<title>Diffusion-cooled large surface area CO2/CO lasers</title>

Abstract

For most of the past twenty years, the development of carbon dioxide laser sources for industrial material processing has seen a strong trend away from diffusion cooling in favour of forms of convective cooling using gas flow techniques. For many years the latter provided the only effective route to high power discharges, and hence to lasers of modest resonator lengths at output powers of a kilowatt and higher. However, recently there has been a revival of interest in the possible use of discharge technologies based on diffusion-cooling for the excitation of lasers for materials processing and other high power laser applications. This possibility has arisen because the potential has been recognised for significant practical improvements in laser performance characteristics, in overall device compactness and in capital and running costs. This renewed interest in diffusion-cooled devices is based on the realisation that, under certain conditions, it is possible to: i) operate transverse capacitative radiofrequency dischar. between metallic electrodes of large area at very high power densities, under conditions where high volumetric power optical extraction may be achieved, even though the gas is static and the cooling is by diffusion; ii) construct lasers where the laser output power scales as the area of the discharge rather than as the discharge length as for conventional carbon dioxide lasers (both diffusion-cooled and convection-cooled devices); iii) design resonators for gain media ofunconventional geometry, with which it is possible to extract laser beams of high optical quality. Although isolated reports ofprevious work exist 1,2,the origins ofthe evolution ofcurrent work on large area lasers lie in research on RF excited lasers in the early 1980s, which itself drew on earlier work on hollow waveguide resonator devices6. This basis, coupled with parallel research on parallel plate transverse RF discharges7' led to research on large electrode area carbon dioxide devices in both planar slab8 and annular9 conflgnrations. Subsequently, veiy compact devices at power levels to 240W were demonstrated using hybrid waveguide-conlocal unstable resonators10, and later this approach was scaled to power levels in excess of 1 kW'1'2 The past few years has also seen commercial exploitation of this technology13'14, as well as the extension ofannular large area RF devices towards 2 kW 15• Inthis paper, a brief review is presented of the principal factors involved in the design of high power area-scaled lasers, including an outline of some of the relevant radiofrequency discharge physics, and a discussion of several types of optical resonators which may be used in combination with large area gain media to produce high quality laser beams