Mitigating effects of nonlinearities in homodyne quadrature interferometers

TL;DR

This paper presents methods to measure, correct, and calibrate nonlinearities in Homodyne Quadrature Interferometers, enhancing their accuracy and applicability in gravitational wave detection and other displacement sensing applications.

Contribution

It introduces real-time correction and post-processing techniques for nonlinearities in HoQIs, improving their performance and expanding their potential uses.

Findings

Nonlinearities can be effectively measured and quantified in HoQIs.

Real-time correction significantly reduces nonlinear effects.

Post-correction further suppresses measurement errors.

Abstract

Homodyne Quadrature interferometers (HoQI) are an interferometric displacement sensing scheme proven to have excellent noise performance, making them a strong candidate for sensing and control schemes in gravitational wave detector seismic isolation. Like many interferometric schemes, HoQIs are prone to nonlinear effects when measuring displacements. These nonlinearities, if left unsuppressed, would substantially limit the use cases of HoQIs. This paper first shows a means of measuring and quantifying nonlinearities using a working HoQI and a mechanical resonator. We then demonstrate a method for real-time correction of these nonlinearities and several approaches for accurately calibrating the correction technique. By correcting in real time, we remove one of the biggest obstacles to including HoQIs in upgrades to future gravitational wave detectors. Finally, we discuss how to post correct data from HoQIs, suppressing even further the nonlinearity-induced errors, broadening the appeal of such sensors to other applications where measurement data can be reconstructed after the fact. We demonstrate all of this on a working HoQI system and show the measured suppression of nonlinear effects from each of these methods. Our work makes HoQIs a more broadly applicable tool for displacement sensing.