A new refractometer by combining a variable length vacuum cell and a double-pass Michelson interferometer

A new refractometer with a variable length vacuum cell has been developed to eliminate errors caused by deformations in optical windows of the cell, The refractive index of air is determined by measuring the changes in the optical path difference between the air of interest and a vacuum as a function of the changes in the cell length, An optical phase modulation technique and a dark fringe detection method are used to get a high resolution in measuring the optical path difference by a double-pass Michelson interferometer. A combined standard uncertainty of 5 x 10(-9) in the measurement of the refractive index of air has been achieved.