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An imploding detonation expansion laser has been built and some preliminary work has been done to determine its properties. Optical and pressure measurements were performed both in the detonation chamber and the nozzle. Gain measurements at CO and CO2 wavelengths were made as a function of starting mixture, throat height and position in the nozzle. A brief attempt to make the system lase was not successful. More effort will be expended after repairs to the machine have been made. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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Pulsed lasers have a wide range of applications in industry, medicine and for scientific research. Many of these devices are expensive and have delicate optics. The nitrogen laser is robust and inexpensive to build and maintain. A short review of the experimental nitrogen lasers is given. A major part of this thesis covers work on increasing the energy output (from30 µJ to 0.3 mJ). The one design of nitrogen laser consists of a pc board etched on the sides and at the centre for the laser discharge. The separated sections are rectangular in shape. However, in the new design the discharge section of the nitrogen laser has a parabolic shape and an inclined laser channel was used instead of a horizontal one to observe the effect on the energy output. Parameters such as the distance between the top and bottom plates, the area of the bottom plate, the area of the parabola and the flow velocity of nitrogen were varied. Both nitrogen gas and cold nitrogen vapour were used as the lasing medium. The substitution of vapour for gas increased the energy 2 fold. Liquid nitrogen was tried unsuccessfully as the medium in the discharge channel. Two large lasers were built giving increased laser energy. A multilayer nitrogen laser was also built increasing the output by a factor of 2.5. The multilayer idea was also tried on the large lasers. The multilayer laser behaves like a small capacitor bank, discharging in parallel into the laser channel. The low pressure electrodes which were used on the large parabolic laser consisted of a pair of flat copper electrodes enclosed in a plexiglass housing and the latter being connected to a vacuum pump. The effect of using the low pressure electrodes on the laser energy output was investigated. Three nitrogen lasers made of aluminium foil were also constructed where transparencies and mylar were used as the dielectric insulator. In addition, a multilayer parabolic N2 laser was made using the same materials. A water wave simulation experiment of the parabolic laser was done which showed that due to the parabolic form, circular waves are converted into planes wave. The spark gap which acts as a fast nanosecond switch must be precisely located at the focus of the parabola. Otherwise the laser does not lase. Michelson Interferometry was carried out to measure the coherence length of the laser Which was found to be longer than that mentioned in the literature. The improved parabolic nitrogen laser was used to obtain fringes in a Mach Zehnder experiment. The laser is now being used by the Durban laser group as a diagnostic tool to measure the refractive index of the gas lens created in a Colliding Shock wire experiment. Carbon dioxide lasers have numerous industrial applications. The Laser group at the Atomic Energy Corporation(AEC), Pretoria are looking into possible industrial applications such as carbon isotope separation, paint-stripping and de-rusting. The author spent sometime at the centre to investigate how the beam quality and energy output of the laser can be improved since a near gaussian profile is very important for many applications. The carbon dioxide laser system basically consists of an oscillator and two amplifiers in series. Measurements of the beam parameters (waist size, pulse shape, divergence angle) along different sections of the laser chain were taken. The laser beam was double passed through one of the amplifiers to observe the effect on the energy output. Burn patterns at several places were taken to observe the beam profile. An investigation into the optical energy losses along the laser chain was made. A device called the Three Element Detector invented by the Laser Group, AEC, was also used to analyse the laser beams. / Thesis (M.Sc.)-University of Natal, Durban, 1999.

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Thesis--University of Florida. / Description based on print version record. Typescript. Vita. Includes bibliographical references (leaves 106-109).

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).

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.

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&deg;C. For a trap temperature of -150&deg;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.