Phase detection of Young’s fringes is applied to a highly precise retardation measurement. A simple common-path polarizing interferometer is used with a birefringent wedge and a polarizer. The birefringent wedge introduces a spatially linear phase difference between orthogonally polarized light and Young’s fringes are formed on an image sensor. The phase difference between the orthogonally polarized components of light is proportional to the phase of Young’s fringes. Thus, the retardation is equal to the Young’s fringes’ phase change before and after insertion of the retarder into the common-path interferometer. The phase of Young’s fringes is calculated from the Fourier cosine and sine integrals of the fringe profile. The experimental results for wave plates, a Soleil-Babinet compensator, and a Pockels cell are presented with error estimates. The accuracy of the retardation measurement is experimentally estimated to be greater than λ/2100.
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