Magnification of mask fabricated fibre Bragg gratings

Bragg gratings have been formed in optical fibres by diffraction of a phase mask. This technique is expanded by allowing for the possibility of shifting the Bragg wavelength for a fixed mask periodicity. This method uses a simple lens to introduce a curvature on to the wavefront of the diffracting light that results in a magnification of the original periodicity formed by the mask. The Letter shows experimentally that the Bragg wavelength is shifted to shorter wavelength by the addition of a converging lens before the mask.