Pulsed 157 nm VUV induced refractive index modification of optical fibres and planar fused silica

This paper reports studies of fibre and planar samples of doped and undoped fused silica irradiated at 157 nm and measurements of the degree to which refractive index changes occur. Low reflectivity fibre Bragg gratings uniformly irradiated at 157 nm have been used to measure the induced refractive index change. Refractive index changes of 3×10−4 for Ge/B co-doped fibres have been observed for laser fluences up to 7 mJ cm−2. Temporary shifts in the reflection wavelength of the grating during irradiation have been attributed to laser heating due to the high absorption of the fibre at 157 nm. Planar samples of fused silica (Heraeus HOQ310) and Ge doped fibre pre-form have been irradiated through contact masks. For the HOQ310, shifts in the Si–O–H infrared absorption band are observed together with reduced transmission in the UV. Atomic force microscopy has revealed an increase in the height of the irradiated area by ∼17 nm.