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Concept
Common-path interferometer
Summary
A common-path interferometer is a class of interferometers in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, Zernike phase-contrast interferometer, and the point diffraction interferometer. A common-path interferometer is generally more robust 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.
Double-path interferometers are highly sensitive to phase shifts or length changes between the reference and sample arms. Because of this, double-path interferometers have found wide use in science and industry for the measurement of small displacements, refractive-index changes, surface irregularities and the like. There are applications, however, in which sensitivity to relative displacement or refractive-index differences between reference and sample paths is not desirable; alternatively, one may be interested in the measurement of some other property.
Sagnac interferometer
Sagnac interferometers are totally unsuited for measuring lengths or length changes. In a Sagnac interferometer, both beams emerging from the beamsplitter simultaneously go around all four sides of a rectangle in opposite directions and recombine at the original beamsplitter. The result is that a Sagnac interferometer is, to first order, completely insensitive to any movement of its optical components. Indeed, in order to make the Sagnac interferometer useful for measuring phase changes, the beams of the interferometer must be separated slightly so that they no longer follow a perfectly common path. Even with a slight beam separation, Sagnac interferometers offer excellent contrast and fringe stability. Two basic topologies of the Sagnac interferometer are possible, differing in whether there are an even or odd number of reflections in each path.
Official source
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