Folded RF-excited CO₂ waveguide lasers

This thesis describes theoretical and experimental work on RF excited CO₂ waveguide lasers and amplifiers.The mode coupling losses at a bend in a folded waveguide have been evaluated as a function of the selectable parameters to determine the low-loss folding geometries. A direct comparison is made between three types of optical arrangement used for folding in a compact, sealed-off, Z-fold CO₂ waveguide laser excited by a transverse RF discharge. In particular, the measured laser output power as a function of discharge conditions and mirror alignment for plane and curved mirror, and partial waveguide folded resonators are compared.The Z-fold laser output power is predicted by incorporating the known and estimated laser parameters into a Rigrod-type analysis. A simultaneous solution of the Rigrod equations predicting the laser powers for different intra-cavity gain lengths is used with the experimental data to derive the discharge and resonator parameters. Experimental results are in good agreement with the theoretical predictions, and suggest that a M% power loss per fold has been achieved with partial waveguide folding. Also, the preliminary theoretical results of a multi-mode resonator model predicting the laser output power as a function of the angular mis-alignment of one of the Z-fold laser folding mirrors are in qualitative agreement with the experimental determinations.Experiments related to laser efficiency and frequency stability are discussed briefly. These include an investigation into an automatic impedance matching scheme for dynamic optimisation of the power transfer efficiency between RF generator and the laser head; the Opto-Hertzian effect (RF equivalent to the opto-Galvanic effect) for laser frequency stabilisation; a novel parallel-resonant distributed inductance excitation technique using a multi-start solenoid; and finally, identification of hooting laser resonator modes responsible for impeding heterodyne measurements Mien a clean RF spectrum is required.In addition, theoretical and experimental studies of laser amplification are presented. The suitability of folded waveguide and non-waveguide structures for power amplification or pre-amplification is assessed to determine their applicability to coherent LiDAR. Optical amplification of wideband transmitter and/or receiver signals is considered a favourable way of improving the discrimination of range and velocity determinations.Finally, as a result of this work, up to 53.4 Watts output power in a high quality fundamental Gaussian beam has been obtained from a compact, sealed-off, Z-fold CO₂ waveguide laser with a 115 cm discharge length, which implies a specific power performance of 0.46 W/cm. Efficiencies (laser output power/RF input power) of up to 9.2% have also been observed