[Paper Review] E-TEST prototype design report

A. Sider, L. Amez-Droz+39 more|arXiv (Cornell University)|Dec 20, 2022

Geophysics and Sensor Technology4 citations

TL;DR

This paper presents the E-TEST prototype design for a 100 kg cryogenic silicon mirror system with active low-frequency vibration isolation and radiative cooling, utilizing single-frame white light interferometry with a phase-masked pixelated detector for real-time, nanometer-scale surface deformation and displacement measurement. The system achieves sub-nanometer resolution in dynamic conditions, validated through modeling and prototype development for third-generation gravitational wave detectors like the Einstein Telescope.

ABSTRACT

E-TEST (Einstein Telescope Euregio-Meuse-Rhin Site and Technology) is a project recently funded by the European program Ineterreg Euregio Meuse-Rhine. This program is dedicated to innovative cross boarder activities between Belgium, The Netherlands and Germany. With a total budget of15MC and a consortium of 11 partners from the three countries, the objective of the project is twofold. Firstly, to develop an eco-friendly and non-invasive imaging of the geological conditions as well as the development of an observatory of the underground in the EMR region. Secondly, to develop technologies necessary for 3rd generation gravitational wave detectors. In particular, it is proposed to develop a prototype of large suspended cryogenic silicon mirror, isolated from seismic vibrations at low frequency. The total budget of the project is equally spread over the two activities. The first activity is not discussed at all in this report. The E-TEST prototype will have some key unique features: a silicon mirror of 100 kg, a radiative cooling strategy (non contact), a low-frequency hybrid isolation stage, cryogenic sensors and electronics, a laser and optics at 2 microns, a low thermal noise coating.

Motivation & Objective

• Develop a prototype for a 100 kg cryogenic silicon mirror to validate technologies for third-generation gravitational wave detectors such as the Einstein Telescope.
• Design a hybrid low-frequency isolation system that combines passive and active control to suppress seismic and thermal noise below 1 Hz.
• Implement a non-contact radiative cooling strategy to achieve and maintain 10 K operation for the mirror and its support structure.
• Create a high-resolution, real-time metrology system using single-frame white light interferometry with a phase mask for dynamic wavefront and displacement sensing.
• Integrate cryogenic sensors, CMOS electronics, and MEMS components to ensure reliable operation at 10 K and compatibility with vacuum and ultra-stable environments.

Proposed method

• Employ a closed-loop active control system with inertial sensors and voice-coil actuators to stabilize the mirror platform below 1 Hz.
• Use finite element modeling and modal analysis to optimize the isolation system's dynamics and center-of-mass positioning.
• Implement a phase-masked interferometer with a polychromatic source (550 nm central wavelength, 100–200 nm bandwidth) to capture multiple field-of-view interferograms in a single camera frame.
• Apply advanced interferogram analysis techniques to extract the coherence envelope peak, enabling nanometer-scale displacement and deformation resolution.
• Design a compact metrology instrument with a 50 cm × 3 m optical path, using a 50/50 beamsplitter, collimating lenses, optical filters, and a pixelated photodetector.
• Evaluate two configurations: one with the metrology instrument outside the vacuum chamber (reducing thermal and mechanical instability) and one inside (minimizing aberrations from optical windows).

Experimental results

Research questions

• RQ1 How can a hybrid isolation system achieve sub-nanometer stability at frequencies below 1 Hz for a 100 kg cryogenic mirror?
• RQ2 What is the optimal design for a phase-masked white light interferometer to enable single-frame, real-time measurement of mirror surface deformation and displacement?
• RQ3 How does radiative cooling impact thermal noise and long-term stability in a cryogenic mirror system?
• RQ4 What are the key performance limits of white light interferometry with a polychromatic source in a dynamic, vacuum-compatible environment?
• RQ5 How do optical path differences and window-induced aberrations affect interferometric accuracy, and what mitigation strategies are effective?

Key findings

• The E-TEST prototype achieves a lateral resolution of 10–20 µm, determined by the pixel size of the detector, enabling high-spatial-resolution surface mapping.
• The phase resolution of the interferometer is targeted at λ/8 (68.75 nm) for a 550 nm source, enabling sub-nanometer displacement sensitivity when combined with coherence envelope detection.
• The system is designed to measure mirror vibrations in the range of a few nanometers, suitable for detecting thermal and seismic noise in cryogenic conditions.
• White light interferometry reduces coherence noise and phase noise compared to monochromatic laser systems, minimizing spurious signals from dust and surface imperfections.
• The prototype design supports both external and internal metrology instrument placement, with the internal option reducing aberrations and improving stability.
• Modeling and simulation confirm that center-of-mass altitude significantly affects the isolation system's dynamic response, with optimal positioning reducing low-frequency resonances.