Boron codoped germanosilicate films with a high concentration of germanium oxygen deficient centres (GeODC) have been fabricated by reduced pressure plasmachemical deposition on a plane silica substrate with the use of hydrogen free gaseous raw materials. Photoinduced changes in the ultraviolet absorption spectra, Ge-ODC triplet luminescence intensity and refractive index of the films have been investigated under excimer laser irradiation (wavelengths of 193 and 248 nm). The value of the induced refractive index is determined from the contrast of probe light interference on a long period phase grating written on the film. The induced refractive index obtained without H2 loading amounted to 5×10 -4 . A strong dependence of all induced effects on the irradiation wavelength has been revealed. Nonmonotonic dose evolution of the contrast of the interference pattern of a phase grating, previously revealed in in-fibre gratings and known as Type IIa photosensitivity, has been observed in films under irradiation at the wavelength of 193 nm. It is shown that the photoinduced refractive index increase can be compensated by the effect of optical length reduction which is due to transverse contraction of the film. The latter effect has been detected experimentally by measuring the film surface relief after ultraviolet light irradiation. Based on estimations, the photoinduced glass contraction is argued to be the cause of the Type IIa photosensitivity.
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Голант et al. (2001) studied this question.