Analysis of the linear electro-optic effect in crystals of classes containing a threefold rotation axis is carried out. It is shown that such crystals display a dual transverse electro-optic effect in which the magnitude of an electric field transverse to the light direction controls the electro-optic retardation and the direction of the field controls the orientation of the fast and slow polarization axes. The most desirable crystals for utilization of this effect are those of cubic class 3m since they are optically isotropic in zero field and are not optically active. Using a crystal of this type, for example, zinc sulfide, analysis shows it is possible to modulate a light beam simultaneously with two independent signals in the form of mutually perpendicular fields both transverse to the light beam. The signals are recoverable at the receiver by photodetectors responding to appropriate polarization components of the beam. If the fields applied to the crystal consist of appropriate combinations of the modulating signals, the latter appear as upper or lower sidebands of the optical carrier. Differences of the polarization states of sidebands and carrier make it easily possible to achieve single-sideband suppressed-carrier transmission. Receiver design for superheterodyne detection of such signals is described. Results of experiments are given demonstrating the modulation and reception techniques described.
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