This thesis describes research into non-linear optical effects in monomode fibres for sensing and signal processing schemes. The work is initially placed into the context of relevant fibre sensor applications for aerospace. Candidate non-linear processes are then described and Stimulated Brillouin Scattering (SBS) targeted as a suitable mechanism. The SBS process in monomode fibres is then fully described, including a theoretical assessment of the magnitude of both pump and Stokes signals with fibre length. The project developed, firstly, an all-optical frequency shifter using SBS which generated output carriers in the 1-70OMHz region, depending on fibre type and system topology. In particular, a single-fibre system was demonstrated for the first time. Limitations of the technique arose through fibre photosensitivity and non-linear dynamical effects which, respectively, increased the input pump power requirements and introduced intensity and frequency instabilities into the output signal. Both processes were investigated fully. Optical fibre and pump characteristics, and system configurations, were therefore identified which minimised their impact. The same SBS heterodyning principles were then used to investigate single-pump sensor systems. Both 'quasi-point' and two-element, multiplexed temperature sensors were demonstrated. A linear relationship between the Stokes frequency and temperature was obtained from 5 to 55°C, determined primarily by the thermal response of the acoustic phonon velocity. The sensing resolution was typically ±2°C, limited by the non-linear dynamical effects, and the spatial resolution was determined by the pump/fibre interaction length. One unique sensing element per optical input was demonstrated. These results are then placed in the context of systems utilising alternative SBS-based strategies, other non-linear effects and fibre ring resonators. Management elements of the project were covered by detailing both the technical and strategic motivations for undertaking the research and, by describing the issues on which a cost benefit analysis of fibre sensors for aircraft can be undertaken.

This research into the feasibility of imaging particulate processes using electrical charge tomography investigates four techniques: the multi-sensing of electrical charge in a cross-section, a neural network based classifier for flow regime identification, cross correlation based velocity determination and spectral analysis of electrodynamic signals. A single charged-particle model is developed to simulate the induction effect on a sensor by a charge. The spatial representation of the voltage induced onto sixteen sensors, placed on the boundary of a circular pipe, gives a flow distribution profile over the cross-section. A two charged-particle model is developed to simulate the electrodynamic effect of two particles on a tomographic sensor configuration. As in the single particle model, a spatial representation of the voltages induced onto the sensors is presented. This voltage profile is due to the combined effects of position and charge of the two particles. A multi-particle model is developed to predict the voltage profile of several flow regimes: full, annular, core, half and stratified. The model is extended to provide the loading and concentration of a given flow. A measurement system is constructed consisting of sixteen sensors equally spaced around the boundary of a circular 100mm pipe. Measurements on a bead drop system are designed to verify the single particle model. A sand flow system, consisting mainly of 300 micron sized particles, is used for measurements of the induced voltages due to different flow regimes. The latter are created artificially by using baffles of different shapes that obstruct the sand flow. The voltage profile from the sixteen sensors gives spatial information about the flow regime. These voltage profiles are normalised into patterns that are presented to a Kohonen neural network for classification. Two regime classification between well differentiated regimes gives an accuracy of identification of 95%. This is expanded to provide classification of three regimes with more variability in the input patterns giving success rates between 50% to 70%. A power spectral density analysis of the measured electrodynamic signals gives observable features for particle characterisation during flow. In full flow, with no baffles obstructing the sand flow, a consistently high frequency spectra of 550Hz is observed. At flow rates above 0.540 kgs-1, the frequency spectra shifts to a lower range of 200Hz. In obstructed flow, such as in stratified regime, an inhomogeneous phase is inferred from the drop in frequency of the power spectra at relatively low flowrates (0.36kgs-1). These results suggest a relationship between the observed spectra and the phenomenon of clustering of particles at higher concentrations. The potential of electrodynamic spectroscopy for particle characterisation in terms of size distribution is discussed. Knowledge of flow regime voltage profile, regime identification and concentration provided a basis for an empirically based image reconstruction algorithm. Finally the achievements of the thesis are discussed and suggestions made for further work.

An ultrasonic imaging technique was to be developed for sizing and characterization of defects in thick sections of steel. The problems generally found with the existing techniques in such applications are low speed, inadequate image quality, large size and high cost.

No description available.

No description available.

The modification of the optical properties of quantum well structures by post growth thermal diffusion techniques is important for the fabrication and integration of quantum well devices for optoelectronic applications. This thesis is concerned with the characterisation of as-grown and thermally interdiffused quantum well structures using room temperature photoreflectance. In particular, all the interband transitions (symmetry 'allowed' and 'forbidden') in the subband of four AlGaAs/GaAs single quantum well structures were determined using photoreflectance. The identification of the transitions was complicated by the presence of Franz-Keldysh oscillations in all photoreflectance spectra which were associated with an interface built-in electric field in the as-grown structures. The source and magnitude of the field on either side of the interface was determined after the sequential etching and photoreflectance analysis of these structures. From the spectra of the etched structures, optical interference effects were found which enabled the layer thicknesses in these structures to be determined. High temperature rapid thermal annealing of these structures resulted in interdiffusion across the well barrier interfaces of the quantum well structures which modified the subband structure, and therefore the optical properties, of the quantum well. This annealing was found to produce limited interdiffusion by correlating the relative 'blue shifts' of the interband transitions with those of a theoretical model. Good agreement with the model was obtained, which allowed temperature dependent interdiffusion co-efficients and an activation energy to be determined. The effects of oxygen ion implantation followed by annealing was also studied and found to significantly enhance the extent of the interdiffusion. In addition, interband transitions of heavy-hole and light-hole character were distinguished from polarisation studies while temporal studies resulted in the identification of unintentional impurities and the determination of photocarrier trap times in the as-grown and implanted structures. The work presented here demonstrates the versatility of photoreflectance for the room temperature characterisation of as-grown and thermally processed quantum well structures for optoelectronic devices, prior to fabrication.

This thesis reports the effects of chromium layers on the vibrational properties of silicon microengineered structures designed as pressure sensors. Both excitation and interrogation have been achieved by optical means. Particular attention has been paid to the optically induced vibrational amplitude, Q factor and phase angle between the motion of the resonator and the incident pulsed laser light used to excite the resonator. Two silicon structures have been investigated; bridges fabricated at Birmingham University and extensively characterised by researchers at Strathclyde University, and devices fabricated at STC Technology Ltd. in Harlow. For both structures the addition of chromium layers onto the surface has had the effect of increasing the value of the amplitude divided by the Q factor. This increase occurs after a layer of chromium of about 15nm thickness has already been deposited onto the silicon. It has also been found that the value of the Q factor of the resonators has decreased with the addition of chromium layers. The phase angle of the motion has also been found to be sensitive to the thickness of the chromium layer. This angle has been observed to increase from approximately zero for bare silicon to about 40° for a few hundred Angstroms, returning towards zero phase for chromium layers of comparable thickness to the silicon thickness.

Time Domain Reflectometry Techniques are applied to measurements in optical fibres. The study covers broadly two areas of interest. Firstly, the Backscatter Technique for attenuation measurements is considered. The resolution requirements of instrumentation are studied and a new 2-channel approach to backscatter waveform analysis is proposed. An optimum operating strategy is described such that the sensitivity and range are maximized. The fundamental accuracy of the backscatter measurement is determined by a comparison with the more-standard cutback technique over a wide spectral range and variations in OH- impurity along the length of a fibre are tracked by measurements at different wavelengths. The effects on the backscattered power of variations in longitudinal fibre parameters are also demonstrated. In particular, backscatter-loss signatures are presented which clearly show correlation with programmed fibre defects and indicate that in many cases diameter variations are the cause of the previously unidentified features in some backscatter waveforms. In addition, the backscatter method is used for the first time to track the state of polarization of a pulse propagating in a monomode fibre. The second application of Time Domain Reflectometry is to the assessment of pulse delay stability against variations in temperature and external stresses. Data from measurements on unjacketed fibres is used to analyse the time delay variations found in jacketed fibres. It is shown that the application of a close-fitting plastic jacket results in a level of residual compressive stress in the fibre and the delay stability is considerably degraded. This may also be influenced by environmental factors.

An optophone is a device that turns light into sound for the benefit of blind people. The present project is intended to produce a general-purpose optophone to be worn on the head about the house and in the street, to give the wearer a detailed description in sound of the'scene he is facing. The device will therefore consist'of an'electronic camera, some signal-processing electronics, earphones`, and a battery. The two major problems are the derivation of (a) the most suitable mapping from images to sounds, and (b) an algorithm to perform the mapping in real'time on existing electronic components. This thesis concerns problem (a). Chapter 2 goes into the general scene-to-sound mapping problem in some detail'and presents the work of earlier investigators. Chapter 3 1- discusses the design of tests to evaluate the performance of candidate mappings. A theoretical performance test (TPT) is derived. Chapter 4 applies the TPT to the most obvious mapping, the cartesian piano transform. Chapter 5 applies the TPT to a mapping based on the cosine transform. Chapter 6 attempts to derive a mapping by principal component analysis, using the inaccuracies of human sight and hearing and the statistical properties of real scenes and sounds. Chapter 7 presents a complete scheme, implemented in software, for representing digitised colour scenes by audible digitised stereo sound. Chapter 8 tries to decide how'many numbers are required to specify a steady spectrum with no noticeable degradation. Chapter 9 looks'at a scheme designed to produce more natural-sounding sounds related to more meaningful portions of the scene. This scheme maps windows in the scene to steady spectral patterns of short duration, the location of the window being conveyed by simulated free-field listening. Chapter 10 gives detailed recommendations as to further work.

In this thesis, several aspects of the optical properties of intersubband semiconductor lasers are studied theoretically, including the waveguiding properties of quantum cascade lasers (QCLs), the anticipated modulation bandwidth, gain and threshold current of intersubband lasers, and the engineering of nonlinear susceptibilities in intersubband quantum well structures. Using two computational solvers, for the Helmholtz and Schrödinger Equations respectively, optical waveguide structures and multi quantum well (MQW) structures are designed for subsequent research. The waveguide design of a QCL reported by the Bell Labs reseachers is analysed and improved upon. A four level rate equation model was used to obtain the population inversion condition and modulation response for a triple quantum well structure (TQW) designed for intersubband lasing. An analytical expression for the modulation response is first obtained, followed by a numerical computation to verify the results. It is demonstrated that there is a unique dependence of the modulation bandwidth upon the output power of the laser, and that the maximum modulation frequency does not increase monotonically with optical output power as is the case with conventional semiconductor lasers. An expression describing the optical gain of intersubband lasers is also derived. Using this, investigations into the predicted achievable gain in mid-infrared (MIR) and near-infrared (NIR) intersubband lasers are conducted. It is found that the NIR gain is at least an order of magnitude higher than that of the MIR case. Self-consistent calculations of the optical gain are also undertaken, where the rate equations and the optical gain equations are solved alternately. An intersubband structure is designed for both triple harmonic generation (THG) and four-wave mixing (FWM). The third order nonlinear susceptibilities of these respective processes in the structure were calculated and found to be comparable to those of structures designed for just one process.