Laser resonator for improving narrow band emission of an excimer laser

1. Field of the Invention An apparatus and method are provided for bandwidth narrowing of an excimer laser to 6 pm or less with high spectral purity and minimized output power loss. Output stability with respect to pulse energy, beam pointing, beam size and beam output location is also provided. The excimer laser includes an active laser medium for generating a spectral beam at an original wavelength, means for selecting and narrowing the broadband output spectrum of the excimer laser, a resonator having at least one highly reflecting surface, and an output coupler. Means for adapting a divergence of the resonating band within the resonator is further included in the apparatus of the invention. The divergence adapting causes the spectral purity to improve by between 20% and 50% and the output power to reduce by less than 10%. A method according to the invention includes selecting and aligning the divergence adapting means.