High-accuracy displacement interferometry in air.
The absolute accuracy of laser interferometers employed in displacement metrology is limited by two dominant factors: uncertainties in the source vacuum wavelength and the refractive index of the ambient air. In this paper we describe an interferometer system designed to minimize these uncertainties. Based on a commercial interferometer, the new system features direct measurement of the vacuum wavelength by frequency comparison with a portable iodine-stabilized He–Ne laser. The refractive index of air is computed from accurately measured values of pressure, temperature, and relative humidity. Combined with a desktop computer, the interferometer system permits the automated field measurement of displacement errors (such as those associated with precision machine tools) with an absolute accuracy of 8.5 parts in 108. Performance of the interferometer in field metrology is illustrated by the results of recent validation testing of the large optics diamond turning machine (LODTM) at Lawrence Livermore National Laboratory. These results highlight the need for new measurements of the absolute refractive index of standard air in order to reduce a limiting uncertainty on such measurements of ±5 parts in 108.