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This paper compares two fibre optic sensing techniques for vibration characterisation: (a) optical fibre Bragg grating (FBG) strain gauges and (b) a novel direct fibre optic shape sensing (DFOSS) approach based on differential interferometric strain measurements between multiple fibres within the same fibre arrangement. Operational mode shapes and frequency measurements of an Airbus Helicopters H135 bearingless main rotor blade (5.1 m radius) were acquired during a series of ground vibration tests undertaken in a controlled laboratory environment. Data recorded by the fibre optic instrumentation systems were validated using commercially available accelerometers and compared against a baseline finite element model. Both fibre optic sensing systems proved capable of identifying the natural frequencies of the blade in the frequency range of interest (0–100 Hz). The data from the FBG sensors exhibited a dependency on their position relative to the neutral axes of the blade, which meant that full characterisation of the flapping and lagging modes required careful consideration of sensor location in the chordwise direction. The DFOSS system was able to identify all structural dynamics, despite being located on the neutral axis in the lagging direction, due to its sensitivity to angle changes, rather than strain, and its biaxial measurement capability. The DFOSS system also allowed the operational mode shapes of the blade to be determined directly, without the requirement for strain transfer from the blade to the sensor and without the requirement for a model of the underlying structure. The accuracy of obtained natural frequencies and operational mode shapes is assessed, demonstrating the potential of the use of both fibre optic sensing systems for determining blade structural dynamics.

The use of optical fibre Bragg gratings (FBGs) and direct fibre optic shape sensing (DFOSS) in the ground vibration testing of a full size helicopter rotor blade has been evaluated, with the performance benchmarked against measurements made using accelerometers. An array of FBGs was used to monitor the amplitudes of the mode frequencies at specific locations along the blade and DFOSS was used to measure directly the shape of the blade and to characterise its modal frequencies. While it was possible to measure modal frequencies using both approaches, DFOSS proved capable of detecting modal frequencies using a single sensor array located along the longitudinal axis of the blade.

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 range-resolved 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 $ \mathbf{40}\;\mathbf{nm}\cdot \mathbf{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.

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.

The use of a range of optical techniques to monitor the process of fabricating optical fiber tapers is investigated. Thermal imaging was used to optimize the alignment of the optical system; the transmission spectrum of the fiber was monitored to confirm that the tapers had the required optical properties and the strain induced in the fiber during tapering was monitored using in-line optical fiber Bragg gratings. Tapers were fabricated with diameters down to 5 μm and with waist lengths of 20 mm using single-mode SMF-28 fiber.

We report the development of a fibre Bragg grating (FBG) interrogation system that is capable of detecting independently and simultaneously the two orthogonally polarized signals reflected from a polarization-maintaining (PM) FBG. This relaxes the constraints on the bandwidth of the FBG spectrum, allowing the use of a shorter PM FBG (larger bandwidth) for sensing applications where a higher spatial resolution is required. It imposes no limitation on the measurable strain range and strain profile. The interrogation system is capable of decoding the complex spectral responses of a PM FBG subjected to a transverse load.

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.

Optical fibre Bragg grating sensors have been field-trialed for the monitoring of dynamic loading of fish-plates, stretcher bars and switchblades on a tram network, with the aim of developing a condition monitoring system. This paper provides preliminary data showing the ability to identify changes in track/component condition.

Optical fibre Bragg grating strain sensors are used to monitor the dynamic loads experienced by a tufting needle installed on a commercial robot-controlled tufting tool during the tufting of a dry carbon fibre preform. The influence of the thread and thickness of the preform lay-up were investigated. Maximum axial compression strains of ~-250 me, and bending strains of 1100 me were measured during the tufting of a 4 ply quasi-isotropic carbon fibre dry preform.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

An effective-soil-pressure sensor for geotechnical applications based on Fibre Bragg Gratings is presented. The sensor simultaneous measures total soil pressure and pore pressure, allowing the calculation of the effective stress of soil. Calibration of the sensor using pressurised air demonstrated a pressure sensitivity of 2.02x10 -3 ± 2.84x10 -5 nm/kPa and 1.87x10 -3 ± 6.88x10 -5 nm/kPa for the total and pore pressure respectively. This corresponds to a pressure resolution of 4.95x10 -1 kPa and 5.46x10 -1 kPa for total and pore pressure using a 1pm interrogation system. Measurements undertaken in two types of soil demonstrated dependence of the total pressure sensitivity on soil density/stiffness. Pore pressure measurements agreed well with the preliminary calibration.

Pile loading test plays an important role in the field of piling engineering. In order to gain further insight into the load transfer mechanism, strain gauges are often used to measure local strains along the piles. This paper reports a case whereby FBG strain sensors was employed in a field trial conducted on three different types of pile loading tests in a glacial till. The instrumentation systems were configured to suit the specific characteristic of each type of test. Typical test results are presented. The great potential of using FBG sensors for pile testing is shown.

The benchmarking of two optical strain measurement techniques, shearography and fibre Bragg grating (FBG) sensors, against theoretical strain calculations and resistance foil strain gauges (RFSG) is described. The test object used for the surface strain measurements was an ABS pipe that was hydrostatically loaded. A multi-component shearography instrument, capable of full surface strain measurement was used to determine the displacement gradients, from which the strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFGSs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Good agreement between theory and the axial and hoop strains determined by the different techniques was found.

Optical fibre Bragg grating (FBG) strain sensor measurements were compared with theoretical strain calculations and measurements from two competing techniques: shearography and resistance foil strain gauges (RFSG). Axial and hoop strains in an ABS pipe, loaded by ch anging the internal pressure were investigated. Six wavelength-multiplexed FBG sensors were used to measure the axial and the hoop strains. RFGSs located on the surface adjacent to the FBGs were used for comparison. A multi-component shearography instrument was also used to perform full-field surface strain measurements on the pipe. Measurements were made using two shear directions, yielding six displacement gradient components in total. The measured Gu/Gx and Gv/Gy components were compared with theoretical hoop and axial strains respectively. Good agreement was found between the three techniques and the theory. Keywords: optical fibre Bragg gratings, shearography, strain measurement 1. INTRODUCTION Modern manufacturing techniques increasingly require advanced techniques for the characterisation of surface strain, for example in materials with anisiotropic properties, such as composites, for the investigation of objects with complex shape, and for strain measurement in components lightened to improve efficiency. This paper addresses the calibration of two such advanced optical techniques: optical fibre Bragg gratings (FBGs) and shearography. FBG sensors have been demonstrated previously for a range of strain measurement applications

In this paper we report the development of a FBG (fibre Bragg grating) interrogation system, that is capable of detecting independently the two orthogonally polarised signals reflected from a FBG fabricated in a PM (polarisation maintaining) fibre. The interrogation system imposes no limitation on the measurable strain. It is possible to detect a strain gradient by measuring independently the spectral broadening of the two polarised signals. The system has been used to fabricate and characterise FBGs in different PM fibres. As a demonstration of the feasibility of the interrogation system for multi-axis strain sensing, the transverse strain sensing characteristics of PM FBGs are presented.