Achromatic combinations of birefringent plates. Part II. An achromatic quarter-wave plate

Abstract

An achromatic quarter-wave plate is obtained by superposing three birefringent plates of the same material; the first and last should have the same retardation 2δ₁ their fast vibration directions being parallel to one another but inclined at a specific angle c to that of the central plate-of retardation π. The desired range of achromatism determines the optimum values of 2δ₁ and c (which, in turn, will determine the orientation of the effective principal planes of the combination). As an example, using mica retardation plates prepared for Hg 5461, the range from 4100 Å to 6800 Å is covered with 2δ₁ = 115° 42' and c = 69° 54'. Further, for a particular wave-length, a birefringent compensator of variable retardation (0 to 2π) is obtained by interposing a half-wave plate that can be rotated in its own plane, between two quarter-wave plates that have their fast vibration directions parallel. The results follow from the Poincare sphere by geometrically compounding successive rotations.