Low coherence interferometric detection of the spectral dependence of the retro-reflection coefficient of an anti-reflective coated interface

TL;DR

This paper introduces a low coherence interferometry method with balanced detection to accurately measure the spectral dependence of extremely low reflection coefficients of anti-reflective coatings, crucial for precision instruments like gravitational wave detectors.

Contribution

It presents a novel interferometric technique that achieves high sensitivity and spectral resolution while eliminating spurious effects from uncoated interfaces.

Findings

Achieved sensitivity of 0.1 ppm in reflection coefficient measurement

Spectral resolution of 0.2 nm demonstrated

Method successfully validated under various experimental conditions

Abstract

The measurement of very low reflection coefficients of anti-reflective coated interfaces has become a key issue for the realization of precision instruments such as the giant interferometers used for the detection of gravitational waves. We propose in this paper a method, based on low coherence interferometry and balanced detection, which not only allows to obtain the spectral dependence of this reflection coefficient in amplitude and phase, with a sensitivity of the order of 0.1 ppm and a spectral resolution of 0.2 nm, but also to eliminate any spurious influence related to the possible presence of uncoated interfaces. This method also implements a data processing similar to that used in Fourier transform spectrometry. After establishing the formulas that control the accuracy and the signal-to-noise ratio of this method, we present the results that provide a complete demonstration of its successful operation in various experimental conditions.