The transverse load and temperature sensitivities of fibre Bragg gratings (FBG) fabricated in a range of commercially available stress and geometrically induced high birefringent (HiBi) fibres have been experimentally investigated. The wavelength reflected by the FBG in each polarisation eigenmode was measured independently and simultaneously using a custom designed interrogation system. The highest transverse load sensitivity, of 0.23 ±0.02 nm/(N/mm), was obtained with HiBi FBGs fabricated in elliptically clad fibre. This was ~25% higher than for any other HiBi fibre, which, coupled with the small diameter of the fibre, makes it a good candidate for an embedded or surface mounted strain sensor. The highest temperature sensitivity of 16.5 ± 0.1 pm/ 0 C, approximately 27% greater than any other fibre type, was obtained with the HiBi FBG fabricated in Panda fibre. HiBi FBG sensors fabricated in D-clad fibre were the only ones to exhibit identical temperature sensitivities for the slow and fast axes (11.5 ± 0.1 pm/ 0 C).

A fibre optic sensor capable of discriminating between temperature and strain, using a single fibre Bragg grating, is presented. The technique exploits the core-cladding mode coupling of a tilted fibre Bragg grating (TFBG). The core and cladding modes exhibit different thermal sensivities, while the strain sensivities are approximately equal. Monitoring the core-core mode coupling resonance and the core-cladding mode coupling resonance of the TFBG spectrum allows the separation of the temperature and strain induced wavelength shifts.

Integrated Optical Fibre (IOF) is a passive optical platform that directly integrates optical fibre to a planar substrate, illustrated in Fig 1(a). It possess advantages associated with optical fibre such a low propagation loss, whilst enabling planar functionality commonplace with integrated optics. Planarization is uniquely achieved through a modified Flame Hydrolysis Deposition (FHD) technique that forms a robust glass alloy between the fibre and substrate. The binding medium is of optical quality and resistant against common solvents, chemicals and elevated temperatures of up to 1000 o C. Furthermore as fibre can be brought seamlessly on-off chip there is no need for glues or cumbersome coupling arrangements, which are typically a point of mechanical weakness when monitoring harsh environments. Thus far, developments in IOF have largely utilised either the environmental stability [1] and/or physical robustness [2] of the platform and not the optical quality of the binding medium. This work reports for the first time and evanescent field based refractometer using IOF, in which the optical mode directly interacts with the deposited FHD glass and the external environment. The concept for design is based largely on so termed thinned-FBGs [3], in which an optical fibre is wet etched using buffered HF. Through applying a similar geometry to the IOF format one can achieve external refractive index sensitivity, but with an enhanced mechanical integrity and capacity for integration. Fig. 1 (a) a cross-section profile of an Integrated Optical Fibre with SMF-28. (b) A cross-section profile of an Integrated Optical Fibre with a wet etched SMF-28 fibre (fibre 8 µm in diameter). (c) The optical response of a Bragg grating in construct (b), under exposure to Cargille refractive index matching liquids. This work reports a proof-of-concept refractometer design, fabricated through wet etching SMF-28 fibre in a buffered HF solution then applying it in an IOF format. For the device depicted in Fig 1 (b), etching is continued until a diameter of 8 µm is reached. This etched fibre is then subsequently transferred and layered-up onto a silicon wafer that has a thick thermally grown oxide (15 µm). The purpose of the oxide is to act as an optical buffer layer (clad), ensuring the final device maintains a guided mode. To form the IOF and maintain as much evanescent field exposure to the external environment a 0.5 µm FHD layer is deposited and consolidated at 1250 o C, leaving the cross sectional geometry shown in Fig 1 (b). The FHD recipe is a silicate, doped with boron and phosphorus that once consolidated has a refractive index of 1.4452 (at a wavelength of 1553 nm). Changes in external refractive index were inferred through use of a Direct UV Written fibre Bragg grating [2]. The grating was inscribed at a fluence of 30kJcm-2 into the fibre core, after the FHD had been consolidated. To enhance UV photosensitivity the chip was hydrogen loaded at 120 bar for 7 days. Cargille refractive index matching liquids (series AA and AAA) were ...

Fibre Bragg gratings (FBGs) fabricated in linearly birefringent fibres were embedded in glass fibre/epoxy composites and in the corresponding unreinforced resin to monitor the effective transverse strain development during the cure process. The optical fibres containing the FBG sensors were aligned either normal or parallel to the reinforcement fibres in unidirectional glass fibre/epoxy prepregs. The chemical cure kinetics of the epoxy resin system used were studied using differential scanning calorimetry, in order to investigate the correlation between the strain monitoring results and the evolution of the curing reaction. A non-parametric cure kinetics model was developed and validated for this purpose. The effective transverse strain measured by the FBGs demonstrated high sensitivity to the degree of cure as a result of the densification of the resin caused by the curing reaction. The effective compressive transverse strain developed during the reaction, and thus the corresponding sensitivity to chemical changes, was higher in the case of the sensing fibre aligned normal to the reinforcement fibres than in the case of the sensor fibre parallel to the reinforcement fibres. Small but measurable sensitivity to cure induced changes was observed in the case of the unreinforced resin.

Dynamic fiber-optic shape sensing, often also referred to as curvature or bend sensing, is demonstrated using fiber segment interferometry, where chains of fiber segments, separated by broadband Bragg grating reflectors, are interrogated using rangeresolved interferometry. In this paper, the theory of interferometric curvature sensing using fiber segments is developed in detail, including techniques to infer lateral displacements from the measured differential strain data and methods for directional calibration of the sensor. A proof-of-concept experiment is performed, where four fiber strings, each containing four fiber segments of gauge length 20 cm each, are attached to the opposing sides of a flexible support structure and the resulting differential strain measurements are used to determine the lateral displacements of a 0.8 m cantilever test object in two dimensions. Dynamic tip displacement measurements at 40 nm • Hz −0 .5 noise levels over a 21 kHz bandwidth demonstrate the suitability of this approach for highly sensitive and cost-effective fiber-optic lateral displacement or vibration measurements.

It is known that optical fiber long period gratings (LPGs) exhibit their highest sensitivity to environmental perturbation when the period is such that the phase matching condition is satisfied at its turning point. The reproducible fabrication of LPGs with parameters satisfying this condition requires high resolution control over the properties of the grating. The performance of an LPG fabrication system based on the point-by-point UV exposure approach is analyzed in this paper, and the control of factors influencing reproducibility, including period, duty cycle, and the environment in which the device is fabricated, is explored.

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry (shearography) and fibre Bragg grating (FBG) sensors, is compared with that of resistance foil strain gauges (RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.

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