Mechanisms of FBG sensor fabrication

There are a lot of defects in the germanium-doped core. The paramagnetic Ge( ) defects, where n refers to the number of next-nearest-neighbour Ge atoms surrounding a germanium ion with an associate unsatisfied single electron, were first identified by Friebele et al. These defects are shown schematically in Fig. 8.3. 

If the optical fibre is treated with high temperature hydrogen, germania will decrease and the concentration of GeO molecules will be enhanced. The reduction process may occur as follows  

Most fibres, if not all, show an increase in the population of the GeE centre (trapped hole with an oxygen vacancy) after UV exposure. This is formed by 

3 A schematic of proposed Ge defects of germania-doped silica. 

4 The capital GeO defect of germania-doped silica, in which the atom adjacent to germanium is either a silica or another germanium. 

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