Running light around a tetrahedron
Page2: Taking the longer way round
Indeed, he and his colleagues hope that the new ring laser will provide answers to a whole series of open questions. For instance, rotation sensors can measure the magnitude of tilting and rotational ground motions, which structural engineers need to enhance the stability of buildings in earthquake zones. Rotation sensors can also provide data that give insights into anomalous magma dynamics in active volcanoes, and thus serve to improve the quality of corresponding modeling studies. In combination with other methods, such measurements permit geophysicists to probe the properties and the dynamics of the Earth’s interior, Igel explains. And that’s not all. ROMY also promises to shed new light on how the world’s oceans interact physically with the planet, causing it to oscillate permanently.
The principle on which the instrument’s operation is based was first demonstrated by the French physicist Georges Sagnac shortly before the outbreak of the First World War: He showed a beam of light is directed around a closed course (with the aid of mirrors), the time it takes to complete a circuit is independent of the direction in which it propagates. However, if the apparatus is rotated, the beam travelling in the same sense as the rotation takes slightly longer for each lap – because it has to cover a greater distance than a beam transmitted in the opposite direction. Due to this difference in path-length, two counter-propagating beams will be phase shifted with respect to one another and, when recombined, they produce a typical interference pattern. In exactly the same way, when two notes that are slightly out of tune are sounded together, they produce a characteristic beat note which varies regularly in pitch. Moreover, the rotation rate can be calculated from the frequency of the beat note produced when the counter-propagating beams are superimposed.
Igel and laser physicist Ulrich Schreiber from the TUM made use of this principle in their design for ROMY to measure spin or tilting motions. In this case, the laser beams are propagated along not one but four axes. Each of the four light paths forms the edges of an equilateral triangle with sides 12 m long, At each apex, the light is deflected by mirrors, whose positions can be adjusted with high precision. Together, the four rings form the faces of a regular, inverted tetrahedron whose apex lies 15 m underground. This set-up enables the scientists to measure rotational motions in all directions.
Five km of optic fiber, tightly wound
“It took us two years to work out how to build it,” Igel says. To ensure high sensitivity, the ring lasers must be shielded from environmental interference. For example, in order to protect the instrument from ground water, it was enclosed in a tetrahedral concrete shell – like a plant in a flower pot. Igel realized early on that he needed to have his colleague from the TUM onboard to make the project a success – for Schreiber had already designed and built several ring-laser systems in Germany, New Zealand, the USA, Italy and elsewhere. ROMY, however, is undoubtedly his masterpiece. Incorporating computer-controlled precision engineering into an instrument with dimensions of 12 m requires a new level of meticulousness.
Meanwhile, the instrument has not only been tested and calibrated, it has already performed a whole series of measurements which will form the basis for several publications. For example, some of the aftershocks observed after the series of earthquakes in Norcia in Central Italy in October 2016 have been characterized, as well as the seismic noise generated by the Earth’s oceans.
Recording the hitherto unquantifiable tilt and rotational motions in the field, i.e. in the vicinity of the seismic focus of an earthquake, will require the use of portable instruments, Igel says – and the researchers responsible for ROMY have already taken a major step towards this goal. They have teamed up with a specialist company in France to develop a portable fibre-optic-based sensor, and the first prototypes were on show at a large geosciences conference held in Vienna in April. These instruments use an extremely thin optic fiber of 5 km in length, which is coiled onto a spool: “A real milestone,” Igel enthuses. The initial measurements performed in Central Italy, and on the volcanic island of Stromboli off the north coast of Sicily “look good,” he says.
The pioneers in Munich hope that others will follow the example set by ROMY. If so, we should someday have a global network of ring-laser seismometers which can finally provide us with a truly comprehensive picture of the dynamics of the Earth’s motions. In such a network, Fürstenfeldbruck’s ring would serve as an essential node – a hotspot, so to speak.
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