With the large ring laser gyros for the geosciences, we enter the extreme high resolution regime of rotation sensing, observing rotation rates of less than 1 picoradian/s. This requires us to look closely at all potential noise sources in the ring laser itself, as well as in the data acquisition process. Our ring laser measurements for space geodesy are also based on ancillary sensors, such as high resolution tiltmeters, ambient pressure sensors, thermometers and an optical frequency comb for the stabilization of the laser frequency. This chapter discusses the required performance of the detection system together with the performance of the ancillary sensors. We also have to examine the reliability of our numerical algorithms, like frequency estimators, both in the time and frequency domains. With everything included and tested, observations of polar motion, solid Earth tides, ocean loading and the Chandler motion are now possible.

This chapter discusses the design properties and operational practices of the helium–neon ring laser gyroscope in fine detail. We look atmonolithic ring laser design and explore the advantage of upscaling, which eventually necessitates a heterolithic cavity structure. This requires us to revisit the scale factor stability, beam wander and strain effects. Topics like backscatter correction, sensor sensitivity and a variety of different operation modes, such as single laser mode or the mode-locked regime, are presented and critically discussed. The mitigation of all sorts of obvious and also a large number of unsuspected ring laser error sources is intensively treated and illustrated with a rich body of measurement examples, before the importance and limitations of the cavity mirrors is addressed. This leads us to explore several alternative transitions of neon, before the design and realization of large sensor arrays is described. With the realization of the ROMY ring laser array in the shape of a tetrahedron, we have recovered the full Earth rotation vector. Furthermore, ROMY is also the first six degree of freedom sensor for teleseismic events.

After looking at the details of ring laser design and operation and putting it into perspective with alternative sensor designs, we now look at the actual areas of application of large ring laser gyroscopes. We start off by discussing ring resonator performance from the viewpoint of an optical frequency source. Then we carry on by summarizing how the application of large ring lasers to high resolution rotation sensing has enabled the research field of rotational seismology, before discussing the achievements obtained with ring lasers for space geodesy over the years. Finally we briefly touch the field of the highly sensitive detection of non-reciprocal phenomena, such as magneto-chiral birefringence, before addressing the applicability of ring laser gyroscopes to tests in fundamental physics. The book concludes with an outlook on the future of large ring lasers.

Over the last century since the first successful demonstration of the Sagnac effect, there have been several attempts to develop an ultra-high-sensitivity device for continuous high resolution rotation sensing, such as the Michelson–Pearson–Gale interferometer in Clearing, Illinois. Apart from the application of a passive optical cavity,active ring laser gyroscopes with the gain medium inside the traveling wave ring cavity have also been used. This chapter briefly introduces the most important activities over the last century and puts them into context with our own large ring laser project.

The essence of a well performing ring laser gyro is the complete reciprocity of the traveling wave oscillator. In addition to this, the scale factor needs to be sufficiently large and above all extremely stable. In an active ring laser gyroscope, the gain medium inside the resonator causes a modification to the scale factor of the instrument, which is important to quantify correctly in order to relate the observed optical beat note properly to the experienced rate of rotation of the sensor. For highly accurate rotation sensing, these effects have to be known with extremely high accuracy. This chapter introduces the important helium–neon-based ring laser gyroscope design features.

This preface of the book provides the driving motivation for the development of ultra-stable and highly sensitive inertial rotation sensing for applications in the geosciences. It identifies the exact observation of the rotation rate and the orientation of the rotation axis of the Earth as the important connecting link between the terrestrial reference frame and the long-term-stable celestial reference frame. The former is important because we navigate in this frame, while the latter is the frame in which navigation satellite motion is defined. With the precise knowledge of Earth rotation, one can transform from one reference frame to the other. At this point in time, space geodesy still awaits a self-contained continuously observing high resolution inertial sensor for this demanding task. Large ring laser gyroscopes are currently the only promising technique for this task.

Ring lasers are not the only devices that allow rotation sensing with high resolution. This chapter looks at alternative rotation sensing concepts and their physical realization, to put the achievements reported in Chapters 3 and 4 into perspective. We briefly introduce passive Sagnac interferometers, small and large fiber optic gyros, helium SQUID gyros, atom interferometry and Coriolis force-exploiting sensors. It turns out that every application has a different set of requirements, and some types of sensors are better suited for the respective purposes than others. Here we illustrate how the purpose ultimately defines the best technical solution. A book like this would not be complete without looking at solid state ring lasers. However, we also show that in terms of sensitivity and stability, the large ring laser gyroscope takes a prominent role in inertial rotation sensing.