NEW FIBRE AND GRATING TECHNOLOGIES FOR LASERS AND SENSORS, pp. 1-62
Authors: John Canning, University of Sydney, Australia
Abstract: Conventional fibre grating technology is extremely well developed as a consequence of the large engineering focus on grating based telecommunication component technologies, both in fibre and planar waveguide form. However, for many non-telecommunication applications such as in sensing and lasers, the limited range of operation is inadequate. In particular, degradation and annealing of the grating at high temperatures is increasingly determining the limits of gratings in these non-telecommunications applications. For example, heating within rare earth doped lasers leads to grating degradation and consequently most gratings have to be attached outside the gain fibre. For many sensor applications, temperature operation above 300°C is common. In response, various attempts have been made to improve their performance, particularly by varying the dopant composition, usually at the expense of other properties such as loss and manufacturing ease. More significantly, however, has been the improved understanding of photosensitivity that has led to new methods of treating and writing gratings in existing standard fibres where keeping dopants to a minimum is a priority. Adding to the challenges for grating science and technology are the increasingly attractive air-structured fibres of various sorts, including photonic crystal fibres, photonic bandgap fibres, Fresnel fibres and air-clad fibres. These have air channels that dictate through their distribution the magnitude of the effective index step and the distribution of the index difference the modal properties of the waveguide to a level that has previously not been possible. For many applications, they have an inherent advantage over conventional fibres since their production does not involve dopant incorporation, usually by modified chemical vapour deposition (MCVD), in order to achieve optical propagation. This can benefit both the production cost of passive fibres since no dopants are necessary, and as well active devices containing rare earth ions that do not perform well with common index-raising dopants such as germanium. On the other hand, the absence of photosensitive dopants such as germanium has limited grating writing until recently when two-photon and higher exponent photon grating writing directly into the band edge of silica was employed. Consequently, for their full potential to be utilised, new grating writing methodologies and alternatives need further investigation. In this work, these new approaches to grating writing in both conventional and air-structured fibres are examined in terms of the technology employed and their potential discussed. A general proposition that a set of design approaches exists to tailor the photosensitive response for a particular fibre and grating application is espoused, and directions for new research outlined.
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