Science1 publisher2 min readPublished
ANU coats two LIGO beamsplitters evenly to within a few atoms across 45 centimeters
Australian National University researchers spent three years coating two LIGO beamsplitters to within a nanometer or two across almost half a meter of glass. Both optics belong to an upgrade meant to give the detector its greatest sensitivity yet.
The Scientist · Science desk
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What happened
- Each beamsplitter is a 45-centimeter disk weighing more than 20 kilograms, made from some of the purest glass in the world.
- One face carries a coating that splits the laser beam precisely in half, and the other an anti-reflection coating ANU says is over 1,000 times more effective than an eyeglass-lens coating.
- The team built eight custom automated systems to clean, measure and handle the optics without anyone touching them.
- The coating work was done in ANU's new Research School of Physics clean room, held to the cleanliness standards of advanced semiconductor manufacturing.
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Why it matters
- exposure If ANU is right that only one other group can make these coatings to LIGO's standard, any spare or future beamsplitter for the detectors depends on just two labs.
- constraint ANU says the back-face coating improves sensitivity by minimizing interference, so any shortfall in it would reduce the gain the upgrade is meant to deliver.
- capability If the upgraded detectors perform as intended, weaker signals from farther away become detectable, giving astronomers more black hole and neutron star events to study.
Gravitational waves were first detected directly in 2015 [15]. Catching one means measuring a change in distance of around a millionth of a billionth of the width of a human hair, using an extremely pure laser beam as the ruler [7]. A passing wave shows up in the resulting light pattern [7].
Robert Ward, director of ANU's Center for Gravitational Astrophysics, described the coating's thickness variation as "a few atoms' difference from one edge to the other" [3]. As a fraction of the part, a nanometer or two over a 45-centimeter disk [5] comes to roughly 2 to 4 parts per billion. One nanometer divided by 450 millimeters is about 2.2 x 10^-9, and two nanometers gives about 4.4 x 10^-9 [1]. It is a tolerance on a manufactured part. "You need exquisite measurement precision, very tightly controlled processes and extreme coating thickness uniformity to build these components," Ward said [4].
Meeting that tolerance meant controlling thickness to almost single-atomic-layer accuracy over the whole surface. The optics also had to stay free of microscopic defects [11]. "There aren't many machines in the world that can meet the requirements, so we had to build our own equipment and find new ways to solve the problems," said Deon Hickey of ANFF OptoFab ACT [12].
In my view the manufacturing result is the solid part of this story. The promise of record sensitivity belongs to the upgrade as a whole [2]. The thing this doesn't tell you is how much of that gain depends on these two optics. Steve Madden, director of the Australian National Fabrication Facility's OptoFab ACT Hub, called them some of the most exacting and precise coatings ever made [13]. The account does not include as-built measurements against LIGO's specification, comparison figures for other coatings, an installation date, or which detectors will receive the beamsplitters.
What to watch
- When the two beamsplitters are installed, and at which LIGO detector sites.
- Published as-built measurements of coating uniformity and back-face reflectivity against LIGO's specification.
- Sensitivity figures from the upgraded detectors' first observing run, and whether LIGO credits any of the gain to these optics.