A differential absorption lidar (DIAL) system, based on a xenon chloride exoimer laser, has been developed and applied to measurements of atmospheric sulphur dioxide. This thesis describes the basis of the technique, the construction and operation of the prototype system, and its theoretical and practical sensitivity to sulphur dioxide concentration. The system, comprising the laser, telescope, detector and data handling equipment was developed for robustness in the field, and novel features have been included for this purpose. The Newtonian type telescope was chosen for its small image size and adequate field-of-view. It uses a "Cassegrain" type primary mirror and a visible reflector at the focus, allowing direct viewing of the scattering target. This feature is used in conjunction with direct viewing through the laser cavity, through a dielectric coated mirror, to permit laser/telescope alignment on the target. The laser itself is pumped by transverse discharge after corona/U,V pre-ionization. Laser characteristics have been measured. Those of primary importance to lidar are the energy per pulse, at 5 to 8mJ, the pulse duration of 32ns, the maximum pulse repetition rate, at 20pps, and the number of pulses to half energy, at 18000 per gas fill. The emission spectrum of the laser, with wavelengths at 307.92nm and 308.17nm, has been compared with the absorption spectrum of sulphur dioxide, showing a difference between the respective absorption coefficients. This indicated the suitability of this particular laser to the differential absorption technique. Unique selection between these wavelengths, per pulse, is desirable for optimum sensitivity in DIAU However, a novel method has been devised for modifying the relative wavelength content of one of the pulses, by inserting an absorption cell of sulphur dioxide into the laser optical cavity. The advantages of this over "distinct" wavelength selection are cheapness and robustness. However, theoretical work has shown a subsequent loss of sensitivity to atmospheric sulphur dioxide concentration, by a factor of about 5, A 3-element birefringent filter has been designed in case a more conventional tuning method is required. Specification of the detector and its operating conditions have been closely defined in order to optimise sensitivity to very low levels of backscattered light, whilst reducing the effects of unwanted background and noise. To this end, a solar-blind photomultiplier has been employed in conjunction with a narrow-band interference filter, centred around the laser emission wavelength. A relatively simple signal handling circuit was built to perform the minimum requirement of measuring the intensity of backscattered radiation. A micro-computer is used to control the circuitry in test and data acquisition modes, and to store data, allowing signal averaging and subsequent data analysis. Program algorithms for data analysis were developed from lidar and DIAL theory. A theoretical investigation of atmospheric scattering properties was carried-out to provide scatter coefficients for application in the lidar equation. The validity of the lidar equation was proved when computer-modelled oscilloscope traces of lidar return signals were found to match, closely, experimental traces of return signals from the smoke plume at Methil power station. A theoretical treatment, using the lidar equation, gave expressions for target gas concentrations as a function of return signal intensities. This was done for the case where unique wavelength pulses are transmitted into the atmosphere and was repeated for the case of "mixed wavelength" pulses, applying to the DIAL experiment. Prediction of sensitivity of the system in measuring sulphur dioxide concentration is based on the noise content of the return signals. The analysis has given a detection limit range of about 10ppm.m to 800ppm.m, for the "mixed wavelength" application and an expected range of 2ppm.m to I60ppm.ra if the laser is tuned conventionally, depending on signal strength and number of averaged pulse pairs. Errors expected in measurements of finite sulphur dioxide concentrations are given. The system was applied to measuring sulphur dioxide, emitted under control, into the path of the laser pulse. Measured peaks of about 120ppm were expected and measurements taken successively are in agreement with expected dispersal rates. Fluctuations of measured sulphur dioxide levels, about a mean, are shown to be within the theoretically-evaluated error limits. This close agreement between theory and experiment allows the theoretical detection limits to be treated as realistic.

A selection of nonlinear materials for use in various frequency-doubled dye lasers is studied. Lithium Formate Monohydrate is operated intracavity in a Coumarin 102 dye laser for the first time. A maximum of 70 microwatts of tunable ultraviolet light was generated over the range 237nm to 24Snm, representing an improvement of an order of magnitude on previously reported work with this material. A pulsed xenon ion laser of high efficiency has been built and the design optimised, giving an output power of 185W. The laser is used to pump a Rhodamine 6G dye laser containing a crystal of Ammonium Dihydrogen Arsenate, to generate 280 milliwatts of tunable UV over the range 291nm to 297nm and this is the first time that a detailed study of this type of laser has been reported. A crystal growing system for water based crystals is described. A crystal of Urea grown in the apparatus has been cut for use in a dye laser and tested with the beam from an argon ion laser. A value of the nonlinear coefficient d14 of 0.90pm/V is reported.

This thesis describes the development of a new generation of pulsed optical parametric oscillators (OPO's) based on two new non-linear materials, urea and beta-BaB2O4 (or BBO), and pumped by a new class of laser pump sources, namely, excimer lasers, to provide broadly tunable coherent radiation in new regions of the electromagnetic spectrum, particularly in the ultraviolet and the visible, which have previously been inaccessible. The laser pump source used during this work was a pulsed ultraviolet XeCl excimer laser operating at 308nm. Because of the stringent demands on the pump beam quality (with regard to both spatial and spectral coherence) for successful operation of OPO's, the pump laser was designed and constructed as an injection-seeded system, to provide a narrow-linewidth, near-diffraction-limited output beam, with sufficiently high peak powers to enable OPO operation. In this way, we were able to obtain an output beam with a linewidth ≤ 0.2cm-1, and a full-angle of divergence as low as 60muR (~3 times the diffraction limit). The maximum energy available from the pump laser was 30mJ, in pulses measuring typically 10ns in duration. The output beam was also linearly polarised to better than 95%, and the pulse repetition rate was 1 Hz. In the early part of this work, we used the constructed pump laser to investigate spontaneous parametric fluorescence in a home-grown urea sample, in order to characterise the crystal, and to compare the observed spectrum with the calculated OPO tuning curves. The results of these experiments were found to be in good agreement with the theoretical predictions. The main thrust of the project, however, was the development of an OPO based on urea as the non-linear medium and pumped at 308nm by the narrowband XeCl excimer laser. We were successful in constructing such an OPO, using an 8-mm-long, home-grown crystal, and were able to generate, continuously tunable output from 572 to 667nm, with a 2.5% energy conversion efficiency. The timing range of the device was later extended to 537-720nm, by utilising a 15-mm-long, home-grown urea sample, and its conversion efficiency was improved to as high as 37% at 90° phase-matching, with ≥ 10% efficiency over a 100-nm range in the visible (from 570 to 670nm). Finally, in an effort to achieve even higher efficiencies, we performed experiments in a 25-mm-long commercial urea crystal, and demonstrated exceptionally high external energy conversion efficiencies of up to 66%, with evidence of even higher levels of pump depletion (as high as 85%) at 90° phase-matching. The latter part of the project was concerned with the design and development of a similar device based on the new non-linear material, beta-BaB2O4, to provide continuously tunable radiation over a much broader tuning range, particularly in the blue and the near ultraviolet, not accessed by the urea OPO. We used a 12-mm-long commercially available beta-BaB2O4 crystal to construct this OPO, and successfully operated this device over the entire wavelength range from 354nm in the near ultraviolet, throughout the visible, to 2.37mum in the near infrared, with an energy conversion efficiency in excess of 10% over the range 450-960 nm. The constructed OPO's were also characterised with regard to several operating parameters, including oscillation threshold, spectral linewidth, as well as spatial and temporal variation and, where appropriate, the experimental results were compared with the predictions of theory.

In this thesis the design, construction and performance of several diode-laser pumped continuous-wave neodymium lasers are described. These lasers were operated both around 1 mum and, by internal frequency-doubling, at 0.5 mum. The main emphasis has been on the assessment of the various laser designs with regard to their potential for efficient, high-power visible operation. A variety of pumping geometries, resonator configurations, gain media and internal frequency-doubling schemes were investigated, and their relative merits explored. Both side-pumping and end-pumping arrangements were employed, with Nd:YAG, Nd:YLF and Nd:YVO4 being used as gain media. Travelling-wave and standing-wave resonator designs were used. The polarisation-rotation effect in non- planar ring resonators was investigated and used to obtain single-frequency output. Single-frequency 0.5 mum powers up to 1.2 W were generated, and the highest 0.5 mum output power achieved was 4 W on two-longitudinal modes spaced by 450 MHz. The highest 1 mum output power achieved was 10 W, with a slope efficiency of 43%. Maximum pump powers for the lasers were in the region 15 - 35 W. A review of diode-laser pumped devices is included, with particular emphasis on the role of the spatial distributions of the pump and signal fields, because this is an important limiting factor in the performance of diode-pumped bulk laser systems. The criteria governing the harmonic output power when internally frequency-doubling are discussed. Issues relating to noise in the harmonic output, and techniques for its avoidance, are also discussed.

Thermal loading models have been developed in order to predict the optimum input powers of free-convection cooled strontium recombination lasers (SRLs) and forced-convection cooled SRLs. Gas temperature models have also been developed for circular bore and rectangular bore SRLs, to complement the thermal loading models, so that the performances of circular and rectangular bore SRLs can be compared. It has been shown, in theory, that rectangular bore tubes offer operation at lower gas temperatures than do circular bore tubes of the same cross-sectional area, with the same input power per unit length. Laser performance is, therefore, expected to be improved through the use of rectangular bores in the construction of SRLs. The results of the thermal loading models are validated by experiments. The need to oven process alumina tubes in forming gas, prior to their use in the SRL, is demonstrated by an increase in output power from tubes processed using this technique. Enhanced radiative heat extraction, accompanied by an increase in average laser output power, is obtained from a free-convection cooled SRL by increasing the emissivity of the outer surface of the ceramic discharge tube. Three rectangular bore water-cooled SRLs, optimising at different input powers, have been constructed, operated and compared in performance to a free-convection cooled SRL, constructed using the same ceramic discharge tube. An average output power of 2.3 W has been obtained from a 40 cm long rectangular bore beryllia discharge tube through the use of active water cooling. This represents an improvement of over 200% when compared to the performance of a free-convection cooled SRL utilizing the same ceramic discharge tube.

The technique of perturbation spectroscopy has been applied to study the argon ion laser discharge. In this technique the laser radiation field present within the active medium is switched on and off, and this modulates the population of the upper laser level. By studying the magnitude of these modulations as discharge conditions within the active medium are changed, the copulation inversions on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 laser transitions have been explored. By studying, in the presence of an applied magnetic field, the polarization induced in the spontaneous emission sidelight through these modulations, the lifetime of the 4p2D5/2 upper laser level has been estimated. By studying the population changes induced on other 4p and 4d levels as a consequence of the population change on the upper laser level, the relative importance of various proposed excitation mechanisms for the 4p upper laser levels have been assessed. The population inversions (1 - g1N2/g2N1, where N1 is the population of the upper laser level and N2. the population of the lower laser level) on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions of Ar II are destroyed by increasing the argon pressure above 0.2 - 0.3 Torr, and for higher pressures the medium exhibits absorption. Both population inversions increase slowly with discharge current. By using a flow graph technique to analyse the population inversions, this latter is shown to imply that the electron temperature in the discharge increases with current. The polarization experiments have been analysed by both a simple rate equation treatment, and by a more complete approach based on Lamb theory. This latter treatment demonstrates that the dependence of polarization ratio on magnetic field can be used to determine the lifetime of the upper laser level. Polarization ratios on the 4p2D5/2 - 4s2P3/2 and 4p4D5/2 - 4s2P3/2 transitions have been measured as a function of discharge parameters and applied axial magnetic field. From the variation of polarization ratio with magnetic field, the lifetime of the 4p2D5/2 level of Ar II under discharge conditions has been estimated. The perturbations in the populations of other 4p and 4d levels of Ar II resulting from the modulation of the upper laser level have been measured as a function of discharge parameters. Their magnitudes and dependence on discharge current provide direct evidence that an important excitation pathway for the 4p upper laser levels of Ar II is step wise excitation through the quartet 4s levels of Ar II.

Spatially and temporally resolved measurements of many of the parameters of a 25mm bore, 800mm active length copper HyBrID laser have been performed. The laser was operated under excitation conditions for maximum efficiency of light generation at all times. The hook method has been used to investigate the population densities in the copper ground state, the upper laser levels, the lower laser levels and one of the quartet levels in copper. The population density in one of the neon metastable levels has also been measured. All of these measurements have been performed with 5 ns time resolution and 2 mm spatial resolution. A two wavelength laser interferometric technique has been used to measure the electron density with nanosecond time resolution and 2mm spatial resolution. The waveforms of the voltage across the laser head and current through the laser tube have been measured. The temporally resolved laser pulse radial profile has been investigated. The results from these investigations have been compared to those for a 42 mm bore CVL previously studied with the same experimental apparatus. Comparison has also been made with a detailed computer model of a 19mm bore CVL. Deductions as to the effect of the remanent electron density prior to the excitation pulse on the current and voltage waveforms have been made. The effect of added HBr has been discussed, relative to the remanent electron density. The laser kinetics are discussed in terms of the electron temperature and the relative rate coefficients of the mechanisms affecting the level populations.

The construction and operation of a small active volume, discharge pumped, rare gas halide excimer laser is described. The values of laser output parameters such as pulse duration, pulse energy and overall efficiency presently achieved with such systems are much less that theoretical studies predict. The performance of this laser when pumped using a novel pulsed power modulator design containing nonlinear capacitors to produce a very fast rise time voltage pulse is contrasted with the performance obtained from a conventional driver circuit. The purpose of this was to determine if such circuits could lead to improvements in glow discharge stability and also laser pumping efficiency by quickly achieving the optimum pumping rates predicted by theory. It was concluded that while the rapid establishment of optimum pumping conditions may be beneficial, too fast a rate of rise of discharge current appears to be detrimental to discharge stability, probably due to skin effects. Having established that premature glow discharge collapse is a serious limiting factor in producing long duration excimer laser pulses, a study is carried out of the factors believed to influence discharge stability. While the effects of halogen donors on discharge stability have received most attention in the past the part played by the other constituents of the laser gas mix tends to have been neglected. A theoretical and experimental study of the role of the rare gas partners, xenon, krypton and argon is presented. It is well known that gas mixes using helium as the buffer gas perform less well than with a neon buffer and this is attributed to the driving of discharge instabilities rather than kinetic factors. A comparison of the relative influences of the buffer gases helium, neon and argon on discharge stability is carried out and claims by other workers for improved laser performance using a mixed helium / argon buffer are tested. Finally, in an attempt to inhibit the mechanisms driving glow discharge collapse, the effects of externally applied magnetic fields on discharge stability and laser performance are investigated.

A unified state-space model of a high repetition rate electrical discharge in a helium-strontium mixture is presented. The atomic (number density of particles in a particular state), optical (number density of photons), thermodynamic (local particle temperature and pressure) and electrical (voltage and current) internal states of the system are the state variables. A generalised circuit analysis program (GCAP) provides a description of the excitation circuit used to power the strontium laser. Rate equations describing the time evolution of the state variables are simultaneously numerically integrated to provide a description of the laser system during the discharge of a capacitor through the laser load and in the immediate afterglow. The effects of parametric variation of the circuit on strontium laser performance are examined. The model predicts that Sr++ ions are formed during the discharge current pulse by step-wise excitation from SrII states lying lower in energy. A population inversion is achieved on the 62S1/2-52P3/2 transition in SrII (lambda=430.5nm) in the current pulse afterglow by rapid three-body recombination of Sr++ ions. A strong recombination flux is established on rapid termination of the discharge current pulse. Impedance matching of excitation circuit to load is essential for stimulated emission under recombination conditions. The results of the state variable analysis are validated by experiment. The operating characteristics of a discharge-heated strontium vapour laser are presented. An average power of 0.3W is obtained in a high heat-loss configuration. The fall-time of the discharge current pulse is reduced by means of a saturable inductor placed in parallel with the laser load. The circuit generates current pulses with peak amplitudes up to 300A and fall-times of less than 70ns. GCAP is used to estimate the theoretical limits on the fall-time of the laser current by state variable analysis of a flux-controlled model of the saturable inductor.

Copper Halide lasers are discussed and the results of experiments reported. It is found that the presence of small quantities of an electron attaching gas (such as bromine) cause discharge instability. Specially designed electrodes which remove excess bromine cure this problem and yield a stable discharge. A 4W copper bromide laser is operated, sealed-off, for 100 hours. This laser has an apertured discharge tube with side-arm reservoirs to control copper bromide vapour pressure, a feature essential to stable operation. The addition of small amounts of hydrogen changes the beam from an annular to a gaussian-like profile. The estimated lifetime of this laser tube is 1,000 hours. Exploiting the theory of metallic walls for discharge confinement, it is found that metal segments shorter than about 1m can support a stable discharge at high pulse repetition rates (5 - 20kHz). On the basis of this, a novel copper halide laser containing cylindrical copper segments is demonstrated. Neon and halogen gases flow through the tube. The reaction between the halogen and the copper segment walls forms copper halide in-situ. Hydrogen bromide, bromine and chlorine have been used. Hydrogen bromide proves to be the most suitable. A one metre long laser tube of this design produces 40W. Pulsed power supplies for metal vapour lasers are discussed. The conventional form of the capacitor-transfer circuit has the peaking capacitor value around one half of the storage capacitor value. It is found that equal capacitor values produce the best results for the gold vapour laser system we describe. The reliability of a gold vapour laser is improved by replacing the hollow anode thyratron (which has a high latch rate) with a solid anode thyratron. The replacement thyratron, in combination with a saturating charging choke, significantly reduces the latch rate. The laser is used for studies of photodynamic therapy of cancer in a local hospital.