Temporal dynamics of Raman fibre lasers tend to have very complex nature, owing to great cavity lengths and high nonlinearity, being stochastic on short time scales and quasi-continuous on longer time scales. Generally fibre laser intensity dynamics is represented by one-dimensional time-series, which in case of quasi-continuous wave generation in Raman fibre lasers gives little insight into the processes underlying the operation of a laser. New methods of analysis and data representation could help to uncover the underlying physical processes, understand the dynamics or improve the performance of the system. Using intrinsic periodicity of laser radiation, one dimensional intensity time series of a Raman fibre laser was analysed over fast and slow variation time. This allowed to experimentally observe various spatio-temporal regimes of generation, such as laminar, turbulent, partial mode-lock, as well as transitions between them and identify the mechanisms responsible for the transitions. Great cavity length and high nonlinearity also make it difficult to achieve stable high repetition rate mode-locking in Raman fibre lasers. Using Faraday parametric instability in extremely simple linear cavity experimental configuration, a very high order harmonic mode-locking was achieved in ò.ò kmlong Raman fibre laser. The maximum achieved pulse repetition rate was 12 GHz, with 7.3 ps long Gaussian shaped pulses. There is a new type of random lasers – random distributed feedback Raman fibre laser, which temporal properties cannot be controlled by conventionalmode-locking or Q-switch techniques and mechanisms. By adjusting the pump configuration, a very stable pulsed operation of random distributed feedback Raman fibre laser was achieved. Pulse duration varied in the range from 50 to 200 μs depending on the pump power and the cavity length. Pulse repetition rate scaling on the parameters of the system was experimentally identified.

Laser beams have a vast range of applications, from optical drives for CDs and DVDs, to large scale laser cutting and drilling processes. Lasers have also been used in the past for surface modification of materials, and laser ablation mechanisms. This work focused on two regimes of laser power intensity, using a Nd:YAG nanosecond pulsed laser for lower power intensity and a picosecond pulsed diode pumped Yb-doped fibre laser for higher power intensity. Using the lower power intensity, studies were undertaken on the surface modification of PP and PEEK material, intended for increased adhesion strength of the materials bonded together in a lap shear configuration. Treated surfaces were examined by CLSM, contact angle analysis, FT-IR, XPS, and ToF-SIMS, and were tested in single lap shear tests. It was found that laser surface treatment improved the surface energy (44.9 mJ m-2 to 72.5 mJ m-2 in the case of PEEK and 32.5 mJ m-2 to 57.5 mJ m-2 in the case of PP) and wettability of the treated surfaces. This lead to improved adhesion strength in the lap shear tests. The higher power intensity provided by the picosecond pulsed laser was used to strip the active layers and coating materials from photovoltaic fibres and energy storage fibres with a copper core conductor, with the intention of exposing the copper for subsequent electrical interconnection. The treated samples were examined by focus variation microscopy, SEM, XPS and electrical continuity measurements. It was found that the coatings could be successfully stripped using a wavelength of 532 nm, pulse repetition rate of 100 kHz, 2 passes, a sample angle of 90°, and a scanning speed of 100 mm s-1. This exposed the copper conductor and maintained electrical continuity.

The work presented is part of a programme for the development of a high mean power pulsed carbon dioxide-nitrogen laser for machining and welding applications in materials used in engineering construction. The system seeks to overcome the power intensity limitations inherent in continuous wave operation which restricts performance, particularly in the case of metals. Herein, the discharge characteristics and the form of the output pulse are studied with the purpose of maximising the efficiency of the laser and optimising the energy coupled with the workpiece. The work is divided into four distinct sections: in the first, a short review is given of the elementary processes occurring in the laser which depend upon the structure of the carbon dioxide and nitrogen molecules present. Mechanisms of excitation are identified and the influence of the electron energy distribution on efficiency is considered using an analytical model. In the second section a suitable geometry for the electrodes is suggested and the optimisation of various physical parameters in order to obtain a uniform, stable glow discharge is considered. Experimental work on gas discharges is reported which relates the ratio of discharge voltage to pressure (which directly affects the vibrational excitation of gases) to the operating current, voltage, gas composition, etc. In the third section a theoretical model is developed relating the characteristics of the input and output for various gas mixture compositions. This theoretical model is based on the four level laser system first developed by Moore, et al ( Ref. [1] ,Chapter III) and subsequently used by various investigators. The fourth section describes the experimental work carried out to relate the input/output characteristics of the laser system, and to make possible a comparison between the experimental and theoretical results. The results are discussed and conclusions regarding the input/output characteristics are made in this section.

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Plastic optical fibres (POF) can be made sensitive to various parameters. Therefore, a successful implementation of tomographic imaging based on POF sensors will open the way to develop imagers for various parameters utilising this inexpensive sensor material. The work reported in this thesis is aiming to implement real time tomographic imaging based on the POF sensor. As the system uses light (photons) guided along the sensor to capture the information and deliver a signal at the periphery of the sensor, this technique is referred to as Photonic Guided Path Tomography (PGPT). As an initial stage of the work, we perform systematic testing of the sensitivity of POF transmission to bending. The results confirm that grooving can enhance this sensitivity of the POF sensor. They also provide an empirical proof of the anticipated changes in transmission loss by positive and negative bending for grooved POF (opening and closing of grooves respectively). We show that small positive bending increases the power loss, while for negative bending the power loss experiences a minimum, corresponding to the best achievable waveguiding recovery because of the closing of grooves. Beyond certain deformation, the sensitivity of the POF sensor is dominated by losses in the un-grooved regions. A bending test for the mechanical integrity of grooved POFs shows that repetitive bending to a small radius results in a quick deterioration depending on the depth of the groove. However for bending radii more than 20mm and groove depths up to 0.3mm, the lifetime of the POF sensor is still acceptable for many targeted sensor application.While it is clear that the grooved POF is more sensitive to small bending compared to the un-grooved POF, the latter offers uncompromised mechanical integrity and is more desirable when the highest possible sensitivity is not a priority. Therefore, in the final stage of the work, both grooved and un-grooved POF are considered as candidates for developing the tomographic imaging modality. Further detail is considered in view of the targeted deliverable, a real PGPT system for footstep imaging, based on POF sensors. This has been successfully achieved by designing, integrating and testing two different PGPT systems: one based on grooved POF with sensor head size of 0.9m x 0.9m incorporating a multiplexed photodetector and another based on un-grooved POF with sensor head size of 2m x 1m, incorporating independent POF sensors. Both are capable of performing the real time imaging task as well as storing the numerical data for alternative processing. The image reconstruction is by applying a median-filtered Landweber method to solve the inverse tomography problem. The frame rate achieved is 2Hz and 1Hz with spatial resolution estimated as 10cm and 3cm for the first and second system respectively

Mid-infrared semiconductor photonics is an emerging field with wide ranging applications. One stream of research is focused on extending the well-developed silicon-based waveguide platforms into longer wavelength regimes because of the inherent transparency window of silicon in the mid-infrared regime as well as its favourable nonlinear properties. Alternative approach is to investigate the optical properties of new materials (i.e. germanium) that offer favourable properties such as broader transparency windows and large nonlinearities, etc. In this thesis, two types of novel mid-infrared waveguide platforms were investigated. The first was the semiconductor optical fibres, an innovative platform that incorporates the functional semiconductors within the robust fibre geometry. A range of different core materials were characterised from the telecommunications band into the mid-infrared regime including polycrystalline silicon, hydrogenated amorphous silicon and hydrogenated amorphous germanium. Particularly, the large nonlinearity of the hydrogenated amorphous silicon core fibres was measured systematically cross this wavelength regime previously unknown for these fibres. With the knowledge of the key nonlinear parameters including nonlinear absorption and refraction, supercontinuum generation was demonstrated in the mid-infrared where the two-photon absorption was negligible. The measurements in the mid-infrared represent the first characterisation of the material beyond 1.55 μm. The second platform was the germanium on silicon waveguides, which can be fabricated using similar techniques to the silicon integrated waveguides and are thus compatible with the widely used complementary metal-oxide-semiconductor platform. The results presented in this thesis represent the first comprehensive linear and nonlinear transmission loss characterisations of this new class of waveguide for selected mid-infrared wavelengths. By exploiting the free carriers and two-photon absorption mechanisms, high speed all-optical modulation was demonstrated across selected mid-infrared wavelengths.

All-optical signal processing schemes are being studied as promising candidates for adoption in future optical transmission systems, where they are hoped to offer benefits such as ultra-fast signal processing, reduced energy consumption and in some cases, multi-channel processing, supporting the deployment of new techniques such as optical burst switching and software defined networks. The topic of this thesis is the all-optical phase and amplitude regeneration of complex signals using four-wave mixing (FWM). Many schemes for all-optical signal regeneration which have so far been demonstrated expose a signal to some undesirable concomitant distortion during regeneration, grossly limiting their practicability. Therefore, the work in this thesis focuses upon eliminating these undesirable effects and pursuing the development of regenerators possessing more ideal performance. To this end, an amplitude preserving phase regenerator is ?first demonstrated using a phase sensitive amplifier (PSA) which functions through the use of an additional phase harmonic beyond that commonly used. The conclusions of this are extended to show that, given a means to coherently add a large number of phase harmonics of a signal, the phase transfer function of a PSA may be tailored exactly as pleased using a method similar to Fourier analysis. Adoption of an exact solution to degenerate FWM allows for the demonstration of phase preservation in a saturated, pump-degenerate FWM-based amplitude regenerator, enabled by adopting a high pump to signal power ratio. Understanding of the phase noise processes present in this amplitude regenerator leads to an alternative scheme for phase preservation being demonstrated, which functions by predistorting the signal using optical nonlinearities, before amplitude squeezing. This technique is then combined with a novel, single stage, wavelength converting idler-free PSA, to realise a system which is capable of regenerating both the phase and amplitude of a signal, and, by making use of the conjugating nature of both stages, allows for the negation of nonlinearity induced phase distortion between the two stages to realise a system which is greater than the sum of its two parts.