In a gas laser a spectrum line, whose upper state is the lower state of the laser transition, may contain a structure due to the laser action. We have observed this structure, which we refer to as ears, on the 6096-Å neon line emitted spontaneously in a 1.15 µ He-Ne laser. The width of the ear was measured interferometrically and compared with the width predicted from the phenomenological width γab/π in the laser transition.
W. G. SCHWEITZER, JR., M. M. BIRKY, and J. A. WHITE, "Ears on the Line Profile of Decay Transitions in a Gas Laser," J. Opt. Soc. Am. 57, 1226-1230 (1967)