New Locking Scheme Using Infrared Light Clears Path for Sharper LIGO Gravitational Wave Mirrors
Australian physicists showed an infrared handover method that works with crystalline mirror coatings that absorb the green light current detectors rely on.
Physicists in Australia have developed and tested a new way to lock the mirrors of gravitational wave detectors, removing an obstacle to the next planned upgrade of LIGO. The method uses infrared light, a color that new low-noise crystalline mirror coatings do not absorb, and it is described in Physical Review D.
LIGO detects ripples in space-time from colliding black holes and neutron stars by measuring changes in distance about 1,000 times smaller than the width of a proton. "To do this, the detector's 40-kilogram (88-pound) mirrors, which sit four kilometers (2.5 miles) apart, must be locked in exactly the right positions using laser light," said lead author Dr. Jian Liu of the University of Western Australia.
Before a detector can start listening, its mirrors must be brought gently to rest and held in place by what is called arm-length stabilization. The current system uses green laser light. The trouble is that LIGO's next upgrade will use crystalline mirror coatings to improve sensitivity, and those coatings absorb green light. "The next upgrade of LIGO will use new crystalline mirror coating technology to improve sensitivity, but the coating absorbs the green laser light the current locking system relies on," Liu said.
The team, drawn from the ARC Centre of Excellence for Gravitational Wave Discovery, or OzGrav, at the University of Western Australia, the Australian National University and the University of Adelaide, built a locking scheme around infrared light at 1596 nanometers. Special crystals convert the frequency of that locking light and the detector's main 1064-nanometer laser into green light, which lets control pass smoothly from the locking laser to the main laser.
In tests, the handover took about 60 seconds, which Liu said is much faster than the current system, and the team repeated it successfully over several days. They estimate the measurement error between the two lasers at about one-tenth the width of an atom, even when measured along a four-kilometer LIGO arm.
Locking matters because a detector cannot observe anything until every mirror is stable and aligned, so any delay or failure in this stage costs observing time. A faster handover that also works with the coated mirrors would help future runs start sooner. The researchers describe the stability of the repeated handovers over several days as a sign the scheme is robust enough for continued development. The findings show a practical locking method exists for detectors that use the new AlGaAs/GaAs coated mirrors, according to the researchers. The new coatings are intended to improve the detector's sensitivity, which is why a locking method that works with them matters.
The study, "Novel arm-length stabilization scheme for gravitational-wave detectors with AlGaAs/GaAs coated mirrors," is published in Physical Review D (2026), DOI 10.1103/6ygv-gxq2, and is on arXiv as 2606.24149.
