A two-color heterodyne interferometer for measuring the refractive index of air using an optical diffraction grating

A new two-color heterodyne interferometer using an optical grating was proposed for measuring the refractive index of air. The heterodyne modulation is realized by moving the diffraction grating. The method allows us to measure the refractive index with high accuracy by using a 1.06 mum YAG laser and its second-harmonic wave because two beams (the fundamental and the second-harmonic wave) are in common path completely. Moreover, good signal-to-noise ratio (SNR) can be obtained by the heterodyne interference. Using the refractive index calculated by Edlen's equation as a reference, that measured by present method was experimentally evaluated having an accuracy of about 1 x 10(-6) within a distance of 100 mm. (C) 2002 Elsevier Science B.V. All rights reserved.