REFRACTOMETER FOR TRACKING CHANGES IN THE REFRACTIVE-INDEX OF AIR NEAR 780 NM

A new system, consisting of a double-channel Fabry-Perot etalon and la ser diodes emitting around 780 nm, is described and proposed for use f or measuring air-refractive index. The principle of this refractometer is based on frequency measurements between optical laser sources. It permits quasi-instantaneous measurement with a resolution of better th an 10(-9) and uncertainty in the 10(-8) range. Some preliminary result s on the stability of this system and the measurements of the refracti ve index of air with this apparatus are presented. The first measureme nts of the index of air at 780 nm are, within an experimental uncertai nty of the order of 2 x 10(-8), in agreement with the predicted values by the so-called revised Edlen equations. This result is, to the best of our knowledge, the first to extend to the near IR the validity of the revised Edlen equation derived for the wavelength range of 350-650 nm. (C) 1998 Optical Society of America.