Absolute Distance Interferometry

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Abstract
A basis for making absolute distance measurements to an accuracy of 0.025 pm over 0-1.5 m intervals is reported. Extensions of this technology will permit distances of 50 m and greater to be measured to the same accuracy. Two-color, synthetic Michelson interferometry using a CO2 laser source capa-ble of generating four sets of R- and P-line pairs is employed. This allows reduction of the very large ambiguity exhibited by conventional Michelson interferometers as well as the resolution of difficulties which would otherwise arise due to instabilities in the measurement arm of the interferometer. This latter effect is a practical rather than a fundamental consideration, but is nonetheless important if the interferometer is to emerge from the laboratory as an effective, workable instrument. Distance is determined in terms of a denumerable number of precisely known wavelengths and fractions thereof.

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