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

Abstract

The self-selection of the transverse mode structure due to a radial variation 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 order up to TEM 15,15) are determined. It is demonstrated that the threshold transverse mode increases in order as increasing pump rate induces a central minimum in the radial gain distribution. Qualitative predictions of the mode structures are made for low and high loss cavities. A 1.6 m long HeNe laser with a supermirror based cavity is described and experimental results from this laser are presented in the context of the theory.