<title>Investigations of a low-pressure free-vortex aerodynamic window for industrial lasers</title>

Abstract

A new free vortex aerodynamic window, developed for industrial high power C02-!asers, is presented. To meet the industrial requirements improvements with regard to a reduction of the total nozzle pressure were necessary. The lower pressure results in reduced turbulence and therefore in less influence on the laser beam. This was verified by interferometric measurements as well as by experiments on an industrial laser. Some results of a numerical simulation of the flow in an aerodynamic window aimed at a better understanding of the phenomena involved are presented.Keywords: aerodynamic window, free vortex, optical quality, high power laser, numerical flow simulation DEVELOPMENT GOALSTo overcome the limitation in laser beam power density, given by the material windows and outcoupling mirrors, aerodynamic windows were proposed and investigated in the past. Calculations and model experiments showed the superiority of the concept 'free vortex aerodynamic window'. Sealing a laser cavity opening by a section of a free vortex flow exerts less influence on the propagating laser beam and needs a smaller mass flow rate than any other concept of aerodynamic window. Free vortex aerodynamic windows have been investigated thoroughly by Guile', Masuda2, Wildermuth3 and by others. These older studies, directed more to scientific bascic research, showed the feasability of such aerodynamic windows. The goal of the work presented here, however, was the development and optimization of a free vortex aerodynamic window for industrial purpose.For the application in a modern industrial rf-exited high power C02-laser an aerodynamic window has to meet some special requirements. From an industrial point of view an aerodynamic window is more than just the sealing flow. It consists of a telescope, a vacuum shutter, the nozzle, diffusor and gas supply of the flow, a device for pressure measuring and a connection to the laser control system. The complete system 'aerodynamic window' has to be as small, cheap and quiet as possible. It should guarantee a good laser beam quality and a high reliability.Further, automatic operation and control is necessary.The presented aerodynamic window was developed with regard to these aspects. The telescope, for example, which produces an intermediate focus in or nearby the flow field to reduce the opening size and the required mass flow rate, was built by two parabolic mirrors of 200 mm focal length. In this way the outcoupling device could be realized very compact (Fig. 1). To have low running costs an air compressor was used as a gas supply instead of compressed nitrogen from bottles. The most important step was the reduction of the total nozzle pressure from usual 10 bar to 5 bar. This reduces the noise and improves the optical quality as will be shown later. O-8194-1860-9/95/$6.OO SPIE Vol. 2502/5S9 Fig. 1: Outcoupling device with telescope, vacuum shutter and free vortex flow module; tested on an industrial 5 kW laser with unstable resonator. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05...