GENERAL LINEAR-DIFFERENTIAL INTERFEROMETERS

We investigate the conditions under which an interferometer's output is a function only of the difference between two optical lengths. In certain applications, such differential behavior, characteristic of the classical Michelson and Mach-Zehnder schemes [M. Born and E. Wolf, Principles of Optics (Pergamon, Oxford, 1980)], improves resilience to source frequency jitter. The question of whether systems of higher sensitivity than the Mach-Zehnder or Michelson schemes can have differential behavior is addressed, and it is proved that no linear differential scheme can have a sensitivity greater than that of the classical Michelson interferometer. It is also proved that the scattering matrix elements for a Linear N port cannot be differential functions of the optical lengths within the network.