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This revue article describes photorefractive Bragg gratings (elaboration, interest, performances) which are planned to be used in optical fibers for telecommunications. This application justifies the effort made presently to increase the photosensitivity of the fibers and the stability of the grating inscribed in them. Since Bragg gratings are based on UV induced refractive index change, underlied mechanisms are described as long as the change of UV absorption at the origin of photo transformation of the glass. Different models invoked to describe UV induced refractive index changes are extensively presented and criticized, leading to conclusions. The different directions of research undertaken to enhance the photosensitivity: H2loading, other dopants, other glasses and other writing wavelengths are explicited. All this leads to a classification of Bragg gratings in terms of the UV induced mechanism involved.

248 nm photosensitivity is studied in thin layers of H2reduced SiO2–GeO2binary glasses deposited onto silica substrates by a flame hydrolysis method. It has been found that several effects take place depending on the time of UV exposure and on the fluence per pulse used. We have called them, `behaviour I, II and III'. The first one corresponds to an increase of volume under irradiation, induced probably by a reoxidation of Ge previously reduced by the pre-irradiation chemical treatment. The second one is a common fusion–ablation process. The third one is still not clear but would imply, a diffusive process from nonirradiated to irradiated regions. These observations contrast with the ones made in nonreduced SiO2–GeO2glass for which densification has been pointed out.

Les métastases extra-osseuses calcifiées peuvent fixer le 99mTc-HMDP et, par conséquent, elles sont souvent visualisées à la scintigraphie osseuse. Nous rapportons le cas d’une métastase cérébrale fixant le 99mTc-HMDP détectée lors d’une scintigraphie osseuse et qui illustre le rôle que peut jouer l’imagerie hybride TEMP/TDM dans la détection et la localisation des métastases extra-osseuses calcifiées.