Refractive index measurement by fiber point diffraction longitudinal shearing interferometry

A refractive index measurement method by measuring the longitudinal displacement of a point light source (PLS) after passing through a plane-parallel-plate sample is proposed. The displacement is derived by applying two point-diffraction longitudinal shearing interferometric measurements if the distance from the PLS to the exit pupil is predetermined. With two fibers to simulate the ideal PLS, an experimental system for solid and liquid sample tests is proposed to verify the principle. The experimental results indicate that its accuracy is in the order of 10(-4). Ways to improve the accuracy are discussed based on the detailed error analysis.