Capability of commercial trackers as compensators for the absolute refractive index of air
A procedure is presented which calibrates a commercial wavelength/refractive-index tracker, so that it can compensate for the absolute refractive index of air within 3e-8*n. The procedure employs ultrahigh-purity helium and argon as reference gases of known n(p, T) to deduce the two unknown parameters in the working equation of the tracker: gas pathlength and pressure-induced distortion error. The performance of the gas-calibration procedure is evaluated by comparing the corrected tracker against a master refractometer based on a Fabry--Perot cavity in nitrogen, a third reference gas of known n(p, T). In nitrogen, the calibrated trackers demonstrate accuracy at the level of 4e-9*n. Testing in a fourth reference gas---water vapor---reveals that the working equation of the trackers must include a third unknown parameter: an end-effect caused by a moisture-dependence of the reflection phase-shift. Correcting for this moisture-related error represents the largest contribution to measurement uncertainty, and explains why performance of the calibrated trackers is an order-of-magnitude worse in moist air than in pure gas. In air, the Fabry--Perot cavity-based refractometer performs within 5e-9*n, but is not a commercially-available device.
Capability of commercial trackers as compensators for the absolute refractive index of air, Precision Engineering, [online], https://doi.org/10.1016/j.precisioneng.2022.04.011, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=933985
(Accessed August 10, 2022)