Grating makers see the light

Commercial fibre Bragg gratings are made by exposing glass to an ultra-violet (UV) laser through a phase mask, requiring expensive equipment and considerable time.

However, the UV laser irradiation of SiO₂–GeO₂ glass to produce planar integrated optical components has been limited because of the low refractive-index change — unless the substrate has been sensitised beforehand. So, new glass compositions must be found that can be processed using a visible source to fabricate lower-cost devices for next-generation metro networks.

Hence, the EC has given EUR100,000 funding to a 12-month EUR166,974 project within its Fifth Framework Programme, initiated in October, called GLAsses for PHOtosensitivity in the VISible (GLAPHOVIS).

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.

The GLAPHOVIS project aims to prepare new glass compositions with an absorption edge in the near UV, then co-dope them with an active element. This will present a band about 1–1.5eV from this edge with the aim of direct writing Bragg gratings by exposure with visible lasers. Visible lasers are less expensive and dangerous than ultra-violet lasers, as well as being tunable.

The project aims 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 it is feasible.

The project will involve preparing new glass compositions, characterising them and then irradiating them with visible (VIS) laser sources. After simulating devices by directly writing gratings into the bulk glasses and characterising the wave-guides produced in terms of photosensitivity.

Tasks involved in the project include:

Preparation and characterisation of host glasses which present a UV absorption edge greater than 250nm and near the visible;
Doping these glasses with europium (Eu) and terbium (Tb) with characteristic absorptions at 464 and 488nm, respectively;
Modifying the glass compositions by adding components that will shift its UV edge towards 464 or 488nm visible wavelengths;
Investigating photosensitivity as a function of change in refractive index under direct write conditions for channel wave-guides or reflectivity of a Bragg grating;
Modelling the photosensitivity observed (transmission properties before and after VIS-IR laser radiation).
Direct writing of channel waveguides and their characterisation by exposure to UV-VIS-IR sources depending upon the optimum conditions for each glass composition;
Characterisation of stability of the directly written waveguides. The samples will be heated at different temperatures and its photosensitivity measured after treatment;
Characterisation of the transformation processes after UV exposure to get an understanding of the nature of photosensitivity.

The GLAPHOVIS project also aims to describe how the energy difference between the UV edge of the host glass and the electronic absorption of the rare earth interacts to influence the photo-sensitivity of the glass.

The prime contractor for GLAPHOVIS is:

Politecnico di Torino in Turin, via its Department of Materials Science and Chemical Engineering (contact Dr Monica Ferraris, e-mail: [email protected]) http://optics.politico.it/smic

Other contractors include:

the Université Paris-Sud in Orsay via the CNRS' Laboratoire de Physico-Chimie de l'Etat Solide (contact Dr Bertrand Poumellec, e-mail: [email protected]);
the University of Southampton's Optoelectronic Research Centre (contact Dr Elizabeth Taylor, e-mail: [email protected]).