Ring-type optical frequency standard

An optical frequency standard was based on a ring helium-neon laser emitting at λ = 3.39 μ. Optical discrimination was provided by the power resonances resulting from the presence of an absorbing methane cell inside the resonator. The use of sealed amplifying and absorbing elements made it possible to achieve a frequency reproducibility of ~2 · 10^–13 at relative stability of ~2 · 10^–14.