Enhancement of Small-Signal Gain and Mode-Volume in a FAF CO2 Laser Amplifier due to Cooling of Wall Temperature

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Abstract
Cooling the wall temperature of a fast-axial flow (FAF) CO2 laser amplifier improved both the small-signal gain and its spatial distribution and increased the effective mode-volume significantly under the turbulent flow condition. Considering the dependences of the gain parameter G and the line-shape function denoted by the Voigt function on the wall temperature T w and the radial distance r, the experimental results of the T w-dependent small-signal gain γ on the axis and its radial distribution are theoretically explained. On cooling T w from room temperature to -30°C, about 17% increase of the gain was obtained and its mode-volume was much improved. The spatial distribution of the small-signal gain becomes almost uniform over the cross section of the plasma tube particularly under the turbulent flow region greater than the gas-flow velocity of 170 m/s.