Negative-index gratings formed by a 193-nm excimer laser

We have demonstrated fast formation [~1500 pulses at ~1 (J/cm(2))/pulse] of fiber gratings with highly negative index modulations (~-3 x 10(-4)). We have found that the maximum negative index modulations that are achieved do not depend on the pulse intensities, although the inverse of the time taken to reach the negative index-modulation maximum varies linearly with the pulse intensities. This prompts us to use a three-energylevel system to model the photosensitivity in boron-doped germanosilicate fiber. All the necessary parameters of the model can be determined from a single growth measurement of the average index change, and the model's prediction fits well the measured index-modulation growth.