Ring-laser gyro provides new geological information
A new ring-laser gyroscope recently completed by Carl Zeiss, (Oberkochen, Germany) is the world`s largest, according to the company, and will be used to measure the rotation of the Earth (see photo). The project was commissioned by the Institute of Applied Geodesy (Frankfurt, Germany) in cooperation with the Technical University of Munich and has a budget of about DM 750,000 ($460,000). The gyro is scheduled for installation in a subterranean cave on the Banks Peninsula in New Zealand early this year.
The gyro is capable of measuring rotation with a relative resolution of 1/10,000,000 over protracted periods of time. As a result, geophysicists will be able to draw conclusions about displacements inside the Earth, continental drift, and earthquakes. Of major importance is the fact that seismographs typically measure only linear seismic shocks, whereas a ring-laser gyro monitors different rotational components of the seismic activity.
Another area of investigation will be climatic research. Changes in the Earth`s rotation are also caused by atmospheric displacements in the form of high and low pressure.
The body of the ring-laser gyro is made of a 1.2 ¥ 1.2-m block of Zerodur glass ceramic material that is 180 mm thick and weighs approximately 600 kg. The ceramic block was produced by Schott Glaswerke (Mainz, Germany) and machined by Zeiss. The Zerodur block is made with four longitudinal bores for a laser beam, each 1 m long. Deflecting mirrors fitted at each of the four corners produce a closed, square resonator in which a laser beam travels in both directions. This optical configuration defines the ring laser.
On rotation of the gyro, a small difference in frequency is produced between the two laser waves traveling in opposite directions through the cavity. This frequency difference is measurable and can be used to determine the Earth`s speed of rotation with extraordinary accuracy. This, however, requires constancy of the light paths. If conventional materials are used for the gyro body, thermal expansion would markedly disturb the measurement. The gyro body must, therefore, be made from a material that is dimensionally stable if temperature changes occur.
Thermal expansion of Zerodur is virtually zero--a property that ensures that the surface encircled by the laser beam is highly stable and unaffected by thermally induced distortion. The machining process at Zeiss involves cutting the edges of the gyro body for attaching the deflecting (beam-steering) mirrors, followed by optical polishing of the oblique surfaces. This process required conversion of a standard optical polishing machine and a special polishing process. The oblique surfaces required polishing to a precision of l/4 so the deflecting mirrors could be attached directly to these surfaces by optical contacting, thereby eliminating use of adhesives.