Abstract: 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.