An active element of cylindrical geometry in a plane–sphere cavity is traditionally used in a helium–neon laser. Part of the active medium in this case does not participate in generating the radiation. However, it is a laborious task to use a gas-discharge laser tube whose geometry would exactly coincide with the field geometry. It is proposed to use a combined element consisting of two or three sections of different diameters with different lengths. A calculation showed an amplification increase by 30% and 40%, respectively, by comparison with cylindrical geometry. Such active elements present no special difficulties in production in this case.
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