A continuous optical maser has been operated at five wavelengths in the near infrared. Such a maser oscillator consists of a medium having optical amplification at the wavelength of interest and a Fabry-Perot interferometer as a resonant cavity.
The optical amplification is provided by maser action in a discharge through a mixture of helium and neon gas. The Fabry-Perot interferometer is constructed within the gas volume using two very flat, and highly reflecting, parallel, silica plates.
The transmission of the Fabry-Perot plates, although small, allows a beam to pass through each end window, with four milliwatts of continuous output power in the strongest transition at 1.153-µ wavelength.
Examination of the beam shows that it is almost diffraction limited for its one-centimeter diameter. The spectral line shape at each transition is made up of three or more components each less than a few hundred cycles in width separated by the spacing of orders in the interferometer.
DONALD R. HERRIOTT, "Optical Properties of a Continuous Helium-Neon Optical Maser," J. Opt. Soc. Am. 52, 31-36 (1962)