This paper reports the development of a new type of interferometer capable of measuring optical path differences of λ/1000 or less. Experiments that verify this capability were performed and are reported. The interferometer exhibits, in addition to a high sensitivity, an advantage of increased environmental stability over more conventional interferometers which depend upon optical path changes manifested as fringe shifts.
Robert E. Kinzly, "A New Interferometer Capable of Measuring Small Optical Path Differences," Appl. Opt. 6, 137-140 (1967)