An Improved Method for Space-Based Gravitational-Wave Measurements
The Laser Interferometer Space Antenna (LISA) will use three laser-linked spacecraft to detect gravitational waves in the millihertz frequency range, which cannot be detected on Earth. LISA’s design won’t change before its planned 2035 launch, but researchers have now shown how to significantly simplify the hardware for later missions while also achieving superior performance [1]. The technique builds on an earlier scheme to streamline the processing of laser signals sent between spacecraft. It also adds new capabilities to more efficiently monitor and correct for drifts of the frequencies of the clocks onboard the spacecraft.
The LISA project will consist of a triangular constellation of three spacecraft—separated by 2.5 million km—exchanging laser beams and performing interferometry to detect ripples in spacetime. These gravitational waves may come from the mergers of supermassive black holes, which can only be detected by space-based observatories. Similar projects are being planned to further expand space-based gravitational-wave detection capabilities, and these schemes all rely on a technique called time-delay interferometry (TDI), explains Kohei Yamamoto of the University of Maryland, Baltimore County. TDI is a method of data processing that accounts for the fact that the arm lengths of the laser interferometer—two sides of the triangle—fluctuate over time during the constellation’s solar orbit. (Earth-based observatories have fixed arm lengths.)
TDI requires precise comparison of clock signals from each spacecraft. In 2015, researchers proposed a way to handle the clock signals that would improve on LISA’s design. The method uses an optical frequency comb (OFC)—a laser source that produces pulses at a frequency of tens of megahertz. The OFC serves as the clock, and it can be precisely synchronized with the fast electromagnetic oscillations of the primary laser beam that is sent out from one spacecraft to another. For LISA, the clock is a separate device whose signal is encoded in the beam but not synchronized with it. With the synchronization, the laser and clock noise would no longer be independent, making it easier to reduce these noise sources.
Now Yamamoto and colleagues have demonstrated that further information can be extracted from the same setup. To detect gravitational waves, the system in each spacecraft combines a local laser beam with one transmitted from another spacecraft, using a standard technique called heterodyning. The result is a megahertz-frequency signal that is very sensitive to differences in the two beams. The researchers showed that—thanks to the clock synchronization—this signal also contains information on the total time the light spent traveling between the two spacecraft and on any slow drifting of the two clock frequencies relative to one another. Uncorrected, such drifts would make it more difficult to eliminate noise and would introduce errors that could swamp the gravitational-wave measurements.
To demonstrate the method, the team performed an experiment using two separate systems that mimicked the laser systems of the spacecraft. Each system featured its own laser, OFC, and phasemeter—a high-precision device that measures the timing (or phase) of the laser signals. The researchers found that the new method could synchronize the clocks and the data-recording hardware of the two systems with an accuracy better than 0.47 nanoseconds, well below the 3.3-nanosecond requirement set for the LISA mission. It could also reduce noise below the level required to detect gravitational waves.
The proposal is a useful extension of the OFC-based technique, says Nan Yu, an expert in precision measurement at the Jet Propulsion Lab in California who codeveloped the 2015 design. “The scheme described in the paper, an alternative to what we originally proposed, offers a likely path to adoption in a future LISA-like mission architecture.”
The new approach comes too late for LISA, Yamamoto says, “but it is important that researchers do not stop brainstorming and still find improvements for future missions.” He also believes that the technique may be of broad use in other space missions deploying multiple communicating spacecraft for purposes of accurate positioning, navigation, or synchronization.
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- K. Yamamoto et al., “Alternative approach to time-delay interferometry with an optical frequency comb,” Phys. Rev. Appl. 25, 054042 (2026).