Photoconductivity measurements in optical fibre preforms

The AC photoconductivity and the photodielectric shift in UV-illuminated germano-silicate preforms and optical fibres has been measured using a noncontact microwave resonator technique at 2.9 GHz. The highest photoconductivity measured was 1.5*10^-6 ( Omega cm)^-1 and the largest measured photodielectric shift corresponded to an increase of the refractive index of 1.6*10^-2. A permanently induced increase of the refractive index, 6.1*10^-3, was also observed. The dark DC conductivity was measured to be 10^-15 ( Omega cm)^-1.