Photosensitivity, chemical composition gratings, and fiber-based components

The different topics of this thesis include high-temperature stable fiber Bragg gratings, photosensitivity and fiber based components. Fiber Bragg gratings (FBG) are wavelength dispersive refractive index structures manufactured through UV exposure of optical fibers. Their applications range from WDM filters, dispersion compensators and fiber laser resonators for telecommunication applications to different types of point or distributed sensors for a variety of applications. One aim of this thesis has been to study a new type of FBG referred to as chemical composition grating. These gratings differ from other types of FBG in that their refractive index structure is attributed to a change in the chemical composition. Chemical composition gratings have shown to be extremely temperature stable surviving temperatures in excess of 1000 C. Photosensitivity of pure silica and germanium-doped core fibers in the presence of hydroxyl groups has also been studied and different types of fiber based components have been developed. The main result of the thesis is a better understanding of the underlying mechanism of the formation of chemical composition gratings and their decay behavior at elevated temperatures. The refractive index modulation is caused by a periodic change in the fluorine concentration, which has been verified through time-of-flight secondary-ionmass spectrometry and through studies of the decay behavior of chemical composition gratings. A model based on diffusion of dopants has been developed, which successfully predicts the thermal decay at elevated temperatures. Studies of the dynamics of chemical composition grating formation have resulted in a manufacturing technique that allows for reproducible grating fabrication. The main results regarding photosensitivity is a method to significantly increase the effect of UV radiation on standard telecommunications fiber. The method, referred to as OH-flooding, has also been applied to pure-silica core fibers resulting in the first report of strong grating formation in such fibers. Finally, research into different schemes for developing fiber-based components has resulted in two types of single fiber integrated Mach-Zehnder interferometers; one passive interferometer that can be used as an optical filter and one active interferometer controlled with internal metal electrodes.