Characterisation of Long Phase Masks for Writing Fibre Bragg Gratings

Abstract

The use of phase masks for writing fibre Bragg gratings was first demonstrated by Anderson et al [1]. These masks are etched in silica (for high transmission at the writing wavelength), and can be fabricated by either photolithographic techniques or electron beam lithography. It was demonstrated afterwards that long gratings can be written by scanning a narrow beam along a phase mask-fibre assembly [2]. The length of gratings is then limited by the length of the mask. Since very long gratings are useful for dispersion compensation, as well as very narrow reflection filters, there is a strong interest in the fabrication of very long phase masks. While in principle there is no limit to photolithographic techniques, the size of the optics required sets a practical limit, and masks greater than 5 cm diameter are prohibitively expensive. On the other hand, to date electron beam lithography can fabricate masks up to 14 cm in length. In this process, the pattern is divided into fields, and parallel lines are scanned with the electron beam in the photoresist across the width of the field. The process is then repeated at a new position until the whole mask is written. Sometimes, this process results in a phase jump between the fields of the grating, called a "stitching error". In this paper, we analyse the consequences of this stitching error, and present experimental characterization of gratings written with long phase masks fabricated by electron beam lithography.