A filter based on an unequal-arm Michelson interferometer is described that will attenuate a strong spectral line by a factor of 100 or more but transmit without reduction a weaker but more interesting line separated by a few hundred MHz from the strong line. The factors limiting the extinction ratio of the filter are considered and a practical instrument that gives results in good agreement with the theory is described.
K. H. LANGLEY and N. C. FORD, JR., "Attenuation of the Rayleigh Component in Brillouin Spectroscopy Using Interferometric Filtering," J. Opt. Soc. Am. 59, 281-282 (1969)