Polarization properties of retroreflecting right-angle prisms

Abstract

The cumulative retardance ⌬ t introduced between the p and the s orthogonal linear polarizations after two successive total internal reflections (TIRs) inside a right-angle prism at complementary angles and 90°Ϫ is calculated as a function of and prism refractive index n. Quarter-wave retardation (QWR) is obtained on retroreflection with minimum angular sensitivity when n ϭ ͑ͱ2 ϩ 1͒ 1͞2 ϭ 1.55377 and ϭ 45°. A QWR prism made of N-BAK4 Schott glass (n ϭ 1.55377 at ϭ 1303.5 nm) has good spectral response (Ͻ5°retardance error) over the 0.5-2 m visible and near-IR spectral range. A ZnS-coated right-angle Si prism achieves QWR with an error of ϽϮ2.5°in the 9 -11 m (CO 2 laser) IR spectral range. This device functions as a linear-to-circular polarization transformer and can be tuned to exact QWR at any desired wavelength (within a given range) by tilting the prism by a small angle around ϭ 45°. A PbTe right-angle prism introduces near-half-wave retardation (near-HWR) with a Յ2% error over a broad ͑4 Յ Յ 12.5 m͒ IR spectral range. This device also has a wide field of view and its interesting polarization properties are discussed. A compact (aspect ratio of 2), in-line, HWR is described that uses a chevron dual Fresnel rhomb with four TIRs at the same angle ϭ 45°. Finally, a useful algorithm is presented that transforms a three-term Sellmeier dispersion relation of a transparent optical material to an equivalent cubic equation that can be solved for the wavelengths at which the refractive index assumes any desired value.