Abstract: A method and apparatus for detection of ultrasonic vibrations using an interferometer built, in its preferred embodiment, with optical fibers is disclosed. The method and apparatus are particularly useful where perturbations and disturbances adversely affecting other interferometer configurations are encountered. Light from a laser is split into two orthogonal polarization modes which travel through different fiber-optic paths and are directed to the object being investigated. A waveplate and a polarization splitter are used to re-direct the reflected light into the optical path opposite from which it arrived. A second polarization splitter combines the returning light and an interference signal is obtained with photodetectors. The bias phase-shift between the interfering beams is fixed with a second waveplate. The detected signal is the result of motion which modulates the phase of the light reflected by the object being inspected.