UV laser radiation of SiO2-GeO2 glass for producing planar integrated optical components has been limited because of the low refractive-index change that this technique permits unless the substrate has been sensitised before. New glass compositions have therefore to be found. Photosensitivity in germanosilicate glasses is traditionally associated with the presence of so-called germanium-oxygen deficient centres, because of the strong absorption band of these defects near 5eV. The distance between the Urbach-edge (9eV) and the absorption peak of germanosilicate glass may be the key factor of photosensitivity. We propose therefore to prepare new glass compositions with an absorption edge in the near UV, then co-doping with an active element, which presents a band around ~1-1.5eV from this edge with the aim of direct writing Bragg gratings by visible laser (less expensive and dangerous, tunable) exposure into these glasses.
We aim to show that photosensitivity can be enhanced by decreasing the distance between the absorption edge and the absorption peak of a doped glass. This is a new concept although there is evidence in the literature that this is likely. We propose to prepare new glasses with UV edge greater than 250 nm. Dope these glasses with Eu and Tb with characteristic absorptions at 464 and 488 nm respectively. Modify the glass compositions by adding components that will shift its UV edge towards 464 or 488 nm. Investigate photosensitivity as a function of change in refractive index under direct write conditions for channel wave-guides or reflectivity of a Bragg grating. Model the photosensitivity observed. We will describe how the energy difference between the UV edge of the host glass and the electronic absorption of the rare earth interact to influence photo-sensitivity.
DESCRIPTION OF WORK
In this project we plan to prepare new glass composition, characterise them and then irradiate them with VIS laser sources. After we simulate devices by directly writing gratings into the bulk glasses and characterise the produced wave-guides in terms of photosensitivity.
The main tasks are:
(1) Preparation and characterisation of the host glasses which presents an absorption edge near the visible;
(2) Investigation of effects of doping with ions which present an absorption peak near the absorption edge of the host glasses;
(3) For each composition, the effect of variations in the glass composition on photosensitivity will be investigated;
(4) Study of the photo-sensibility: transmission properties before and after VIS-IR laser radiation;
(5) Direct writing of channel wave-guides and their characterisation by exposure to UV-VIS-IR sources depending upon the optimum conditions for each glass composition;
(6) Characterisation of stability of the directly written wave-guides. The samples will be heated at different temperatures and its photosensitivity measured after treatment;
(7) Characterisation of the transformation processes after UV exposure to get an understanding of the nature of photosensitivity.
Funding SchemeACM - Preparatory, accompanying and support measures
75794 Paris Cedex 16
91405 Orsay Cedex
SO17 1BJ Southampton