Researchers develop glass sensors for the Einstein Telescope

From 2035, the Einstein Telescope will be able to study gravitational waves with unprecedented accuracy. For the telescope, researchers from Jena have manufactured highly sensitive sensors made entirely of glass for the first time.

Gravitational waves are distortions of space-time caused by extreme astrophysical events, such as the collision of black holes. These waves propagate at the speed of light and carry valuable information about such events throughout the universe. In the future, the Einstein Telescope will measure these waves with unprecedented precision, making it a world-leading instrument for detecting gravitational waves.

In order to minimize the impact of noise on the measurements, the telescope is to be built up to 300 meters underground. But even there, there are still mechanical vibrations, caused, for example, by distant earthquakes or road traffic above ground. Highly sensitive vibration sensors will measure these remaining vibrations.

Researchers from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have developed and built these vibration sensors for the Einstein Telescope in collaboration with the Max Planck Institute for Gravitational Physics, Hanover (Albert Einstein Institute AEI).

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,” explains Dr. Pascal Birckigt, the responsible sub-project manager at Fraunhofer IOF in Jena. The resonator was built in Jena and the laser was added in Hanover. “The mechanical resonator is the part of the sensor that converts the vibrations from the environment into a measurable movement, similar to a tuning fork.”

Researchers at Fraunhofer IOF have created something that has never been seen before: a filigree mechanical resonator made of pure silica glass (>99.8% SiO2). It combines a low natural frequency of 15 Hertz with a high-quality factor (>100,000) and a compact size of just 5 centimeters in diameter.

“In the future, the vibration sensors will be placed in the immediate vicinity of the approximately 200-kilogram mirrors in the Einstein Telescope’s gravitational wave detectors,” Birckigt continues. There will be three sensors per mirror. “Thanks to our resonators, the sensitivity of the sensors will be so high that they will be able to make the water waves of the Atlantic Ocean, which is prospectively 200 kilometers away from the telescope’s location, clearly visible as peaks in the seismic spectra.”

Complex sensor requirements: glass is the solution

The fact that the resonators are made entirely of glass is due to the complex requirements for the sensor. “There is very little space available for the sensors in the Einstein Telescope,” explains Birckigt. “At the same time, the sensors have to be particularly powerful.” Only with glass as a material could the requirements for compactness and low natural frequency be combined with high sensitivity. The reason for this is the so-called leaf springs inside the resonator.

The leaf springs are the heart of the resonator. They enable its low natural frequency, that is, the frequency at which the system begins to react to vibrations. This is necessary because the Einstein Telescope wants to measure low-frequency waves in the range between 3 and 30 Hertz. “There are two technical options to make this possible,” explains Birckigt. “Either a large test mass is installed inside the resonator, which reacts to the external vibrations, or long, elastically deformable bending beams, known as leaf springs, are attached to the test mass.

A large test mass is not possible due to the required compactness of the sensor. So the only solution was to use leaf springs, which the researchers made from glass: “Glass is a material characterized by its particularly high rigidity,” explains Birckigt. “It shows practically no plastic deformation. It is therefore possible to produce paper-thin leaf springs from glass.”

In this case, paper-thin means that a single spring is 0.1 millimeters thick, 7 centimeters long and weighs just 34 milligrams. A total of six such springs hold the 3-gram test mass stably and aligned inside the resonator.

Special bonding process for manufacturing the glass resonator

The production of such a delicate yet powerful resonator is a complex process. It includes milling and polishing work as well as laser processing methods. Furthermore, a special plasma-activated bonding process is used to create a bond at atomic level between the glass surfaces of the resonator.

“From now on, the two individual parts form a monolithic, that is permanent, unit,” explains Birckigt, who was specifically responsible for the bonding processes for the production of the glass component in the project. “This makes the resonator extremely stable and precise.”

The researchers at Fraunhofer IOF want to further develop this special method of joining glass without any additional intermediate layer in the future. Their aim is to create even more complex, three-dimensional structures.

Application potential for space and semiconductor manufacturing

In the future, the new glass resonators can be used wherever systems need to be monitored with a number of compact acceleration or position sensors. In addition to gravitational wave research, this is the case with satellites, for example, for determining their orbits, measuring the earth’s surface or inertial navigation. The resonators can also be used to improve the measurement accuracy of atomic interferometers and in EUV lithography systems for processing semiconductors.