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A tunable diode laser (TDL) operating in the region of 2150 to 2350 cm⁻¹ wavenumber is used in this work to investigate the dynamics of level populations in CO₂ lasers. The wide tunability of the TDL is exploited in determining the populations in any level of the CO₂ molecule. Thus, the vibrational population distributions in a CW CO₂ laser discharge under both lasing and non-lasing conditions are measured and are compared with the mode temperature model. The relaxation processes associated with the upper and lower levels of CO₂ lasers are also investigated. In particular, the electron excitation and de-excitation rates of the upper laser level in typical CO₂ discharges are determined directly from the experiment, and it is shown that electron de-excitation is responsible for the saturation of v₃ mode temperature in electrically excited CO₂ lasers at high discharge currents, and imposes a fundamental limitation on the gain attainable in CO₂ lasers. The relaxation of the lower laser level is complicated by the occurrence of several competitive vibration-vibration (V-V) processes. The TDL measurements enable us, for the first time, to monitor separately the population in all levels of concern to the relaxation of the lower laser level and to determine the rate constant of each of the available decay channels. It is shown that the O2²O level plays an important role in the relaxation of the lower laser level. Doctor of Philosophy (PhD)

The major purpose of the work described in this thesis is the experimental and theoretical study of the amplification of the spontaneous emission in dye amplifiers. This work falls into three distinct sections: (a) the study of high-current short-pulse flashtubes operating in (or near) the ablation regime; (b) the study of the amplification process of the spontaneous emission in a homogeneously broadened medium, such as a dye material, in the thin cell case, where there is no variation of the pumping across the cell; and, (c) the study of the amplification of the spontaneous emission in the thick cell case, where gain and index of refraction inhomogeneities exist across the cell thickness. In each case, we have developed theoretical models and compared their predictions with the experimental results. In particular, for the ablation-type flashlamps, it is shown that their behaviour is well represented by an analysis in which the discharge plasma is supposed to radiate as a blackbody at the temperature of the plasma. This model accounts for the fact that the light output from a flashlamp saturates whereas the current does not, provides a more accurate value for the plasma resistivity as a function of temperatures and gives a useful design criterion for determining the physical dimensions of an ablation-type flashlamp. With respect to the amplification of the spontaneous emission in a thin cell, it is mainly shown that (i) the spontaneous emission propagating along the gain medium is subject to narrowing along with shifting of its peak wavelength; (ii) even at moderate pumping the population of the excited singlet state, which determines the gain of the system, is non-uniform along the cell; (iii) it is possible to calculate the unsaturated gain by calculating the intensity of a particular wave-length and its spectral narrowing both as a function of the exciation and compare them with the same quantities measured experimentally; (iv) it is also possible to develop a semianalytic system independent method to measure the gain approximately. The theory developed to account for the propagation of the amplified spontaneous emission (ASE) has been partly extended to cover the case of the propagation and spectral narrowing of the ASE in a thick cell, where non-uniformities of the index of refraction and the gain are important along with the diffraction losses. In this particular case, it is shown that a model, which assumes an index step variation across the cell, and solves the Maxwell's equation for the field propagation along the cell, can account for the spatial distribution of the field across the cell and its manifestation at the far field. Most of the predictions of the model are verified experimentally. In particular, it is shown that (i) in a thick cell, part of the ASE propagates outside the gain medium, because of the lensing effect caused by heating of the lasing material; (ii) the field across the cell can exhibit stable spatial modes, which can produce a far field far from the amplifying cell axis; (iii) it is possible to improve the spectral narrowing of the ASE since saturation can be delayed to higher pumping than before and most of the narrowing occurs below saturation of the gain; (iv) it is also possible to measure the temperature and the index of refraction change due to heating by measuring the far field angle, and estimate the diffraction losses involved in the propagation of the ASE in a thick cell. Finally, a number of suggestions on further research are proposed. Doctor of Philosophy (PhD)

The major purpose of the work described in this thesis is to characterize the basic properties and evaluate the potential of optical fibre filters. As it turns out, these filters have a spectral bandwidth in reflection which can be varied over a very wide range, from as narrow as 0.01 to several angstroms (A). As a result, they are potentially very important for applications as multiplexers and demultiplexers in Wavelength-Division-Multiplexing (WDM) in Optical Communication Systems (OCS) (WDM is a means of increasing the data handling capability of a given fibre system through the use of multiple propagating wavelengths). Up to the present moment, gratings in the form of slabs have invariably been employed as the multiplexers and demultiplexers in WDM which greatly restricts the number of wavelengths which can be used since a large spectral bandwidth separation (>100 Ǻ) is required between adjacent channels is such systems. The optical fibre filters we report here can have a spectral bandwidth narrower than 1 Ǻ and consequently, employing them as multiplexers and demultiplexers in WDM allows a lot more channels to be transmitted. Other than this particular application, optical fibre filters can equally well function as resonant reflectors in a laser cavity, distributed feedback devices. tunable filters or external wavelength selective reflectors for heterojunction injection laser by end butting them to the latter. Furthermore, they have the potential to be used for equalizing material dispersion in OCS employing single mode fibres. Lastly, they can be tailored to give multifilters, comb filters or in general, filters that perform a variety of specified functions. The approach to the characterization of optical fibre filters is divided into two parts. The first part deals with the development of a theoretical model that can be used to predict and describe the performance of the optical fibre filters. The second part describes the experiments for the actual fabrication of the filters and subsequent extraction of some of their physical parameters. By comparing the theory with the experimental results, we are able to deduce the range of filters available when using an Argon Iaser as the means of writing filters and subsequently evaluate their full potential in OCS. Also, a very important relationship between the optical writing power applied to a fibre and the resultant index perturbation in the fibre or equivalently, the resultant filter characteristics, is established so that the actual fabrication of filers designed to satisfy specific function can be performed. Master of Engineering (ME)

It was the purpose of the work described in this thesis to develop a basic understanding of the passive mode-locking behaviour of TE CO₂ lasers employing the saturable absorbers SF₆ and germanium (Ge). An understanding of the dynamical processes, which occur in the gain and loss media on sub-ns to μs time scales, is necessary in order to predict the output characteristics of mode-locked pulse trains. In the case of SF₆ the complex dynamical behaviour was not known previous to our work. Therefore, a large portion of this thesis is devoted to investigating such processes. Transmission measurements, using ~200 ns TE CO₂ laser pulses, are used to obtain the first complete measurements of the saturation characteristics of SF₆. The experiments are performed for a range of CO₂ wavelengths which characterize the entire SF₆ absorption at the 10.4 μm band (P(12) - P(28)) and for a range of SF₆ gas pressures which are most frequently used in SF₆-CO₂ mode-locking systems. These experiments demonstrate that, for low J-value CO₂ lines (less than P(24)), the rise in transmission at increased pulse intensities is due to intensity saturation effects. The pressure dependence of the saturation curves indicates that the level relaxation rates scale with SF₆ pressure. However, for CO₂ lines greater than P(22) (longer wavelength), the transmission curves and the transmitted pulse shaping cannot be accounted for solely by intensity saturation effects. It is shown that a multi-vibrational and rotational level treatment for the absorption of CO₂ radiation by SF₆ can fully account for the discrepancies between theory and experiment observed at longer CO₂ wavelengths. The model treats all the SF₆ vibrational and rotational levels as belonging to a bath of levels characterized by a single virbational and a single rotational temperature. Energy absorbed for a laser pulse is rapidly distributed to the bath of levels, establishing new vibrational and rotational populations characterisitics of a higher bath temperature. This type of absorption process depends on the energy rather than intensity, of the laser pulse. Such a model, although expected to apply only at high SF₆ pressures (where fast equilibrium times exist), can in fact be used to predict double-resonance signals observed at low SF₆ pressures (1-10 Torr). Infrared double-resonance experiments with mode-locked pump pulse trains and with ~200 ns duration pump pulses demonstrate that the success of the model at these low pressure is due to very fast vibration-to-vibration and rotational energy transfer rates. The observed rates are much faster than those reported in the literature. It is shown that this heating model, combined with the previously mentioned intensity saturation processes, fully describes the transmission data for the wide range of CO₂ rotational lines and SF₆ pressures employed. This thesis also demonstrates that the mode-locking of a TE CO₂ laser using an SF₆ saturable absorber, can, for the first time, be understood from a knowledge of the above intensity saturation and vibrational bath heating effects. The mode-locking with SF₆ is reported over the widest range of CO₂ rotational lines and with the shortest pulse durations obtained to date. Pulse narrowing across the mode-locked pulse train (usually necessary to obtain very short pulses) is absent in the SF₆-CO₂ mode-locking system. It is indicated that processes such as multi-level saturation, vibrational bath heating and multiple-phonon absorptions may prevent such narrowing from occuring. The mode-locking behaviour of p-type Ge is also investigated in this thesis. Computer simulations of the evolution of mode-locked pulse from noise are presented using a 2-vibrational level model, which includes rotational coupling, to describe the CO₂ amplifier. A steady-state inhomogeneously broadened saturation response is shown to be appropriate for the Ce absorber. A set of density equations is used to describe the growth of the pulse intensities in the amplifier. For the first time a theory is presented which quantitatively predicts the experimentally observed pulse narrowing across the mode-locked pulse trains, the pulse intensities and shape of the mode-locked train envelope. It is also shown that there is a progression from deterministic mode-locking at laser pressures of ~300 Torr to statistical mode-locking at pressures substantially in excess of this pressure. Furthermore it is demonstrate that there is an optimum value in how close to lasing threshold one can operate to achieve repeatable, clean, short-duration, high intensity mode-locked laser pulses. In addition, it is shown that too large a linear loss cavity and/or too low a ratio of the non-linear to linear loss is detrimental to the production of short-duration pulses. It is also indicated that a laser cavity which diverges the laser beam at the absorber must be used to obtain optimum mode-locking for high pressure lasers (e.g., 5 atm). The choice of the correct cavity is the single most important consideration necessary to achieve stable short-duration ( Doctor of Philosophy (PhD)

The work described in this thesis concerns the investigation of diode laser dynamics on the time scale of 100 picoseconds or less. Predictive models based upon the diode laser rate equations are developed. A requirement of such predictive models is that the necessary parameter values be determined by methods independent of the comparison between model predictions and experimental verification. For this reason, a great deal of effort has been directed towards the careful utilization of existing techniques for parameter determination and the development of new techniques where existing techniques were lacking or inadequate. Two dynamical operating regimes are investigated. The first deals with single current pulse pumping of the laser. A time domain model (JLASER), allowing explicitly for wavelength dependence of system parameters and dynamics, was developed and is shown to generate good agreement with experimental measurements of the relaxation oscillation response to the current pulse excitation. The spontaneous emission and the multi-mode nature of the laser are shown to be of particular importance in determining the diode laser pulsed response. The other operating regime investigated is active modelocking of the diode laser in an external cavity. This mode of operation allows extremely short pulses to be obtained, with a much milder current modulation requirement and therefore better spectral control, than in the case of single current pulse pumping. A frequency domain model, incorporating the effects of chip facet reflectivity and gain saturation, is developed. Good agreement is found between this model and experimental measurements of the modelocked pulse characteristics. The detailed characterization carried out on the external cavity coupling efficiency is shown to be critical for accurate model predictions. Doctor of Philosophy (PhD)

The work described in this thesis concerns the investigation of the absorption properties of SF₆ in the 10.4 µm band of CO₂ lasers. Although SF₆ has been used frequently as a saturable absorber in CO₂ laser systems, the complex dynamical behaviour of its absorption not understood prior to our work. A model is developed which allows for the multilevel nature of the absorption processes. This model treats all the SF₆ vibrational and rotational levels as belonging to a bath of levels characterized by single vibrational temperature and a single rotational temperature. Energy absorbed from a laser pulse is rapidly distributed throughout this bath, establishing a new vibrational population distribution characteristic of the higher vibrational temperature. The vibrational equilibrium is readily maintained at high SF₆ pressures. It is demonstrated that in this case the model successfully predicts the transmission and pulse shaping of high power CO₂ laser pulses. In particular, the rapid decrease in absorption at high incident intensities is shown to be due to vibrational heating and not to any intensity saturation process. Dissociation which occurs as a result of sufficient vibrational heating is discussed. The effect of dissociation on single pulse transmission is determined through simple modifications of the model. The model is also shown to have application to low SF₆ pressures. Previous investigations concerning vibrational relaxation rates in SF₆ would indicate that vibrational equilibrium cannot be maintained in such a situation. More recent results show that in fact sufficiently rapid vibrational energy exchange can occur. The final portion of this thesis discusses the dependence of these rates on the vibrational energy of the molecules. An approximate, but conceptually simple, method of incorporating these effects into the model is described. In conclusion, it is shown that the model is capable of determining the SF₆ absorption behavior for a wide range of SF₆ pressures and CO₂ laser wavelengths when both intensity saturation and vibrational heating effects are included. Master of Science (MS)

The work described in this thesis concerns the investigation of the properties of a new type of CO₂ laser, one which operates in the wavelength region near 4.3 μm rather than in the 9-11-μm region where most CO₂ lasers operate. Lasing at 4.3 μm is achieved by using a 10-μm sequence CO₂ laser to optically pump CO₂ molecules which have been excited in an electrical discharge. The laser is based on existing CO₂ laser technology, and therefore it is potentially a very useful source of coherent radiation in a spectral region where there are presently few lasers suitable for widespread use. Preliminary experiments are described which serve to identify the various physical processes involved in 4.3-μm Iaser action. Efficient optical pumping is demonstrated and peak output powers of 1 kW are observed. A theoretical model of the laser is developed. The model can accurately predict the observed 4.3-μm output and it is shown that the lifetime of the upper laser level is the critical factor in determining the gain dynamics of the laser system. Two principal operating regimes are identified. The first is a high repetition rate Q-switched mode suitable for the production of high average power. A Q-switched 4.3-μm laser is constructed, employing continuous discharge tubes and SF₆ is used to passively Q-switch the sequence oscillator. The average output power at 4.3 μm is 120 mW. The other operating regime is a high power pulsed mode. This involves scaling the laser to higher operating pressures, which requires the use of high levels of sequence pumping power. To this end, a TEA sequence CO₂ laser having output energies of up to 6 J per pulse was developed, and design criteria for such lasers are presented. The scalability of the 4.3-μm laser, using the TEA sequence laser as the pump source, is discussed. The factors which limit the maximum output power attainable are identified, and guidelines for the construction of high power 4.3-μm lasers are presented. Doctor of Philosophy (PhD)

Most semiconductor laser configurations have an active region width comparable to the wavelength of the radiation giving rise to potentially important diffraction effects. In this work, optically confining mechanisms which can affect the magnitude of those diffraction effects are investigated in electron-beam-pumped GaAs bulk material and GaInAs hetero-epitaxial layers of various thicknesses. One major aspect of the laser behaviour is the time delay between the start of the excitation and the onset of lasing. It has been reported that various semiconductor lasers can exhibit time delays to threshold which exceed by as much as two orders of magnitude the time required for the gain or the radiation to build up within the resonator. In the first part of this thesis, the dependence of the time delays on the electron-beam voltage and current is studied experimentally and theoretically in GaAs bulk material. A model is proposed which accounts for the long time delays observed on the basis of transient waveguiding. According to this model, there is a growing refractive index difference between the active region and the underlying passive region which progressively confines the radiation thereby reducing the cavity losses and eventually bringing about lasing threshold. The spatial and temporal variations of the refractive index are attributed to corresponding variations in the temperature and the gain. As an integral part of this model, minority carrier concentration and temperature profiles resulting from the differential energy loss profile of the electron beam are derived. To test the above hypothesis, the measurements were repeated on GaInAs hetero-epitaxial layers. Incorporated in such structure is a permanent refractive index difference between the epitaxial layer and the substrate which confines the radiation within the epitaxial layer. As suggested by the model, for layer thicknesses of the order of the electron-beam penetration depth, only short time delays to threshold are observed. This demonstrates that a confining structure can affect dramatically the lasing behaviour. In the second part of this thesis, the transverse distribution of the radiation within hetero-epitaxial layers is studied in more detail. An electromagnetic model for such a cavity is introduced in which the spatial variation of the refractive index within the epitaxial layer is approximated by a step profile. The importance of the permanent confining surface is shown to depend on the cavity parameters such as refractive index difference between active and passive region, absorption in the passive region and thickness of the epitaxial layer compared to the electron penetration depth. The specific conditions under which transient waveguide effects take preponderance over the permanent optical confinement are derived explicitly. It is found that the spatial distribution can be very sensitive to the permanent waveguide effects. Accordingly, the far-field diffraction patterns of the lasing radiation were measured on hetero-epitaxial layers of various thicknesses and on bulk material under various pumping conditions. The theoretical fit to the experimental data shows that multimode lasing is a common occurence in epitaxial layers. Moreover, the time-resolved spatial measurements on the bulk material confirm that long time delays to lasing threshold are effectively associated with optical confinement of the radiation. There is evidence that waveguiding mechanisms as described here are also important factors in the behaviour of other semiconductor laser configurations. Time-resolved measurements of the far-field patterns could be used to verify this proposition. Doctor of Philosophy (PhD)

The work presented in this thesis concerns the development and characterization of the first optically-pumped continuous (cw) lasers in the mid-infrared. Ammonia is chosen as the active lasing medium, and this research describes experiments in which NH₃ is pumped both off- and on-resonance with a CO₂ laser using several innovative techniques. The new lasers provide efficient, powerful and line-tunable cw radiation in a region (10 to 30 μm) where such sources are scarce. Initial experiments were performed with several off-resonantly pumped 12-μm NH₃ transitions and, at low pumping powers, a Raman two-photon process was identified as the dominant mechanism responsible for the gain. The two-photon process results from the coherent interaction of two laser fields with a three-level system and can create gain in the absence of a population inversion. This property was essential to the successful operation of the first optically-pumped cw laser in the mid-infrared as the available pump intensity was insufficient to produce a population inversion. A detailed understanding of this new cw laser was achieved by probing the small-signal gain with a tunable diode laser in an amplifier cell. A theoretical model based on a density matrix formalism approach was developed and good agreement was found between experiment and theory. The tunable diode laser measurements emphasized the importance of the pump intensity in determining the magnitude of the Raman gain. Following this study, an optimized 12-μm cavity was constructed in which the pump intensity was maximized by using a small capillary tube. Quantum efficiencies as high as 45% and output powers up to 10.5 W were obtained. Line-tunability between 10.7 and 13.3 μm was accomplished with a novel pumping scheme. The frequency of the CO₂ laser is shifted into coincidence with the line center of an NH₃ transition using acoustooptic modulators. This on-resonance pumping enabled us to create a vibrational inversion between the NH₃ ground state and the upper vibrational level v₂=1. By adding a buffer gas, gain was obtained in the entire P- and Q-branches. A simple model which assumes thermalization among the rotational levels fully accounts for the lasing behavior. Doctor of Philosophy (PhD)

A fast, simple photodetector which is compatible with optical integration techniques can be produced using a design consisting of an interdigital metal -- semiconductor -- metal (MSM) photodiode. The general operation of these devices is discussed with emphasis placed on basic device characteristics such as IV, CV, and steady state and pulsed light response. Ideas and models are presented to allow predictions of generic device performance as well as design and optimization of specific devices. These theoretical aspects are validated through comparison with experimental results. Doctor of Philosophy (PhD)

The group Ill-V alloy quarternary semiconductors, such as Ga(₁₋y)InyAs(₁₋ᵪ)Pᵪ have been extensively employed in lattice matching different semiconducting layers (at specific bandgaps) to form heterojunction electro-optical devices. However, these cover only a limited set of direct bandgap/lattice constant combinations. The analogous pentenary alloys, consisting of the ternary chalcopyrite groups l-lll-VI₂ and II-IV-V₂, have the potential of similar applications as they cover a even wider band/lattice range. As, a prototype of such alloys, samples the pentenary Cu(₁₋y)AgyInS₂(₁₋ᵪ)Se₂ᵪ have been synthesize and studied. Samples were prepared by reacting stoichimetric powder mixtures at about 900ºC. X-ray diffractometry tests suggest the compounds maintained complete powder solid solubility throughout the system in the chalcopyrite crystal structure. The intrinsic conductivity type of the alloys appear to follow a trend towards n-type for silver and sulfur rich compounds, while forming p-type for copper and selenium rich materials. The bandgap of these samples were measured using cathodoluminescence techniques, which generally have some ambiguity in their resulting estimates. To generate better values of the band parameters extensive computer modeling for emission spectra from heavily doped direct bandgap materials was done. The effect of band tails and Gaussian impurity states on the luminescence spectra was studied for changes in doping densities, temperature and carrier injection levels. Formulae were derived from these models to obtain better estimates of the bandgap and impurity activation levels. Algorithms were developed to obtain the impurity spreading energy of a tailed or Gaussian band, and the quasi-Fermi energy levels for injected current in a material with a specific band structure. Cathodoluminescence measurements were made at 300 and 77 K on the samples. As predicted by the models, it was found easier to generate good estimates from the 300 K results due to the obscuring effects of the impurity bands at cooler temperatures. Using these band estimates least squares fits were made on the data to generate the topological bandgap/lattice constant versus compositional value maps for the studied alloy. Doctor of Philosophy (PhD)

Title: Pulse Delays and Gain Characteristics in TEA CO2 Lasers, Author: John Reid, Location: Thode A novel method is reported for studying the time-dependent gain and cavity losses in a TEA CO2 laser. The technique involves an investigation of the time delay of the onset of lasing with respect to the current pulse in such a laser. Detailed measurements of this time delay as both the laser gain and the cavity loss are varied, in conjunction with a simple theoretical analysis, permit the evaluation of parameters describing the time-dependent gain and the cavity loss. A refinement of the technique, using two amplifier tubes, allows an accurate measurement of the decay time of the gain. Thesis Master of Science (MS)

Title: Gain Saturation and Pulse Solutions for a CO2-HE T.E.A. Laser, Author: Terence Tricker, Location Thode The saturation behaviour of various gain models for the CO2-HE T.E.A. laser was investigated. The corresponding laser pulse shapes for an oscillating cavity were generated by computer and compared to experimentally observed output pulses. Good agreement is obtained for low gain using a simple two-level model but at high gain, vibrational coupling between levels must be included. Thesis Master of Engineering (ME)