Common-path interferometer

A common-path interferometer is class of interferometer in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, the point diffraction interferometer, and the Zernike phase contrast interferometer. A common-path interferometer is generally more resilient to environmental vibrations than a "double path interferometer" such as the Michelson interferometer or the Mach-Zehnder interferometer. Although travelling along the same path, the reference and sample beams may travel along opposite directions, or they may travel along the same direction but with the same or different polarization.