The centrosymmetry of amorphous silica can be broken through the technique of thermal poling, whereby the application of an electric field at elevated temperatures introduces an effective second-order nonlinearity. Quasi-phase-matching is realised through periodic ultra-violet erasure of the uniformly induced nonlinearity. A truly all-fibre laser system operating in the visible was envisioned to replace the crystalline frequency doublers currently employed. Despite the lower second-order-nonlinearity in poled silica fibres, the longer interaction length for comparable acceptance bandwidths, the higher damage threshold and straightforward integration makes them a very attractive substitute. In this thesis, the routes taken to induce the maximum possible nonlinearity and the optimisation of the quasi-phase-matching technique to achieve the highest possible normalised conversion efficiency are reported. Periodic ultra-violet erasure for quasi-phase-matching allows scalability to longer interaction lengths in comparison to the photolithography technique previously employed. Further circular twin-hole fibres can be used instead of D-shaped fibres. A greater than 16 times improvement in the normalised conversion efficiency over previous results was obtained through the enhancement of key parameters of interaction length, stability and reproducibility. An average conversion efficiency of ~15% was demonstrated in a ~32cm long periodically poled fibre using a fibre laser source with just ~200W of peak power. The functionalities of all-fibre frequency converters have been extended to include broadband wavelength tunability of 45nm and chirped period poling for precise control of the acceptance bandwidth.

No description available.

Tunable diode laser absorption spectroscopy (TDLAS) is an optical gas sensing technique in which the emission frequency of a laser diode is tuned over a gas absorption line of interest. A fraction of the radiation is absorbed by the sample gas and this can be determined from measurements of initial intensity and the intensity transmitted through the sample. The amount of light absorbed is related to the gas concentration. Additional modulation techniques combined with phase sensitive detection allow detection of very low gas concentrations (several parts per million). The advantages of using TDLAS for trace gas sensing include; fast response times, high sensitivity and high target gas selectivity. However, the sensitivity of many practical TDLAS systems is limited by the formation of unintentional Fabry-Perot interference fringes in the optical path between the source and detector. The spacing between the maxima of these fringes, in particular those generated in gas cells, can be in the same wavelength range as Doppler and pressure-broadened molecular line widths. This can lead to (1) interference fringe signals being mistaken for gas absorption lines leading to false concentration measurements or (2) distortion or complete obscuring of the shape and strength of the absorption line, such that the sensitivity of the instrument is ultimately limited by the fringes. The interference fringe signals are sensitive to thermal and mechanical instabilities and therefore can not be removed by simple subtraction techniques. Methods that have been proposed by previous workers to reduce the effects of interference fringes include careful alignment of optical components and/or mechanically jittering the offending components. In general the alignment of the optical components is critical. This often leads to complex and fragile designs with tight tolerances on optical component alignment, and can therefore be difficult and expensive to maintain in field instruments. This thesis presents an alternative approach based on the deliberate use of diffusely scattering surfaces in gas cells as a means of eliminating spurious signals due to Fabry-Perot etalons. However, their use introduced laser speckle that contributed an intensity uncertainty to gas detection measurements. A methodology for investigating the laser speckle related intensity uncertainty has been developed and confirmed. The intensity uncertainty has been quantified for the different gas cell geometries employing diffusely scattering surfaces including integrating spheres. Methods for reducing the speckle related intensity uncertainty were also investigated and are presented. It has been shown that under the right circumstances robust gas cell designs that do not suffer from Fabry-Perot etalon effects and are relatively easy to align can be realised. The performance was found to be comparable to a conventional cell design (e.g. 3ppm detection limit for a 10cm standard cell and 11ppm for a 10cm diffusive cell). The technique could potentially simplify instrument design, thereby aiding the transfer of technology to industry.

The demodulation of a Wavelength Division Multiplexed FBG sensor array by a matching array of holograms hosted within a Volume Holographic (VH) material is considered within this thesis. The FBG sensor elements possess separate quiescent wavelengths and operate within different wavelength ranges. The edge of the transfer function of the demodulating holographic element is aligned with the operating range of the matching sensor element. The holographic element then diffracts a fraction of the sensor signal depending on its instantaneous wavelength. The signals from each of the sensor elements are also diffracted through separate angles to matching detectors so de-multiplexing the sensor array. A scheme using narrow bandwidth holographic transfer functions to demodulate a two element strain sensor array fabricated 4nm apart is reported. The transfer functions and the hysteresis within the PZT actuator, applying the strain, are represented mathematically and used to process results. These are compared with a normalised saw-tooth voltage waveform applied to the PZT to achieve a high Pearson correlation factor of 0.9992. The holograms however possessed poor diffraction efficiency <1% so severely degrading strain resolution. The crosstalk between the sensors’ channels is measured as -8.3dB. The demodulation scheme is intensity based so is susceptible to fluctuations in source intensity and fibre bend losses. An intensity reference scheme is therefore demonstrated using two holograms to demodulate a single FBG strain sensor. The sensor’s signal is divided by the two holograms and the intensity of the respective parts recorded on matched photo-detectors. Ratiometric detection is then used to identify changes in applied strain while disregarding fluctuations in source intensity and fibre bend losses. The standard difference over sum equation for ratiometric detection however is modified to take account of the respective holographic transfer functions.

Until recently, quantum photonic architecture comprised of large-scale (bulk) optical elements, leading to severe limitations in miniaturization, scalability and stability. The development of the first integrated quantum optical circuitry removes this bottleneck and allows realization of quantum optical schemes whose greatly increased capacity for circuit complexity is crucial to the progress of experimental quantum information science and the development of practical quantum technologies. Integrated quantum photonic circuits within Silica-on-Silicon waveguide chips were simulated, designed and tested. Hundreds of devices have been fabricated with the core components found to be robust and highly repeatable. Amongst these demonstrations, all the basic components required for quantum information applications are shown. The first integrated quantum metrology experiments are demonstrated by beating the standard quantum limit with two- and four-photon entangled states while providing the first re-configurable integrated quantum circuit capable of adaptively controlling levels of non-classical interference of photons. The tested integrated devices show no limitations to obtain high quality performances. It is reported near-unity visibility of two-photon non-classical interference and a Controlled-NOT gate that could in principle work in the fault tolerant regime. It is demonstrated the realization of a compiled version of Shors quantum factoring algorithm on an integrated waveguide chip. This demonstration serves as an illustration to the importance of using integrated optics for quantum optical experimentsThe first integrated optical circuits fabricated in the laser direct-write technology are reported in this Thesis. The quality quantum effects, together with a rapid turnaround process and the capability of writing complex 3D structures are promising for future quantum optical networks. The advent of integrated quantum photonics is necessary for the progression of quantum information science. The results reported in this Thesis provides fundamental building blocks from which future quantum devices will be constructed and presents high-fidelity quantum optics platforms for fundamental investigation

Vision occupies a prime position in how we understand and respond to our immediate environment, and it is because this that it is essential that advanced wearable devices incorporate some sort of visual competence. The importance of wearable visual sensors is not just in the wide spectrum of information that they can recover such as world structure, object properties or human activities, but also because of its unique position. Computational power required to process visual signals is also becoming less incompatible with wearable cpu's and therefore, a number of applications are appearing possible. Wearables have mostly viewed the world through conventional, narrow view, passive cameras designed for non-wearable applications. Although ideal for recovering ambient information such as lighting levels, colour histograms and so on, for more speci c tasks the resulting imagery is too dependent on the wearer's exact posture and motions. The aim of this thesis has been the advancement of wearable vision, and to do so it marries techniques from active vision with wearable computing to produce a novel human-computer interaction interface. This results in the ability to decouple the wearer's motion and attention from that of the visual sensor and our experience is that this is not only feasible but desirable. By gaining a degree of independence from the wearer, the visual sensor can work in one of three set of reference frames. The first set relates to the wearer's surrounding, for example, sensing the manipulative space in front of wearer's chest, or pointing where the wearer's head is attending. The second set includes frames aligned with the static surroundings, for example the camera becomes aligned with the horizon; and third is the set of frames attached to independent moving objects, particularly relevant to object tracking. The thesis addresses a number of important issues for visual wearables, such as the how to select sensor placement around the person, and the kind of sensor to use. The design and implementation is developed from objective methodologies and the hardware presented in repeatable detail. Several core wearable visual tasks such as camera stabilization, object tracking, 3D localisation and mapping and gesture identification are considered, alongside discussions of potential applications and future work.

A new monolithic semi-insulating (SI) GaAs sensor design for X-ray imaging applications between 10-100keV has been proposed. Monolithic pixel detectors offer a number of advantages over hybrid bump-bonded detectors, such as high device yield, low costs and are easier to produce large scale arrays. In this thesis, an investigation is made of the use of a SI GaAs wafer as both a detector element and substrate for the epitaxially grown High Electron Mobility Transistors (HEMTs). The design of the HEMT transistors, optimised for this application, were produced with the aid of the Silvaco' Virtual Wafer Fab' simulation package. It was determined that the device characteristics would consist of a small positive threshold voltage, a low off-state drain current and high transconductance. The final HEMT transistor design, that would be integrated to a pixel detector, had a threshold voltage of 0.17V, an off-state leakage current of ~ 1nA and a transconductance of 7.4mS. A number of test detectors were characterised using an ion beam induced charge technique. Charge collection efficiency maps of the test detectors were produced to determine their quality as a X-ray detection material. Prom the results, the inhomogeneity of SI GaAs, homogeneity of epitaxial GaAs and granular nature of polycrystalline GaAs, were observed. The best of these detectors was used in conjunction with a commercial field effect transistor to produce a hybrid device. The charge switching nature of the hybrid device was shown and a sensitivity of 0.44?C/?Gy mm2, for a detector bias of 60V, was found. The functionality of the hybrid sensor was the same to that proposed for the monolithic sensor. The fabrication of the monolithic sensor, with an integrated HEMT transistor and external capacitor, was achieved. To reach the next stage of producing a monolithic sensor that integrates charge, requires further work in the design and the fabrication process.

Much progress has been made in the development of active silicon opto-electronic devices over the last 15 years. This is primarily due to the widely accepted belief that the free carrier effect is the most efficient optical modulation and switching mechanism in silicon, along with the potential advantages of combining optical and electronic devices onto a single silicon substrate rather than using discrete components. A study of the scientific literature shows that whilst numerous devices have been reported, few have been seriously optimised. In the literature, devices have consisted primarily of two or three terminal devices based around a rib waveguide. The three terminal devices are fewer in number but generally perform better. Conversely, two terminal devices have received a little more attention in terms of producing faster devices. Therefore, this work provides an in depth analysis of the performance of p+-i-n+ diodes when configured as optical modulators, with the aim of improving both the device DC and transient performance characteristics. The primary DC performance characteristic is the current required to achieve a given phase change and the transient performance characteristics are measured in terms of the device rise and fall times. These characteristics have been studied with variations in geometrical and fabrication based parameters such as the position and doping concentration of the contacts, the aspect ratio of the rib waveguides, and the overall dimensions. The key result from the modelling is that the most efficient multi-micron size device is a three terminal device with high doping concentration, constant doping profiles and large diffusion depth doped regions located close to the rib edge. A theoretical device of this nature required a current of only 2.7mA for a ? radian phase shift with rise and fall times of 22ns and 2ns respectively. The best previously achieved was a device which theoretically required 4mA for a ? radian phase shift. Additionally, by including isolation trenches on either side of the doped regions the DC performance characteristics can be further improved by up to 74%. There are also advantages in reducing the dimensions of the devices to 1 micron or less. At these dimensions the DC and transient performance characteristics are improved by more than a further order of magnitude, hence requiring fractions of 1mA for a ? radian phase shift. Two of the most promising designs have been fabricated and experimentally analysed. Due to fabrication constraints the most efficient device was not fabricated. However, both two and three terminal devices were fabricated. The best device tested experimentally was a three terminal device that required a current of 14mA for a ? phase shift. The modelling and experimental results agree well therefore validating the modelling. Therefore we can be confident that the additional theoretical results for devices that could not be fabricated are reliable, and hence significant further improvements could be made by fabricating these devices. Likely roles for these types of devices are medium bandwidth modulators/switches and a variety of sensor applications.

Bio and optoelectronics systems are different systems in their composition and application but they have surprising similarities in terms of their spatio-temporal dynamics and dynamics control. Both systems are complex systems with many degrees of freedom that are spatially structured on micro and nano scales. Their behaviour can easily switch between stable regime to chaos by changing internal and external influencing factors. Historically, optoelectronics systems have been studied for more than 30 years. Due to novel biotechnological applications bio systems have more recently gained importance. Today, many natural and technological systems are composed of mesoscopic bio and optoelectronic devices or elements. In nature, many biological and chemical processes controlling the functioning of a system or a specific process (e.g. photosynthesis) occur in complex cellular and molecular ensembles. In technological systems, improvements in the development and design of novel materials on an atomic level have opened the way to efficient devices suitable for mass production. This work focuses on the control and analysis of two mesoscopic model examples: molecular motors and semiconductor lasers. In recent years they have been of particular importance due to their complex dynamics. These active systems have many features in common. Both are open systems which use an energy source to generate motion or light. Molecular motors are nanometer-scaled functional biomolecular structures that convert chemical energy into directed motion. Semiconductor lasers convert an electrical pump current into coherent light emission. Due to underlying complex interaction processes both systems show instabilities under the influence of parameters. In a molecular motor ensemble, external forces, diffusion of free and bound motors, and geometrical arrangement of the microtubule substrate tend to bring instabilities into the molecular motor system. In semiconductor lasers, the interplay of carrier diffusion, light diffraction and geometrical constraints given by the width of the laser (typically in the ?m regime) lead to complex interplay of longitudinal and transverse modes and to instabilities and chaos. The aim of this work is to fundamentally analyse the mechanism relevant for the complex dynamics of the two active systems and to derive schemes to control them. For this purpose I have performed complex numerical simulations. The theoretical description of the molecular motor systems is based on Fokker-Planck equations. Space-time simulations of the dynamics of molecular motors reveal the influence of the diffusion constant, arrangement of filaments, number and separation between filaments and external forces on the spatio-temporal dynamics of molecular ensemble. The theoretical analysis of semiconductor lasers is done on the basis of multi-mode Maxwell-Bloch equations. Simulations of the spatio-temporal dynamics in this system demonstrate that the application of a delayed optical feedback realised by a suitable external resonator configuration can lead to an efficient emission control. Various feedback configurations are discussed. The thesis is organised as follows. Chapter 1 gives a general introduction to molecular motors and semiconductor lasers. Chapter 2 focuses on molecular motors. This includes a review of theoretical models, molecular motor properties and simulation results on the complex spatio-temporal dynamics of molecular motor complexes. In Chapter 3 of this thesis, broad area semiconductor lasers with delayed optical feedback will be investigated. The analysis will concentrate on laser systems with unstructured and structured delayed optical feedback. Concluding remarks and future work are given in Chapter 4.

In this dissertation, we explore the optical properties of various opal-based photonic crystal structures. Particular attention is paid to the tunability of the optical properties, especially the photonic band gaps (PBG), with the motivation to apply these opal-based photonic crystals (PCs) to the design of functional surfaces and switchable windows. After reviewing the basic optical properties of inverse opals, two different types of opal-based photonic crystals, namely the Double-Shell Photonic Crystal (DSPC) and the Double-Inverse-Opal Photonic Crystal (DIOPC) are successively introduced and throroughly studied. In the DSPC structure, each sphere in the periodic photonic crystal structure is made of a hollow core, along with an accompanying shell of a different dielectric material; the resulting spherical structure is embedded in a high-index dielectric background. By contrast, the DIOPC is designed with an inverse opal backbone, in which the air pores are partially filled with a dielectric core sphere. In our work, two types of photonic bandgap tuning are examined, namely geometrical/positional, and tailoring of the materials' properties. A comparison of the proposed structures regarding their potential for experimental realization is also performed. Considering that the air shells in the DIOPC structure allow for relative movement of the dielectric cores inside them, we propose and study a completely novel approach to obtain a switchable complete band gap achieved, by shifting the spheres inside the air shells. After demonstrating that the complete photonic band gap is open for certain core sphere positions and is closed for others, we propose ways to optimise this new switching process. This optimisation leads to a maximum switching of the complete photonic band gap of 3.5%.Taking into account that in real-world situations, all photonic crystals possess a certain amount of structural imperfections, the last step of the study concerns the effects of disorder on the optical properties of opal-based photonic crystals. After discussing different types of disorder, in both the backbone and the core spheres, we conclude that to retain a complete PEG in the DIOPC structure, and thus the switching process, a maximum backbone disorder of 1 % should be reached experimentally, whereas the disorder on the core sphere does not affect the PEG as much. These investigations have been performed using both a plane-wave expansion method and a finite-difference time-domain method.