Selection and self-locking of modes in a He-Ne laser with nonlinear absorption

Experimental investigations of a long 6328-Å He-Ne laser with a neon absorption discharge cell inside the laser cavity are presented. A considerable difference between the strong field saturation parameters for absorption and gain in the medium leads to a new principal of behavior in the long laser. At small absorption the self-locking of modes and spectrum "weeding out" are observed. The oscillating locked-mode interval reaches with increase of the absorption. When the absorption value is about 1 percent per pass, single-mode operation takes place. The maximum single-mode intensity is about 80 percent of the total output in the absence of absorption. At higher field amplitude in the absorption cell as compared to the gain cell, the increase of absorption leads to spectrum narrowing of the self-locked mode down to the selection of a single mode. In the last case only the mode that is located near the absorption line center oscillates. In addition various hysteresis phenomena are observed.