No description available.

No description available.

No description available.

This thesis presents an investigation into the application of optical fibre sensors to a tomographic imaging system for use with gas/water mixtures. Several sensing techniques for measurement of two component flow using non-intrusive techniques are discussed and their relevance to tomographic applications considered. Optical systems are shown to be worthy of investigation. The interaction between a collimated beam of light and a spherical bubble is described. Modelling of different arrangements of projections of optical sensing arrays is carried out to predict the expected sensor output voltage profiles due to different flow regimes represented by four models. The four flow models investigated are: a single pixel flow, two pixels flow, half flow and full flow models. The response of the sensors is based on three models: optical path length, optical attenuation and a combination of optical attenuation model and signal conditioning. In the optical path length model, opaque solids or small bubbles, which are conveyed, may totally or partially interrupt the optical beams within the sensing volume. In the optical attenuation model, the Lambert-Beer's Law is applied to model optical attenuation due to the different optical densities of the fluids being conveyed. The combination of optical attenuation model and signal conditioning is designed to improve the visual contrast of the tomograms compared with those based on the optical attenuation model. Layergram back-projection (LYGBP) is used to reconstruct the image. A hybrid reconstruction algorithm combining knowledge of sensors reading zero flow with LYGBP is tested and shown to improve the image reconstruction. The combination of a two orthogonal and two rectilinear projections system based on optical fibres is used to obtain the concentration profiles and velocity of gas bubbles in a vertical column. The optical fibre lens is modelled to determine the relationships between fibre parameters and collimation of light into the receiver circuit. Modelling of the flow pipe is also carried out to investigate which method of mounting the fibres minimises refraction of the collimated light entering the pipe and the measurement cross-section. The preparation of the ends of the optical fibre and design of the electronics, which process the tomographic data, are described. Concentration profiles obtained from experiments on small bubbles and large bubbles flowing in a hydraulic conveyor are presented. Concentration profiles are generated using the hybrid reconstruction algorithm. The optical tomographic system is shown to be sensitive to small bubbles in water of diameter 1-10 mm and volumetric flow rates up to 1 1/min, and large bubbles in water of diameter 15-20 mm and volumetric flow rates up to 3 1/min. Velocity measurements are obtained directly from cross correlation of upstream and downstream sensors' signals as well as from upstream and downstream pixel concentration values. Suggestions for further work on optical tomographic measurements are made.

The measurement of humidity is rapidly increasing in importance in physical, chemical and biological processes as industry attempts to improve quality and production rates by better control of relative humidity (RH). A novel optical fibre humidity sensor suitable for use in an optical fibre multi-point distributed sensing system has been developed, thus for the first time allowing multipoint measurements of humidity along an optical fibre which can be distributed within an industrial plant or machine without the need for electrical power supplies. The novel point sensor is based on the principle of using the absorption spectrum of a colorimetric reagent (cobalt chloride) immobilised on the surface of the core of a multimode optical fibre by employing a thin gelatin film. The single point sensor has an insertion loss of less than 0.2 dB. Two-wavelength detection is employed to provide referencing to eliminate common mode intensity variation. The basic design, construction and testing of an experimental sensor in the humidity region 20 to 80% RH and temperature range 25 to 50 °C is described. Resolution and Repeatability can be better than 2% RH with a time constant of 0.5 second. A two-wavelength optical time domain reflectometer (OTDR) has been developed and a four-sensor network has been built. The resolution of measurement on the network can be better than 4% RH, but because of noise in the OTDR the resolution value is dependent on averaging time in the instrument.

No description available.

No description available.

This thesis describes a detailed study of advanced fibre optic sensors and their applications for label-free biochemical detection. The major contributions presented in this thesis are summarised below. A self-assembly based in-situ layer-by-layer (i-LbL) or multilayer deposition technique has been developed to deposit the 2D material nanosheets on cylindrical fibre devices. This deposition technique is based on the chemical bonding associated with the physical adsorption, securing high-quality 2D materials coating on specific fibre cylindrical surface with strong adhesion as well as a prospective thickness control. Then a " Photonic-nano-bio configuration", which is bioprobes immobilised 2D-(nano)material deposited fibre grating, was built. 2D material overlay provides a remarkable analytical platform for bio-affinity binding interface due to its exceptional optical and biochemical properties. EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) and NHS (NHydroxysuccinimide) were used to immobilise bioprobes. This kind of configuration is considered to have many advantages such as: enhanced RI sensitivity, enrich immobilisation sites, improved binding efficiency, selective detection. Followed by this configuration, several label-free biosensors were developed. For example, graphene oxide coated dual-peak long period grating (GO-dLPG) based immunosensor has been implemented for ultrasensitive detection of antibody/antigen interaction. The GO-LPG based biosensor has been developed for label-free haemoglobin detection. Apart from biosensors, the black phosphorus (BP) integrated tilted fibre grating (TFG) has been proposed, for the first time, as BP-fibre optic chemical sensor for heavy metal (Pb2+ ions) detection, demonstrating ultrahigh sensitivity, lower limit of detection and wider concentration range. Ultrafast laser micromachining technology has been employed to fabricate long period grating (LPG) and microstructures on optical fibre. The ultrafast laser micromachined polymer optical fibre Bragg grating (POFBG) has been developed for humidity sensing, showing the significant improvement with the reduced response time.

Freestanding self-written polymer waveguides were fabricated between two single mode fibres by using free radical photopolymerization technique. Photo curing enables modification of the initial liquid monomer at the illuminated part of the photopolymerizable system to form a permanent solid polymer waveguide between the cores of the fibres. Three compound acrylate based monomer (PETA) and Norland optical adhesives (NOA) were used to fabricate polymer bridges. The two systems are cured by visible and UV respectively. The optical, mechanical and nonlinearity properties of freestanding self-written polymer waveguides were investigated. Polymer waveguides up to 600 μm long between two fibres were fabricated by unidirectional illumination. The optical power required for photopolymerization is sufficiently low that it is possible to use incoherent light source instead of laser, which also allows investigation of the optical transmission over a wide range of wavelengths from visible to NIR. Optical characterization showed that PETA waveguides have better optical transmission and insertion loss of about 1.2 dB was measured for the bridges up to 600 μm long at 1550nm. Some limitations prevented us to extend waveguide’s length beyond 600 μm, the bridges also showed poor adhesion quality particularly at the polymer/fibre interfaces which needed an action. In order to improve mechanical properties of polymer waveguides also to overcome bonding failure between fibre and polymer, the fibres were treated with adhesion promoter. The treatment together with bidirectional curing from both fibres improved mechanical and optical properties of the polymer waveguides. The insertion loss was reduced to 0.6 dB associated with bi-directionally cured 600 μm long waveguide. Nonlinear response of polymer waveguides was measured by coupling a high power ultra short pulse laser. Using bidirectional curing and inserting fibre ends into capillaries millimeter long waveguides with minimum loss of 1.1dB were fabricated. A long interaction length of polymer waveguide allows spectral broadening and self-phase modulation features to occur in response to the high power laser propagation through the polymer bridge. The spectral broadening in polymer waveguide was much broader than that of 1.5m plain fibre. The comparison of the results associated with maximum phase shift occurs in polymer with plain fibre revealed that the nonlinear coefficient of polymer material is about 1000 times larger than that of silica fibre.

The main topic of this thesis is realization and characterization of fibre Bragg gratings. A novel versatile grating fabrication technique is developed and a number of gratings are realized, showing the potential of the system. Arbitrarily-shaped gratings are sequentially imprinted in the fibre by a moving interference pattern created with a continuous-wave ultraviolet (UV) source. This scheme allows for a very good control and stability of the grating shape, which is also shown experimentally. As opposed to most other present fabrication techniques, the proposed method offers a total control over the grating parameters by software, enabling simple implementation of new designs. Different kinds of error sources when stitching long gratings are identified and investigated regarding impact on the final grating result. Another important question within this field is how to characterize gratings. We propose a new characterization method based on optical low-coherence reflectometry (OLCR). A new interferometer design allows for simple simultaneous detection of the reflection response from two different points in the interrogated grating, so that differential measurements can be performed. The advantage of this is that the sensitivity to noise caused by e.g. thermal fluctuations in the system is substantially reduced. Several test gratings have been investigated and a very good agreement to the expected results is noted. A second characterization technique using interferometric detection of the side diffraction from the grating under test is investigated both theoretically and experimentally. With aid of two-dimensional theory for wave propagation, it is shown that there is a linear relation between the detected phase and modulation depth and the corresponding grating properties. The technique is evaluated with a novel scheme of implementation where the UV source provided in a fabrication setup is used assource for the side probe. This approach results in a very simple implementation and opens for an integration of the characterization and fabrication systems. Finally, a tuning method for transmission filters based on local heating of linearly chirped fibre Bragg gratings is analysed and further developed to allow for fully software-controlled operation. The potential of this technique is illustrated by some promising initial experimental results. / QC 20100812