Highly Photosensitive Germanosilicate Fibre Codoped with Nitrogen

In-fiber grating writing based on the photorefractive effect is now attracting considerable interest [1]. However, photosensitivity of standard germanosilicate fibres with a moderate concentration of germanium in the core (5 ÷ 7 mol.% GeO2) is not large and does not allow efficient writing of refractive index gratings. To increase the writing efficiency, one has to increase the germanium concentration and/or load the fibre with hydrogen [2]. Increasing the germanium concentration results in a reduction of the mode spot size and in additional losses when the fibre is spliced with a standard telecommunication fibre. Hydrogen loading complicates the grating technology and raises losses in the IR region owing to the OH-group absorption. Therefore, optimization of the grating writing technology and development of novel types of photosensitive fibres are now high on the agenda.