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LIGO’s next mirror coatings are designed to reduce thermal noise, but they absorb the 532-nanometre green light used by the detector’s current arm-length-stabilization system. Researchers have demonstrated a workaround: a 1596-nanometre infrared locking laser can stabilize the interferometer first, then transfer control to LIGO’s 1064-nanometre science laser through phase-linked frequency conversion.
Why the mirror coating creates a locking problem
LIGO measures passing gravitational waves by tracking extraordinarily small changes in the lengths of two perpendicular, four-kilometre arms. Each arm ends in a mirror weighing about 40 kilograms. The mirrors must remain in precisely controlled positions while the detector looks for distance changes roughly one thousand times smaller than the width of a proton, according to a 2026 explanation from The University of Western Australia (UWA).
Before the detector can make science measurements, its control system must acquire and hold the optical resonances in the arms. The existing arm-length-stabilization (ALS) method uses 532 nm green laser light to establish the initial lock and coordinate that lock with the 1064 nm laser used for gravitational-wave observations.
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LIGO’s planned upgrade introduces low-noise crystalline mirror coatings. Their lower thermal noise could improve sensitivity, but the coating absorbs the green wavelength on which the current ALS method depends. Absorption can heat the coating and test mass, making the existing locking process unsuitable for those mirrors.
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The engineering requirement is therefore specific: find a locking wavelength that the crystalline coating does not absorb, while preserving a precise phase relationship with the 1064 nm science laser.
How the 1596 nm method works
An infrared auxiliary laser avoids the coating’s absorption
The proposed system uses a 1596 nm auxiliary laser for arm-length stabilization. The infrared wavelength is selected because the new crystalline coating does not absorb it in the way it absorbs 532 nm green light. That lets the detector establish control without introducing the same coating-heating problem.
Nonlinear crystals connect the locking and science lasers
The auxiliary laser cannot simply replace the 1064 nm science laser: the detector ultimately has to hand control to the science wavelength. In the A# implementation described by OzGrav, nonlinear frequency-conversion crystals provide that link. The system includes second-harmonic generation and sum-frequency generation stages, the 1596 nm auxiliary source and science-cavity mirrors.
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These conversions create optical signals whose phase can be compared with and locked to the 1064 nm laser. Once the infrared system has acquired a stable arm lock, control can be transferred smoothly to the science laser rather than forcing the detector to acquire lock directly at a wavelength the coating absorbs.
What the researchers demonstrated
Stable hand-over in about one minute
The team reported a stable transition from the auxiliary locking laser to the main laser in about 60 seconds. UWA described that as faster than the current hand-over procedure. The transition was repeated successfully over several days, demonstrating that the result was not limited to a single successful sequence.
Sub-atomic-scale agreement between the lasers
From the measured relationship between the two lasers, the researchers estimated an inter-laser measurement error of about one-tenth the width of an atom when the measurement is projected along a four-kilometre LIGO arm. This is an estimate reported by UWA for the demonstrated system, not a claim that LIGO has already detected a gravitational wave with that exact uncertainty.
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Lead author Dr Jian Liu said the 40-kilogram mirrors must be locked in exactly the right positions despite being four kilometres apart. The result addresses the locking transition needed to use future crystalline test masses without sacrificing the 1064 nm measurement laser.
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Existing green ALS versus the infrared approach
| Comparison point | Current ALS approach | 1596 nm approach for crystalline coatings |
|---|---|---|
| Locking wavelength | 532 nm green light | 1596 nm infrared light |
| Interaction with planned crystalline coating | The coating absorbs the green wavelength, creating the upgrade obstacle (UWA, 2026) | The coating does not absorb the selected infrared wavelength, according to the reported design (UWA, 2026) |
| Relationship to the science laser | Uses the existing stabilization scheme with eventual control by the 1064 nm laser | Phase-related to the 1064 nm laser through nonlinear frequency conversion, enabling control transfer |
| Hand-over result | The current procedure is slower than the demonstrated infrared transition; an exact baseline time was not stated in the UWA report | About 60 seconds in the reported demonstration (UWA, 2026) |
| Repeatability | The UWA announcement does not give a comparable multi-day figure | Repeated successfully over several days (UWA, 2026) |
| Compatibility with future low-thermal-noise test masses | Limited by absorption of 532 nm light in the planned crystalline coating | Designed to work with the proposed crystalline-coating technology |
Where the upgrade stands
Progress recorded by OzGrav
OzGrav’s annual report records a 532 nm beat signal in August 2025, followed by initial phase locking between the 1596 nm and 1064 nm lasers in November 2025. The A# system uses the auxiliary laser, frequency-conversion crystals and science-cavity mirrors as an integrated control chain.
Suspended-cavity demonstration planned for 2026
The report lists a 2026 goal of demonstrating the locking transition in a suspended cavity and publishing the experimental paper. A suspended-cavity test matters because it moves beyond a purely optical or laboratory setup toward the mechanical conditions relevant to an operating interferometer. The annual report describes planned work; it does not establish commercial availability of a finished upgrade.
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Collaborating institutions
The UWA announcement identifies OzGrav nodes at The University of Western Australia, the Australian National University and the University of Adelaide. It says the study was published in Physical Review D.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why crystalline coatings could improve sensitivity
Mirror coatings are a major thermal-noise source in precision interferometers. A coating that contributes less thermal motion can make smaller mirror displacements easier to distinguish from noise. The trade-off is that optical performance at one wavelength does not guarantee low absorption at another, so every candidate coating must be characterized at the wavelengths used for locking and measurement.
As an example of the optical performance being pursued, an MIT report from 2020 described a tested LIGO mirror sample with 99.9995% infrared reflectivity. That reflectivity figure belongs to the reported sample and date; it is not a specification for every future LIGO coating or proof that a coating is suitable for the full locking system.
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Nicholas Demos, an MIT physics graduate student, said coatings LIGO wants to use will first be characterized by the team’s experiment. That screening is important because the infrared workaround depends on the coating remaining sufficiently reflective and sufficiently low-absorption at the wavelengths used by the auxiliary system.
What this result does—and does not—establish
- It demonstrates: a phase-linked 1596 nm locking laser can hand control to the 1064 nm science laser in about 60 seconds, with successful repetition over several days.
- It supports: the use of crystalline coatings that would otherwise absorb the 532 nm light used by the current ALS method.
- It does not yet establish: that the complete method has been deployed in a full observing LIGO detector, that every candidate coating will meet the required optical specifications, or that the planned suspended-cavity demonstration has already been completed.
- It does not change: the science wavelength. The gravitational-wave measurement system still uses the 1064 nm main laser after the locking transition.
Why the hand-over matters for future observing runs
A detector upgrade is useful only if it preserves reliable lock acquisition and recovery. A coating that lowers thermal noise but prevents the interferometer from being locked would not improve observing time. The 1596 nm method separates those jobs: infrared light handles the coating-compatible lock acquisition, while the established 1064 nm laser remains responsible for the science measurement.
The reported speed and repeatability also matter operationally. A transition that takes about a minute can reduce the time needed to return an arm to science operation after lock loss, although the ultimate benefit will depend on performance in the suspended-cavity and detector environments still being evaluated.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →In practical terms, the work removes a wavelength conflict rather than replacing LIGO’s optical architecture. The proposed path is: acquire lock with 1596 nm light, maintain a phase relationship through nonlinear crystals, and hand control to 1064 nm light for observation. If the planned suspended-cavity tests confirm the laboratory result, the approach could allow lower-thermal-noise crystalline mirrors to be integrated without giving up the existing science laser.
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