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Research towards the development of a gas-discharge fibre laser using noble gases, with target emission wavelengths in the mid-IR. Additional and separate work on gas treatment methods for managing the formation of photo-induced defects in silica glass.

This thesis describes the theoretical and experimental investigation into many aspects of circuit and component design for high average power short pulse systems. The primary objective of this work is to develop both circuit design criteria and pulsed components for long life rare gas halide lasers. A pulsed system consists of three major components, energy storage, switches, and load. This investigation considers the type of circuit which uses capacitive energy storage in combination with a closing switch to transfer electrical power to a load. Specific loads are not addressed but the implications of load characteristics that affect circuit/component design and life are considered. The investigation reported, describes the physical and electrical characteristics and analysis of phenomena that adversely affect the performance and life of pulse duty components. In the area of capacitive storage, lifetimes of one particular design was improved by 3 orders of magnitude and a means of detecting the imminent failure of oil filled capacitors was devised and patented. In the area of switching, methods are described by which hydrogen thyratrons can be operated in parallel with equal current sharing without the need for inductive or resistive ballasting. Finally, the design and testing of a 200 watt XeCl laser modulator is presented.

This thesis reports the first comprehensive comparison of the operating regimes of the three major types of Cu halide laser, which oscillate on the 510.6 nm and 578.2 nm resonance-metastable transitions of atomic Cu in pulsed discharges at 10-50 kHz pulse recurrence frequency. The three lasers had similar active volumes (36.8-43.5 cm3) and bores (12.5-13 mm), were excited using the same power supply and circuit and monitored using the same diagnostic apparatus. The CuBr-Ne laser produced an annular output beam, weighted towards the yellow transition, with a maximum average output power of 3.55 W and a maximum efficiency of 0.71 %. When H2 gas was added to this laser at a level of ~5%, the output beam developed an axial (central) peak in intensity, the beam was less constricted, the balance of green and yellow powers was improved, the output power rose to a maximum of 11.4 W and the maximum efficiency reached 1.47 %. In both of these lasers, the CuBr vapour was generated by heating a sidearm of the discharge tube. The vapour was entrained in a flow of Ne buffer gas to seed the active volume. A Cu hybrid laser, where CuBr is generated in the tube in situ by reaction of the discharge products of a Ne-HBr buffer gas with Cu pieces in the tube, has been compared to the two conventional CuBr lasers. The Cu hybrid laser also produced an output beam with a central maximum, little or no constriction and a good balance of green and yellow powers. Maximum average output power reached 12.8 W and the maximum efficiency was 1.66%. In terms of specific average output power, the hybrid laser was clearly superior to the other two, with values of 82 mW.cm-3 (CuBr), 262 mW.cm-3 (CuBr-H2) and 348 mW.cm-3 (Cu hybrid). The specific output power of the Cu hybrid laser obtained in these studies is a record value for any Cu laser (including elemental Cu lasers) of tube bore ~12.5 mm. This result and the general dependences of output power on buffer gas pressure, additive (H2, HBr) pressure, pulse recurrence frequency and charging voltage and capacitances are discussed in detail in terms of the fundamental processes and chemical reactions. The most important processes responsible for the high powers and efficiencies and the Gaussian-like beam profiles in the presence of hydrogen are dissociative attachment of HBr in the interpulse period and at the beginning of the pulse, and the reduction of CuxBrx polymers and monomers by H2 to free Cu atoms in the active volume. This is the first time that the importance of hydrogen reduction in these lasers has been identified. Without it, the filling in of the annular output beam cannot be explained. The mechanism of Cu seeding of the hybrid laser has also been studied in detail, as it is the most obvious difference between the Cu hybrid and conventional CuBr lasers. The basic reactions of the seeding process are described, and it is found that in addition to Cu3Br3 and Cu4Br4 polymers there must be a substantial amount of CuH in the discharge to account for the large density of Cu atoms in free form and locked up in molecular forms. This is the first time that CuH has been suggested as a major Cu-bearing species. The process of Cu dendrite formation in the tube is also discussed. Finally, the properties of the hybrid laser have been considered from the point of view of scaling to very high average output powers. It has been shown that average output powers of 1 kW are possible using current technology.

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.

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.

A high resolution study of magnetic-field-induced second harmonic generation in sodium vapour was performed using a single-frequency continuous-wave dye laser. Resonant enhancement was obtained by tuning the laser to the frequency of the 3S-40 two-photon transition of the sodium atoms. Coherent parametric emission of the second harmonic radiation ocurred in the presence of a symmetry-breaking magnetic field by means of a coherent electric-quadrupole emission at twice the laser frequency. A theoretical model of the second harmonic generation (SHG) was developed, and includes the roles played by atomic selection rules, Zeeman splitting and eigen function mixing in a magnetic field, and the effects of homogeneous and inhomogeneous broadening. The use of a single-frequency laser and an atomic nonlinear medium allowed an experimental investigation of a SHG process in unprecedented detail. This provided astringent test of the theoretical model developed to predict the properties of the nonlinear interaction in terms of fundamental atomic parameters. The excellent agreement obtained between theoretical and experimental results vindicated the theoretical methods used in the modelling.

Infrared, ultraviolet and mass spectrometric techniques have been used to investigate the short gas-life of discharge excited XeF lasers, for He/Xe/NF3 mixtures. Infrared absorption studies provided initial information on the changes which occur in the laser gas composition during pulsing. The information was used to complement those of mass spectrometric studies, in which the chemical composition of laser gas mixtures were determined as a function of the number of laser pulses. Ultraviolet absorption spectroscopy was used to study optical absorption at the laser wavelength in fresh and used gas mixtures. The effects of several contaminants on laser performance were studied by adding small concentrations of these contaminants to the basic gas mixture of He/Xe/NF3. The results provided information on the identity of the main contaminants. Cold traps were used to extend the gas-life, and to identify dominant contaminants. The laser device used in this work is excited by a conventional blumlein circuit, which is triggered by a pressurised spark-gap switch. For a single gas fill of the basic mixture (He/Xe/NF3), the number of laser pulses to half-energy is approximately 150/litre atm. Infrared and mass spectrometric studies show that the fast deterioration of laser performance is due to the depletion of NF3, and to the accumulation of contaminants in the laser. The contaminants have been identified as N2, O2, NO, NO2, N2O, CO, CO2, NF2, N2F2, HF and CF4. Of these, NO2 absorbs at the laser wavelength (351nm), but the absorption coefficient in used gas mixtures is small compared to small signal gains of laser devices similar to the one used in this work. There is strong evidence that water vapour, which is one of the main impurities in fresh gas mixtures, may be the source of oxygen in the formation of oxides of nitrogen (NO, NO2, N2O) and carbon (CO, CO2) For fresh gas mixtures, the laser pulse energy is insensitive to the addition of small concentrations of N2, H2 and CF4. In contrast, the addition of 0.05% of CO2, CO and O2 results in approximately 60, MO and 20% reductions in the laser pulse energy respectively. The estimated change in laser output after 1000 shots due to the accumulation of CO2, O2 and CO is 20, 10 and 5% respectively. The addition of small concentrations ( < 1 torr) of N2 CO2, CO, O2 and H2 results in negligible changes in the gas-life. However, when 2 torr of CF4 is added to the basic mixture of He/Xe/NF3 a threefold increase in the gas-life is observed. The improved gas-life is attributed to lower rates of formation of O2, NO2 and NO. After using He/Xe/NF3/H2 mixtures, the gas-life of the basic mixture increased by a factor of five to about 700 shots/litre atm. Mass spectrometric analysis of the gas mixture before and after lasing shows that the improvement in the gas-life is mainly due to lower levels of O2, NO2 and NO, and to a significant reduction in the rate of depletion of NF3. The eventual deterioration of laser performance is mainly attributed to the accumulation of CO and CO2 in the laser. For the laser device and gas mixture used in this work, the optimum trap temperature lies in the region of -100 to -150°C. For a trap temperature of -150°C, the gas-life is 1500 shots/litre atm for a single gas fill. This is about 2.5 times the best result obtained without the use of cold traps. The eventual termination of laser action is due to NF3 depletion and the accumulation of contaminants in the laser. By boiling-off the contaminants sequentially, CO2 has been identified as a major contaminant.

The optical perturbation of discharge current voltage characteristics (optogalvanic effect or OGE) has been investigated for CO2 and CO laser discharges. A quantitative power perturbation model is constructed and a series of experiments show, a close agreement for changes of all the major operating parameters of the CO2 laser. The theory consists of an evaluation of the microscopic kinetic relaxation processes leading to the changes in thermal balance of the discharge that occur due to the absorption and amplification of the resonant laser radiation. A generation of compact and efficient cw CO2 lasers has been developed which produces higher stable output powers per unit length than previously reported, and these have been actively stabilized by OGE to provide a high degree of frequency stability (<50 kHz) and amplitude stability (< 0.5%) which is a six orders and two orders of magnitude improvement over the passive resonator capability, respectively. New optogalvanic effects have been discovered both at high frequencies (up to 100 kHz) and for sequence (00°2) laser transitions in CO2 and also in the cw CO laser. Preliminary investigation of optogalvanic detection of standing wave saturation resonances in low pressure discharged CO2 have been carried out and some analysis of discharge noise has been necessary to evaluate the ability of such a narrow band detector for laser stabilization. The empirical evidence provided by the temporal response of OGE combined with the gas composition dependence (including N2 free mixtures) proves conclusively that no major ionization mechanism described so far can be responsible for the effect. A thermal explanation of the effect due to modified kinetic cooling of the laser gas has been developed from existing qualitative explanations. This proposed "gas temperature" power perturbation model provides for the first time an accurate (~20%) prediction of perturbations.(amplitude, phase, and frequency) due to the resonant interaction of a CO2 laser beam over a wide range (up to 4 orders of magnitude) of detailed parametric changes, with a CO2 or laser mixture discharge.

This thesis deals with gas contamination problems in atomic fluorine and KrF lasers. Five different versions of transverse electric discharge lasers were constructed using different materials, geometries, and discharge circuits. Three of these were investigated in detail for lifetime performance of a single fill of He-F2 mix (atomic fluorine laser) and of He-Kr-F2 mix (KrF laser). This was done using an on-line quadrupole mass spectrometer. The evolution of the various gas components appearing in the mix was calculated from the mass spectra of the gas mixture recorded at intervals when the laser was operated at one pulse per second. The impurity or contaminant species (ie species other than He, Kr, F2) were found to be the same in all the lasers, differing only in their concentrations. The major contaminants found were CO2, N2, O2, COF2, CF2, SiF4, HF, CO, NO, SF6, and H2O. Of these, CO2 was identified as the most deleterious impurity for both atomic fluorine and KrF lasers, reducing their output energy and the fluorine content, thereby reducing the single fill lifetime of the mix. A simple cold trap decreased the partial pressures of most of the detected impurities in the gas mixture and resulted in a marked increase in the number of shots to half energy and a decrease in the depletion rate of fluorine by half in both KrF and atomic fluorine lasers. To isolate and quantify the effects of individual contaminant species, they were deliberately added singly to the pure gas mix. These experiments confirmed that CO2 was the most important impurity and that H2 and CF4 were the least harmful. Since the impurities were found to influence arcing of the discharge, they must affect preionization and/or discharge processes. Absorption effects were found to be insignificant in the regions of the laser wavelengths (248 nm, 730 nm). In order to quantify the effect of the impurities an effectiveness constant for each contaminant has been defined and used in a simple model which successfully calculates the laser output energy. This model has circumvented the problems arising from the physical complexity of the system and the lack of data on the various kinetic processes. The calculations show that CO2 impurity depletes the energy output of the KrF laser at the rate of 0.9 mJ/ppt(wrt He).

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.

The work described in this thesis covers various aspects of pulsed copper and gold vapour lasers. The work is divided into four main parts : a computer model of the kinetics of the copper vapour laser discharge; construction and characterization of a copper vapour laser and a gold vapour laser system (to be used for photodynamic cancer treatment); analysis of the thermal processes occurring in the various forms of thermal insulation used in these lasers; and studies of the use of metal walls to confine a discharge plasma. The results of this work were combined in the design of the first copper vapour laser to use metal rather than an electrically insulating ceramic material for confinement of the discharge plasma. Laser action in copper vapour has been achieved in a number of metal-walled designs, with continuous lengths of metal ranging from 30 mm, in a segmented design, to 400 mm, where the discharge plasma was confined by two molybdenum tubes of this length. A theoretical explanation of the behaviour of plasmas in metal-walled discharge vessels is described.

This thesis presents the results of a study of the mechanisms responsible for limiting the laser pulse duration obtainable in xenon chloride lasers which are excited by UV-preionized, self-sustained gas discharges. The xenon chloride laser system, the principal emission band of which is centred around 308 nm, belongs to the class of high pressure gas lasers known as 'rare-gas halides'(RGH). RGH lasers are now well known for their high peak power output at a number of wavelengths from 193 nm to 353 nm in the ultraviolet region of the spectrum. To date, however, they have only been operated in the pulsed mode with laser pulse durations of ~1000 ns for devices employing electron beam excitation and ~30 ns for devices employing transverse discharge excitation. There is no a priori kinetic limitation which prevents RGH lasers from operating in the CW mode, and an attempt to extend the duration of the laser pulse would enable the quality of laser output to be improved. The laser pulse duration of a discharge excited XeCl* laser was extended by about one order of magnitude - to 270 ns FWHM - by the use of a distributed resistance electrode to stabilize the discharge. The typical gas mixture used in the laser was ~2 atm of Ne (buffer gas), ~25 mbar of Xe, and 2.5 mbar of HC1. However, the laser pulse duration obtained was considerably shorter than the 500 ns duration, 2000 A peak current, discharge excitation pulse. The cause of this difference between the duration of the laser output pulse and the discharge current pulse was found by carrying out a comprehensive parametric study of the laser, combined with a detailed spectroscopic analysis and the results of a semi-empirical computer model. Two interrelated factors were identified as being responsible for the short duration of the laser output: namely, a temporal collapse of the discharge volume and a spatially non-uniform depletion of the HCl within this volume. The experimental results presented here contradict an earlier theory which ascribed the onset of discharge instabilities in RGH lasers to step-wise ionization of the minority rare gas atoms, and which attributed stability enhancement properties to the electronegative halogen gases used in RGH lasers.