Multimode-optical-fiber Michelson interferometer

A multimode-optical-fiber Michelson interferometer is constructed by utilizing a fiber-optic directional coupler as a beam-splitting/recombining element, and is used to measure an amplitude of sinusoidal vibration in a homodyne detection scheme. The mixing efficiency in interferometry is discussed on a simplified assumption of the propagating field and related to the number of speckles received by a photodetector. In order to achieve a considerable S/N in the beat photocurrent, the aperture to limit the number of speckles should be as small as a few percent of the radiation pattern in area. Theoretical considerations are followed by experiments.