A KrF laser has been recognized as an attractive medium for the amplification of ultrashort pulses, because it has high gain and wide spectral width in the UV region. However, amplified spontaneous emission (ASE) significantly degrades the performance of the short pulse amplification. Suppression of ASE is the most critical issue for designing ultrashort puise excimer lasers. The low divergence ASE from the front end of multistaged amplifiers makes it possible to use tight spatial filters. Thus most of the research has been done in low gain conditions to eliminate prepulse target iliumination. © 1989 Optical Society of AmericaPDF Article