Ring Laser Design

I was pleased to read Alexander Hellmans' article “Ring laser senses Earth's spin” (Research News, [5 Sept., p. 1435][1]), which describes the use of large ring lasers in high-precision geodetic measurements. The article states that scientists from the Federal Office for Cartography and Geodesy in Frankfurt, Germany (IfaG), the Technical University of Munich (Germany) (TUM), and the University of Canterbury in New Zealand “built” the ring laser C-II and that I “helped” to design it. The company Schott/Zeiss (Mainz, Germany, and Oberkochen, Germany) also participated in building the device, and I designed it alone. TUM/IfaG commissioned it, and the University of Canterbury has been testing it since February 1997. The picture accompanying the article shows C-II in its final stages of construction at Zeiss, with Zeiss employees. It was not at the University of Canterbury. The “adaptive optics” consist of one mirror mounted on a piezo element to correct for path-length variations by moving it in and out (not by “bending” it). Also, the 1.2-meter × 1.2-meter [× 0.18-meter] “glass block” mentioned is actually a very-low-expansion glass ceramic called Zerodur, manufactured by Schott. A good part of the attraction of C-II is due to its mirrors, which approach reflection loss of 1 part per million. This accounts for its extraordinarily high cavity quality factor. The precision of this ultrahigh-resolution instrument is expected to surpass that of Very Long Baseline Interferometry in measuring the short-term (less than 1 week) fluctuations of Earth's rotation rate. [1]: /lookup/doi/10.1126/science.277.5331.1435a