A helium-neon laser amplifier operating at 1.15 µ is described A discharge tube of rectangular cross section was employed, allowing a large cross section of discharge without impairing its efficiency for laser action. The optical arrangement provided multiple transits of the signal beam through the discharge, enabling the use of long optical paths. A low-level gain of 400 and a saturation output power of 20 mW were obtained. The output power was limited by inefficient use of the discharge volume. The system was also operated as an oscillator and a pulsed amplifier.
L. E. S. Mathias and N. H. Rock, "A Helium-Neon Laser Amplifier," Appl. Opt. 4, 133-135 (1965)