A rotating-field phase shifter is described which permits continuous electronic tuning of birefringent filters over extended spectral regions. The use of such phase shifters reduces the conditions for tuning synchronism between all filter stages to a simple relationship between the electrical driving frequencies. Materials which exhibit a dual transverse electrooptical effect and no natural birefringence may be employed, and a number of suitable candidates are discussed. Also proposed is a modification of the classical Lyot filter which utilizes reflection-type elements between crossed polarizers. Such a design considerably reduces the number of optical components required by an adjustable filter of conventional form. The appropriate theory is given for both the simple-element and the split-element cases. It is shown that a filter of this type can be tuned successfully over a wide range of wavelengths if only certain of the phase shifter wave plates are achromatic. Fresnel rhombs are shown to be applicable as simple achromatic λ/4 plates. A mechanically tuned four-stage filter has been constructed with reflection-type elements and Fresnel rhombs. Its performance was found to be in agreement with theory. The feasibility of electronic tuning is demonstrated by the results of frequency modulation experiments with a nitrobenzene Kerr cell and a circularly polarized electric field.
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