Abstract: A laser includes a lasing medium having a slab geometry, a waveguide arrangement either side of the slab, mirrors forming an unstable sub-resonator acting in a plane perpendicular to the waveguide, and a concave mirror arrangement acting on light incident on it launched from the waveguide, and thereby transformed from waveguide light to free space light, to redirect and retransform a portion of said light to be re-entrant waveguide light constituting the unstable sub-resonator round-trip self-replicating light. The unstable sub-resonator preferably includes a folding mirror to direct light towards the concave mirror, and the concave mirror is preferably partially transmissive.

Abstract: An RF-excited discharge laser includes a pair of electrodes and an RF power source, both electrodes being ungrounded, the power source being adapted to apply an RF voltage to both electrodes, the respective voltages applied to each plate being out of phase. A balanced discharge is thereby established in which the inter-electrode voltage is unchanged whilst the potential difference between each electrode and the surrounding grounded surfaces is halved. Thus, the trade off between discharge power and discharge escape likelihood is overcome. The voltages applied to the electrodes are preferably 180 degrees out of phase, or nearly so. One way of delivering the RF energy to the electrodes is to provide a power splitter to divide the output of the RF supply into two outputs, one of which is delivered to one electrode and the other of which is delivered to the other electrode in substantial anti-phase.