An extremely stable Michelson interferometer with an optical path difference up to several thousand meters has been developed for the precise investigation of the frequency behavior of lasers. The theoretical value for the spectral resolution is of the order of 1012. The spectral resolution of a real system is determined by external disturbances and was measured to be close to 101l. The capability and the characteristics of the system are discussed.
H. Welling and B. Wellegehausen, "High Resolution Michelson Interferometer for Spectral Investigations of Lasers," Appl. Opt. 11, 1986-1990 (1972)