A method and apparatus for detecting small shifts in optical wavelength is provided. Light having a known wavelength is directed towards a target volume in the atmosphere. Light reflected from the target volume is received and mixed with differentially delayed copies of itself to produce three interference signals. The interference signals are taken at delays spanning one-half the wavelength of the transmitted signal. The intensity of the signal at the three different delays is measured simultaneously, from which the wavelength of the reflected signal can be determined.
