Design of phase-controlled coatings to correct skew-ray depolarization in LCOS projectors

Abstract

LCOS projectors usually employ tilted beam-dividing coatings that see both bright and dark polarization states. The optics must then be designed to eliminate polarization mixing at these coatings, which ordinarily arises whenever the S/P planes of different rays are not aligned with one another. (This non-parallelism is a consequence of the compound incidence angles with which a beam of finite NA intercepts a tilted coating.) We show how to correct this effect using phase-controlled coatings that exploit the double-pass symmetry of the Plumbicon tri-prism geometry, reducing cross-polarized leakage to ∼1 × 10-3. In lowest order, any polarization ellipticity that the coatings may introduce in the light valve illumination will cancel in the return pass through the optics (while induced rotation will double). The condition that must be satisfied to correct rotation will only constrain the coatings in a single collective degree of freedom (for each color channel); this condition depends on both the orientations and phase shifts of the coatings. Even beyond this first order result, we show that for NA ≲ 0.2 the computation involved in calculating beam contrast can be reduced to the equivalent of tracing a single ray (in double-pass), despite the fact that contrast along different rays in the beam will vary as a function of both the skew component and incidence angles of the rays at coatings. The efficiency of this algorithm makes it practical to refine the design of all coatings in the system simultaneously to take higher order terms into account. © 2002 Elsevier Science B.V. All rights reserved.