Different types of semiconductor diode laser sources were tested in a range of spectroscopic and metrological applications to demonstrate the versatility of our laser set-up implementations. Two main topics were pursued in this study: (a) experiments on absorption spectroscopy were carried out using external cavity laser diode modules in the wavelength range (410 – 1550 nm) while vertical cavity surface emitting lasers (VCSELs) were used in the experiments involving opto-galvanic spectroscopy and laser frequency stabilization at 800.6 nm. Absorption experiments were performed for the quantitative detection of (atmospheric) trace gases such as H₂O, CO_, and CO₂. The Tuneable Diode Laser Absorption Spectroscopy (TDLAS) was realised without the need for sophisticated detection electronics (e.g. lock-in amplifiers, etc.). A notebook-data acquisition system in conjunction with dedicated software developed during this study was fully adequate and allowed us to generate “any” desired shape of the modulation signal; and – after the data had been acquired – post-collection processing could be carried out (like averaging, noise removal, signal normalization, trend plots, etc.). In order to implement the opto-galvanic (OG) spectroscopy experiment, the opto-galvanic driver, control and detection unit was designed and built in-house. This electronic device was one of the centrepieces of the set-up for locking the laser diode frequency to an atomic transition in a hollow cathode discharge lamp. An integrated device such as the one required in this study is not available commercially. The observed OG signal revealed a series of sub-structures in the form of doublets (0.0074 nm or 4 GHz apart). The doublet frequency splitting results from the fact that VCSEL light is composed of two clearly-resolved spectral components, both linearly polarized, associated with transverse mode oscillation. Locking the laser frequency in the OG signal of the Argon transition at 800.6 nm was achieved using modulation of the injection current of the laser diode. This was done by feeding the OG signal into the data acquisition and control card to generate the suitable error signal (in magnitude and sign) to the laser current driver. This process was entirely controlled by a software program written during the course of this work. Incomplete attempts were made to couple the Fabry-Perout (FP) laser radiation to a fibre Bragg grating (FBG) due to some technical problems. Specifically, the fabricated FB didn’t match fully to any diodes available to us during the time of the experiments. Thus, only very weak feedback on FBG side-bands was observed which was insufficient to push the laser into single-wavelength oscillation.

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The main objective of the work was to investigate the measurement errors in an interferometer system which arise from manufacture or from the level of skill of the operator. A computer model using a ray-trace was used to simulate the use of the interferometer in calibrating a rotary table. This application of an angle measuring interferometer is currently being developed by Renishaw Transducer Systems, Ltd. as an extension to their existing product. The errors associated with this application have been investigated both in isolation and in combination. The theoretical results indicate that without compensation, errors an order of magnitude larger than the required accuracy of 0.1 '' may be introduced. However, the use of a pre-calibration cycle in conjunction with a compensating data processing routine can reduce the measurement errors to within 0.1 ''. Experimental data taken with the interferometer and a rotary table of known accuracy confirmed the order of magnitude and general form of the results. Data from the calibration of a rotary table showed that the errors from the table were much larger than those seen with the interferometer and accurate table alone. Other errors due to thermal effects on the interferometer components, and to the method of supporting the components were also investigated, by finite element analysis and experiment, respectively.

This thesis describes some studies of ion emission from a laser-produced plasma. The plasmas were produced by focussing a light pulse of wavelength 1.06 um and duration 35 ps from a multistage high power (GW) Neodymium-in-Glass laser system onto a solid plane target. An ion analyser mounted normal to the target, with its axis pointing directly at the site of the plasma, received a small fraction of the ions emitted from the plasma and provided data regarding their atomic nature and charge state. Preliminary measurements of ion emission using an oil-pumped vacuum chamber revealed that the targets were severely contaminated with an impurity of a hydrocarbon nature. Pre-cleaning of the target with a prior laser pulse was not possible since it was found that the impurity was rapidly re-deposited. The installation of a new turbomolecular pumping system and the meticulous cleaning of the vacuum chamber and plasma diagnostics alleviated these problems and allowed, for the first time, plasmas to be produced and studied that either contained or did not contain ions of a hydrocarbon impurity, depending upon the nature of the target site. The nature of the target site irradiated was either 'Fresh' or 'Cratered'; a fresh target site was an area of the target not previously irradiated with laser light whereas a cratered target site was an area previously irradiated with laser light. It soon became clear that plasmas produced in the new clean vacuum chamber from a 'cratered' target site contained ions of a much higher charge state than seen from a 'fresh' contaminated target site. This observation was further substantiated when the limited resolution of the original ion analyser was greatly increased by the use of an electromultiplier system of much wider bandwidth. The refinements made to the previous state of this project have made possible studies of a number of important features of the plasma, for example, fast ions where their true nature and charge state could be established. As a consequence it should be possible in the future to obtain satisfactory estimates of the electron temperature in the plasma corona.

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This thesis is concerned with the design, construction and investigations of hollow cathode helium-cadmium lasers, which are capable of oscillating simultaneously on five lines, two red, two green and one blue, i.e. three primary colours, which results in the production of a 'white light' laser. For the first time simultaneous oscillation on seven cadmium lines is reported in the present study, which includes the afore-mentioned 5 lines together with two near infrared lines. The physical processes responsible for the creation of the population inversions between the twelve energy levels of the Cd<SUP>+</SUP> ion are discussed in detail. The applications of white light lasers are in areas such as colour holography and colour printing, and a description of the basics of colour science and its application to these multicolour lasers is presented. Following a critical review of related investigations by other workers, which includes details of the hollow cathode designs used to produce the hollow cathode discharges, a discussion on the glow discharge and the hollow cathode effect is presented. The designs of lasers constructed in the present work, including details of the discharge tubes and optical resonators, are described in detail. The theoretical basis for designing hollow cathode discharge tubes for laser production is also presented. A description of the ancillary apparatus and the method used to align the white light laser is given. Measurements on gain and cavity optimization were made for each colour during multi-line operation of the laser, and these results are presented and discussed. Following a discussion on the general observations of the operating characteristics of the laser, a detailed parametric study is described, in which the effects of changing helium pressure and discharge current are investigated. On the basis of these measurements the fundamental processes involved in the creation of the population inversions are discussed. Of particular interest are the results obtained for the green transitions, which suggest that at pressures below ~ 15 mbar the population inversion is created by radiative cascades through the red laser lines (which are populated by charge transfer reactions with the He^+ ion), while, in contrast, at higher pressures the population inversion is produced via charge transfer reactions in collisions between ground state cadmium atoms and He_2^+ molecular ions. Spectroscopic studies of the discharge and the laser ouput were made and these results are presented and discussed. Finally in the light of the experience gained during the present work, suggestions for further work are given.

This thesis describes an investigation of the physical processes which occur when intense laser radiation is focused in a gas to produce ionization and breakdown. The ionization processes are treated numerically. The numerical analysis which is developed in the first part of the thesis is based on a solution of the rate equation of electron concentration when both multiphoton and cascade ionizations are the only active processes when ruby and Nd3+ lasers with short durations produce breakdown in argon at pressure range 8 x 102 - 105 torr. Solutions of this equation have been developed to provide the threshold intensities as a function of gas pressure. Reasonably good order of magnitude agreement between measured and calculated threshold intensities is obtained for ruby laser. A study is developed of laser-induced gas breakdown through a cascade process in which free electrons gain a constant amount of energy of ( -.l eV) at each time step from the electromagnetic field of the laser beam, eventually causing exciting and ionizing collisions with ground state and excited state atoms. An estimate of the photo-ionization cross-section of the excited helium atoms, when irradiated by ruby, is obtained, where the continuity equations which represent the rate of change of the electron energy distribution function, the excited atoms, and the newly-produced electrons with zero energy, are solved analytically. This provides an estimate for the three ruby photons photo-ionization absorption coefficient for helium, and breakdown threshold intensities as a function of pressure. A reasonable agreement for calculated and measured thresholds has been obtained, over the pressure range used in experiments previously carried out in this laboratory, if aberration is present in the optical system used. Improvements in the computing techniques have been introduced which are based on the assumption that electrons gain a variable amount of energy from the field, rather than the fixed amount which is exactly equal to the separation between two steps. Interpolation methods are then used to find the number of electrons at these fixed steps. A reasonable ionization growth is attained, but unexpected oscillation in the energy distribution of electrons is observed. The concept of energy diffusion of electrons in energy space is introduced into the energy gain equation of electrons. Smoothing of the energy distribution function, as well as a large amount of ionization growth, is achieved. Order of magnitude agreement between measured and calculated threshold intensities is obtained for helium irradiated by 40 n sec. ruby radiation at the pressure range 103.2 x 104 torr. An indication of the importance of a step-wise ionization process on the ionization growth and breakdown has been deduced, and the time and excited atom concentration at which this process starts to be significant, has been disclosed. Computations based on the combined effects of 3-photon absorption and electron collisional ionization of excited atoms agree with experimental values of breakdown threshold intensity provided the 3-photon absorption coefficient B is -10-21, corresponding to a photon absorption cross-section o-,r10-16 cm2.

No description available.