The C-II ring laser project

The intensive use of Very Long Baseline Interferometry (VLBI), as well as Satellite and Lunar Laser Ranging (SLR, LLR) and the GPS-System allows for a precise monitoring of the current variations in earth rotation. The precision for the VLBI-system is as high as one part in 109 for a 24 hour measurement campaign. Since all these techniques are based on external reference frames, it is desirable to develop inertial sensors to the same resolution. Ring laser gyroscopes have the potential to be used for that purpose. The basic idea is to scale up the area surrounded by the two counter-rotating laser beams in order to gain higher sensitivity for the instrument with respect to the rotation rate. The Canterbury Ring Laser (C-I) consisting of a square of area 0.755 m2 has shown that ring lasers with a perimeter more than 60 cm are possible, in that the occurrence of more than one longitudinal mode of operation could be avoided by reducing the power of the circulating beams (Bilger et al., 1993). The next instrument C-II with an area of 1 m2 demonstrated many of the mechanical requirements for a stable instrument. It was constructed from a monolithic block of Zerodur, so that the size of the area does not change under temperature variations. A successfully built prototype for a large ring (G0) finally proved that it is possible to build a ring laser gyroscope with at least 14 m length of cavity (Rowe et al., 1999). This means that most of the demands for a highly sensitive inertial earth rotation sensor are met. On this basis a high resolution and highly stable instrument, suitable for geophysical applications, can be constructed.