| 摘要 |
A method and apparatus for measuring fluctuations in the refractive index of a gas, such as air, in a measurement path (66) may be used to measure displacement of an object (67) independent of these fluctuations. A coherent source of light (1, 4) provides two source light beams (11, 12) having source wavelengths ( lambda 1, lambda 2) along the measurement path (66) which are substantially harmonically related to each other. The beams (11, 12) are electronically processed (98, 981, 982, 983, 984, 985) to provide modified heterodyne phase shifts based on the provided source wavelengths which are similarly harmonically related to the harmonically related source wavelengths for providing a superheterodyne modulation phase substantially insensitive to motion along the measurement path (66).
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