Achromatic phase-shifters for broadband interferometry

Many interferometric measurements require the introduction of a controllable phase shift in one of the beams. This is commonly done by changing the optical path length by means of a tilting glass plate or a mirror mounted on a piezo-electric translator, but the change in the phase of the beam is then strongly dependent on the wavelength. An alternative way to shift the phase of a beam is by means of the Pancharatnam phase. This is the phase shift experienced by a light beam as a result of a cyclic change in its state of polarization. We show how the Pancharatnam phase can be exploited to produce a series of achromatic phase-shifters. One such device uses mica half-wave and quarter-wave retarders and produces a phase shift proportional to the rotation of one element which is independent of the wavelength from 450 nm to 700 nm. Another uses two quarter-wave retarders and two ferroelectric liquid-crystal devices which can be switched to produce phase shifts of 0 degrees, 90 degrees, and 180 degrees which are nearly independent of the wavelength over a wide range of wavelengths