Experimental observation of unstable resonator mode evolution in a high-power KrF laser

Abstract

For a pulsed high-power laser with unstable resonator, the mode evolution time is important from the viewpoints of beam focusability and laser-energy usefulness. We observed temporal and spatial behavior of the mode evolution in a several-tens-joule KrF laser with a positive branch confocal unstable resonator, which is pumped by electron beams with a pulse width of 65 ns (FWHM). The beam divergence angle was decreased by a factor of 1/M2 (M:magnification) per round trip in the resonator and resulted in a full angle divergence of 0.6 mrad.