Geophysics

Dr. Thorsten Döhring, Optics Division, SCHOTT GLAS, Mainz

Precise Monitoring of the Earth’s Rotation

Geometrical stability in the measurement set-up is crucial for the world’s biggest and most precise ring laser gyroscope. Clear arguments for using “ZERODUR®” as the base material.

Wettzell Fundamental Research Station, June 7, 2000. The crane gradually lowers a huge “ZERODUR®” glass ceramic disk millimeter by millimeter into the partially completed underground laboratory. The giant disk, 4.25 meters in diameter, 25 centimeters thick and weighing 10 tonnes, forms the heart of the “G” ring laser, the largest instrument of its type in the world. After several hours, the “ZERODUR®” block finally comes to rest eight meters under the surface of the Earth. There it is positioned on a concrete foundation which itself sits on a solid rock foundation 12 meters below. “It’s a really strange feeling”, says Dr. Ulrich Schreiber of Munich Technical University, “to see the whole experiment hanging in the balance from the hook of a crane.” The experiment in the Wettzell geodesic observatory has in fact been commissioned jointly by the Bundesamt für Kartographie und Geodäsie (BKG or Federal Cartography and Geodetics Bureau) of Frankfurt/Main and the Forschungseinrichtung Satellitengeodäsie (FESG or Satellite Geodetics Research Institute) of Munich Technical University. Its ambitious goal is to demonstrate short-term variations in the Earth’s rotation.

Procedures used previously are based on determining the rotation of the Earth using radio telescopes and reference objects such as distant radio stars or quasars. The spatially fixed reference system of the quasars can be linked on the basis of these measurements with reference systems on the Earth as it rotates – important information for satellite-supported navigation systems such as the “Global Positioning System” (GPS) for example. However, these measurements are relatively costly and they can only be carried out in conjunction with several radio telescopes at different locations on the Earth. That is why independent alternative procedures are important.

Ring lasers are such an alternative – although not of the type that is used in compass systems for airplanes. The sensitivity of these laser gyroscopes needs to be increased by a factor of more than a thousand. The realization of the measurement principle and the technical requirements were investigated in a project study carried out jointly by the BKG, the Munich TU and the University of Canterbury, New Zealand. For this purpose a one-square meter prototype was produced by Carl Zeiss using a single piece of “ZERODUR®” made by SCHOTT. This instrument was installed and tested in 1997 in an underground laboratory in New Zealand.

“ZERODUR®” is the material of choice

At this time the decision had already been made for an even bigger ring laser. In 1998 Carl Zeiss in Oberkochen received the green light from the BKG to build the large “G” ring laser – again using a “ZERODUR®” block made by SCHOTT. But why was this material chosen? In order to demonstrate the minute differences in travel time of the laser beams, no changes can be tolerated in the actual measurement array and consequentially the travel time of the laser light. “ZERODUR®” with its extremely low thermal expansion appeared to us to be the most likely base material amongst several choices to offer the possibility of success”, argues Dr. Ulrich Schreiber. However, the thermal expansion of the 4.25 meter diameter monolith for the Wettzell ring laser had to be smaller than that of the prototype.

That was not yet foreseeable in 1992 when the huge glass-ceramic block – originally weighing 20 tonnes and now destined to serve as the core of the ring-laser – was cast at SCHOTT as a mirror blank for astronomy. Double the amount of care and preparation was, therefore, required at every stage of manufacture. First the raw disk was cut into two halves – a procedure which took a week to complete – with continuous quality monitoring as it was converted to its present form. To reduce its thermal expansion to a fifth of the previous “ZERODUR®” value, the blank was subjected to further heat treatment – known as post-ceramization – which took three months. As a result of this treatment, the block now only expands 60 nanometers per degree Kelvin temperature variation.