BEST PAPER-2004 CONFERENCE

摘要

The refractive index of helium at atmospheric pressure can be calculated from first principles with a very low uncertainty, on the order of 10~(-10) Furthermore, the low refractive index of helium puts minimal demands on the pressure and temperature measurements required to determine the refractive index of a given sample of helium gas. Therefore helium can serve as a practical, theory-based standard of refractive index that might be used in place of air for ultra-high accuracy interferometric length measurements. Because its index of refraction is known, helium can also be used to characterize and correct errors in a gas refractometer. We have built two refractometers based on a laser locked to the transmission maximum of a Fabry-Perot cavity, and we use helium to measure and correct pressure-induced distortions of the refractometers. We have also characterized other sources of error in our Fabry-Perot refractometers, such as errors associated with the effect of humidity on the mirror coatings. When our two refractometers simultaneously measure the refractive index of a common nitrogen sample, we find that the two systems agree with each other within a few parts in 10. These measurements are sufficiently encouraging that one might speculate on the possibility of developing pressure standards based on refractometry.