Polarisation Spectroscopy (PS) has been used as a novel approach for measuring the collisional removal of bulk rotational orientation and alignment in OH (X 2Π3/2, v = 0, j = 1.5 – 6.5, e) and OH (A 2Σ+, v = 1, N = 1 – 5, f1). Both one-colour degenerate PS (OCPS) and two-colour PS (TCPS) have been exploited. TCPS provides a route to measuring thermal rate constants (298 K) for the collisional evolution of rotational polarisations in unique rotational quantum states. For OH (X), the dependence on the decay of the PS signal was investigated with a number of collision partners: He, Ar, Xe N2 and O2. The ability to remove PS signals increases across the series of noble gases He < Ar < Xe. In all cases the measured rate constant for loss of alignment is larger than that for orientation. This provides conclusive evidence that elastic depolarisation (the elastic redistribution of mj-sublevels) contributes to the loss of PS signals. The efficiency of this contribution is found to be modest for He, but significant for Ar and Xe. Comparison of the PS measurements with quantum scattering calculations assists in the evaluation of the magnitude of elastic depolarisation for the atomic partners and also provides a rigorous test of the potential energy surfaces describing their interaction. Intriguing differences are found between the kinematically similar N2 and O2. A detailed cross comparison between colliders, comparison of the measurements to the potential energy surface and independent theoretical work suggest that the weaker long-range attractive forces play a significant role in elastic depolarisation. The dynamics of OH (A) was measured with He and Ar only. PS experiments were compared to independent experimental and theoretical work carried out in parallel to this research. Distinctively different j-dependent trends for the removal of bulk rotational polarisations are observed for OH (X) + Ar and OH (A) + Ar. This comparative study emphasises the role of the attractive limbs of the potential energy surface in mediating elastic depolarisation.

A source-demand in the near- and mid-IR wavelength spectrum exists for various applications such as waveguide inscription, multiphoton imaging, and nonlinear spectroscopy. All of the applications seek for higher repetitions rates for faster processing speed, better signal to noise ratios or to improve the results for applications like laser waveguide inscription. This is in contrast to the high pulse energies, required to drive the nonlinear processes involved with these applications. Available systems are either based on low-energy, high-repetition-rate optical parametric oscillators or high-energy, low-repetition-rate optical parametric amplifiers. In this thesis a sources was developed that can bridge the wide gap between these two extremes, providing sufficient energy to drive nonlinear processes, with repetition rates in the MHz domain. This was achieved by introducing three techniques previously employed for energy scaling in laser cavities. Firstly an exchange from the conventionally used Ti:sapphire pump to a commercial high power Yb:fibre laser system readily scaled the usable pump energy. This was combined with a technique known as cavity-length extension, which allows a lowering of the cavity roundtrip time offering the build-up of pulses with increased energy. In a final stage, cavity-dumping on basis of an acousto-optic modulator was introduced into the a redesigned cavity. The combination of these three techniques, novel to synchronously pumped optical parametric oscillators, enabled the extraction of record-high pulse energies and peak powers

This thesis presents a study on the use of low temperature co-fired ceramic (LTCC) material as a new platform for the packaging of multiple broad area single emitter diode lasers. This will address the recent trend in the laser industry of combining multiple laser diodes in a common package to reach the beam brightness and power required for pumping fibre lasers and for direct-diode industrial applications, such as welding, cutting, and etching. Packages based on multiple single emitters offer advantages over those derived from monolithic diode bars such as higher brightness, negligible thermal crosstalk between neighbouring emitters and protection against cascading failed emitters. In addition, insulated sub-mounted laser diodes based on telecommunication standards are preferred to diode bars and stacks because of the degree of assembly automation, and improved lifetime. At present, lasers are packaged on Cu or CuW platforms, whose high thermal conductivities allow an efficient passive cooling. However, as the number of emitters per package increases and improvements in the laser technology enable higher output power, the passive cooling will become insufficient. To overcome this problem, a LTCC platform capable of actively removing the heat generated by the lasers through impingement jet cooling was developed. It was provided with an internal water manifold capable to impinge water at 0.15 lmin-1 flow rate on the back surface of each laser with a variation of less than 2 °C in the temperature between the diodes. The thermal impedance of 2.7°C/W obtained allows the LTCC structure to cool the latest commercial broad area single emitter diode lasers which deliver up to 13 W of optical power. Commonly, the emitters are placed in a “staircase” formation to stack the emitters in the fast-axis, maintaining the brightness of the diode lasers. However, due to technical difficulties of machining the LTCC structure with a staircase-shaped face, a novel out-plane beam shaping method was proposed to obtain an elegant and compact free space combination of the laser beam on board using inexpensive optics. A compact arrangement was obtained using aligned folding mirrors, which stacked the beams on top of each other in the fast direction with the minimum dead space.

This thesis addresses the strong need for efficient and compact techniques for brightness enhancement of laser diode arrays and responds to the challenges created for high performance optics and techniques for laser characterisation. A novel optical inter-leaving method for a 7-bar stack of single-mode emitters, providing a nearly 2-fold improvement in the slow axis beam parameter product, enabling fibre-coupling, is demonstrated. A laser-written dual-axis optics approach is used to perform challenging slow axis collimation combined with fast axis correction for closely-packed 49-single-mode emitter bars, to provide low-loss collimation with high pointing accuracy of less than 3% and 10% of a beam divergence in the fast and slow axis direction, respectively. This produces excellent source for application beam-combined laser diode systems. An emitter-by-emitter simultaneous analysis is used to provide spectra and far field pointing for all emitters and evaluate the performance of various external cavity configurations with Volume Holographic Gratings (VHGs). For the ultra-collimated bars, high efficiency VHG-locking is shown to be maintained over enhanced range of temperatures (>17˚C) and large laser-VHG distances (>110 mm). Highly effective feedback enables the use of a folded cavity configuration for wavelength selection over a range of 8 nm for the full 49-emitter bar, giving a prospect for multi-wavelength single-VHG-locking of bars for cost-effective spectral combining. An innovative technique of wavelength stepping by individually-formed folded cavities for 5 and 7 sections along the bar demonstrates a potential to produce a source for high performance dense spectral beam combining. In a VHG-based Talbot cavity, eight emitters are coherently locked with a highvisibility interference pattern at 1W of output power. The results of phase-locking for full 49-emitter bar show that the slow axis pointing variation of ± 2mrad produces different supermodes, for a fixed alignment of the cavity, thus it must be additionally corrected for further improvement.

A study of some key areas in which ultrafast laser inscription may usefully be employed is presented. The thesis includes waveguide inscription in a variety of substrates including passive glass, doped glass and a nonlinear crystal. The work contained can be split into three studies, with some overlap between them. Firstly fused silica glass is used, both in planar substrates and as flat fibre, for the inscription of two sensing elements. The planar substrate is used for a device similar in design to a side-polished fibre and the flat fibre is used for the fabrication of a Bragg grating waveguide array. In the second study, waveguides are inscribed in the nonlinear crystal monoclinic bismuth borate, and used for guided mode second harmonic generation. A novel waveguide design is employed to increase overlap between the pump and second harmonic waveguide modes. The remainder of the thesis investigates the applicability of ultrafast laser inscription to the fabrication of compact modelocked lasers. Lasing is demonstrated, both continuous wave and modelocked, using a laser inscribed erbium doped bismuthate glass waveguide as the gain element. A study is then undertaken into methods of integrating carbon nanotubes, used as saturable absorbers to modelock lasers, into laser inscribed waveguides.

The use of palladium coated optical fibres containing an in-fibre long period grating (LPG) structure for the sensing of low concentrations of hydrogen has been investigated. Previous proof of principle experiments were refined and extended, demonstrating sensitivities of up to 60 pm for 1% hydrogen at an operating temperature of 70°C. Characterisation of the Pd deposition technique (RF sputter coating) including XPS and SEM analysis to investigate surface roughness and contamination were carried out and are discussed. These measurements were prompted by the need to characterise and eliminate sensor drift associated with delamination of the sensor layer. Particular care was taken to characterise the optical properties of Pd thin films, both in the presence of hydrogen and without, through ellipsometry and surface plasmon resonance, since values gained from the literature showed wide variations. The results presented in this thesis differ from published Pd refractive indices but are specific for the conditions used in this work. Techniques to directly measure the refractive index profile within the core of an LPG were investigated and a possible solution is presented. Theoretical models to calculate the transmission spectrum of a LPG, both with and without a Pd layer are presented, discussed and the implications due to the poor characterisation of the LPG are considered. The potential use of higher order double cladding modes (with up to 20 times the sensitivity of the lower mode orders) as a sensing regime is investigated and the practical limitations discussed.

High precision miniature and micro products which possess 3D complex structures or free-form surfaces are now being widely used in industry. These micro products require to be fabricated by several machining processes and the integration of these various machining processes onto one machine becomes necessary since this will help reduce realignment errors and also increase the machining efficiency. This thesis describes the development and testing of several hybrid machining approaches for machines which are typically used to produce micro products such as micro fluidic moulds, solar concentrator moulds, micro grooves in brittle materials and micro structured milling cutters. These are: (a) micro milling and laser deburring; (b) micro grinding involving laser pre-heating; (c) micro milling and laser polishing. The hybrid micro milling/ laser deburring process was tested during the fabrication of a micro fluidic injection mould. Micro burrs on the channel of micro fluidic mould generated during micro milling were completely removed by developed laser deburring process. This approach can achieve a good surface finish on a micro fluidic mould. The hybrid laser assisted micro grinding process was investigated by fabricating a set of micro grooves on brittle materials, including Al2O3 and Si3N4. The workpiece was pre-heated by laser to increase its temperature above that of the brittle to ductile transition phase interface. It was found that lower cutting forces were apparent in the grinding process when used to machine brittle materials. It was also found that laser assisted grinding helped achieve a very good surface finish and reduced subsurface damage. The final hybrid machining approach tested involved micro milling and laser polishing to fabricate solar concentrator moulds. Such a mould requires a good surface finish in order to accurately guide light focusing on a target. The laser polishing process was successfully used to remove any unwanted cutting marks generated by a previous micro milling process. Abstract iii As a novel extension to this hybrid machine world, a focussed ion beam (FIB) fabrication approach was researched regarding the generation of microstructures on the rake faces of milling cutters with the aim of reducing cutter cutting forces and increasing tool life. The tool wear resistance performance of these microstructured tools was evaluated through three sets of slot milling trials on a NAK80 specimen with the results indicating that micro structured micro milling cutters of this kind can effectively improve the tool wear resistance performance. A microstructure in a direction perpendicular to the cutting edge was found to be the best structure for deferring tool wear and obtaining prolonged tool life. This approach can potentially be further integrated into a hybrid precision machine such that micro structure cutters can be fabricated in-situ using a laser machining process. The conceptual design of a 5-axis hybrid machine which incorporates micro milling, grinding and laser machining has been proposed as a test-bed for the above hybrid micro machining approach. Through finite element analysis, the best configuration was found to be a closed-loop vertical machine which has one rotary stage on the worktable and another on machining head. In this thesis, the effectiveness of these novel hybrid machining approaches have been fully demonstrated through machining several microproducts. Recommendations for future work are suggested to focus on further scientific understanding of hybrid machining processes, the development of a laser repairing approach and the integration of a controller for the proposed hybrid machine.

This work addresses the nature of the delay experienced by pulsed Stokes radiation when amplified by stimulated Brillouin scattering (SBS), topically referred to as “slow light in SBS”. The term “slow light” refers to the propagation of a light pulse in a medium in which the group velocity of the pulse is considerably lower than the phase velocity of light. A comprehensive review of the literature on “slow light” has revealed a range of inconsistencies in attributing experimentally observed pulse delays to the group velocity effect. For the case of SBS the controversies are resolved through analytic solutions of the basic coupled SBS equations in both the frequency and time domains. The solutions provide the first mathematically rigorous and physically non-contradictory description of the temporal, spectral and energy characteristics of the Stokes radiation and of the induced acoustic wave in an SBS amplifier. Based on these solutions, a theoretical model of Stokes pulse propagation through a CW-pumped SBS medium is developed, the so called “inertial” model. The solutions are verified experimentally through study of the Stokes pulse dynamics in a set of fibers with different inhomogeneous SBS bandwidths and acoustic wave relaxation times. The results obtained confirm that the delay, shape and amplitude of the output Stokes pulse follow the predictions of the “inertial” model and that, contrary to popular opinion, the phenomenon of group delay, or “slow light”, is irrelevant to the observed delays.

In this thesis design, development and realisation of a substrate emission electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) emitting in the 980 nm wavelength range is discussed. Chapter 1 provides a literature review of the relevant VCSEL and (OP-VECSEL) technology required for the design of an EP-VECSEL. In chapter 2, different areas of the device design are highlighted, including electrical and optical performance of the distributed Bragg reflectors (DBRs), active region design, device detuning and antirefiective coating design. Chapter 3 provides a description of the method used to fabricate EP-VECSEL devices and focuses on optimisation of different process steps, namely the trench etch profile and depth, as well as the contact metalisation. A method for characterising the detuning of a wafer is also presented. In chapter 4 measurements of fabricated EP-VECSEL are presented, with a method for the characterisation of the EP-VECSEL material by modulating the output coupler mirror reflectivity demonstrated. This method is then used to examine the affect of different substrate dopings on device performance. Data is also presented on beam quality, power scaling and thermal properties. Chapter 5 investigates methods for improving electrical aspects of device operation, with improved nand p DBR designs proposed. In addition, analysis of SIMS data for an EP-VECSEL and n-DBR are presented, along with an investigation of the top contact geometry. In chapter 6 a discussion of the QW active region is provided, first by analysing the epitaxial material used in chapter 4 and then proposing improvements to the growth process. A comparison of a 3, 6 and 9 QW active region is then presented, where the trade offs in the optimum number of QWs are examined. Finally, this thesis is summarised and a new device design is proposed from the findings.

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