Ring Laser Measures Earth's Rotation

R. Winn Hardin

CHRISTCHURCH, New Zealand -- In an underground cave, deep within the Cashmere peninsula, the musical note E-flat has taken on special meaning.
Researchers from the University of Canterbury have installed a giant ring gyroscope to keep track of minute fluctuations in the Earth's rotation. Cast in a square, 1.2-m slab of Zerodur glass, the C-II gyroscope acts as a resonance cavity for a HeNe laser. Inside the glass, a pair of counterpropagating laser beams with wavelengths of 633 nm travel around the cavity, creating a standing wave pattern. The wave pattern stands still in absolute space, so that as the Earth spins on its heavenly journey, individual bright points, or nodes, march by a fictional viewing window. Normally the nodes pass any point on the gyroscope at a rate of 79.4 nodes per second, the frequency of the musical note E-flat.

New Zealand researchers have turned military technology into true science with the inclusion of adaptive optics and finely turned mirrors capable of maintaining a perfect standing wave around a 4.8-m optical path.

The ring laser uses radio waves to produce stimulated emission. High-precision mirrors at each corner direct the beam; a very stable reference laser provides data on temperature and pressure fluctuations. A computer then uses that data to control actuators attached to one mirror to keep the beam path constant.
Researchers planted the gyroscope deep within the coastal cavern to reduce temperature fluctuations as much as possible. However, even these measures make it difficult for scientists to keep reliable counts of the moving nodes. To detect the millisecond fluctuations in the Earth's rotation, the device must be accurate to within one node in 10 million.
Creators of the C-II ring gyroscope, the Institut für Angewandte Geodaesie of Frankfurt and the Technische Universität München, hope to build an even larger gyroscope by 2002. The 4-m ring gyroscope, to be placed at an underground site in Wettzell, Germany, would offer even greater accuracy.