New adaptive optics to support gravitational-wave discoveries

Gravitational-wave detection technology is poised to make a big leap forward thanks to an instrumentation advance led by physicist Jonathan Richardson of the University of California, Riverside. A paper detailing the invention, published in the journal Optica, reports the successful development and testing of FROSTI, a full-scale prototype for controlling laser wavefronts at extreme power levels inside the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

LIGO is an observatory that detects gravitational waves - ripples in spacetime caused by massive accelerating objects like merging black holes. It was the first to confirm their existence, supporting Einstein's Theory of Relativity. LIGO uses two 4-km-long laser interferometers in Washington and Louisiana to capture these signals, opening a new window into the universe and deepening our understanding of black holes, cosmology, and extreme states of matter.

LIGO's mirrors are among the most precise and carefully engineered components of the observatory. Each mirror is 34 cm in diameter and 20 cm thick and weighs about 40 kg. The mirrors must remain perfectly still to detect distortions in spacetime smaller than 1/1,000th the diameter of a proton. Even the smallest vibration or environmental disturbance can overwhelm the gravitational wave signal.

"At the heart of our innovation is a novel adaptive optics device designed to precisely reshape the surfaces of LIGO's main mirrors under laser powers exceeding 1 megawatt - more than a billion times stronger than a typical laser pointer and nearly five times the power LIGO uses today," said Richardson, an assistant professor of physics and astronomy. "This technology opens a new pathway for the future of gravitational-wave astronomy. It's a crucial step toward enabling the next generation of detectors like Cosmic Explorer, which will see deeper into the universe than ever before."

Did someone say FROSTI?

FROSTI, short for FROnt Surface Type Irradiator, is a precision wavefront control system that counteracts distortions caused by intense laser heating in LIGO's optics. Unlike existing systems, which can only make coarse adjustments, FROSTI uses a sophisticated thermal projection system to make fine-tuned, higher-order corrections. This is crucial for the precision needed in future detectors.

Despite its icy name, FROSTI works by carefully heating the mirror's surface, but in a way that restores it to its original optical shape. Using thermal radiation, it creates a custom heat pattern that smooths out distortions without introducing excess noise that could mimic gravitational waves.

Why it matters

Gravitational waves were first detected by LIGO in 2015, launching a new era in astronomy. But to fully unlock their potential, future detectors must be able to observe more distant events with greater clarity.

"That means pushing the limits on both laser power and quantum-level precision," Richardson said. "The problem is, increasing laser power tends to destroy the delicate quantum states we rely on to improve signal clarity. Our new technology solves this tension by making sure the optics remain undistorted, even at megawatt power levels."

The technology will help expand the gravitational-wave view of the universe by a factor of 10, potentially allowing astronomers to detect millions of black hole and neutron star mergers across the cosmos with unmatched fidelity.

Looking Ahead: LIGO A# and Cosmic Explorer

FROSTI is expected to play a critical role in LIGO A#, a planned upgrade that will serve as a pathfinder for the next-generation observatory known as Cosmic Explorer. While the current prototype was tested on a 40-kg LIGO mirror, the technology is scalable and will eventually be adapted to the 440-kg mirrors envisioned for Cosmic Explorer.

"The current prototype is just the beginning," Richardson said. "We're already designing new versions capable of correcting even more complex optical distortions. This is the R&D foundation for the next 20 years of gravitational-wave astronomy."

Richardson was joined in the research by scientists at UCR, MIT, and Caltech.

The research was funded by a grant to Richardson from the National Science Foundation.

The title of the paper is "Demonstration of a next-generation wavefront actuator for gravitational-wave detection."