Photorefractive effect and photoinduced quadratic nonlinear susceptibilityin germanosilicate fibres fabricated in nitrogen and helium atmospheres bythe MCVD technique

Single-mode optical fibres were fabricated from a germanosilicate glass by the method of modified chemical vapour deposition (MCVD), which used sinter ing of a porous glass in a reducing (helium or nitrogen-containing) atmosph ere, The optical fibres exhibit a high photoinduced change in the refractiv e index and a high efficiency of recording quadratic nonlinear susceptibili ty compared to a standard germanosilicate fibre. Sintering, both in nitroge n and in helium atmospheres, was shown to increase the concentration of ger manium oxygen-deficient centres in glass. It is likely that nitrogen enters into a germanosilicate glass in the concentration that is sufficient to mo dify the glass structure and to additionally increase its photosensitivity. The replacement of oxygen or silicon in the close vicinity of an oxygen va cancy by nitrogen may play a key role in the photosensitivity enhancement o wing to the formation of additional valence bonds and blocking of recombina tion processes.