New Type of Birefringent Filter

Abstract

A new type of birefringent filter is proposed which uses a single birefringent plate placed between two Faraday-rotation elements within the cavity of a Fabry–Perot interferometer. The interferometer is placed between crossed polarizers whose axes are parallel to the principal directions of the plate. Such an instrument, called a gyromagnetic polarizing interferometer (GYMPI), has an instrumental function which is essentially an Airy function whose finesse is approximately equal to the reflective finesse of the étalon. The argument of the Airy function is the birefringent phase shift of the retardation plate rather than the phase shift of transit between the mirrors. Thus the passband of the filter has that relative independence of angle of incidence that characterizes all birefringent filters. The instrument is readily tunable over a free spectral range by making the retarder a Soliel compensator or incorporating an electro-optic element. This advantage, along with the greater versatility and lower cost of construction, should make the instrument of interest despite the low peak transmittance which is the order of the reciprocal of twice the reflective finesse. The optimum of Faraday rotation is only a few degrees which means that the magnetic-field requirements are not severe.