Effects of radial gain variations on single transverse and longitudinal mode operation in a low loss 1.6 m helium-neon laser

The self-selection of the transverse mode structure due to a radial variati on of the population inversion density is investigated in a HeNe laser with no mode discriminators. Radial distributions of gain that are dependent on pumping rate and gas composition are used as a basis for calculations. The effective small-signal single mode gains of Hermite-Gaussian modes (of ord er up to TEM15.15) are determined. It is demonstrated that the threshold tr ansverse mode increases in order as increasing pump rate induces a central minimum in the radial gain distribution. Qualitative predictions of the mod e structures are made for low and high loss cavities. A 1.6 m long HeNe las er with a supermirror based cavity is described and experimental results fr om this laser are presented in the context of the theory. (C) 2000 Publishe d by Elsevier Science B.V.