# Highly sensitive frequency metrology for optical anisotropy measurements 

Gilles Bailly, Rapha¨ el Thon, and C´ ecile Robilliard ∗ 

Universit´e de Toulouse-UPS; Laboratoire Collisions Agr´egats R´eactivit´e, 

IRSAMC; F-31062 Toulouse, France. 

CNRS, UMR 5589; F-31062 Toulouse, France. 

(Dated: November 11, 2018) 

## Abstract 

In this paper we present a novel apparatus aimed at measuring very small birefringences and anisotropies, based on frequency metrology and not on polarimetry as usual. In our experiment, a very high finesse resonant cavity is used to convert the phase difference into a resonance frequency difference, which can then be measured with very high accuracy. We describe the set-up and present the results of experimental tests which exhibited a sensitivity δn ≃ 2 × 10 − $^{18}$, allowing for the measurement of long-predicted magneto-electro-optical effects in gases. Since the shot- noise limited sensitivity of our apparatus lies well below the state-of-the-art sensitivity, frequency metrology appears as a promising technique for small birefringence measurements. 

PACS numbers: 33.57.+c, 42.25.Lc, 42.62.Eh, 06.30.Ft 

