An optical power beam transmission systems, with a directional light transmitter and receiver. The transmitter contains an amplifying laser medium, and this together with a retroreflector in the receiver, forms a laser resonator. When lasing sets in, the receiver can extract optical power through an output coupler and convert it to electrical power. The gain medium may be a disc having a thickness substantially smaller than its lateral dimensions. The laser resonator is operated as a stable resonator to ensure safe operation. This is achieved by use of an adaptive optical element, for reducing the diameter of the energy beam impinging on the gain medium, thereby increasing the overlap between the energy beam and the gain medium. As the transmitter-receiver distance is changed, such as by movement of the receiver, the adaptive optical element focal length changes to ensure that the cavity remains within its stability zone.