

Recently, a very efficient backscattering technique called layer-peeling has been applied to the FBG domain for designing new kinds of gratings that exhibit special features, for example zero dispersion properties [1-30, 1-31]. The layer-peeling method is based on the causality principle and therefore strongly depends to the FBG impulse response. This indicates that the OLCR measurements and the layer-peeling reconstruction method form a promising pair to locally characterize FBGs.

In this work, a new OLCR interferometer has been conceived and realized to accurately measure the complex impulse response of a FBG (amplitude and phase). The reconstruction of different types of FBGs has been performed using the layer-peeling method. The local characterization of an axial strain field has also been determined by the combination of a FBG gauge, OLCR measurement and layer-peeling.

Other sensing applications have been studied, including transversal strain measurement (FBG gauges written in polarization maintaining fibers), humidity and temperature measurements (polyimide coated FBG gauges) as well as vibration amplitude measurements (OLCR interferometer technique).

Chapter 2 presents the fundamentals of fiber Bragg gratings : definition and properties,  fabrication and characterization methods used in this work. A new writing set-up is presented that allows the writing of FBGs with different Bragg wavelengths using the same phase mask.

Chapter 3 describes the theoretical simulations of the spectral response using the T-matrix method, and the theoretical reconstructions of the grating distributions from the grating complex spectral response using the layer-peeling method. Several simulations have been made to study the important parameters concerned in the reconstruction process. An evolution of the T-matrix and the layer-peeling algorithm is proposed, which takes account propagation losses in the grating.

Chapter 4 focuses on the OLCR fundamentals, the development of the new OLCR set-up and the experimental reconstruction of several homogeneous and non-homogeneous FBGs.

Chapter 5 presents the combined use of FBGs and the reconstruction method (OLCR & layer-peeling) for distributed axial strain field characterization; the behavior of FBGs subjected to transversal strain fields is also studied for gratings written in low-birefringent or in polarization maintaining fibers.

Chapter 6 resumes the analysis conducted on the sensitivity of polyimide coated FBGs to temperature or relative humidity changes.

Chapter 7 describes the vibration amplitude sensor developed for the control of SNOM tips (Scanning Near-field Optical Microscopy).