Single sideband suppressed carrier optical modulation

A brief review of the electro-optic effect in crystals as applied to light modulation will be given, then followed by a description of the technique developed at our laboratories for single-sideband modulation of light by this effect. Aside from applications to superheterodyne optical communications, this technique can be used to shift, by a specified amount, the inherently fixed frequency of a laser beam without introducing additional frequency components. One method uses two electro-optic crystals in a tandem arrangement and is capable of shifting about 45% of the input light energy onto a sideband when a sinusoidal voltage is applied. The remaining energy is absorbed in the device. Principles of operation will be analyzed and the evolution of a simpler embodiment, which uses only one crystal, will be discussed. This latter device can completely convert an incident beam to a sideband and is easily adaptable to broadband operation far into the microwave region. These modulators were demonstrated with coherent light obtained from a helium-neon gas laser using audio-frequency modulating signals. Two tone tests showing the expected intensity envelopes and confirming single-sideband suppressed-carrier modulation will be described.