Influence of acoustic waves on TEA CO 2 laser performance

Abstract

In this paper we present results on the influence of acoustic waves on the output laser beam from high repetition rate TEA CO2 lasers. We show that acoustic waves generated inside the cavity lead to deterioration in beam quality, decreased output energy, and an increase in pulse to pulse energy variation. We investigate the impact of gas mix on the acoustic behaviour, and present experimental results on laser performance across a range of gas mixtures. Solutions to the acoustic wave problem are presented together with experimental results. The influence of acoustic damping measures on laser gain are demonstrated showing a significant improvement in gain and output power at high repetition rates. The link between the pre-ionisation method employed and the acoustic wave impact on laser performance is discussed. In this paper we report on acoustic wave experiments on two different laser systems. The first is a high repetition rate system capable of operating at repetition rates of up to 2000 Hz and delivering multi kW laser output, which was originally developed for molecular laser isotope separation. The laser employed a wind tunnel type flow loop with a centrifugal fan driven by a variable frequency drive which provided continuously variable flow speeds up to 90 m/s and provided sufficient clearing ratios for repetition rates in excess of 2000 Hz. The electrode structure consisted of 800 mm long discharge electrodes, separated by 20 mm. The laser employed spark pre-ionisation by two arrays of sparks placed up- and down-stream of the discharge electrodes with a spark separation of 25 mm. Flow profiles, approximating a flow nozzle, provided uniform flow in the electrode region. Arrays of current return feed-throughs were placed up- and down- stream spaced by 50 mm. The total energy supplied to the discharge electrodes was 20 J resulting in a specific energy deposition of 90 to 130 J/l atm, depending on the gas mixture. The system was equipped with a number of diagnostics tools to monitor gas dynamics as well as discharge and laser performance. The gas flow velocity was measured with a Pitot tube, while time resolved pressure measurements were made using fast miniature piezoelectric pressure transducers. Perturbations of the gas density in the inter electrode region were performed using a Schlieren system. Visual observations of the discharge, both in the gas flow direction and in the direction of the optical axis were carried out by CCD video cameras through dedicated observation ports. Recordings were made at 24 frames per second; therefore each video frame was averaged over many discharges. The second system studied is a commercially available TEA CO2 laser designed by SDI (Pty) Ltd and used extensively for the non-destructive testing of composite materials. The laser is designed for 400 Hz operation with very short time pulses (more than 80% of the energy in the first 100 ns). The