One photon and two photon process in photo-decomposition of germanium oxygen deficient centres

Abstract

There has been much debate on the fundamental mechanism of photosensitivity in germanium-doped silica optical fibres. The importance of the effect, which was discovered by K.O.Hill in 1978, has been well demonstrated by the still rapidly increasing application areas that it has in fibre optics. Photo-induced index changes as high as ~10 in H₂ loaded fibres have enabled very strong gratings to be written conveniently into optical fibres to produce filters, wavelength defining reflectors, dispersion compensators, and sensors. However, the connection between the photosensitivity and the germanium-related oxygen deficient centre (GODC), first proposed by Hand and Russell, is well recognised. Despite uncertainty on the exact microscopic structure of the GODC, the energy levels of the centre is well understood through spectroscopic studies of the centre. A diagram of such an energy level system is depicted in figure 1, with singlet states marked by S and triplet states T. Despite the fact there has been much work been done on the photo-decomposition of GODCs, there is still work to be done for a thorough understanding the process. Potentially there are two possible reaction path ways. One is a direct two photon reaction producing a photoelectron in the conduction band. Most of the photoelectrons will recombine with no net effect and some will be trapped at various sites to create a net change. The second one is a single photon process involving the long lived triplet state and a near by trapping site, probably situated at the next co-ordination sphere. The two situations are shown in figure 1. Here we report an experimental study on the reaction path way of the GODCs, demonstrating that there are both single photon and two photon process existing under different conditions.

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