Photorefractive index gratings in SnO2:SiO2 optical fibers

Permanent photorefractive gratings with considerable refractive index modulations (~3×10–4) were written in silica optical fibers doped with very low levels of SnO2(~0.15 mol %) using ultraviolet (UV) laser radiation at 248 nm. The photoinduced refractive index change does not show any sign of saturation even for long exposure times (i.e., total fluence >20 kJ/cm2). A comparison with GeO2:SiO2 fibers shows that under similar UV intensities saturation takes place for shorter exposure time and comparable refractive index changes are obtainable for GeO2 concentrations ~10 mol %.