Ghost Beam Suppression in Deep Frequency Modulation Interferometry for Compact On-Axis Optical Heads.

Oliver Gerberding,Katharina-Sophie Isleif

doi:10.3390/s21051708

Oliver Gerberding, Katharina-Sophie Isleif

Open Access

https://doi.org/10.3390/s21051708

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Abstract

We present a compact optical head design for wide-range and low noise displacement sensing using deep frequency modulation interferometry (DFMI). The on-axis beam topology is realised in a quasi-monolithic component and relies on cube beamsplitters and beam transmission through perpendicular surfaces to keep angular alignment constant when operating in air or in a vacuum, which leads to the generation of ghost beams that can limit the phase readout linearity. We investigated the coupling of these beams into the non-linear phase readout scheme of DFMI and implemented adjustments of the phase estimation algorithm to reduce this effect. This was done through a combination of balanced detection and the inherent orthogonality of beat signals with different relative time-delays in deep frequency modulation interferometry, which is a unique feature not available for heterodyne, quadrature or homodyne interferometry.

Highlights

Frequency ModulationLaser interferometric displacement sensing is a central tool in precision metrology, inertial sensing, quantum technologies and prominently, gravitational wave detection experiments [1]
Its major feature is the multi-fringe dynamic range phase readout with single-beam, unequal-arm-length interferometer topologies, which are enabled by creating a deep phase-modulation-like interferogram from which the desired phase information can be extracted in real time [14]
1 μrad/ Hz and this means that we can evaluate the effect of the ghost beams on our readout purely with regard to the phase dynamics of the actual phase φL, which is the test mass motion in combination with the average laser frequency variation, though the latter can be stabilised to negligible values. This is true for the analysis of all other ghost beams discussed below as well, and we argue that ghost beams occurring in our optical head are fully negligible for the readout of phase variations in applications with very low signal dynamics

Summary

Introduction

Laser interferometric displacement sensing is a central tool in precision metrology, inertial sensing, quantum technologies and prominently, gravitational wave detection experiments [1]. An important second feature is the ability to reduce the effect of in-band laser frequency noise by using a fixed length, ultra-stable interferometer, or optical head, as a frequency reference to implement either an active stabilisation or a noise-subtraction algorithm [10,11,12]. The latter approach is made possible by a third feature, an additional readout of the macroscopic arm-length difference, or to be more precise, delay difference, from the interferogram that provides absolute ranging information.

On-Axis Optical Head Design

Ghost Beam Analysis for Deep Frequency Modulation Interferometry

Findings

Conclusions and Outlook