Starting in 2035, the Einstein Telescope will study gravitational waves with unparalleled accuracy. As part of this groundbreaking project, researchers from Jena have developed highly sensitive sensors made entirely of glass for the first time.
Researchers from Jena have manufactured highly sensitive resonators made entirely of glass for the vibration sensors of the Einstein Telescope. Image Credit: Fraunhofer IOF
Gravitational waves are ripples in space-time caused by extreme astrophysical events, such as black hole mergers. These waves travel at the speed of light, carrying invaluable information about these cosmic phenomena. The Einstein Telescope will measure these waves with unprecedented precision, establishing itself as a leading global instrument for gravitational wave detection.
To minimize interference from environmental noise, the telescope will be constructed up to 300 meters underground. Even at that depth, mechanical vibrations—caused by distant earthquakes or traffic above—can still affect measurements. To counter this, highly sensitive vibration sensors will be deployed to detect and compensate for these disturbances.
In collaboration with the Max Planck Institute for Gravitational Physics in Hanover (Albert Einstein Institute AEI), researchers from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have developed and built these advanced vibration sensors.
Resonator for Vibration Sensors Made Entirely of Silica Glass for the First Time
Such a vibration sensor consists of two core components: a movable resonator and a laser that reads the movement of the resonator.
Dr. Pascal Birckigt, Sub-Project Manager, Fraunhofer Institute for Applied Optics and Precision Engineering
The resonator was manufactured in Jena, while the laser was integrated in Hanover.
Birckigt said, “The mechanical resonator is the part of the sensor that converts environmental vibrations into a measurable movement, similar to a tuning fork.”
For the first time, researchers have created an intricate mechanical resonator made entirely of pure silica glass (> 99.8 % SiO2). This innovation combines a low natural frequency of 15 Hertz with a high-quality factor (>100,000) while maintaining a compact five-centimeter diameter.
“In the future, the vibration sensors will be positioned in close proximity to the approximately 200-kg mirrors in the gravitational wave detectors of the Einstein Telescope,” Birckigt added.
Each mirror will have three sensors.
Thanks to our resonators, the sensitivity of the sensors will be so high that they will be able to detect the water waves of the Atlantic Ocean, which is potentially 200 km away from the telescope's location, as distinct peaks in the seismic spectra.
Dr. Pascal Birckigt, Sub-Project Manager, Fraunhofer Institute for Applied Optics and Precision Engineering
Complex Sensor Requirements: Glass is the Solution
The choice of glass for the resonators stems from the demanding design requirements.
There is very little space available for the sensors in the Einstein Telescope. At the same time, the sensors must be particularly powerful.
Dr. Pascal Birckigt, Sub-Project Manager, Fraunhofer Institute for Applied Optics and Precision Engineering
Glass allows for a compact design while maintaining a low natural frequency and high sensitivity—achieved through the use of ultra-thin leaf springs inside the resonator.
Leaf springs are crucial to the resonator’s function, enabling it to respond to low-frequency waves between 3 and 30 Hertz.
Birckigt explained, “There are two technical approaches to achieve this. Either a large test mass is placed inside the resonator to respond to external vibrations or long, elastically deformable bending beams, known as leaf springs, are attached to the test mass.”
Given the telescope’s spatial constraints, a large test mass was not feasible. Instead, researchers designed ultra-thin leaf springs made of glass.
Birckigt stated, “Glass is a material known for its exceptional rigidity. It exhibits virtually no plastic deformation, allowing for the production of paper-thin leaf springs.”
In this context, “paper-thin” refers to springs that are 0.1 mm thick, 7 cm long, and weigh only 34 mg. A total of six such springs securely hold the three-gram test mass within the resonator.
Special Bonding Process for Manufacturing the Glass Resonator
Producing these delicate yet powerful resonators requires highly specialized techniques, including milling, polishing, laser processing, and an advanced plasma-activated bonding process. This method creates atomic-level bonds between glass surfaces, resulting in a monolithic, highly stable structure.
“From now on, the two individual parts form a monolithic, that is permanent, unit. This enhances the stability and precision of the resonator,” explained Birckigt, who oversaw the bonding processes for the glass component in the project
The researchers at Fraunhofer IOF aim to further refine this bonding technique to develop even more complex three-dimensional glass structures in the future.
Application Potential for Space and Semiconductor Manufacturing
Beyond gravitational wave research, these glass resonators have potential applications in various fields. They could enhance satellite systems by improving orbital tracking and Earth surface measurements, support inertial navigation, and increase the precision of atomic interferometers. Additionally, they may contribute to advancements in semiconductor manufacturing, particularly in EUV lithography systems.
Commissioning of the Einstein Telescope Planned From 2035
Development of the Einstein Telescope has been ongoing since 2008. As a third-generation gravitational wave detector, it will be up to ten times more sensitive than current instruments. Construction is set to begin in 2026, with operations expected to commence in 2035. The proposed site for the telescope is the Euregio Meuse-Rhine region, spanning Germany, Belgium, and the Netherlands.
The vibration sensors were developed as part of the "Glass Technologies for the Einstein Telescope" (GT4ET) project, a collaboration between research teams in Jena and Hanover.