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In this paper we report on experiments aimed at the development of a highly responsive sensor based on long period gratings (LPGs) operating at high order cladding modes which exhibit a turn-around-point in their phase matching diagrams. These high order cladding modes are accessed by decreasing the period of the grating within standard optical fiber according to theoretical simulations. The sensors tested detect hydrogen through a Pd coating on the fiber cladding. As expected from simulations these sensors exhibit a near eight fold increase in response to 1% hydrogen when operated near the phase turning point. This increase in response is, however, not selective to hydrogen and a combination of increased thermal noise and broadened resonance condition serves to decrease the signal to noise ratio such that any gain in theoretical sensitivity is counter-acted by a loss in fidelity of the recovered (or recorded) signal. The highest sensitivity (shift of a LPG lossband) recorded was in the order of 470 pm for an exposure to 1% H-2 at 70 degrees C, which represented a shift of similar to 1/150 of the FWHM of the LPG loss band.

In this paper, we describe a technique to characterize long period fiber gratings (LPG) by use of a correlation between the measured transmission spectrum and a theoretical model. The model is robust enough to require only a-priori knowledge of the fiber design and the inscription period. We demonstrate the application of this technique on an example LPG resulting in the calculation of the dispersive core index, length, and strength of the grating to better than experimental error. The technique is further expanded to allow for the modeling of metal jacketed long period gratings by use of an extended version of Erdogan's model.

Smart materials with integrated sensing capabilities are now ubiquitous in many structures and devices manufactured from composite materials and they offer enhanced safety, reliability and efficiency in such smart devices. This paper explores the application of embedded sensors to components manufactured using additive layer manufacturing (ALM) technology. ALM offers the ability to create physical parts with little or no restriction in shape and complexity. In this paper, optical fiber sensors incorporating fiber Bragg gratings are embedded inside a component made by, and during a powder-bed-based, layer-by-layer, additive manufacturing process. A commercial EOS P730 system is used, where a laser is employed to sinter the polymeric powder into a 3D component. The fiber embedding approach is based upon insertion of a “fiber-carrier” component, which replaces a removable “place-holder” component during an interruption of the ALM build process. Tensile test specimens fabricated this way are subjected to extended cyclic tensile loading trials at low strain levels of up to 580 μe . The test specimens demonstrate stable and reproducible responses over a period in excess of 720 days and 311000 load cycles. Polyimide and acrylic jacketed fibers are trialled, and the resulting deformations of the component through internal stresses depending on the fiber jacket type are discussed.

We report a compact two-dimensional accelerometer based upon a simple fiber cantilever constructed from a short length of multicore optical fiber. Two-axis measurement is demonstrated up to 3 kHz. Differential measurement between fiber Bragg gratings written in the multicore fiber provides temperature-insensitive measurements.

Optical fibre pressure sensors have potential performance advantages over electrical sensors in measuring rapid transients such as shock waves from explosive blasts. We report the development of micromachined optical fibre Fabry–Perot pressure sensors using a silicon dioxide or nitride diaphragm and detail the fabrication stages of the sensor body and diaphragm. The planar technology used is based on silicon deep etching and direct fusion bonding of silicon wafers. Test results for both types of diaphragm are presented. Sensors with rise times better than 3 µs, range 0.1 to 1 MPa and resolution ~500 Pa have been demonstrated in explosives trials. Despite the difference in the sign of stress for the two diaphragm types, both demonstrated excellent high-speed response to explosively generated air shocks.

Hydrogen sensing at low concentrations in the order of tens to hundreds of parts per million (ppm) over extended periods requires highly sensitive and stable sensor technology. Palladium and its alloys are widely used in hydrogen sensing as they show a high and selective affinity for hydrogen. On exposure to hydrogen, Pd experiences a volumetric expansion, roughly proportional to the hydrogen concentration in the environment. This volumetric expansion can be measured either directly by dimensional monitoring or through monitoring of secondary physical effects. Optical sensor technology is a preferred technique for hydrogen sensing because of its inherent safety for use in a potentially explosive environment, and several optical hydrogen sensors have been reported. However, the majority have been designed to measure hydrogen concentrations of a few per cent in air, whereas the sensors described here are capable of operating in concentrations down to hundreds of ppm. Fibre Bragg gratings (FBGs) and interferometric length monitoring are shown to be able to monitor the dimensions of a palladium sensor element. Hydrogenation not only induces physical strain but also changes the electronic configuration resulting in a change in refractive index which can be monitored using a long-period grating (LPG). The paper discusses the respective characteristics of these different optical measurement techniques using Pd and Pd/Ag alloys as sensor elements for hydrogen concentrations between 100 ppm and 1%.

In this paper, we report the first demonstration of multiplexed fibre Bragg grating strain sensors in a multicore fibre for shape measurement and their application to structural monitoring. Sets of gratings, acting as strain gauges, are co-located in the multicore fibre such that they enable the curvature to be determined via differential strain measurement. Multiple sets of these gratings allow the curvature to be measured at several points along the fibre. In this paper, the multicore fibre is configured to measure the deflection of a simple mechanical beam arising from the displacement of concrete tunnel sections. Laboratory tests are presented in which the system was demonstrated capable of displacement measurement with a resolution of ±0.1 mm over a range of several millimetres. © 2006 IOP Publishing Ltd.

We report the first use of a multicore fibre incorporating fibre Bragg grating strain sensors in each core as a fibre optic pitch and roll sensor. A length of four-core fibre supported at one end forms a cantilever. The differential strains between opposite grating pairs depend on the fibre's orientation in pitch (in the vertical plane) and roll (azimuth) with respect to gravity. Resolutions of +/-2degrees in roll and +/-15degrees in pitch were measured.

We have investigated the effects of ionizing radiation-induced changes on dispersion and refractive index in photonic crystal fibre, boron co-doped photosensitive fibre and standard SMF-28 fibre up to a level of 7 MGy of 60Co γ-radiation. We have been unable to observe any changes within an experimental error of ~1% for the dispersion value and

A novel dual-fibre Bragg grating sensor structure is described, incorporating integrated mechanical stress amplification and temperature compensation by common mode rejection. Stress amplification allows the measurement of very low actuator forces, such as generated by small cross section bellows, for operation as a barometric pressure sensor. We demonstrate a pressure sensor range of 800–1100 hPa and a resolution of 0.1 hPa.

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical-fiber pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m squared, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m-thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a single mode fiber sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by three wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of 1.6 radbar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 8 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.

We describe the calibration of a technique allowing quantitative measurements of industrial coatings with thicknesses as small as 10-20 nm. Wax films doped with fluorescent rhodamine dye have been deposited by an electrospray method onto an optically flat surface of aluminium-coated glass. The films were of 220-450 nm peak thickness, which we measured with an optical profilometer using laser triangulation. Possession of a set of films allowing an absolute calibration of the fluorescence intensity versus thickness to be achieved for application in trials of the fluorescence method for measuring coating thicknesses of 10-20 nm under industrial process conditions.

A sensor to measure rapidly changing pressures in an explosive air blast requires high bandwidth and high spatial resolution. For such an application a low-cost, electrically isolated sensor is particularly attractive. We describe an optical fibre-based pressure sensor that meets these requirements. The sensor is subjected to an experimental explosive-blast measurement test and the results are discussed with respect to the sensor performance and compared with a simple blast-wave model and conventional pressure transducers.

We describe a fibre Fabry-Perot heat-transfer sensor that is capable of performing the high-spatial-resolution measurements required in turbomachinery research. The all-fibre cavity operates as a calorimeter embedded perpendicular to the test surface. The cavity incorporates a Bragg grating as the internal reflector, producing a mechanically robust sensor suitable for aerodynamic measurements. We present experimental results from an impinging flow rig, comparing the optical sensor with a thin-film resistance gauge subjected to a step increase in heat flux of 30 kW m-2.

We examine the application of an artificial neural network to classification of tool wear states in face milling. The input features were derived from measurements of acoustic emission during machining and topography of the machined surfaces. Five input features were applied to the back-propagating neural network to predict a wear state of light, medium or heavy wear. We present results from milling experiments with multi- and single-point cutting and compare the neural network predictions with observed cutting insert wear states.

We describe an electronic speckle pattern interferometer (ESPI) system that has enabled non-harmonic vibrations to be measured with μs temporal resolution. The short exposure period and high framing rate of a high-speed camera at up to 40,500 frames per second allow low-power CW laser illumination and fibre-optic beam delivery to be used, rather than the high peak power pulsed lasers normally used in ESPI for transient measurement. The technique has been demonstrated in the laboratory and tested in preliminary industrial trials. The ability to measure vibration with high spatial and temporal resolution, which is not provided by techniques such as scanning laser vibrometry, has many applications in manufacturing design, and in an illustrative application described here revealed previously unmeasured “rocking” vibrations of a car door. It has been possible to make the measurement on the door as part of a complete vehicle standing on its own tyres, wheels and suspension, and where the excitation was generated by the running of the vehicle's own engine.

Development of pressure sensors for the instrumentation of experimental aerodynamic facilities has traditionally concentrated on electrical techniques. An improvement in the currently attainable temporal and spatial resolution in pressure measurement would be beneficial in the characterization of turbulent flows behind turbine rotor stages, for example. We present results obtained in a turbine test rig from a fiber optic pressure sensor based upon the interferometric response of an extrinsic cavity formed between the interrogation fiber and a reflective diaphragm. We discuss the design trade offs, optical interrogation and temperature sensitivity of such a configuration, and demonstrate the success of the design in small-scale shock tube experiments. We then describe the application of the sensor in a full scale turbine test facility in which pressure signals with frequency components exceeding 200 kHz were obtained.

The development of the Sagnac interferometer to monitor the machining process in high-value-added machining processes is described. The instrumental requirements are non-contact measurement of acoustic emission, in the form of 0.1–1.0 MHz out-of-plane vibration, with amplitudes in the subnanometre range. The instrument was designed to detect acoustic emission by probing the surface of the rotating tool holder on the machine tool shaft. Experiments leading to the instrument design, face milling and slot milling machine tests and a comparison with earlier instrument designs are described.

We demonstrate the proof of principle for a novel fibre optic sensor for the detection of high humidity. The device uses single-mode fibre at telecommunications wavelengths, making it particularly suitable for application in passive optical networks. The sensing mechanism relies on a humidity-induced refractive index change in an organic film coated onto the cladding of a bent fibre. The sensor was shown to respond at relative humidities >62% and >80% using gelatine and polyethylene oxide films respectively, by the onset of a strong interaction between the LP 01 mode and a whispering gallery mode.

We describe a set of wear tests in which machined surfaces were produced by single point face milling of an aluminium alloy. The objectives were firstly to identify surface characteristics associated with tool chipping, and secondly to demonstrate that a fibre optic surface profiling probe capable of in-situ operation on the machine tool had the ability to provide the same wear information as a metrological interference microscope. Spectral energy of the surface profiles decreased significantly with insert damage in a spatial frequency band around the tooth passing frequency of the cutter and also at higher spatial frequencies.

We describe a systematic series of wear tests in which surface profiles were measured at intervals during face milling of En24 steel, an aluminium alloy and a stainless steel. Profiles were measured by a stylus instrument and, in the case of the aluminium alloy, by a novel optical fibre instrument mounted on the machine headstock. Spatial frequency analyses are presented that show the long wavelength amplitude increases with insert wear, with a similar tendency in the high spatial frequencies (above the feed per tooth). Variation in tool set affected insert wear rate and the evolution of surface profile. Insert chipping damage was seen to increase the low spatial frequency content. The Ra parameter correlated with insert wear only for single point cutting.

An experimental determination of the thermo-optic effect of titanium dioxide is described in which the test element is a thin film deposited onto the end face of an optical fibre. The film acts as an etalon. The reflectivity of titanium dioxide films was measured as a function of temperature experimentally over the range 20 °C to 500 °C, and modelled theoretically to yield a value of the thermo-optic coefficient of − 1.49 × 10−4 K−1. The use of the data in the design of a thin-film optical fibre temperature sensor is described.

We describe a fibre interferometer for making non-contact differential height measurements of machined metal surfaces without removal from the machine tool. The system comprises a probe which produces two small laser spots ( diameter, 3 mm apart) on the surface, an optical fibre addressing lead and signal processing unit incorporating an optical source and detector. The probe is an extrinsic fibre optic Michelson interferometer whose arms terminate in the laser spots on the test surface, and the signal processing unit calculates the difference in local surface height between the two spots. Differential height profiles of machined aluminium surfaces and optical mirror surfaces are presented.

Acoustic emission (AE) has previously been shown to be a useful technique in monitoring the state of wear of cutting tools. The piezo-electric transducers conventionally used for AE detection are contacting devices with a limited bandwidth. This paper describes the use of a robust fiber optic interferometer for the in-process measurement of AE during the face milling of annealed En24 steel to provide tool wear information via analysis of the rms AE signal. The interferometer displayed an improved diagnostic capability over a conventional piezoelectric sensor due to its advantages of being non-contacting, broadband with a flat frequency response to displacement, and providing absolute calibration.

This paper describes the design, operation, construction, and demonstration of a new type of high-bandwidth unsteady temperature sensor based on fiber optics, and capable of operating in a high-speed multistage research compressor with flow representative of jet engine conditions. The sensing element is an optical coating of zinc selenide deposited on the end of an optical fiber. During evaluation in aerodynamic testing, a 1 K gas temperature resolution was demonstrated at 9.6 kHz and an upper bandwidth limit of 36 kHz achieved.

Interferometric temperature sensors comprising a zinc selenide film on the end of a single-mode optical fiber have been incorporated into rugged probes for measurement of gas temperature fluctuations in aerospace turbomachinery test rigs. Two-wavelength illumination provides signals in quadrature for determination of optical phase change of the signal reflected from the film which is exposed to the gas flow. We present ensemble-averaged results showing 15 K amplitude gas temperature fluctuations up to 70 kHz with a resolution of 1 K. >

We investigate the phase matching conditions and sensitivities of higher order metal jacketed long period gratings (LPGs). These higher order modes have been previously demonstrated to have flatter, and therefore more sensitive, phase matching conditions leading up to the phase matching turning point. We demonstrate this increased sensitivity as applied to a Pd jacketed LPG hydrogen sensor illustrating an improvement in both the refractive index and temperature sensitivity (of the 17th order mode) of an order of magnitude over the lower order (1st-9th) modes. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A sensor for high-frequency measurement of gas temperature has been developed: the sensing element is a very short length (40 mu m) optical fibre Fabry-Perot interferometer, fabricated using a technique based on fusion of optical fibres coated with titanium dioxide to form partially reflective splices. The interferometer is thermally sensitive and small in size. Thermal response is demonstrated experimentally to 3 kHz in vortex shedding from a heated wire in air flow. The observed sensitivity is in agreement with heat transfer and aerodynamic modelling of the sensor and with the thermo-optic coefficient of the fibre core.

Bend loss in buffered single mode optical fibre is an oscillatory function of wavelength. In previous work, these oscillations have been described by a two-beam interference model between a core guided mode, and a whispering gallery mode propagating in the cladding and buffer. It is shown that higher order recoupling occurs between the two modes, such that the shape of the oscillations is not given by a simple raised sinusoid. It is demonstrated theoretically and experimentally that the shape of the bend loss oscillations is given by a superposition of the harmonics of a sinusoid, and that the distribution of power amongst these harmonics depends on the bend length.

A novel type of interferometric fibre optic sensor is described, based on a curved section of a buffered single-mode optical fibre. Interference occurs between the core-guided mode and the 'whispering gallery' mode guided within the fibre buffer. The fibre buffer coating thus serves as the sensing element. The technique is illustrated by its application to temperature measurement. A linear output is produced by using a two-wavelength phase-stepping technique.

Observations of oscillations in bending loss as a function of illuminating wavelength in monomode optical fibers are presented. A model is developed based on the interference between the core-guided mode, and a "whispering gallery" (WG) mode established by reflection at the buffer-air interface. It is shown that the relative phase of the modes is a function of the dimensions and refractive index of the buffer coating. The phase is determined in experiments where the bend radius and wavelength of illumination are varied, allowing the buffer diameter and refractive index to be determined by a best fit to experimental data. The practicality of this method for the characterisation of buffer properties is discussed. >

Additive layered rapid manufacturing offers the ability to create physical parts with little or no restriction from shape complexity. Modelling and validation of this new technology requires embedded sensors which challenges conventional sensor technology. In this work, fibre Bragg gratings [FBG] have been embedded inside components made from a polymer during the manufacturing process of layer-by-layer additive manufacturing technology by laser sintering of powder layers using a commercial EOS P730 system. The technique is based on the insertion of a fibre carrier component, replacing a removable ‘place holder’ during an interruption of the build process. Tensile test specimens have been subjected to extended cyclic tensile loading trials at low strain levels of up to 400 µ-strain. The test specimen show stable and reproducible response over a period in excess of 330 days and 142000 load cycles. Polyimide and acrylic jacketed fibres have been trialled and the effect of minor deformations of the component through internal stresses depending on the fibre jacket type is discussed.

Acoustic emission has previously been shown to be a useful technique in monitoring the state of wear in machine tools. The piezoelectric transducers conventionally used for acoustic emission detection are contacting devices with a limited bandwidth. Operationally, a non-contacting transducer of broader bandwidth would be preferred. The authors have accordingly developed a technique based on optical interferometry, using fibre optics, to produce a miniature and robust probe for detecting acoustic emission by measuring the small amplitude ( approximately 0.1 nm), high frequency (0.1-1 MHz) surface vibrations that it produces. They demonstrate the technique in measurements made probing both the workpiece and the rotating tool holder during face milling of mild steel.

A novel optical arrangement is described for an interferometric optical fibre sensor of the extrinsic type. Based on a Michelson interferometer, it combines a bend insensitive downlead with the availability of antiphase outputs without insertion loss, and provides isolation of the source.

Short length ( approximately 2 mm) fibre Fabry-Perot interferometers embedded in test models have been used as calorimeter heat transfer gauges in transient wind tunnel experiments. Four sensors were multiplexed to a single laser diode source by a fibre optic coupler network. Results are presented from wind tunnel evaluations with comparison against conventional thin film resistance thermometers in the measurement of heat fluxes of approximately 50 kW m-2 and 20 ms duration.

The construction and evaluation of miniature interferometric fibre optic sensors for thermal measurements are described. The sensors are of the Fabry-Perot or Fizeau configuration and comprise a short length of single mode optical fibre with a reflective coating at each end. One end is coated with titanium dioxide, and is fusion spliced to an arbitrary length of similar fibre, which serves to address the sensor, thus facilitating remote operation. The steady state response of the sensors to temperature cycling, and their transient response to pulse heating were investigated. Sensors were constructed with lengths from 0.2 to 20 mm and mean reflectivities ranging from 1% to 25%, leading to visibilities of interference of 0.6 to 0.9. Temperature cycling was carried out from ambient to about 500 degrees C, although operation at temperatures of up to 800 degrees C is considered feasible. The sensors were also demonstrated as heat flux gauges of the calorimetric type.

The authors describe a new type of differential laser Doppler velocimeter based on the use of a modified fibre optic Sagnac interferometer. A feature of the technique is that the interferometer phase is dependent, not on the target displacement, but on its velocity. Unlike other interferometers used to measure a Doppler frequency shift directly, the practical use of the Sagnac is not restricted to very high velocities. The authors have noted its particular value in the measurement of high frequency oscillatory velocities, where the direct response to velocity effectively discriminates against unwanted low frequency components. To optimize the sensitivity of the interferometer, they have introduced a pi /2 phase bias between the two beams, using a passive technique based on control of the birefringence of the fibre loop. The authors describe illustrative experiments using a vibrating target, but note other designs of optical probe for use in other applications.

A novel intrinsic optical fibre sensor for detection of acoustic emission (AE) has been designed, built and tested. This sensor is intended for use in monitoring machine tool wear. An optical fibre is held between the transducer backing and the surface being probed. The coupling of AE waves through the fibre into the backing causes a phase change in the light transmitted by the fibre. This phase change is detected using a Mach-Zehnder interferometer, locked in quadrature with a phase servo. Acoustic emission during cutting is conveyed to the sensing fibre on the machine bed via either the tool or workpiece. Sensing of AE may then yield information on the state of wear of the tool.

Wind tunnel experiments to investigate flow-induced vibration by vortex shedding from both multimode and monomode optical fibres are described, covering Reynolds numbers from 30 to 240. The fibre vibration frequency was a linear function of air flow speed over the range 2 to 8 m s-1. Multimode fibre vibration was detected by the modulated bend loss at the fibre clamping points. A fibre Mach-Zehnder interferometer enabled the strain, and hence vibration amplitude, of a monomode fibre to be determined under similar flow conditions. It was thus shown that for vortex shedding flowmeters with fibre sensing elements, interferometric techniques offer no significant performance advantage over simpler bend loss modulated devices.

We describe a prototype sensor for surface heat transfer measurements based on a miniature fibre Fabry-Perot (FFP) interferometer. These sensors are being developed for a particular application where heat transfer data are currently obtained using conventional platinum thin film resistance thermometers. The specification that the sensors must exceed is: (i) temperature resolution of 25 mK over a 50 K range; (ii) temporal response of 10 μs; (iii) an ability to operate as a calorimetric heat transfer gauge. The sensor consists of a short length of single mode optical fibre (∼ 3 mm) to which low reflectivity coatings have been applied at each end. It is illuminated and interrogated by an arbitrary length of addressing fibre. A laser diode is used as the source and we have exploited the facility to frequency modulate the diode in a novel signal processing scheme. To determine the performance of the sensor, short duration heat pulses derived from a pulsed Nd: YAG laser were applied to one end of the FFP. The response time was found to be 8 μs and the sensor operation as a calorimeter was verified.

Bend loss in monomode optical fibers exhibits oscillations as a function of wavelength due to a whispering gallery mode. The period of these oscillations has been used to measure the mean refractive index and thickness of the buffer material on two optical fibers in situ. The mean refractive index of the buffer material was measured to an accuracy of ±0.0025, which is sufficient to distinguish two nominally identical fibers with buffer surface cures of 86% and 96%.

Strain and thermal sensitivities of germanate and tellurite glass fibres were measured using a fibre Fabry-Perot (FFP) interferometer and fibre Bragg gratings (FBG). The strain phase sensitivity for germanate and tellurite fibre were 5900×103 rad/m and 5600×103 rad/m respectively at a central wavelength of 1540nm using FFP interferometer, which is consistent with the value of 1.22pm/µepsilon obtained for a germanate fibre FBG. The Young's modulus for germanate and tellurite fibre were also measured to be 58GPa and 37GPa. The thermal responses of germanate fibre were examined as 24.71 and 16.80 pm/°C at 1540nm and 1033nm wavelength using the FBG.

We demonstrate for the first time true distributed sensing of scale and location of transverse loading along an extended length of polarisation maintaining [PM] fibre with high spatial resolution using Optical Frequency Domain Reflectometry [OFDR]. Transverse loading of PM fibres induces local and differential changes in refractive index in slow and fast axis, depending on scale and direction of load relative to the axes. The change in refractive index is detected by monitoring the change in local periodicity of the statically distributed Rayleigh scatter for each PM fibre axis simultaneously using OFDR.

The bend-induced optical loss in a monomode fiber has been studied as a continuous function of wavelength in the range 1.2-1.6 microm. The loss was observed to be an oscillatory function of wavelength. These observations were explained by using a model based on interference between the guided core mode in the fiber and a "whispering gallery" mode ejected from the core by the bend. The wavelengths of the minima in the oscillatory function were accurately predicted by the model when the propagation of optical power in the fiber buffer coating was taken into account.

A three-core tellurite glass fiber having different combinations of rare earth oxide dopants in each core has been fabricated using shaped die-extrusion. Three cores, doped with Ho(3+)-Tm(3+)-Yb(3+), Er(3+)-Ce(3+), and Tm(3+)-Yb(3+) respectively, exhibited visible upconversion (blue, green and red) and infrared emissions at 1.4 microm, 1.5 microm, 1.9 microm and 2.05 microm when pumped at a wavelength of 980 nm. The prospects for multi-band amplifiers and lasers are discussed.

The temperature coefficients of optical phase have been measured at 1536 nm wavelength for short fiber Fabry-Perot cavities of tellurite and germanate glass fibers spliced to silica fiber. The results are consistent with the thermal expansion and thermo-optic coefficients of the bulk glasses.

The invited paper explains the transmission properties of a range of near-, mid-, and far-IR optical fibres for their applications in chemical and biological sensing. Methods for the fabrication of single and multiple-core mid-IR fibres are discussed in view of controlling the thermal and viscosity properties for fibre drawing. In particular, the need for removing impurity bands in the 5000 to 1000 cm-1 range is explained. The importance of engineering multi-core fibres is also discussed for simultaneous measurements of Raman, IR and surface plasmon enhanced modes together with say, temperature using a mid-IR transmitting tellurite fibre e.g. in a chemical process. The paper explains the principles and advantages of evanescent wave coupling of light at the resonant frequency bands for chemical sensing using a fibre evanescent wave spectroscopic sensor having a GeTeSe chalcogenide fibre. Using fibre based techniques, measurements for Cr6+ ions in solution and As3+ and As5+ in solids have been characterized at visible and mid-IR regions, respectively. In this paper we also explain the importance of using mid-IR fibres for engineering novel laser and broadband sources for chemical sensing.

Experimental results for the oscillation of bend loss with wavelength and temperature in buffered monomode optical fibre are reported. The oscillations have a compound structure characterised by two distinct periods. The observations are explained on the basis of interference between the core guided mode and two “whispering gallery” modes, one of which propagates by reflections at the buffer-air interface, the other by reflections at the cladding-buffer interface.

A passive signal processing technique is described for the recovery of the frequency of large amplitude oscillatory measurements from interferometric fibre optic sensors. The technique was developed for vortex shedding flowmeters, and was evaluated in this application and compared with other signal processing schemes. The flow-induced vibration frequency of one arm of a fibre Mach-Zehnder interferometer was measured in wind-tunnel experiments, demonstrating a linear frequency flow characteristic over the range 2.5-8 m s-1. Values for the fibre strain amplitude were also obtained.

A multicore fibre (MCF) sensor to measure the radial deformation of a compliant cylinder under compression is presented. The sensor is connectorised and need not be permanently bonded to the test object. A differential measurement technique using FBGs written into the MCF makes the sensor temperature insensitive. FBG measurement of axial strain of a cylinder under compression is also reported.