Abstract
Radiation patterns obtained with He–Ne masers employing mirrors external to the discharge region are described. Operation was obtained with discharge tubes having Brewster angle windows, windows perpendicular to the optical axis, two spherical, one flat and one spherical, and two flat mirrors. Mode selection by using obstacles interposed in the cavity is described. It is shown that the radiation pattern is established within a very small region of the cavity. Comparisons of lasers operating with external and internal mirrors show that the lack of azimuthal symmetry of the radiation patterns is intrinsic to the spherical mirror geometry and not to the Brewster angle windows.
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