Abstract:
A common-path interferometer that is intended for observation of a faint off-axis light source against the background of an intense on-axis source is suggested. The on-axis source and its replicated image acquire an achromatic phase shift of 180$^\circ$ and interfere in antiphase. The achromatic phase shift is due to the geometrical phase in the configuration of a 3D interferometer. Interference spatially separates the dark and bright fields of the on-axis source by directing them to different sides of a beam splitter. At the same time, interference does not attenuate the field of the off-axis source, directing it to different sides of the beam splitter with equal intensities. Such a configuration of the common-path interferometer provides mechanical stability.
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