[en] Laser systems in which efficiency and high repetition rate operation are obtained utilize an active laser medium consisting of pentaphosphate with high concentration doping of a rare earth such as neodymium and a pumping means in optical contact with the active medium which produces pump radiation confined to a band which essentially overlaps the absorption band of the laser medium. An active mirror configuration is described with thin layer of pentaphosphate active medium on one side of a substrate which is transmissive to the pump radiation. The pump radiation is provided by an excimer laser or by a semiconductor converter layer which is excited by flash lamps. The laser medium may be segmented with the space between segments occupied by material absorptive of amplified spontaneous emission wavelengths. The laser system is especially useful in laser fusion systems