B.S.T.J. Briefs: The Use of Wollaston Prisms for a High-CapacityDigital Light Deflector

A digital light deflcctor was recently proposed by T. J. Nelson 1 in which n optical modulators and n uniaxial crystals were used to provide 2" positions of the beam. Each uniaxial crystal was used to deflect a beam of light into either of two beams, with the light at the output remaining parallel to the input but displaced by an amount proportional to the thickness of the crystal. For a large number of positions it was found that a lens had to be employed in order to focus the beam into a small spot. The use of the lens requires that converging or diverging light must pass through the uniaxial crystal. In this brief we point out that this converging beam, when passing through the crystal as an extraordinary ray, is subjected to an index of refraction which varies rapidly with angle. The digital light deflector using this type of deflection therefore has appreciable image distortion, and the limiting spot densities that can be achieved are less than would be predicted if only diffraction were important. A system which has less image distortion than the above can be constructed by using Wollaston prisms 2 (similarly Rochon or Senarmont prisms) and parallel light. (See Fig. 1.) Only the first two prisms are shown in Fig. 1. A Wollaston prism has the property that a collimated beam incident to its first face will be deviated, depending on its polarization, into either of two collimated beams. In this case the output beams will have an angular separation, whereas in the Nelson proposal the two output beams were laterally displaced without a change in angle.