preferred embodiments of a purge gas port , laser beam attenuating input window , and laser shutter constitute subsystems of a uv laser optical system in which a laser beam is completely enclosed to reduce contamination of the optical system components . purge gas is injected through multiple locations in a beam tube assembly to ensure that the optical component surfaces sensitive to contamination are in the flow path of the purge gas . the input window functions as a fixed level attenuator to limit photopolymerization of airborne molecules and particles . periodically rotating optical elements asymmetrically in their holders reduces burn damage to the optics .