Single transverse mode oscillation is realized in a conventional HeNe laser outside the stability region of the optical resonator. Depending on the mirror separation different spatial modes can be generated. The mode volume of these modes is laterally limited by the diameter of the discharge capillary rather than by the beam waist of a stable Gaussian mode. Numerical solution of the Maxwell equations with appropriate boundary conditions shows good agreement with the observations. Such modes could potentially facilitate single transverse mode operation of waveguide lasers and fiber lasers.
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