An unexpected phenomenon has been observed which triggered an investigation into the lensing effect of a CO2 laser plasma. This effect, so far thought to be negligible in a conventional CO2 laser of, for example, 2-m length, produces a focal length in the order of magnitude of — 20 m. In view of this experimental observation, the focal length of the plasma lens, as well as the stability condition for an optical resonator with a plasma lens within its plane concave mirror system, are determined and expressed in terms of plasma and resonator characteristics as well as of the electrical power dissipated in the plasma. The analysis reveals that the semiconfocal configuration is most adverse for a frequency-stabilized laser. The overall result of this investigation suggests that the optimum configuration of a conventional CO2 laser for maximum output power is obtained when the negative focal power of the plasma lens precisely compensates for the positive focal power of the slightly curved mirror.
H. K. V. Lotsch and W. C. Davis, "The Lensing Effect of CO2 Laser Plasma," Appl. Opt. 9, 2725-2728 (1970)