Double pass technique for fabricating compact optical fibre rocking filters

Abstract

By using the photosensitivity of Ge-doped birefringent fibres, efficient narrowband rocking filters have been fabricated that, to the authors' knowledge, are the shortest in length per unit beat length to date. This has been achieved by using a point-by-point external writing process in which a UV beam periodically exposes sections of an elliptically-cored fibre. Alternate sections are exposed from opposite sides of the fibre, effectively doubling the coupling strength per beat length. Alignment with the fibre axes is assisted by use of a keyed optical fibre.