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A study of the design and development of transversely excited atmospheric pressure carbon dioxide lasers has been carried out. Three lasers were constructed with output energies ranging from a few millijoules to several joules. Damage in transmitting output mirrors is one of the limiting factors in the development of high energy density carbon dioxide lasers. One form of damage in uncoated germanium components is characterised by regular patterns. A detailed examination of these damage patterns on several mirrors has been carried out using both optical and electron microscopy. A semi-quantitative physical model describing the damage structure formation is proposed, and good agreement with experimental observation is found. The thesis goes on to examine in depth particular aspects of the model with the object of making the analysis fully quantitative. The thesis concludes by preparing the necessary data base for quantitative computer modelling. A full description is prepared of the behaviour of all the relevant physical parameters characterising germanium from room temperature to melting point temperature (300K - 1210K). The technique adopted is to use a theoretical (or sometimes empirical) framew6rk within which to extrapolate from available published data into parameter ranges not hitherto available. For germanium, numerical values have been assigned to all relevant physical parameters (thermal, electrical and optical) over the full temperature range. The way is now open for computer simulation of. the interaction between laser radiation and germanium under a wide variety of circumstances. A significant refinement of the model describing damage structure has already occurred. When the magnitudes and temperature variation of Ε' and Ε" for germanium were established, the particular model behaviour of germanium suggested a generalised view of the behaviour of other materials. At the time of writing, the model is being adapted to account for the observations of other workers on a wide variety of other materials.

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The work presented in this thesis is involves two distinct topics. The first area is the main theme of the thesis, and is an investigation of the contribution made by the gain to the temperature sensitivity of long wavelength semiconductor lasers. The second topic is in a separate but related area and consists of an experimental determination of the valence band deformation potential, b, whose magnitude is found to be in good agreement with theoretical prediction. The thesis first presents an overview of the equipment used in the measurements, followed by a review of the available methods for making gain measurements on semiconductor lasers. It is concluded that the Hakki-Paoli method, in conjunction with the Cassidy method, provides the most suitable technique, but the measurement system must be very carefully set up to ensure valid results. The gain-current relationship is then measured in three quantum well lasers with 1.55μm tensile, compressive and unstrained active region respectively, and in two 1.3μm devices, with tensile and compressive quantum wells. It has been observed experimentally that the modal gain, G, varies linearly with the log of the drive current, I, in many quantum well lasers. This relationship was expressed by McIlroy as G= Go in011/0 ). It has been suggested that the strong temperature sensitivity of the threshold current in long wavelength lasers is due to strong temperature dependence of the gain characteristics. We show that the Go parameter is virtually independent of temperature in the 1.5μm devices studied, close to that expected for an ideal laser, while the 1.3μm lasers depart from the ideal case to some degree. The 1.54m devices all have a characteristic temperature, To, of = 70K, in good agreement with what would be expected if non-radiative phonon assisted Auger recombination, with an activation energy of 25meV, dominates the current. The lower To values in the 1.3μm devices of 42K and 50K respectively are consistent with an additional temperature dependence of the differential gain above that predicted in an ideal laser. In both cases it is concluded that Auger recombination makes the dominant contribution to the temperature sensitivity. In the second topic considered, photo voltage measurements are used to determine the energy splitting of the light hole and heavy hole valence subbands in a set of tensilestrained lasers. Using these measurements it is shown that a theoretical model, using the interpolated strain deformation potential determined by Krijn, gives good agreement with experiment.

This thesis presents the design, fabrication, characterisation and theoretical modelling of oxide confined GaAs/AlGaAs VCSELs operating around 850nm wavelength. Three VCSEL structures utilising different oxide aperture thickness were designed. The oxide apertures contained in these VCSELs can be easily formed through the selective wet oxidation process. Detailed experimental investigations were performed on these devices with emphasis placed on the dependence of the resonant wavelength characteristics, the threshold and modal properties on the aperture size and oxide thickness. A feasibility study of polarisation control through using nonsymmertical apertures was also carried out. These experimental activities were supplemented by theoretical modelling which included calculation of resonant wavelengths and eigenmode sizes using a scalar variational method that assumed Hermite-Gaussian transverse field variations. In addition, the light-current characteristics of the first two eigenmodes were also simulated by solving the photon and carrier rate equations, after their modal reflectance and transmittance had been calculated. The theoretical predictions were then compared with the experimental findings and ways of improving the model have also been suggested. The oxide apertures in these VCSELs provides both current constriction and optical confinement. The resultant index guiding strength is dependent on the thickness as well as on the longitudinal position of the oxide. Devices with stronger guiding have smaller mode sizes which results in lower modal reflectance, larger blue shifts in resonant wavelengths from that of plane wave values, and larger wavelength separations between the fundamental mode and the higher order modes. Devices with smaller aperture sizes also have larger spectral separations and blue shift in resonant wavelengths. When the symmetry in aperture shape is broken, the wavelength degeneracy between the TEM, o and TEM0, mode is also broken. Among the three VCSEL structures, the one with the strongest guiding provides single mode operation at larger aperture size and is less affected by thermal lensing effects. However, lower threshold currents and higher efficiencies can be obtained from devices with weakest guiding. Despite being more susceptible to thermal lensing effects, single mode power up to 1.72mW and threshold current as low as 350μA can be obtained from devices with weakest guiding design.

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