This version is available at https://strathprints.strath.ac.uk/42768/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.

We describe a novel technique for measurement of absolute order of interference in multifrequency interferometry. An optimization criterion is introduced that leads to frequency selection formulations that are optimized with respect to the minimum number of frequencies required for achieving the maximum target dynamic range. The method is generalized to N frequencies and gives a definition of measurement reliability. We demonstrate the technique by means of coherent fringe projection for nonintrusive, full-field profilometry. Experimental data for three frequencies are presented.

We report on the development of hollow-core photonic bandgap fibers for the delivery of high energy pulses for precision micromachining applications. Short pulses of (65ns pulse width) and energies of the order of 0.37mJ have been delivered in a single spatial mode through hollow-core photonic bandgap fibers at 1064nm using a high repetition rate (15kHz) Nd:YAG laser. The ultimate laser-induced damage threshold and practical limitations of current hollow-core fibers for the delivery of short optical pulses are discussed.

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...

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