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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)

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)

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)

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)

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)