FLUID DYNAMICS IN CLOSED CYCLE PULSED LASERS

Abstract

S types of repetitively pulsed gas lasers use closedcycle flow systems to minimize gas consumption, eliminate exhaust, and provide minimum weight laser systems for long run time These lasers span the spectrum from infrared to ultraviolet with output pulse lengths from tens of milliseconds down to tens of nanoseconds. Pulse repetition rates as high as 1500 Hz have been demonstrated. A closed cycle is used with CO2 lasers to minimize gas consumption and system weight for long run times. Axial discharge CO2 lasers up to 5 kW output power with recirculated axial flow are commercially manufactured for industrial However, this paper will concern only transverse flow lasers whose flow, optical, and excitation axes are orthogonally disposed. Several pulsed electric discharge CO2 lasers have been reported with recirculating transverse flow systems. Commercially manufactured pulsed CO2 closed-cycle lasers with transverse flow are used as laboratory sources. Pulsed CO2 closed-cycle lasers have also been developed for Lidar applications. Closed-cycle recirculation systems have also been added to pulsed HF/DF chemical lasers to reduce gas consumption and eliminate noxious effluents. These discharge-initiated lasers incorporate chemical scrubbers to remove chemical reaction products. Closed-cycle gas recirculation systems are used to allow high output power and to conserve expensive rare gas mixtures in pulsed excimer lasers. Several closed-cycle excimer laser systems have been described in the literature, and small closed-cycle ArF, KrF, XeCl, XeF lasers are commercially available. These excimer lasers are used for spectroscopic studies, to pump dye lasers, and for materials processing with ultraviolet radiation. A closed-cycle recirculation system must efficiently supply laser gas with the correct properties and adequate medium homogeneity to the repetitively pulsed laser cavity. The system must remove detrimental chemical contaminants and replace chemical components consumed in the laser reaction or in wall reactions. Flow system component reliability and gas lifetime are critical issues for long run The flow system must provide flowing gas to the laser cavity with sufficient medium homogeneity to allow the desired output optical quality and to assure discharge or reaction uniformity. Index of reflaction disturbances resulting from density or composition nonuniformites exponentially degrade optical quality and Strehl ratio. The density nonuniformity must be maintained below a level on the order of 10 ~ to provide a 90% Strehl ratio over a i m path length in a typical XeCl mixture. This homogeneity requirement scales with the ratio of the wavelength to the path length. Electrical discharge stability is sensitive to disturbances in the E/n parameter caused by density nonuniformities in the discharge cavity. Density nonuniformities must be kept below 1% to assure discharge stability in a typical discharge geometry. High laser system efficiency requires efficient movement of the lasing gas around the closed loop. The gas mixture, cavity volume, repetition rate, and optical length are determined by the desired laser output requirements. Discharge or excitation requirements specify the cavity pressure and excitation height. However, fluid mechanical design controls the cavity flow velocity and the power required to circulate the flow. The pulsed power system controls the efficiency of laser systems with small output apertures or low pulse repetition rates. Fluid dynamic design becomes more important for larger laser devices. For a given aperture width, the efficiency of a flow system is directly related to its size since the individual component pressure drops are proportional to the inverse square of the local flow area. A well-balanced flow system design allocates volume and pressure drop to best obtain particular design goals.