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We report what we believe to be the first highly symmetric first-order IR femtosecond laser fiber Bragg gratings within the telecommunications C band in free-standing optical fiber, fabricated with a relatively low NA lens and without use of oil immersion techniques. This grating features the smallest dimensions for a point-by-point fiber grating reported so far (to our knowledge). This achievement paves the way to rapid mass manufacturing of highly efficient and stable Bragg gratings using ultrafast lasers in any type of fiber. Mastering this femtosecond grating inscription technique also allowed the fabrication of the first Bragg gratings with direct near-IR femtosecond inscription in photonic crystal fibers, and without the use of techniques that rely on the compensation of the holey structure.

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We describe recent research into devices based on fibre Bragg gratings in polymer optical fibre. Firstly, we report on the inscription of gratings in a variety of microstructured polymer optical fibre: single mode, few moded and multimoded, as well as fibre doped with trans-4-stilbenmethanol. Secondly, we describe research into an electrically tuneable filter using a metallic coating on a polymer fibre Bragg grating. Finally we present initial results from attempts to produce more complex grating structures in polymer fibre: a Fabry-Perot cavity and a phase-shifted grating.Keywords: Polymer optical fibre, fibre Bragg grating, sensor, filter. INTRODUCTIONAt the moment there is considerable interest in the use of polymer optical fibre (POF) in a variety of applications. In several countries POF is seen as a good candidate as a digital transmission medium in fibre-to-the-home applications and for home networking. POF is also increasingly being used in automotive data buses, while one of the largest markets, in terms of the sheer length of fibre used, is in illumination applications. All of these markets are possible because POF based systems are seen as being low cost; this applies to both the intrinsic production cost of the fibre itself as well as the cost of its installation, including the making of connections. The communications applications mentioned all involve short transmission distances where the much higher loss of POF in comparison with silica fibre and its high intermodal dispersion are not serious disadvantages. The high intermodal dispersion in these systems arises because of the predominant use of very large core multi-mode fibres, which makes it easy and therefore cheap to couple to low cost, large-area sources and to make fibre-to-fibre connections.A number of workers have developed techniques for sensing using POF which also follow this low cost paradigm 1 and some of these devices are beginning to be commercialised. At the same time, there have been some recent technological developments which open up new possibilities for POF based sensors and devices. These are:• the development of single-mode step index optical fibre; 2• the demonstration of the recording of fibre Bragg grating (FBG) filters in POF; 3• the production of microstructured or photonic crystal polymer optical fibre (mPOF). 4 Whilst, as we shall explore in this paper, these technologies offer tremendous scope for the realisation of novel devices, for the foreseeable future they are unlikely to offer very low costs. This is partly due to the underlying cost of production but also often due to the need for sources compatible with single mode fibre, and the difficulty (and hence expense) of connecting single mode polymer fibres. To justify work in this area, one then has to look for other advantages over the very much more mature silica fibre technology. Fortunately it is not difficult to find several major factors that can act in POF's favour for certain applications.