Permanent gratings (refractive index modulation /spl sim/3/spl middot/10/sup 4/) were written in tin-doped silica optical fibres with low levels of SnO/sub 2/ (/spl sim/0.15 mol%). In Ge-doped silica, /spl sim/10 mol% GeO/sub 2/ is required to produce a comparable result under similar conditions.
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