Ring Laser Accuracy: Unprecedented Daily Measurement and Mapping of Earth's Rotation

Scientists at the Technical University of Munich have made significant progress in measuring the Earth's rotation with unprecedented accuracy. Now, the ring laser from the Wettzell Geodetic Observatory can be used to capture data at a quality level unmatched anywhere in the world. These measurements are crucial for determining the position of the Earth in space, assisting climate research, and improving the reliability of climate models.

Advanced ring laser technology
Want to quickly walk to the basement and see how fast the Earth has been spinning in the past few hours? Now, you can visit the Wettzell Geodetic Observatory. TUM researchers have improved the ring laser there so that it can provide daily current data, which is currently impossible at comparable quality levels.

What exactly does a ring laser measure? During the journey through space, the speed at which the Earth rotates around its axis varies slightly. In addition, the axis of planetary rotation is not completely stationary, it is a bit wobbly. This is because our planet is not completely solid, but composed of various components, some solid and some liquid. Therefore, the interior of the Earth itself is constantly moving. These mass changes can accelerate or brake the rotation of planets, and these differences can be detected using measurement systems such as TUM ring lasers.

The rotating waves are not only important for astronomy, but we also urgently need them to create accurate climate models and better understand weather phenomena such as El Ni ñ o. The more accurate the data, the more accurate the predictions, "said Professor Ulrich Schreiber, who led the project at the TUM Observatory.

Technical improvements and challenges
When repairing the ring laser system, the team prioritizes finding a good balance between size and mechanical stability, as the larger the device, the more sensitive the measurements it can make. However, size means making compromises in terms of stability and accuracy.

Another challenge is the symmetry of two relative laser beams, which is the core of the Wettzell system. Accurate measurements can only be made when the waveforms of two backpropagation laser beams are almost identical. However, the design of the device implies that there is always a certain degree of asymmetry. In the past four years, geodetic scientists have successfully captured these system effects using theoretical models of laser oscillation, so that they can be accurately calculated over a long period of time and thus eliminated from measurements.

Improve accuracy and application
The device can use this new calibration algorithm to accurately measure the Earth's rotation to 9 decimal places, equivalent to a fraction of a millisecond per day. In terms of laser beams, this is equivalent to an uncertainty starting from 20 decimal places after the optical frequency and stabilizing for several months. Overall, within approximately two weeks, the observed fluctuations reached a value of up to 6 milliseconds.

The improvement of lasers has now greatly shortened the measurement cycle. The newly developed correction program enables the team to capture current data every three hours. Urs Hugentobler, a professor of TUM satellite geodesy, said: In Earth science, such a high level of temporal resolution is absolutely novel for independent ring lasers. Compared to other systems, lasers operate completely independently and do not require reference points in space. In traditional systems, these reference points are created by observing constellations or using satellite data. However, we are independent of this kind of thing and very precise. Data captured independently of stellar observations can help identify Do not compensate for system errors in other measurement methods. The use of various methods can help to make the work particularly detailed, especially in situations with high precision requirements, such as ring lasers. In the future, there are plans to further improve the system to achieve shorter measurement cycles.

Understanding Ring Lasers
A ring laser consists of a closed square beam path, with four mirrors completely enclosed in a Ceran microcrystalline glass body, known as resonators. This can prevent the length of the path from changing due to temperature fluctuations. The helium/neon mixture inside the resonator can achieve clockwise and counterclockwise laser beam excitation.

If there were no motion of the Earth, light would travel the same distance in both directions. However, due to the device moving with the Earth, the distance of one of the laser beams is shorter because the Earth's rotation brings the mirror closer to the beam. In the opposite direction, the distance of light propagation is correspondingly longer. This effect creates a difference in the frequency of two light waves, and their superposition produces a beat note that can be measured very accurately. The higher the speed of Earth's rotation, the greater the difference between the two light frequencies. At the equator, the Earth rotates 15 degrees eastward every hour. This will generate a signal of 348.5 Hz in the TUM device. The fluctuation of a day's length is expressed as 1 to 3 millionths of a hertz.

Powerful and precise infrastructure
Each side of the ring laser located in the basement of the Wezel Observatory is four meters long. Then, the structure is anchored to a sturdy concrete column located on a sturdy bedrock in the Earth's crust, with a depth of approximately six meters. This ensures that the Earth's rotation is the only factor affecting the laser beam and excludes other environmental factors. This structure is protected by a pressurized chamber that can compensate for changes in air pressure or required temperature of 12 degrees Celsius, and automatically compensate for these changes. In order to minimize these influencing factors as much as possible, the laboratory is located at a depth of five meters below the artificial mountain. Nearly 20 years of research work have been invested in the development of measurement systems.

Source: diodelaser net