PHOTOSENSITIVITY IN TANTALUM-DOPED SILICA OPTICAL FIBERS

Photodarkening and photoinduced index changes are reported for the first time to our knowledge in tantalum-doped silica optical fibers exposed to KrF excimer laser emission at 248 nm. The UV absorption of the tantalum-doped preform features the tail of a strong absorption band below 200 nm. The 248-nm-induced absorption in the UV also features the tail of a band below 200 nm. Photodarkening at 900 nm shows signs of saturation after several excimer pulses and some recovery after each pulse. Using an external holographic writing technique at 248 nm, we also formed Bragg gratings at 1.55 mu m in these fibers. The grating growth saturated after several pulses and displayed a fast decay component when writing was performed with a higher pulse rate. The decay is well fitted by a stretched exponential with beta = 0.35.