Femtosecond UV long-period fibre grating fabrication with amplitude mask technique

Using femtosecond 264 nm laser pulses and a photosensitive Ge/B co-doped fibre, we fabricated long-period fibre gratings (LPFGs) with 475 μm period and 3 cm length using two different methods: the amplitude mask (AM) technique and, for comparison, the step-by-step (SBS) inscription method. In order to rule out the possible impact of high-intensity UV radiation on LPFG inscription efficiency, the comparison was done at fixed focussing conditions. Using the irradiation intensity of 200–300 GW/cm2, we found that both methods bring about the same value of light fluence per creation of one LPFG groove (2.3–2.4 J/cm2). At the same time, we observed that the use of AM technique leads to the expenditure of a much smaller amount of incident light energy than in the case of SBS method (2.4 J and 12 J, respectively). We demonstrated that the last result is related to the simultaneous fabrication of multiple LPFG grooves in the case of the amplitude mask.