Possibilities of achromatization of coaxial asymmetric phase shifters with an even number of reflections

Abstract

Two types of coaxial phase shifters (PSs) are considered. They are designed for spectral ellipsometry, where achromatism is improved using a pair of parallel Al mirrors oriented at small angle θ 2 with respect to the incident laser beam. In a phase device based on a fused silica Fresnel rhomb, a high degree of achromatism (Δ = 440° ± 0.4° in the wavelength range of 250–1000 nm) is obtained with the aid of two Al mirrors coated by a native oxide layer about 5 nm thick and tilted at θ 2 = 18°. The achromatism of four-mirror PSs can be improved using two mirrors with a thin dielectric coating (Al 2 O 3 or MgF 2 ) 20–80 nm thick, for which phase shift Δ is close to 180° at small angles θ 2 and there are fragments of spectrum Δ(λ) where Δ decreases with an increase in the light wavelength.