An extension of the two-wavelength interferogram technique is proposed that will enable selective measurements to be made of the variation in the density of a specific atomic population. One wavelength should be chosen to closely coincide with the wavelength associated with an absorption line of the species of interest, whereas the other wavelength should be displaced by several angstroms from the line. The difference between the fringe shifts on the two interferograms would then enable the density variation of the specific atomic population of interest to be measured with high sensitivity. Two criteria are established that ascertain for any system the minimum density that could be detected, and the range in density variation for which the interferograms should be discernible.
R. M. Measures, "Spectral Line Interferometry: a Proposed Means of Selectively Measuring the Change in the Density of a Specific Atomic Population," Appl. Opt. 9, 737-741 (1970)