Bragg Gratings Written in Photonic Crystal Fibres with a High-Index Germanosilicate Core

Bragg gratings are straightforwardly written into a 10-ring photonic crystal fibre (PCF) with a highly Ge³⁺-doped core by means of a 193nm ArF excimer laser. The grating inscription efficiency is improved, due to enhanced
photosensitivity and the large overlap between the fundamental mode and the photosensitive core. The exposure duration only lasts less than 180s. In addition, the high-index core changes the modal properties of the PCF and two groups of higher order mode resonance dips are therefore obtained. A strong cladding mode resonance is observed, because the LP₀₂-like mode has a large overlap integral with the fundamental mode.