A very narrow band KrF excimer laser capable of producing 10 mJ laser pulses at 1000 Hz with a bandwidth of less than 0.6 nm (FWHM). The very narrow bandwidth is achieved by reducing fluorine partial pressure to less than 0.08 and by increasing the reflectance of the output coupler to greater than 25 percent. In a preferred embodiment, prior art fused silica beam expansion prisms used in the prior art line narrowing module were replaced with calcium fluoride prisms.
