In-line broadband 270° (3λ/4) chevron four-reflection wave retarders

The net differential phase shiftΔt introduced between the orthogonal p and s linear polarizations after four
successive total internal reflections inside an in-line chevron dual-Fresnel-rhomb retarder is a function of
the first internal angle of incidence φ and prism refractive index n. Retardance of 3λ=4 (i.e., Δt ¼ 270°) is
achieved with minimum angular sensitivity when φ ¼ 45° and n ¼ 1:900822. Several optical glasses with
this refractive index are identified. For Schott glass SF66 the deviation ofΔt from 270° is ≤ 4° over a wavelength
range of 0:55 ≤ λ ≤ 1:1 μm in the visible and near-IR spectrum. For a SiC prism, whose totally reflecting
surfaces are coated with an optically thickMgF2 film,Δt ¼ 270° at twowavelengths: λ1 ¼ 0:707 μm
and λ2 ¼ 4:129 μm. This coated prism has a maximumretardance error of ≈ 5° over three octaves (0.5 to
4:5 μm) in the visible, near-, and mid-IR spectral range. Another mid-IR 3λ=4 retarder uses a Si prism,
which is coated by an optically thick silicon oxynitride film of the proper composition, to achieve retardance
that differs from 270° by 0:5° over the 3–5 μm spectral range