New technique enables LIGO to peer farther into the distant universe

Scientists at the University of California, Riverside have developed a new way to help gravitational-wave observatories see farther into the universe by solving one of their biggest challenges: tiny heat-induced distortions in the massive mirrors at the heart of the detectors.

The technique, described in a paper published in Classical and Quantum Gravity, uses thermal imaging to reveal microscopic distortions caused by powerful lasers. By measuring those distortions more precisely, observatories such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) could improve their sensitivity and detect weaker, more distant gravitational-wave events.

"The goal for the next generation of gravitational-wave detectors is to achieve about 10 times the sensitivity of today's instruments," said Jonathan Richardson, an associate professor of physics and astronomy at UC Riverside who led the study. "One of the key obstacles to achieving that is reducing the fundamental quantum mechanical noise that limits the precision of the measurements."

LIGO detects gravitational waves - ripples in spacetime created by accelerating massive objects - by measuring changes in distance smaller than the width of a proton. To make those measurements, lasers circulate through the 4-kilometer-long observatory with powers approaching one megawatt.

Although LIGO's mirrors are among the purest optical components ever made, they still absorb a tiny fraction of that laser light. The resulting heat changes the mirrors' shape by just a few nanometers, slightly distorting the laser beam and reducing the detector's sensitivity.

Researchers already know how to counteract those distortions by applying carefully controlled heating patterns to the mirrors. The challenge has been determining exactly how the mirrors are distorted in order to apply the optimal heating pattern.

The new method combines infrared thermal images of the mirror surface with existing wavefront measurements and computer models of heat flow to reconstruct the mirrors' optical distortion across their entire diameter.

"You can think of it like taking an infrared picture of a car engine," Richardson said. "An engineer can look at the temperature pattern on the outside and infer what's happening inside the engine. We're doing the same thing with LIGO's mirrors."

The idea grew out of an unexpected experimental result while Richardson's group was testing new adaptive optics on a full-scale, 40-kilogram LIGO mirror. The experiment was designed to precisely reshape the surfaces of LIGO's main mirrors.

"We found that if you know the surface temperature and have a good model of how heat flows through the optic, you can accurately reconstruct the optical distortions inside it," Richardson said. "That surprising result is what led us to develop this new sensing technique."

Unlike many LIGO upgrades, the approach relies on commercially available thermal imaging cameras mounted outside the interferometer rather than entirely new instrumentation.

"It only requires a thermal imaging camera that already meets today's commercial standards," Richardson said. "It doesn't require any new technology development, which is almost unheard of for solving a LIGO instrumentation problem."

The researchers estimate the technique could improve the strain sensitivity, the smallest fractional change in length that an instrument can detect, of the upcoming LIGO A+ upgrade by as much as 31%, allowing the observatory to detect binary neutron star mergers about 10 megaparsecs (roughly 33 million light-years) farther away on average.

"When you make the detector more sensitive, you don't just see a little farther - you observe a much larger volume of the universe," Richardson said. "Because the detectable volume scales with the cube of the distance, even a modest increase in sensitivity can produce a much larger increase in the number of gravitational-wave events we detect."

The technology is also expected to become part of the baseline design for Cosmic Explorer, the proposed next-generation U.S. gravitational-wave observatory. Richardson serves as the system design lead for its mode sensing and control system.

"LIGO will serve as the pathfinder," Richardson said. "We'll develop and validate these techniques in the next upgrades and then use that experience to enable Cosmic Explorer."

According to Richardson, the work fills an important gap in the technology needed for future observatories.

"Our research provides not only a way to correct these optical distortions, but now also a practical way to measure exactly how they should be corrected in the first place," he said. "That capability will be essential for the next generation of gravitational-wave observatories and will allow us to see farther into the distant universe than ever before."

The paper's co-authors are Liu Tao, a former postdoctoral researcher in Richardson's laboratory, and Pooyan Goodarzi, a doctoral student in the UCR Department of Physics and Astronomy.

The research was supported by the National Science Foundation.

The paper is titled "Error signals for overcoming the laser power limits of gravitational-wave detectors."

Header image shows researchers in Richardson's group testing a novel adaptive optics device designed to precisely reshape the surfaces of LIGO's main mirrors. (LIGO Laboratory/Arnaud Pele)