ACTIVE STABILIZATION OF THE BEAM POINTING OF A HIGH-POWER KrF LASER SYSTEM

An active beam-pointing stabilization system has been developed for a high-power KrF laser system to eliminate the long-term drift of the directional change of the beam in order to have a stable focusing to a high intensity. The control of the beam direction was achieved by a motor-driven mirror activated by an electric signal obtained by monitoring the position of the focus of the output beam. Instead of large sized UV-sensitive position sensitive detectors a simple arrangement with scatter plates and photodiodes are used to measure the directionality of the beam. After the beam stabilization the long-term residual deviation of the laser shots is similar to 14 mu rad, which is comparable to the shot-to-shot variation of the beam (similar to 12 mu rad). This deviation is small enough to keep the focal spot size in a micrometer range when tightly focusing the beam using off-axis parabolic mirrors.