Interferometric refractometer with a variable-length vacuum cylinder

A new design for a variable length vacuum path air refractometer is developed. The device has some advantages as compared with traditional gas refractometers. It measures directly the refractive index of laboratory air. Hence it is not necessary to move air to or from any separate cavity. The optical construction of the device is simple. The device utilizes two Michelson type interferometers. One interferometer has fixed mechanical lengths in both arms. The optical length of one arm can be altered by changing the proportion between air and vacuum lengths of the beam path. This is realized by a variable length vacuum cylinder. The other interferometer measures the displacement of the vacuum window of the cylinder. The interference signals caused by the change of optical path change and by displacement of the vacuum window are digitized and analyzed to give the refractive index of ambient air. The Abbé error is eliminated by using a glass corner cube with a hole through the center that allows symmetric alignment of the beams of the two interferometers. Difference of refractive index values measured by the refractometer and calculated with updated Edlén's formula from experimental data was 1.2× 10-8 when standard deviation of the difference was 4×10-8.