PHOTOSENSITIVITY IN TANTALUM-DOPED SILICA OPTICAL FIBERS

Photodarkening and photoinduced index changes are reported for the fir st time to our knowledge in tantalum-doped silica optical fibers expos ed to KrF excimer laser emission at 248 nm. The UV absorption of the t antalum-doped preform features the tail of a strong absorption band be low 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 s aturation after several excimer pulses and some recovery after each pu lse. Using an external holographic writing technique at 248 nm, we als o formed Bragg gratings at 1.55 mu m in these fibers. The grating grow th saturated after several pulses and displayed a fast decay component when writing was performed with a higher pulse rate. The decay is wel l fitted by a stretched exponential with beta = 0.35.