LIGO optical coatings pose new challenges in materials research
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LIGO optical coatings pose new challenges in materials research Rachel Berkowitz
W
hen the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first ever detection of a gravitational wave in September 2015, one common question was “when will they find the next one?” The answer came quickly, with a second detection on December 25, causing excitement throughout the multinational
collaboration that designed and built the intricate optical instruments. Gravitational waves, or “ripples in spacetime,” radiate throughout the universe for billions of light-years, and were predicted a century ago by Einstein’s general theory of relativity. They are a byproduct of vast amounts of energy generated by massive, accelerating bodies such as supernovae or merging black holes. Not only has the discovery opened a new era of astronomy, it has also unveiled new horizons for optical and materials sciences. In the search for gravitational waves, LIGO’s optics system plays a critical role. The interferometer amplifies a laser beam and splits it into two beams that are sent down 4-km-long orthogonal vacuum tubes, or arms. The beams build power by resonating between some of the world’s highest-quality mirrors, or “test masses,” suspended at either end of each arm. Specially designed coatings on the surfaces of the fused-silica test masses make them highly reflective, thus controlling the laser Two LIGO test masses after coating in the ion-beam sputtering beam’s path within deposition chamber at the Laboratoire des Matériaux Avancés (LMA) in the interferometers. Lyon, France. Credit: LMA.
When the split beams recombine, they produce a signal that is observable at the photodetector. Any minute change in one arm’s path length produces a shift in the detected light power. Thus a difference in arm length can be measured with unprecedented precision—the system is capable of measuring a relative length shift less than one thousandth the diameter of a proton. The passing of a gravitational wave, as detected simultaneously by two facilities in Livingston, La., and Hanford, Wash., creates a spatial shift on the order of just 10 –18 meters. However, vibrations and environmental noise reduce sensitivity of the entire system. Actual strain on the instruments at all frequencies must be distinguished from gravitational-wave signals, and the dielectric coatings which give the test masses their reflectivity and low optical loss contribute to this noise. “A better understanding of coatings that will not jeopardize optical properties has great potential to improve LIGO sensitivity,” says Fred Raab, Director of the LIGO Hanford Observatory. At frequencies below 15 Hz, LIGO’s sensitivity is limited by seismic motion. Above 200 Hz, “shot” noise, or the quantum fluctuations in the detected laser power, is the limiting factor. But the mid-frequency range from ~30 to 200 Hz, where many gravitational-wave signals propagate and other noise sources are minimal, is limited by thermal noise from the coatings. Thermal noise refers to the materia
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