Phase conjugate, common path interferometer

Abstract

A phase conjugate interferometer has a partially reflective conventional mirror placed in front of and in close proximity to a phase conjugate mirror using internally self pumped phase conjugation in barium titanate or other photorefractive material. The reflective surface may be the surface of the phase conjugate mirror. An optical system under test is illuminated with coherent light and the wavefront at the exit pupil thereof is imaged through a beam splitter onto the reflective surface (imaging being unnecessary if the distance between the exit pupil and the reflective surface is sufficiently small that negligible diffraction occurs over that distance). Part of the wavefront is reflected at the reflective surface of the partially reflecting conventional mirror. The transmitted portion of the wavefront is incident on the phase conjugate mirror where a wavefront reversed replica of the incident wave is produced and reflected. The two reflected waves (from the conventional and from the phase conjugate mirror travel back towards the exit pupil. The interference pattern which characterizes the incident wavefront is formed at the reflective surface of the conventional mirror. The interferometer is an essentially perfect common path interferometer for the following reason: the only path that is not common to the two waves is the round trip path between the conventional and phase conjugate mirrors, which has zero effective path due to the wavefront reversal properties of phase conjugate mirrors. The pattern is deflected by the beam splitter to an observation plane. The pattern is imaged, as by a relay lens, on the observation plane where the interference fringes of the pattern can be examined or recorded on photographic film or displayed with a television camera. That portion of the optical path between the reflective surface and the phase conjugate mirror (which provides the reference arm) of the interferometer is not subject to environmental effects (turbulence, thermally induced pathlength changes, etc.) and the path through which the reflected object beam travels to the location where the interference pattern with the reference beam is formed is extremely small and not affected by environmental effects. Accordingly, the interferometer is robust and can be made extremely compact.