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The Wettzell ring laser has been continuously improved since its commissioning. Credit: Technical University of Munich

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The Wettzell ring laser has been continuously improved since its commissioning. Credit: Technical University of Munich

Researchers from the Technical University of Munich (TUM) have succeeded in measuring the Earth’s rotation more precisely than ever before. The Wettzell Geodetic Observatory’s ring laser can now be used to capture data with an unparalleled level of quality anywhere in the world. The measurements will be used to determine Earth’s position in space, benefit climate research and make climate models more reliable.

Do you want to quickly go down to the basement and see how fast the Earth has rotated over the past few hours? This is now possible at the Wettzell Geodetic Observatory. TUM researchers there improved the ring laser so that it can provide daily current data, which was until now not possible with comparable quality levels.

What exactly does the ring laser measure? During its journey through space, the Earth rotates on its axis at slightly varying speeds. Additionally, the axis around which the planet rotates is not completely static, it wobbles a bit. Indeed, our planet is not completely solid, but is made up of various components, some solid, others liquid. So the interior of the Earth itself is constantly in motion. These mass changes speed up or slow down the planet’s rotation, differences that can be detected using measurement systems like the TUM ring laser.

“Rotation fluctuations are not only important for astronomy, we also urgently need them to create accurate climate models and better understand weather phenomena like El Niño. And the more precise the data, the more accurate the forecasts “, explains Professor Ulrich Schreiber. , who led the project at the TUM Observatory.

Revised sensors and correction algorithm

When redesigning the ring laser system, the team prioritized finding a good balance between size and mechanical stability, because the larger such a device, the more sensitive the measurements it can make. However, size implies compromises in terms of stability and therefore accuracy.

Another challenge lay in the symmetry of the two opposing laser beams, the heart of the Wettzell system. Exact measurement is only possible when the waveforms of the two inversely propagating laser beams are almost identical. However, the design of the device means that some asymmetry is always present.

Over the past four years, geodesists have used a theoretical model of laser oscillations to successfully capture these systematic effects, to the extent that they can be accurately calculated over a long period of time and thus eliminated from measurements.

Device measurements are significantly more precise

The device can use this new corrective algorithm to measure the Earth’s rotation with an accuracy of 9 decimal places, which corresponds to a fraction of a millisecond per day. For laser beams, this amounts to an uncertainty starting only at the 20th decimal place of the light frequency and stable for several months.

Overall, the observed upward and downward fluctuations reached values ​​of up to 6 milliseconds over approximately two weeks.

Improvements to the laser now also make it possible to significantly reduce measurement periods. Newly developed patch programs allow the team to capture current data every three hours.

Urs Hugentobler, professor of satellite geodesy at TUM, says: “In geosciences, such high levels of temporal resolution are absolutely new for autonomous ring lasers. Unlike other systems, the laser works completely independently and does not require reference points in space. With conventional systems, these reference points are created by observing the stars or using satellite data. But we are independent of this stuff and also extremely precise. »

Data captured independently of stellar observation can help identify and compensate for systematic errors in other measurement methods. The use of different methods makes the work particularly thorough, especially when precision requirements are high, as is the case with the ring laser. Further improvements to the system, allowing even shorter measurement periods, are planned in the future.

Ring lasers measure interference between two laser beams

Ring lasers consist of a square, closed beam path with four mirrors completely enclosed in a certain body, called a resonator. This prevents the path length from changing due to temperature fluctuations. A helium/neon gas mixture inside the resonator allows the excitation of the laser beam, one clockwise and one counterclockwise.

Without the movement of the Earth, light would travel the same distance in both directions. But because the device moves with the Earth, the distance for one of the laser beams is shorter, because the Earth’s rotation brings the beam’s mirrors closer together. In the opposite direction, light travels a proportionately longer distance.

This effect creates a difference in the frequencies of the two light waves, the superposition of which generates a beat note that can be measured very precisely. The higher the speed at which the Earth rotates, the greater the difference between the two optical frequencies. At the equator, the Earth rotates 15 degrees eastward every hour. This generates a 348.5 Hz signal in the TUM device. Fluctuations in the length of a day manifest themselves with values ​​​​of 1 to 3 millionths of a Hz (1 to 3 microhertz).

Each side of the laser ring located in the basement of the Wettzell Observatory measures four meters. This construction is then anchored to a solid concrete column, which rests on the solid bedrock of the earth’s crust at a depth of approximately six meters. This ensures that the Earth’s rotation is the only factor impacting the laser beams and excludes other environmental factors.

The construction is protected by a pressure chamber, which compensates for changes in air pressure or the desired temperature of 12 degrees centigrade and automatically compensates for these changes. To minimize these influencing factors, the laboratory is located five meters deep under an artificial hill. Nearly 20 years of research were devoted to developing the measurement system.

The study is published in the journal Natural photonics.

More information:
K. Ulrich Schreiber et al, Variations in the Earth’s rotation speed measured with a ring laser interferometer, Natural photonics (2023). DOI: 10.1038/s41566-023-01286-x

Journal information:
Natural photonics

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