An efficiency near unity can be achieved in optical pumping devices for lasers, where unabsorbed light is refocused at the light source, leading to multiple passes of pumping energy through source and laser. In properly dimensioned elliptical mirrors this can, in principle, be done by placing source and laser outside of the focal points. Furthermore, in these “exfocal” elliptical designs the light density of the source is transformed into the laser at a ratio of 1:1, resulting in very low-threshold energies. The lowest value observed for ruby lasers up to 7.6 cm in length was 50 W-sec at room temperature in a rotational-ellipsoidal mirror where the axis of source and laser are oriented in the rotational axis of the ellipsom, while their lengths are equal to the distance between focus and wall. Alternative designs are exfocal elliptical and circular cylinders. In exfocal ellipsoids the pump light distribution is of exactly rotational symmetry which leads to symmetrical absorption of pumping light in the laser. As a result, quasi-periodic relaxation oscillations of 5000-μsec duration and a component of continuous emission have been observed at room temperature with 300 W-sec pumping energy.
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