Density Homogeneity Control in Repetitively Pulsed Gas Lasers

Abstract

Numerous laser applications require a highly collimated output beam. To achieve such collimation in a gas laser, the gas density variations must be carefully controlled. Depending on the type of laser to be used, its operating pressure and application, the permissible spatial density variations inside the laser cavity, at the time of lasing, may not exceed 1×10−5 to 1×10−3 of the mean gas density. This is particularly demanding in repetitively pulsed gas lasers where large cavity pressure and density disturbances are inherent. Three flow elements are typically used to achieve the desired density uniformity in repetitively pulsed lasers. These are: (1) a thermal homogenizer, which suppresses the temperature inhomogeneities in the gas feeding the cavity; (2) a contoured cavity entrance region, with boundary layer control, that maintains a uniform flow towards the laser cavity; and (3) an acoustic wave suppressor, which attenuates both the high-frequency pressure waves generated in the laser cavity and the lower frequency waves generated in the flew system, outside the cavity. Vie have built a complete closed-loop flow system, with e-beam generated pulsed energy deposition in the test section, to simulate a repetitively pulsed laser flew loop. All three medium homogeneity control elements, mentioned above, were incorporated in this loop. Loop performance, in terms of cavity pressure and density uniformity, was characterized. Where available, results were compared with model predictions. Significant insight was gained as to the origin of cavity density variations and the means required to control them. The loop tested achieved a 1×10−4 level of density uniformity. Approaches to further reduction in cavity medium nonuniformities in repetitively pulsed gas lasers are outlined.