A heterodyne refractometer for air index of refraction and air density measurements

We briefly describe a heterodyne refractometer developed at the BIPM in collaboration with the BNM/INM conservatory. The heart of the refractometer, a double Fabry–Perot interferometer, is placed inside the balance case of a very sensitive 1 kg mass comparator, the FB2 balance. Comparisons between methods using refractometry and the NPL revised Edlén formulas, carried out for a period of nine months, yielded a difference in air index of refraction of ${\text{4×10}}^{\text{−8}}$ with a standard deviation of ${\text{1×10}}^{\text{−8}}.$ The variation of air index of refraction was about ${\text{1.5×10}}^{\text{−5}}$ during the study. Precise determinations of the short-term and long-term stability of the Fabry–Perot cavity, made of Zerodur, were also achieved. For monitoring air density, results obtained with the refractometry method were compared with those deduced from two other methods: the CIPM formula for the density of moist air and the use of buoyancy artifacts. The response characteristics for the three determinations were comparable and the agreement among the air density determinations was within ${\text{1×10}}^{\text{−5}}\hspace{0.17em}{\mathrm{kg m}}^{\text{−3}}.$