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An array of three long period gratings (LPGs) fabricated in a single optical fibre and multiplexed in the wavelength domain was used to measure simultaneously temperature, relative humidity (RH) and volatile organic compounds (VOCs). Each LPG sensor was designed to optimize its response to a desired measurand. The LPGs were fabricated with periods such that they operated at or near the phase matching turning point. The sensors were calibrated in the laboratory and the simultaneous measurement of the key indoor air quality parameters was undertaken in laboratory and office environments. It was demonstrated successfully that the data produced by the LPG sensor array under real conditions was in a good agreement with that produced by commercially available sensors. Further, the potential application of fibre optic sensors for VOCs detection at high levels has been demonstrated.

The use of optical fibre Bragg gratings (FBGs) to monitor the Interface Pressure Distribution (IPD) on an automotive disc brake pad under a variety of loading conditions is studied. The results demonstrate successful strain transfer from the brake pads to the attached FBG sensors under static loading, with a linear response to increasing pressure, and with the measured IPD showing good agreement with that recorded using pressure sensitive paper. Results are also presented demonstrating that changes in the IPD as a result of torque acting on the brake pads can be monitored by the FBG sensors.

The process of fabricating tapered long period gratings (TLPGs) using a CO 2 laser is described. TLPGs with a period spacing of 378 μm, were fabricated by optimization of the taper waist diameter and careful control of the duty-cycle and its uniformity along the length of the grating. The 6-period TLPGs exhibited a pass-band insertion loss of 0.6 dB, resonance band extinction values of 3 dB and had a physical length of 2.27 mm.

The use of an optical fibre long period grating (LPG) as a soil moisture sensor is reported. Characterization of the device in both clay and sandy soils revealed a sensitivity to moisture levels in the range 10-50%, and the results were compared with the output from a Theta probe, the standard soil moisture sensor, which measures the impedance of the soil. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Laser spectroscopy provides the basis of instrumentation developed for the diagnosis of infectious disease, via quantification of organic biomarkers that are produced by associated bacteria. The technology is centred on a multichannel pulsed quantum cascade laser system that allows multiple lasers with different wavelengths to be used simultaneously, each selected to monitor a different diagnostic biomarker. The instrument also utilizes a hollow silica waveguide (HSW) gas cell which has a very high ratio of interaction pathlength to internal volume. This allows sensitive detection of low volume gas species from small volume biological samples. The spectroscopic performance of a range of HSW gas cells with different lengths and bore diameters has been assessed using methane as a test gas and a best-case limit of detection of 0.26 ppm was determined. The response time of this cell was measured as a 1,000 sccm flow of methane passed through it and was found to be 0.75 s. These results are compared with those obtained using a multi-pass Herriot cell. A prototype instrument has been built and approved for clinical trials for detection of lung infection in acute-care patients via analysis of ventilator breath. Demonstration of the instrument for headspace gas analysis is made by monitoring the methane emission from bovine faeces. The manufacture of a hospital-ready device for monitoring biomarkers of infection in the exhaled breath of intensive care ventilator patients is also presented.

The development of an ammonia sensor, formed by the deposition of a functionalised titanium dioxide film onto a tapered optical fibre is presented. The titanium dioxide coating allows the coupling of light from the fundamental core mode to a lossy mode supported by the coating, thus creating lossy mode resonance (LMR) in the transmission spectrum. The porphyrin compound that was used to functionalise the coating was removed from the titanium dioxide coating upon exposure to ammonia, causing a change in the refractive index of the coating and a concomitant shift in the central wavelength of the lossy mode resonance. Concentrations of ammonia as small as 1ppm was detected with a response time of less than 1min. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Detection of methane at 3.3μm using a DFB Interband Cascade Laser and gold coated integrating sphere is performed. A 10cm diameter sphere with effective path length of 54.5cm was adapted for use as a gas cell. A comparison between this system and one using a 25cm path length single-pass gas cell is made using direct TDLS and methane concentrations between 0 and 1000 ppm. Initial investigations suggest a limit of detection of 1.0ppm for the integrating sphere and 2.2ppm for the single pass gas cell. The system has potential applications in challenging or industrial environments subject to high levels of vibration.

A range-resolved interferometric signal processing technique using sinusoidal optical frequency modulation is applied to fibre segment interferometry. Here, six optical fibre segments of gauge length 12.5 cm are used as interferometric strain sensors and are formed between seven weak, broadband fibre Bragg gratings, acting as in-fibre partial reflectors. In a very simple and cost-effective optical setup using injection current modulation of a laser diode source, interferometric measurement of acoustic wave propagation in a metal rod is used to demonstrate the capabilities of the technique.

An optical fibre long period grating (LPG) coated with an inorganic molecularly imprinted thin film based on TiO 2 was used to measure in real time the photodecomposition of an organic target compound. The TiO 2 film was deposited onto the LPG using the aqueous liquid phase deposition method and its photo-catalytic properties were employed to decompose a porphyrin. The principle of operation is based on the measurements of the refractive index (RI) change of the TiO 2 thin film at the binding and removal of the imprinted organic compound. Keywords: long period grating (LPG), molecularly imprinte d polymers (MIPs), selective photodecomposition. 1. INTRODUCTION Optical techniques are considered widely to be powerful tools for the development of chemical and biological sensors, covering a wide range of applications [1]. Sensing techniques based upon the use of optical fibre long period grating (LPG) devices to probe the opti cal characteristics of nanomaterials that exhi bit changes in their optical properties upon exposure to targeted chemical species are particularly attractive, due to their potential high sensitivity, the ability to multiplex arrays of sensors, and the prospect for remote sensing [2]. Molecularly imprinted materials provide a universal platform for the creation of artificial receptors that can be well controlled and tailored to achieve precise molecular recognition [3]. The technique is very versatile as any compound with functional groups can in theory be imprinted in different porogens (either water or organics). The basic concept of molecular imprinting is based on the creation of imprints of the template (i.e. chemical molecules or biological species that need to be detected – the analyte) into a matrix. The matrix is most often polymeric, with molecular imprinting achieved by incorporation of the template during the polymerization step, which is subsequently removed to leave behind a cavity specific for the template material, as illustrated in figure 1. Inorganic and hybrid inorganic-organic molecularly imprinted materials have advantages over those prepared from organic materials. Such materials are highly stable and reproducible, which is esp ecially important in real world sensing applications. Inorganic MI has been achi eved successfully using various metal alkoxi des, most often using silica to form the inorganic matrix via a sol-gel process that incorporates the template molecule [3]. Other methods that have been employed for the preparation of the thin imprinted films include organically modi fied sol-gels [4], gas-phase surface sol-gel deposition [3], liquid phase deposition [5 ] and spin coating [6]. One of the disadvantages of using metal alkoxides is the need to use organic solvent which limits the ability to use water soluble templates in the MI technique. In this work we propose a different approach for the fabrication of molecularly imprinted thin inorganic films using water soluble compounds. The approach involves the deposition onto the LPG of the TiO

A method for the preparation of a sensor consisting of an optical fibre long period grating coated with human hemoglobin is described. The utility of this sensor in detecting dissolved oxygen in phosphate buffered saline solution, by the conversion of the coated hemoglobin from deoxyhemoglobin to oxyhemoglobin, is described. The sensor shows good repeatability with a %CV of less than 1% for oxygenated and deoxygenated states and no drift or hysteresis with repeated cycling.

An optical fibre long period grating (LPG) modified with a thin film of ZIF-8, a zeolitic immidazol framework (ZIF) material, a subgroup of the metal organic framework (MOF) family, was employed for the detection of organic vapours. ZIF-8 film was deposited onto the surface of the LPG using an in-situ crystallization technique. The sensing mechanism is based on the measurement of the refractive index (RI) change induced by the penetration of the chemical molecules into the ZIF-8 pores. An LPG modified with 5 growth cycles of ZIF-8 responded to exposure to methanol and ethanol vapours.

An optical fibre long period grating (LPG) modified with a mesoporous film infused with a calixarene as a functional compound was employed for the detection of a mixture of volatile organic compounds (VOCs). The sensing mechanism is based on the transduction of the refractive index change induced by the complexion of the VOCs with calixarene into a change in the form of the transmission spectrum of the LPG. An LPG, modified with a calixarene-infused coating comprising 5 cycles of silica nanoparticles/poly(allylamine hydrochloride) polycation (SiO 2 /PAH), was exposed to mixture of VOCs emitted from paint at conditions simulating ISO standards test (16000-10).

A Long period grating (LPG) with a period of 111 μm was fabricated in the highly birefringent (Hi-Bi) optical fibre with the aim of developing a sensor for chirality measurements. The LPG sensor was exposed to different concentrations of glucose D(+) and fructose D(-) in water, which have similar structures but exhibit opposite optical rotations, i.e. chirality. The behaviour of the resonance bands of the submodes corresponding to the two orthogonal polarization states was different depending on the chirality of the compound, thus allowing discrimination between two compounds.

Two highly sensitive ammonia sensors, formed by depositing coatings composed of titanium dioxide (TiO2) onto the cladding of an optical fibre sensing platform, are evaluated. A long period grating (LPG) of period 111 μm was fabricated in the core of an optical fibre so that the LPG operates at or near the phase matching turning point (PMTP). The first coating that was investigated was composed of TiO2 nanoparticles deposited by liquid phase deposition. The sensor showed high sensitivity and allowed low concentrations of ammonia in water (0.01 ppm) to be detected with a response time of less than 60 sec. The second coating was composed of TiO2 with subsequent layers of poly (allyamine hydrochloride) (PAH), and SiO2 nanospheres infused with a sensitive element composed of porphine. The ammonia adsorption to the porphine compound led to the changes in the LPG’s transmission spectrum and allowed 0.1 ppm of ammonia in water to be detected with a response time of less than 60 sec.

The use of bacteriorhodopsin (Br) coatings to create photosensitive optical fibre long period gratings (LPGs) is described. The response of the coated LPGs both sustained and pulsed illumination at a wavelength of 532 nm is monitored. The results show a clear response to the illumination and full recovery of the optical properties of the coating. This technique could allow the use of LPG for typifying photosensitive compounds and to develop optically controlled chemical sensors.

This paper describes an extended theory of the displacement of the objective speckle pattern resulting from displacement and/or deformation of a coherently illuminated diffuse object where the influence of the surface shape is included via the linear surface gradients. An experimental system capable of measuring the translational scaling factors, the ratios of speckle shift to object translations, to an accuracy of ± 0.02 and a repeatability of approximately ± 0.008 is described which was used to experimentally measure the speckle shift for a range of detector positions and surface gradients. The original expressions developed by Yamaguchi1 and the new extended expressions2 are then compared with experimental results for measurements on zero surface gradients, i.e. the mean surface lying in the x-y plane. The divergence of Yamaguchi’s expressions from experimental results for off-axis detector positions that was first observed by Světlík3 was confirmed, and the new expressions shown to successfully predict translational scaling factors for off-axis positions. The new expressions are then compared to the experimental results for a range of surface gradient magnitudes and directions, as well as detector positions both on and off-axis, and shown to successfully predict the observed speckle shift. EPSRC

We introduce a novel approach to continuous in-situ pathlength calibration of an integrating sphere based gas cell. Using two light sources and two detectors, a four beam ratiometric scheme is constructed, which compensates for component variation and sample chamber contamination. By applying the scheme to both on and off gas line measurements, changes in pathlength due to cell wall contamination can be identified and corrected. In this way the gas absorption coefficient can be determined continuously without needing to recalibrate the sphere. Results are presented for detection of methane at 1651nm. This method has the potential for extension to other gases such as CO2, CO, H2S, NOx.

A polling topology that employs optical switching based on the properties of erbium-doped fibres (EDFs) is used to interrogate an array of FBGs. The properties of the EDF are investigated in its pumped and un-pumped states and the EDFs’ switching properties are evaluated by comparing them with a high performance electronically controlled MEM optical switch. Potential advantages of the proposed technique are discussed. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

An optical fibre refractometer using a ball lens as a sensor head has been developed and characterised. Light from a superluminescent diode is directed to an optical fibre sensor head and the intensity of the returned beam gives a measure of the refractive index of the medium at the ball lens fibre tip. A second beam is used to reference the intensity measurements. The system is capable of detecting changes in refractive index with a resolution of 0.003 RIU. The ball lenses have been coated with an 80nm thick layer of palladium and the potential use of this system as a micromirror hydrogen sensor is demonstrated. This technique offers a simple sensor head arrangement, with a large signal sampling area compared with that of a bare fibre.

A differential extrinsic optical fiber Fabry-Perot based pressure sensor has been developed and benchmarked against a conventional piezoresistive Kulite pressure sensor. The sensors were placed on the fuselage of a 1:10/3 sub-scale model of a Scottish aviation Bulldog, which was placed in a wind-tunnel. Pressure tappings that surrounded the sensors aided the mapping of pressure distribution around this section of the fuselage. The results obtained from the fibre optic pressure sensor are in good agreement with those obtained from the Kulite and from the pressure tappings.

The response to temperature and refractive index (RI) of cascaded long period gratings operating at or near the phase matching turning point was studied. Temperature- and RI-induced changes of the phase of the channelled spectrum and wavelength shift of the long period grating’s resonance band envelope were observed for two conditions; when the length of fibre separating the long period gratings was perturbed and when the whole length of the device was perturbed. It is shown that when the central section of the device is perturbed the phase of the channelled spectrum changes and the envelope remains fixed, while when the entire device is perturbed the phase and central wavelength of the envelope shift at the same rate. Thus measurement of changes in the channelled spectrum with respect to the centre of the resonance band envelope offers a means for developing sensors whose response is compensated for temperature and RI.

Copyright ©2014 Society of Photo-Optical Instrumentation Engineers. This paper was published in the Proceedings of SPIE and is made available with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic electronic or print reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. An optical fibre long period grating (LPG), modified with a coating of silica gold (SiO2:Au) core/shell nanoparticles (NPs) deposited using the layer-by-layer (LbL) method, was employed for the development of a bio-sensor. The SiO2:Au NPs were electrostatically assembled onto the LPG with the aid of a poly(hydrochloride ammonium) (PAH) polycation layer. The LPG sensor operates at the phase matching turning point to provide the highest sensitivity. The SiO2:Au NPs were modified with biotin, which was used as a ligand for streptavidin (SV) detection. The sensing mechanism is based on the measurement of the refractive index change induced by the binding of the SV to the biotin. The lowest detected concentration of SV was 19 nM using an LPG modified with a 3 layer (PAH/SiO2:Au) thin film.

In-fibre microcavity Fabry-Perot interferometers were constructed by splicing single mode fibre to polarisation maintaining photonic crystal fibre (PCF), with the air in the PCF pressurised to 5.000±0.005bar. The response to transverse load was characterised, along with the influence of rotational orientation and the repeatability of the fabrication process. It was found that the features of the channelled reflected spectrum exhibited a blue wavelength shift with increasing applied transverse load.

Formaldehyde is a volatile organic compound that exists as a gas at room temperature. It is hazardous to human health causing irritation of the eyes, nose and throat, headaches, limited pulmonary function and is a potential human carcinogen. Sources include incomplete combustion, numerous modern building materials and vehicle fumes. Here we describe a simple method for detecting formaldehyde using low resolution non-dispersive UV absorption spectroscopy for the first time. A two channel system has been developed, making use of a strong absorption peak at 339nm and a neighbouring region of negligible absorption at 336nm as a reference. Using a modulated UV LED as a light source and narrowband filters to select the desired spectral bands, a simple detection system was constructed that was specifically targeted at formaldehyde. A minimum detectable absorbance of 4.5 × 10-5 AU was estimated (as ΔI/I0), corresponding to a limit of detection of approximately 6.6 ppm for a 195mm gas cell, with a response time of 20s. However, thermally-induced drift in the LED spectral output caused this to deteriorate over longer time periods to around 30 ppm or 2 × 10-4 AU.

Conventional singlemode semiconductor DFB and VCSEL lasers used in high resolution spectroscopy are often required to operate at specific, custom wavelengths, such as those associated with gas absorption lines. We present the results of work to develop alternative sources in the 1550nm and 1650nm regions, the latter coinciding with an absorption line of methane. Custom wavelength Bragg gratings have been used to stabilize the output of external cavity lasers implemented in both optical fiber and planar silica-on-silicon integrated circuits, using commercially available semiconductor gain chips, to give laser output at 1648 and 1649 nm, respectively. Thermal expansion or mechanical strain of the Bragg grating offers a suitable wavelength tuning mechanism. Results are presented including the wavelength tuning range, output power, relative intensity noise (RIN), side-mode suppression and linewidth of devices for application in high resolution gas spectroscopy. The different methods of writing Bragg gratings in optical fiber and planar silica-on-silicon allow a high degree of flexibility in the choice of emission wavelength.

The response of optical fibre long period gratings (LPGs), fabricated with precise control of the grating period to operate at or near the phase matching turning point (PMTP), to temperature was studied. The effect of the grating period on sensor performance was studied. The sensitivity of the LPG operating at the phase matching turning point to temperature was 0.99 nm/°C for 111.5 μm grating period that is ca. 7 times higher than that of an LPG operating far from turning point. The possibility of using LPG sensors to determine the thermo-optic coefficient of the surrounding material was explored.

The infusion, flow and cure of RTM6 resin in a carbon fibre reinforced composite preform have been monitored using a variety of multiplexed fibre optic sensors. Optical fibre Fresnel sensors and tilted fibre Bragg grating (TFBG) sensors were configured to monitor resin infusion/flow in-plane of the component. The results obtained from the different sensors were in good agreement with visual observations. The degree of cure was monitored by Fresnel sensors via a measurement of the refractive index of the resin which was converted to degree of cure using a calibration determined from Differential Scanning Calorimetry. Fibre Bragg grating sensors fabricated in highly linearly birefringent fibre were used to monitor the development of transverse strain during the cure process, revealing through-thickness material shrinkage of about 712 μe and residual strain of 223 μe. An alternative approach to infusion monitoring, based on an array of multiplexed tapered optical fibre sensors interrogated using optical frequency domain reflectometry, was also investigated in a separate carbon fibre preform that was infused with RTM6 resin.

A continuously chirped long period grating (CCLPG) was used to perform directional flow sensing of a UV-cured epoxy resin and to subsequently monitor the resin cure via the measurement of the change in refractive index. The asymmetric nature of the CCLPG led to a change in the shape of the attenuation bands of the transmission spectrum dependent on the direction of flow of the resin along the fibre. The transmission spectrum of a CCLPG was monitored during the resin cure and exhibited a wavelength shift of 1.1 ± 0.1 nm. The change in the refractive index of the resin during cure was determined in the same experiment using a fibre optic Fresnel based refractometer, and correlated well (correlation coefficient 97 %) with the response of the CCLPG.

Coherent optical frequency domain reflectometry is used to multiplex a serial array of tapered optical fibre sensors. By monitoring changes in the Rayleigh backscattered signal from tapered regions of a single mode optical fibre, the refractive index of the medium surrounding each taper can be determined. The approach is used to demonstrate a liquid flow sensor.

A novel optical signal processing scheme for fibre sensors is proposed, which combines interferometric phase measurements with range multiplexing. The scheme is based on single-sideband signal processing and uses continuous-wave pseudo-random range encoding. The potential of the technique for cost-effective dynamic quasi-distributed strain sensing is explored by applying it to an array of fibre segments. In its current implementation dynamic strains in segments of 3.4 m gauge lengths can be measured with a resolution well below microstrains at a bandwidth of 100 kHz. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We present the construction of an external cavity laser (ECL) diode using an InP semiconductor gain chip and a fibre Bragg grating (FBG), designed to have an emission wavelength that coincides with an absorption line of methane, to be used for spectroscopic gas sensing. The FBG was employed as a wavelength selective and feedback element, which will potentially provide the laser with wavelength tuning capability. Narrow linewidth lasing output of less than 5 MHz was achieved. To our knowledge, this is the first FBG based ECL in the 1.65 µm region.

Imaging fibre bundles and a conical mirror have been used to construct a passive, endoscopic OCT probe with no scanning components at the probe tip. Circular scanning of the beam projected onto the proximal face of the imaging bundle results in a corresponding circular scan at the distal end of the probe. A 45 o base-angle conical mirror turns the output light to produce a radially-directed beam that permits circumferential OCT scanning in quasi-cylindrical ducts. Keywords: Optical coherence tomography, OCT, conical mirror, imaging fibre bundles 1. INTRODUCTION Endoscopic optical coherence tomography (OCT) systems using imaging fibre bundles have been previously reported 1-5 . Incorporating imaging bundles into an OCT endoscope enables a 2-D region of the sample to be imaged, as light can be coupled sequentially into multiple fibre cores within the bundle, removing the need for miniaturised scanning components at the probe tip. The technique is compatible with both time-domain (TD-OCT) and swept-source (SS-OCT) systems. Imaging is achieved by coupling light in rapid succession into each element of a straight or curved row of bundle cores. Samples can be positioned either on-axis, with the sample surface normal to the system axis, or to one side of the probe tip, with the sample surface parallel to the optical axis and a turning mirror used to rotate the beam through 90

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.

Imaging fibre bundles are widely used in image transmission systems in the visible spectral range 1 . Their image transmission capabilities and flexibility make them interesting for OCT endoscopy 2 . However, cross-talk occurring between individual fibres and multimode transmission cause deterioration of the images obtained with OCT systems. In this study, a model of light propagation in a fibre bundle is developed using the Finite Difference - Time Domain (FDTD) method in order to quantify these effects. The simulated bundle output power density distribution is compared to experimental images.

The transmission spectrum of a fibre optic long period grating (LPG) coated with a porous multilayer coating of thickness of order 400 nm formed from silica nanospheres is shown to exhibit a strong sensitivity to the infusion of a functional, chemically sensitive material into the coating. Subsequently, the transmission spectrum of the LPG shows sensitivity to changes in the properties of the functional material when exposed to a particular chemical species in an aqueous solution. The operation of such a device as an ammonia sensor is demonstrated, exhibiting 1 ppm sensitivity. The sensing mechanisms are discussed.

Species selective vapour sensing has been demonstrated using a coated Long Period Grating with a period and coating thickness that gives rise to a dual resonant response. A calixarene coating with nanostructured features was deposited using the Langmuir-Blodgett technique. A nanoscale cavity in the calixarene molecule adsorbs the analyte molecule causing an increase in the density and thereby the refractive index of the coating. Preliminary vapour sensing experiments were performed and indicate the sensor's selectivity, demonstrating strong responses to toluene and benzene, while exhibiting lower sensitivity to hexane and cyclohexane.

The response of the transmission spectrum of a fibre optic long period grating operating near the phase matching turning point to the deposition of a nanostructured coating is investigated. The dual-resonant bands characteristic of operation in this regime are observed to show a high sensitivity to the thickness of the coating, but with opposite signs. Appropriate design of the device, based on the grating period, the refractive index and thickness of the coating and the fibre composition, can allow the sensitivity of the device to the optical thickness of the coating to be optimised.

OCT probes for endoscopy typically use a single optical fibre to deliver light to the measurement region. 2D imaging is achieved using mechanical scanning components at the probe tip. We are investigating an OCT system in which fibre imaging bundles replace the single probe fibre, allowing many thousand measurement points to be addressed without any mechanical scanning at the probe tip, over a region measuring a few millimetres in each dimension. This allows a small-diameter, electrically passive probe to be engineered using offthe- shelf scanning optics. Images from a single-fibre, conventionally-scanned probe and from a bundle-based probe are presented, using swept-source processing centred at 1330 nm.

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 manufacturing and commissioning of superconducting magnet coils was monitored using embedded optical fibre Bragg grating sensors (FBG) fabricated in highly linearly birefringent (HiBi) fibre. Strain developed in the coils was monitored from the resin impregnation and curing process, through cool down of the magnet in a helium cryostat from room temperature to 4 K, to energising the coils. The development of multiple components of strain in the coils when the magnet quenched was monitored, revealing phenomena that it had not been previously possible to measure using other sensor technologies.

Experimental results of static pressure measurements from specially packaged π-phase shifted optical fibre Bragg gratings (FBGs) that were installed into a 2D, 3 element high lift wing are presented. The static pressure measurements from the fibre sensors are found to be in good agreement with theory. Further pressure calibrations are currently under investigation.

Quantitative surface strain measurement is demonstrated on dynamic test objects using a shearography instrument comprising four measurement channels. The measurement channels are formed using four observation directions and a single illumination direction. Images from the four measurement channels are transported to a shearing interferometer using fibre-optic imaging bundles after which they are spatially multiplexed onto the quadrants of a single CCD camera. This facilitates the simultaneous acquisition of data from the four measurement channels. A pulsed laser source was used to effectively freeze the motion of the dynamic surface at two positions in its cycle. The phase variation caused by surface deformation in the time between the two recordings was calculated using the spatial carrier technique. The orthogonal displacement gradient components which characterize the surface strain of the object were calculated from the unwrapped phase maps from each measurement channel using a matrix transformation. Two test objects were investigated. The first was a thermally loaded PTFE plate rotating at 610 rpm. Images were recorded a frequency of 10 Hz, corresponding to the repetition rate of the laser. The second object was a speaker cone that was set to vibrate at frequencies in the range of 1.9-4.5 kHz. Phase measurements were made from images recorded 1.6 μs apart using dual pulsed illumination in combination with a dual-framing CCD camera.

Quantitative, full-field surface strain measurement is achievable using shearography instrumentation that employs at least three measurement channels. The system presented here possesses four measurement channels consisting of four views of the object. Images from the four views are ported to the shearing interferometer using fibre-optic imaging bundles. The use of fibre-optic imaging bundles simplifies the experimental arrangement considerably, allowing full-field surface strain measurements to be made using just one CCD camera and one shearing interferometer. Simultaneous capture of data from the four measurement channels using a pulsed laser source allows the measurement of surface strain on dynamic objects. In this paper the use of the instrument to make measurements of the surface strain profile of a PTFE plate rotating at 600 rpm is described. The use of the spatial carrier technique allows the deformation induced phase variation to be calculated across the field of view from just two images recorded from the object on subsequent rotations.

A chirped long period grating (LPG) is used as a directional flow sensor by monitoring the change in the transmission spectrum upon immersion in an oil of refractive index higher than that of the fibre cladding. The change in profile of an attenuation band is shown to depend on the direction of immersion, indicating the possibility of using such devices in applications such as resin flow sensing in the manufacture of fibre reinforced plastic composite structures.

Nanostructured coatings have been deposited onto tapered optical fibres using the Langmuir Blodgett technique. The response of the transmission spectrum of the tapered fibres to changes in thickness of the coating has been analysed. The effects observed are shown to be dependent on the diameter of the tapered region.

Quantitative surface strain measurement using shearography requires the calculation of six components of displacement gradient. This is done using shearography instrumentation with at least three measurement channels combined with two orthogonal shear directions. These channels take the form of either multiple illumination or observation directions. The system presented here is based on the illumination of the object of interest using a pulsed Nd:YAG laser and the observation of the object from four separate positions arranged in a square around the illuminating beam. Images from the four observation positions are transported to a shearing interferometer using coherent fibre-optic imaging bundles, where they are spatially multiplexed onto the sensor of a single CCD camera. Displacement gradient measurements from a static test object are presented and compared to the results of a computational model. Phase analysis is carried out using two approaches, temporal phase stepping and the carrier fringe technique, with the aim of extending the application of the instrument to the monitoring of dynamic loading events.

Optimum performance from advanced composites requires careful control of the resin matrix during cure. This is to ensure there are no cure induced voids and to minimise the build up of internal stresses. Careful control of the process is also necessary to reduce wastage. Traditional resin inspection techniques are bulk or sample oriented and thus cannot provide data about critical component parts. Optical fibre based sensors however, allow for in-situ monitoring techniques to be deployed in components without effecting their structural integrity. In this work, two fibre optic grating techniques are demonstrated as process monitoring sensors and are compared with a Fresnel refractometric method. The change in refractive index of a resin has previously been used as a means for assessing the degree of cure. The central wavelength of an attenuation band of a long period grating (LPG) was monitored during the cure of a resin. In parallel the spectral resonances of a tilted fibre Bragg grating (FBG) are also monitored. The two techniques are shown to correlate well with the Fresnel based method in both detecting the resin and monitoring the state of cure, indicating the potential of the techniques for online production monitoring.

This paper will present recent developments in optical fibre sensors and optical fibre based instrumentation research undertaken at Cranfield University. New sensor techniques based on nanoscale molecular coatings deposited on singlemode fibres containing long period gratings and the use of singlemode fibres and coherent imaging fibre bundles in full-field speckle interferometry and planar Doppler velocimetry will be presented.

The work presented here describes a method that allows three component velocity measurements to be made, quickly and non-intrusively, across a plane in a flow defined by a laser light sheet. The method, two frequency planar Doppler Velocimetry (2v-PDV) is a modification of the planar Doppler velocimetry (PDV) technique, using only a single CCD camera, and sequential illumination of the flow using two frequencies, separated by about 700 MHz. One illumination frequency lies on an absorption line of gaseous iodine, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapour cell onto the camera. The beam at a frequency off the absorption line is not affected by passage through the cell, and provides a reference image. The other beam encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Use of a single camera ensures registration of the reference and signal images, which is the major problem in any spilt image system such as a two-camera imaging head, and cost efficiency is improved by the simplification of the system. A 2v-PDV system was constructed using a continuous-wave Argon ion laser and acousto-optic modulators to produce two frequencies of illuminating laser light. This was combined with multiple imaging fibre bundles, to port three different views of the measurement plane to a CCD camera, allowing the measurement of three velocity components.