AbstractWe calculated ASE propagation in a Cassegrain-type amplifier. The code used in the calculations is applicable to a multipass amplification system. Output fluence of the ASE at the exit of the system can be decreased by optimization of design parameters. Ratio of diameter of main mirror hole to sub mirror diameter is the most sensitive parameter, and should be larger than 1.2. It is suitable to set gain medium (laser glasses) as near to the main mirror as possible. We found optimal values for a new amplifier chain in GXII laser facility which can deliver ultrashort and high power pulses for the basic research on fast ignition (Fujita et al., CLEO '97, CFK 3, Baltimore MD, 1997; [1]).