Beam-splitters for the Division-of-amplitude Photopolarimeter

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Abstract
Beam-splitters for the division-of-amplitude photopolarimeter (DOAP) are described. The polarization-independent beam-splitter (PIBS) makes DOAP equivalent to the rotating-analyser ellipsometer with no moving parts. If tan ψr exp (jΔr) and tan ψt, exp (jΔt) are the ratios of the complex parallel and perpendicular reflection (r) and transmission (t) coefficients of the beamsplitter, DOAP becomes a complete polarimeter, capable of measuring all four Stokes parameters of light simultaneously, if ψr p ψt p 0 or 1/2π and Δr - Δt p 0 or ±π. A simple beam-splitter which satisfies these conditions consists of a transparent (low-index) thin film on a transparent (high-index) substrate. The film thickness must not equal an integral multiple of a quarter-wave, and a high angle of incidence φ is needed for appreciable reflectance. Examples are presented of such beam-splitters made from MgF2/ZnS and ThF4/Ge film/substrate systems at φ = 70° for the 0.6328μm (He-Ne) and 10-6 μm (CO2) laser wavelengths, respectively.