Intensity resonances in a double-mode HeNe/CH4 laser with synchronized modes

4 gas laser with an intracavity absorbing cell is studied experimentally and theoretically at a modulation frequency approximately equal to the intermode beat frequency. It is revealed that in the vicinity of the absorption-line centre the intensity of every mode for certain system parameters displays resonance structures narrower than the homogeneous line width of the absorption line. This occurs when the range of mode synchronization is near the absorption-line centre and its size is less than the homogeneous width of the absorption line. Received: 29 February 1996/Revised version: 4 February 1997