Experimental and theoretical study of stable negative index gratings formed at 193 nm

We have demonstrated fast formation (~1500 pulses at ~1 J/cm2/pulse) of fibre gratings with high negative index modulations (~ -3x10-4). These gratings were found to be far more stable than the gratings with positive index modulations formed at the early stage of the grating growth. We have also found that the maximum negative index modulations achieved do not depend on the pulse intensifies, 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 energy level system to model the photosensitivity in the boron-doped germanosilicate fibre. 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. A complex grating decay process is also observed at elevated temperatures as predicted by the three energy level model. The thermal stability of both positive and negative index gratings in a Boron-co-doped germanosilicate fibre is characterised at fixed temperatures, so that the stability of such grating can be accessed for any writing fluence