Jena researchers manufacture highly sensitive resonators made entirely of glass for the vibration sensors of the Einstein telescope. Credit: Fraunhofer IOF
Starting in 2035, Einstein Telescope will be able to study gravitational waves with unprecedented accuracy. For telescopes, Jena researchers produced the first highly sensitive sensor made of glass.
Gravitational waves are space-time distortions caused by extreme astrophysical events such as black holes collisions. These waves propagate at the speed of light, carrying valuable information about such events throughout the universe. In the future, the Einstein telescope will measure these waves with unprecedented accuracy and make it the world’s leading instrument for detecting gravitational waves.
To minimize the effect of noise on measurements, the telescope will increase the underground up to 300 meters. But even there, there are still mechanical vibrations caused by, for example, distant earthquakes and road traffic on the ground. A highly sensitive vibration sensor measures these remaining vibrations.
Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have collaborated with the Max Planck Gravitok Institute of Physics at Hanover (Albert Einstein Institute AEI) to develop and build these vibration sensors for the Einstein telescope.
Resonator for vibration sensors made entirely of silica glass
“Such a vibration sensor consists of two core components: a laser that reads the movement of the resonator and the movement of the resonator,” explains Dr. Paskarbil Kigut, responsible subproject manager at Fraunhofer IOF at Yenna. The resonator was built in Jena and the laser was added to Hanover. “Mechanical resonators, like tuning forks, are part of a sensor that converts vibrations from the environment into measurable movement.”
Fraunhofer IOF researchers have created something they have never seen before. It combines a low eigenfrequency of 15 Hertz with a high quality coefficient (>100,000) and a compact size with just 5 centimeters in diameter.
“In the future, vibration sensors will be located very close to the roughly 200-kilometer mirror of the Einstein Telescope’s gravitational wave detector,” continues Birckigt. Each mirror has three sensors. “Thanks to our resonators, the sensors are very sensitive and can create water waves in the Atlantic Ocean.
Complex Sensor Requirements: Glass is the Solution
The fact that the resonator is made entirely of glass is due to the complex requirements of the sensor. “There is little space available for sensors on the Einstein telescope,” explains Birckigt. “At the same time, the sensor must be particularly strong.” Only glass as a material can combine compactness and low natural frequency requirements with high sensitivity. The reason for this is the so-called leaf springs in the resonator.
The leaf spring is the center of the resonator. They may be their low natural frequencies, the frequency at which the system begins to respond to vibrations, or the frequency at which it is. This is necessary because the Einstein telescope wants to measure low-frequency waves in the HERTZ range of 3-30. “There are two technical options to make this possible,” explains Birckigt. “A large test mass is attached inside the resonator, either responsive to external vibrations or a long, stretchable, deformable bending beam known as leaf springs, is attached to the test mass.
Due to the compactness required of the sensor, large test masses are not possible. Therefore, the only solution was to use leaf springs made from glass by researchers. “Glass is a material that is particularly strong in its rigidity,” explains Birckigt. “It does not show practically any deformation of the plastic. Therefore, it is possible to produce thin paper leaf springs from glass.”
In this case, thin paper means one spring thickness is 0.1 mm, 7 cm long, and weighs only 34 milligrams. A total of six such springs will stably stabilize the test mass of 3 grams and align it within the resonator.
Special coupling process for fabricating glass resonators
The production of such delicate and powerful resonators is a complex process. This includes milling and grinding operations, as well as laser treatment methods. Additionally, a special plasma-activated coupling process is used to create bonds at the atomic level between the glass surfaces of the resonator.
“From now on, the two separate parts form a permanent unit, a monolithic,” explains Birckigt, who was particularly responsible for the coupling process for the production of glass components in the project. “This makes the resonator very stable and accurate.”
Researchers at Fraunhofer IOF want to develop further this special method of bonding glass without additional middle layers in the future. Their goal is to create even more complex, three-dimensional structures.
Space and potential for semiconductor manufacturing applications
In the future, new glass resonators can be used if the system needs to be monitored with a large number of compact acceleration or position sensors. In addition to studying gravitational waves, this is for example to determine orbits and measure the Earth’s surface or inertial navigation in the case of satellites. Resonators can also be used to improve the measurement accuracy of atomic interferometers and EUV lithography systems for processing semiconductors.
Provided by Fraunhofer-Institutfür Angewandteoptik und feinmechanik iof
Citation: The researchers have developed the Einstein Telescope glass sensor obtained from March 13, 2025 from https://phys.org/2025-03-03-Glass-sensors-einstein-telescope.html from March 13, 2025.
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