The high power laser includes a high gain medium (>80%) such as a lightly doped Nd:YAG rod contained within an unstable optical resonator of the type wherein power flows outward from the optical axis of the resonator. The output beam is coupled out of the resonator by means of a coupling device having a high coupling factor (>55% of the circulating power inside the optical resonator) and in a preferred embodiment the coupling factor is as high as 80-90%. The optical resonator is preferably a confocal resonator employing one concave mirror and one convex mirror. The convex mirror forms the output coupling device for providing a high degree of diffraction output coupling around the periphery of the output mirror. The output beam is collimated in the region of the output mirror. The curvatures of the two mirrors forming the optical resonator are dimensioned to take into account the positive focusing lens effect of the high gain medium produced by a transverse thermal gradient in the high gain medium encountered in use when pumped by flash lamps. The laser is Q-switched by means of a linear polarizer and a pulsed KD*P quarter wave plate. The output beam has 25 megawatts of peak power with a pulse width of 10 nanoseconds, a pulse repetition rate of 10 pulses per second. The output beam has a power density of 75 megawatts per square centimeter.