To study the distortion of a gaussian light beam in a long gas lens beam waveguide, a light pulse was shuttled back and forth more than 200 times in a 25-m optical cavity containing a gas lens. From the decay of the pulse train, a conservative upper limit of the optical loss of the gas lens was found to be 0.1%. By scanning the intensity of the beam on various round trips, the beam distortion could be measured. When the center of the beam remained near the optic axis of the lens, no appreciable beam distortion was measured after more than 400 trips through the lens. If the beam center did not hit the lens at the same transverse point on each pass but undulated, some distortion was observed. The largest distortions occurred when the beam undulated in the lower part of the lens where the lens has the greatest aberration caused by gravity. The larger the undulation amplitude, the faster the beam broke up. The largest distortion was observed when the lens with the greatest aberration due to gravity was used.
William H. Steier, "Optical Shuttle Pulse Measurements of Gas Lenses," Appl. Opt. 7, 2295-2300 (1968)