Spectral measurement of gaseous optical masers has been made by photomixing techniques and noise distribution measurement. Particularly, low-frequency beat components were verified to occur between spatially independent transverse modes of the same order in an experiment of mode control. Frequency-spread of beat components is also discussed.
Teiji Uchida, "Frequency Spectra of He-Ne Optical Masers with External Concave Mirrors," Appl. Opt. 4, 129-131 (1965)