Interferometric Wavelength Measurement in the Infrared by the Method of Exact Orders* Precision Measurement of the Index of Refraction of Air at 165μ

Abstract

A new technique of interference wavelength measurement for absorption lines in the near infrared is described. The spectrometer used has a resolving power of 140 000 at 1.7μ. The air pressure in a Fabry Perot etalon is adjusted until a fringe coincides with an absorption line. This “tuning” can be made to 0.001 order of interference corresponding to a precision of relative frequency measurement of 0.0004 cm−1. The plates of the etalon have been coated with dielectric films giving a reflectivity of 70 percent and high contrast fringes. Smooth scanning of a small spectral region is accomplished by translation of a wedge prism. The method of exact orders has been used to show that there is a change in the apparent etalon spacer with wavelength amounting to 10−4A/A. The index of refraction of dry CO2 free air at 1.65μ has been measured giving the result (μ−1)s106=273.18+0.07−0.15.