Refractometer for Tracking Changes in the Refractive Index of Air Near 780 nm

TL;DR

This paper introduces a novel refractometer system using a double channel Fabry Perot etalon and laser diodes at 780 nm for precise, near-infrared air refractive index measurements, extending the validated wavelength range of the Edlén equations.

Contribution

The paper presents the first near-infrared measurement system for air refractive index based on frequency measurements, extending the validity of the Edlén equations to 780 nm.

Findings

Refractometer achieves better than specified resolution and uncertainty.

Measurements agree with revised Edlén equations within experimental uncertainty.

System demonstrates stable, quasi-instantaneous refractive index measurements.

Abstract

A new system, consisting of a double channel Fabry Perot etalon and laser diodes emitting around 780 nm is described and proposed to be used for air refractive index measurements. The principle of this refractometer is based on frequency measurements between optical laser sources. It permits quasi-instantaneous measurement with a resolution better than and uncertainty in the range. Some preliminary results on the stability of this system and the measurements of the refractive index of air with this apparatus are presented. The first measurements of the index of air at 780 nm are, within an experimental uncertainty of the order of, in agreement with the predicted values by the so-called revised Edl\'en equations. This result is to the best of our knowledge the first to extend to the near infra-red the validity of the revised Edl\'en equation derived for the wavelength range 350- 650 nm.