One of the main challenges for fibre optic based sensing is robust operation in the mid-infrared (mid-IR) region. This is of major interest because this wavelength region is where the characteristic absorption spectra for a wide range of molecules lie. However, due to the high absorption of silica (above 2 μm), mid-IR sensors based on solid core silica fibres are not practical. Of the many alternatives to solid silica fibres, hollow core microstrutured optical fibres are being explored and show great promise. One relatively new fibre, the hollow core negative curvature fibre (NCF) is promising for novel optical devices due to the simple structure (in comparison to other microstructured fibres) in combination with a hollow core which enables low loss mid-IR infrared guidance in a silica based fibre. In this paper, an all silica NCF that is post-processed with a fs laser, in order to increase access to the hollow core, is presented with acceptable loss and significant potential for mid-IR gas sensing.
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.
We investigate the optical properties of Pd thin films of the thickness 20-100nm deposited on Si wafer via RF sputter
coating. The Pd samples are characterised using white light interfermometry for thickness and ellipsometry for
refractive index. We demonstrate the independence of refractive index on film thickness above 20nm. Considerable
discrepancy is found between our measurement and previously published complex refractive indices for both bulk and
RF sputter coated Pd, indicating a high degree of dependence on deposition technique.
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.
We report an accelerometer based upon a simple fibre cantilever constructed from a short length of multicore fibre
(MCF) containing fibre Bragg gratings (FBGs). Two-axis measurement is demonstrated up to 3 kHz.
Palladium and its alloys show a high and selective affinity for hydrogen, resulting in a volume expansion. We report on development work using fibre Bragg gratings attached to palladium tubes to monitor strain resulting from hydrogen uptake. The technique is aimed at low concentration hydrogen monitoring where H2 is the result of ageing of polymer materials.
The sensitivities of type I and IIA fibre Bragg gratings written to different reflectivities in SMF-28 and B/Ge fibres to ionizing radiation up to 0.54MGy are investigated. The Bragg wavelength shows a small and rapid increase at the start of irradiation followed by either a plateau (type I) or a decrease (type IIA).
In this paper we describe absolute distance measurement through an optically dense transparent polymer, using a fibrebased, broadband interrogated, Fabry-Perot interferometer. The dual fibre sensor demonstrates a system that provides 'on-line' compensation for temperature and stress effects through recovery of refractive index information. Over a typical sensing range of 1mm distance measurements with sub-micron accuracy are reported.
Sensors based on optical fibre materials will be required to tolerate a restrictive combination of physical and environmental parameters for several remote monitoring applications at AWE. These include changes in atmospheric pressure, temperature, humidity, vibration, shock and acceleration, with the sensors being required to operate reliably for periods of up to 30 years with minimal intervention for maintenance. In addition, it is necessary that the sensors can function in the presence of ionising radiation. The sensors are being developed for a number of challenging in-situ physical and chemical measurements. These include remote gas composition analysis, monitoring shape change in compliant materials and the movement of metallic and polymeric components using sensors based on fibre Bragg gratings and interferometric techniques. Reliability issues include the long-term mechanical and optical performance of standard and novel glasses, optical fibres and cables, connectors, couplers, optical switches and Bragg gratings. The durability of materials used in the construction of fibre optic sensing components also requires to be assessed in addition to the epoxy and metallic coatings used to bond these components to a variety of material substrates.
We describe an optical system to monitor small long-term changes in the shape of a surface by using a network of optical fibre Bragg grating strain gauges, for applications in which space does not permit the use of techniques such as photogrammetry or structured light methods. Gratings are bonded to copper beryllium strips held under tension in contact with the test surface. The copper beryllium strips enable sufficient force to be transferred to the optical fibre from the compliant surface. Shape changes are revealed as strain changes in the sensor strips, inferred from wavelength shifts in the Bragg peaks. The optical signals are obtained in reflection by illuminating the sensor fibres with a broadband source and using a scanning Fabry-Perot filter to generate the spectrum with a wavelength resolution of 0.3 pm over the range 1530 to 1570 nm. Laboratory tests show that a strain resolution of 8 microstrain can be achieved with temperature compensation over the range 20 to 50 C, with a multiplexing capability of between 11 and 16 temperature - strain sensor pairs, depending on temperature gradients on the test surface. We present experimental measurements on a cylindrical test object subject to diametral loading, and show a comparison with a finite element model.
This paper describes a simple, low cost technique for producing a micro-oxygen sensor based on indicator chemistry, and results obtained using this device to monitor dissolved oxygen concentration are reported. The technology is based on the use of submicron optical fiber tips which have a fluorescent chemical reagent immobilized on their tip surface. Fiber tips were formed by drawing single-mode optical fiber in a fusion splicer. Sol-gel films doped with a fluorescent, oxygen sensitive chemical indicator were then deposited on the surface of the fiber tip using dip-coating techniques. The sensors that we have developed using these coating technologies are reversible and have response times of a few tenths of a second.
Over the last four years, the authors have been involved in research in the broad area of optoelectronic and optical fibre chemical and biomedical sensing. The research projects have differed quite considerably in their applications and the technologies
A simple, low cost technique is described for producing miniaturized fiber optic chemical sensors, and results obtained using these devices to monitor pH within the micro- chemical environment of individual mammalian cells (e.g. mouse embryonic fibroblasts) is reported. The technology is based on the use of submicron optical fiber tips which have fluorescent chemical or biological reagents immobilized on their tip surface. Fiber tips (0.1 - 1 micrometers ) were formed by drawing single-mode optical fiber (125 micrometers outer diameter) in a commercial fusion splicer. Fluorescent dye-doped sol- gel films and polymers were then deposited on the surface of the fiber tip using dip-coating techniques and photochemical synthesis respectively. Laser light from an argon ion laser was coupled into the fibers untapered end, and the fluorescence light equipped with a cooled photomultiplier detection system. The sensors that have been developed using these coating technologies are reversible and have response times of a few tenths of a second. We are currently expanding this sensing technique to monitor other biomedical parameters (e.g. oxygen concentration), by utilizing alternative chemical indicators that are compatible with the immobilization techniques described.
Miniaturized fiber optic sensor probes continue to be developed in a number of research laboratories around the world. Such fiber optic probes originate from work undertaken on the development of nanometric probes used in scanning near-field optical microscopes. The probes can be converted to sensors for highly localized chemical measurements by attaching the appropriate chemical indicators to the probe tips. We use the term 'micro- optrode' to describe fiber-optic chemical sensors possessing such high spatial resolution. This paper presents a brief summary of the progress that has taken place in the development of the micro-optrode and includes recent results obtained in our laboratory with such sensors.
The near-field interaction between an optical aperture of sub-micrometer diameter and a sample surface can be exploited to spectroscopically probe a biological medium with a resolution unattainable with traditional far-field techniques. A simple, low-cost technique is described for producing a chemical sensor which is based on the principles of near-field optics, and preliminary results obtained using this device to measure pH are reported. Singlemode optical fibers are drawn into sub-micrometer optical fiber tips and then coated with aluminum, to form a subwavelength aperture. By using laser illumination through this aperture, samples can be studied with a resolution better than the wavelength of light. Sub- micrometer pH sensors have been prepared by incorporating the pH sensitive dye fluorescein into a silica based sol-gel glass which is then coated in the form of a thin film onto the fiber tip surface. These sensors were used to monitor the pH of buffer solutions inside micron-size holes in a polycarbonate membrane, and to probe the intracellular environment of mouse embryonic fibroblast cells. It was found that the pH response is reversible in the range 3 - 10. In addition the design potential of the sol-gel process for optimization of the sensor by careful control of the coating thickness is demonstrated.
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