We have measured the optical phase sensitivity of fiber based on poly(methyl methacrylate) under nearsingle-mode conditions at 632.8 nm wavelength. The elongation sensitivity is 131± 3 ϫ 10 5 rad m −1 and the temperature sensitivity is −212± 26 rad m −1 K −1 . These values are somewhat larger than those for silica fiber and are consistent with the values expected on the basis of the bulk polymer properties. © 2005 Optical Society of America OCIS codes: 060.2300, 060.2370 Fiber optic strain sensors offer advantages that include insensitivity to electromagnetic fields, light weight, and minimal intrusiveness 1 compared with conventional strain gauges. Fused silica, the material of choice for the majority of optical fibers, has near-ideal mechanical characteristics for many strain-sensing applications. However, fused-silica fibers have an upper strain limit of approximately 3-5% and in general are reliable only to ϳ1% strain after selection of fibers by proof testing.2 In highly loaded engineering structures such as highway bridges, buildings, and aircraft wings, transverse loading can result in large bending strains, which can induce locally high strains, so monitoring structural strain is becoming increasingly important. With the advent of new engineering materials, such as composites, the acceptable range of applied strain can exceed the breaking strain of fused-silica fiber, precluding the use of standard fiber-based strain gauges. The inherent fracture toughness and flexibility of polymer optical fibers (POFs) makes them much more suitable in high-strain applications than their glass-based counterparts. In addition, fiber Bragg gratings have recently been written into POFs, broadening their potential applications. 3POF sensors for strain and curvature measurement have been reported in the literature. 4 However, these measurements have generally been intensiometric measurements made with multimode fibers. Whereas robust sensors have been demonstrated to use intensity modulating mechanisms, they are susceptible to unwanted intensity losses, for example, bend loss or connector loss, and to variations in source power. These losses may be addressed technically, by use of additional reference power measurements, for example, but an alternative to intensity measurement is desirable. Interferometry offers a potential alternative, in which the sensing element is a length of fiber between a pair of partially reflecting splices 5 or a pair of matched Bragg gratings. 6 To produce these designs, single-mode fiber rather than conventional multimode POF is required. The advent of single-mode POF offers the potential for highstrain fiber sensors to exploit the advantages of interferometry. We report here interferometric measurement of the optical phase sensitivities of single-mode POF versus strain and temperature changes.The POF (Paradigm Optics) used in these experiments had a cladding diameter of ϳ125 m (commercial acrylic with n = 1.4905) and a core diameter of 6 m [poly(methyl methacrylate) (PMMA) doped with Ͻ3% polysty...
A Long Period Grating was fabricated in endlessly single mode photonic crystal fibre using a spatially-periodic electric arc discharge. The sensing characteristics of the grating were studied and it was found to possess an insensitivity to temperature, a bend sensitivity of 3.7 nm.m and a strain sensitivity of -2.0 pm/με.Introduction: Long period gratings (LPGs) consist of a large scale (hundred's of microns)
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.
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