Throughout his career, laser physicist Prof. Ulrich Schreiber of the Technical University of Munich (TUM) has been pursuing the objective of using a local measuring device to detect minimal variations in the movement of the earth and thus to establish new measurement methods for geodesy. Schreiber is head of the "Optical Technologies" group at the Geodetic Observatory in Wettzell. Heiner Igel, Professor of seismology at Munich's Ludwig Maximilian University (LMU) wants to measure ground movements in ever more detail. These are the ideal prerequisites for a partnership which has existed since 2001. The results speak for themselves: The ring laser ROMY (Rotational Motions in Seismology) has now officially been put into operation in Fürstenfeldbruck, near Munich. According to an article in the journal "Science", the new ring laser is one of the most sophisticated devices of its kind in the world.
The precision device, one hundred cubic meters in size, has been buried in the ground and will measure the earth's rotational motions in more detail than any other device has been able to do in the past. Our planet is constantly in motion, orbiting the sun while rotating around its own axis. But the path is not always the same; it is subject to minimal variances. The earth wobbles a little bit like a top does: The axis varies a little, the drive is not consistent. Strong winds in the atmosphere, ocean currents and magma movements in the earth's interior pull at the globe, which is shaken by earthquakes.
Measurement results in a matter of seconds
"It's not possible to model these influences," explains Schreiber. "We have to measure them." For example, any GPS system can only work properly in the long term when such deviations are included in positioning calculations. The currently conventional method for doing this is Very Long Basis Interferometry (VLBI) in which a worldwide network of radio telescopes uses quasars in the farthest reaches of the universe, millions of light-years away, as fixed points to determine the position of the earth in space. But this method is complicated and takes several days to return results, and is also dependent on the fixed points in space. The scientists hope the ring laser in Munich will turn out to be at least as accurate as this method, but significantly faster. The measurement results are expected to be available in a matter of seconds. "One objective is combining the two methods in order to benefit from their respective advantages."
The ring laser employs the principle of the laser gyroscope, also used to stabilize aircraft in case of poor visibility. "The airplane can move along three axes," Schreiber explains: along the pitch axis when the nose of the plane rises or falls, the yaw axis when the nose moves to the left of right, and along the roll axis when one wing rises and the other wing falls.
The earth also moves in three dimensions, in a manner of speaking it is the airplane, although a fairly large one. "If we want to use this principle to measure the irregular movements of the earth, the instrument has to be millions of times more sensitive than the aircraft gyroscope." In his post-doctoral dissertation, Schreiber worked together with colleagues from the University of Canterbury in New Zealand to develop such an instrument. At the end of the 1990s construction began on the most stable ring laser in the world at the Geodetic Observatory in Wettzell, which is operated by the TU Munich and the German Federal Agency for Cartography and Geodesy.
In highly simplified terms, the principle works like this: Two laser beams travel in opposite directions along a square path with mirrors at the corners, forming a closed circuit (thus the term ring laser). If this system turns, the laser beam travelling in the direction of movement has a longer path than the beam travelling in the opposite direction. The beams alter their wavelengths accordingly and the optical frequency changes. Based on this difference, conclusions can be reached about the speed at which the system is turning. In Wettzell it's not the ring laser which rotates, but the earth itself. The structure itself, measuring four by four meters, is anchored in massive concrete columns, themselves resting on the solid rock of the earth's crust at a depth of around six meters. This means that nothing other than the rotation of the earth can affect the laser beams.
The way in which the rotation of the earth affects the light depends on the location of the laser: "If we were located at one of the poles, the axis of rotation of the earth and the axis of rotation of the laser would be identical and we would detect a one-to-one rotational speed," says Schreiber. "On the other hand, at the equator the beam of light would not detect the fact that the earth is turning at all." The scientists therefore have to take the position of the laser in Wettzell on the 49th parallel into account. If the axis of the earth's rotation changes, then the researcher's view of the rotation speed changes as well. The changes in the behavior of the laser light thus indicate the fluctuations in the earth's axis.
The official opening is only the beginning
The scientists still weren't satisfied with the results. The original Wettzell ring laser consists of only one component and can therefore only measure the movements of the earth along one axis. In order to cover all three spatial dimensions, the new ring laser doesn't have a single square ring, but rather four large triangular laser systems with an edge length of twelve meters, which are opposed to one another in order to form a tetrahedron.
Before construction could begin, the scientists had already worked together with the University of Canterbury to build a simple prototype. The greatest challenge in the development of the ring laser was achieving both high resolution and high stability at the same time, Schreiber observes. "We moved from one insight to the next." And the official opening of the new ring laser ROMY, which belongs to the LMU, also marks the beginning of further optimizations. Now the researchers can test how good the measurements of the individual components fit with one another and how stable they are in relation to one another.