Double pass technique for fabricating compact optical fibre rocking filters

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