Another Nobel Prize for photonics

The team headed by Rainer Weiss, Barry Barish and Kip Thorne who recently received the Nobel Prize, needed a great deal of patience. Over a period of nine years, their first laser interferometer costing many millions of dollars did not detect even a trace of a gravitational wave. Although their existence was unproven, the team insisted on a second even more sensitive and expensive measuring device: the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO).

When building the machine, the scientists and engineers pushed the boundaries of what is possible. The huge laser interferometer consists of steel tubes each 4 kilometers long and 120 cm in diameter. They are arranged at right angles, absolutely horizontally and are mathematically round. The latter is important because of the extreme vacuum conditions (10-9 torr) in the tubes to keep sound waves out. In addition, concrete tunnels help to protect the measuring tubes against outside influences.

High-tech isolation against earthly influences

The building measures alone were not enough to protect the ultra-sensitive sensors from the conditions outside. The optical systems in the interferometer are also protected against thermal and seismic influences. In the initial LIGO, mirrors weighing 11 kg were suspended on fine steel cables. This was not enough. In order to achieve success, it took a 140 kg construction with a quadruple pendulum in which mirrors weighing 40 kg and additional weights are suspended on quartz fibers 0.4 mm in diameter. In addition to the passive seismic isolation system, an active sensor-actuator system is used. This monitors the location of the optical systems and keeps them in position—accurate to a trillionth of a meter. This is the only way to ensure that the laser sensor system can measure the minimal expansions and compressions in the space resulting from gravitational waves.

In order to carry out measurements, a laser beam is directed at a beam splitter at the start of the tubes. This splits the beam and directs the individual beams into the tubes, where after 4 km they hit the high-tech mirrors which are coated with nanometer precision. They are then reflected with virtually no losses (the mirrors absorb every 3.3 millionth photon). Back at the beam splitter, the absolutely synchronous light waves fuse and are directed at a photodetector.

High-end lasers used

If gravitational waves compress and expand the space, this leads to minimal shifts in the light waves which are otherwise completely synchronous. These shifts can be only one ten thousandth of the size of a proton. Given the magnitudes involved, it becomes clear that the team of researchers must eliminate any form of “noise” in order to be able to prove the existence of gravitational waves. Another essential factor is the quality of the laser which needs to provide light at an absolutely stable frequency and power over long measuring periods. The team solved this challenge with a 4 watt (W) laser diode (808 nm wavelength). Its light is initially converted into a 2 W seed beam with a wavelength of 1,064 nm in a solid body – specifically a non-planar ring oscillator (NPRO).

This is amplified to 200 W in two stages. The light first runs through solid rods 5 cm in length and based on neodymium, yttrium and lithium. Excited by the laser, their molecules emit iden-tical photons that join the laser beam and amplify it. This process is repeated four times until the beam has a power of 35 watts. In the next step, the light runs through HPOs (high power oscillators) and fibers. During this process, it is boosted to the 200 W power required. But this is only the start. In order to detect gravitational waves, the researchers had to increase the stability of the seed laser by a factor of 100 million—while eliminating the natural fluctuations in frequency. A control system which measures and adjusts the beam quality with a frequency of 16 kilohertz, i.e. every 0.000061 seconds, is used for this.

Destined for a measuring station in space

In 2015, the researchers were able to record gravitational waves for the first time and thus confirm Albert Einstein’s theoretical assumptions. Since then, the aLIGO has been able to do this a number of times. In the middle of October 2017, the team announced a new scientific sensation: they registered gravitational waves that were from a neutron star collision rather than from black holes. The signals were so clear that the weight of the stars and the distance at which the collision occurred could be determined.

In order to carry out more in-depth investigations into gravitational waves, the European Space Agency (ESA) is planning a laser interferometer which will completely avoid interference-causing earthly influences. From 2034, the LISA mission (Laser Interferometer Space Antenna) will install this machine 70 million kilometers from the Earth. It will consist of three satellites arranged in a triangle between which the laser measuring beam will travel a path around 2.5 million kilometers in length.

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