Refractive index engineering in silica glass

The thesis covers research performed durint the last eight years by the author in collaboration with members of his group within the field of UV-written gratings and poling of silica-based materials. The subjects cover several steps on the value chain from basic physics and chemistry via component technology to the fabrication and testing of final components. Most of the components have found applications within telecommunications, but a significant minority has been used for optical sensor systems. Several of the developed components have gone into production. The research results are described in detail in chapters 4,5 and 6. Chapter 4 describes the semi-classsical model developed by the author to describe the basic UV-induced processes in germanium-doped silica. The idea behind the model is that oxygen-deficient germanium centres in the glass work as gates for the UV-photon energy, which is later transferred non-radiatively to other sites in the lass, where index-modifying defects may be formed through conversion to internal energy. The non-radiative processes are predominantly dipole-quadrupole process and the model is therefore named the UV dipole-quadrupole model". The model gives a good description of the majoriy of experiments with UV-irradiation of silica glass and one of its dditional advantages is that it can be used to make a more rigorous definition of the term "photosensitivity". The model can also be used to predict a range of UV-induced effects such as new types of luminescence and UV-induced dispersion. Chapter 5 describes the development of Bragg grating technolgy. It covers the way from the inscription of simple gratings in fibres and optimisation of photosensistivity via the use of Bragg gratings in DFB fibre lasers and planar waveguides to the development of a new technologically sophissticated method to control grating writing using the polarisation of the UV-light. This chapter also contains a description of some specail and surprising properties of UV-written gratings. Some aspects of the DFB fibre laser production and the entire new grating writing method have een patented and part of it is already used for component production. Chapter 6 describes research on poling. The aim of poling silica is to obtain non-linear optical properties. Two methods have been applied. The first is negative poling where the glass properties are only slightly modified, in particular by freezing a semi-permanent electric field into the glass. The non-linear properties obtained this way are very stable and show small dispersion with wavelength. Therefore they are potentially useful for applications ranging from electro-optic switching and modulation to wavelength conversion. Unfortunately the effects are quite small, so it is only realistic to use them for switching in the near future. The other type of poling is called positive ppoling. In brief, this method uses the reservese electric field compared to negative poling, whereby migration of positive ions into the glass is facilitated. This way the glass undergoes significant physical and chemical changes and in some cases very large non-linear effects have been demonstrated. This includes a non-linear coefficient around 22 pm/V in a wavelength range near 800 nm. The author believes this is due to the combined action of silver nano-cluisters with large third-order non-linear coefficients and frozen-in fields in the glass. In the future it will be a great challenge to extend or shift the wavelength range and improve the figure of merit so the large non-linear effects can find practical applications.