Spatial coding of laser beams by optically biasing electro-optic

Abstract

Apparatus for encoding a laser beam with information indicative of position in the beam. A linearly polarized laser beam passes through a birefringent wedge which encodes a continuously varying polarization across one dimension thereof. The polarization-encoded laser beam is applied to an electro-optic modulator which induces a second harmonic component of an applied modulation signal to appear in the beam due to optical biasing by the polarization encoding. On beam center, only the fundamental appears, while going away from beam center the second harmonic appears, having a varying magnitude and phase. A polarizer transmits a linearly polarized component of the encoded beam. To encode a second dimension of the beam orthogonal to the first, a second wedge and modulator are provided. To provide discrimination between the two, the second modulator operates at a different fundamental modulation frequency. The doubly-encoded beam is applied to a polarizer which transmits a linearly polarized component of the laser beam. A receiver detects and separates the encoded signals by frequency, and the fundamental signals are frequency-doubled and compared with the second harmonic signals. A signal is developed which is a function of the magnitude and phase of the second harmonic of each encoded dimension. These signals are indicative of the receiver position in the beam, and may be used as error signals, as in a missile guidance system or the like.