The number and relative power of simultaneously oscillating modes in 6328-Å He-Ne optical gas masers (lasers) were measured by means of a technique utilizing Fabry-Perot interferometers. In one case 28 dominant modes spanning a frequency range of 1080 Mc/sec out of the 1500 Mc/sec half-width of the 6328-Å Ne line were found. The relative amplitudes of the modes can be fitted generally to a Gaussian-shaped curve. The technique, which utilizes a scanning Fabry-Perot, is described and discussed.
THOMAS G. POLANYI and WILLIAM R. WATSON, "Interferometric Investigation of Modes in Optical Gas Masers," J. Opt. Soc. Am. 54, 449-453 (1964)