To meet this challenge, a KAIST led collaboration has implemented a reference signal system that sends an optical frequency comb laser directly into radio telescope receivers, using laser light to lock the timing and phase of observations with ultra high precision.
An optical frequency comb differs from an ordinary laser because it emits tens of thousands of evenly spaced colors whose exact frequencies are known and can be tuned with the accuracy of an atomic clock, forming what researchers describe as an ultra precision ruler made of light.
Very Long Baseline Interferometry, or VLBI, depends on aligning the phases of radio signals collected by widely separated telescopes as if they were being measured against a single precise ruler, but conventional electronic reference signals become harder to calibrate as observation frequencies increase and phase stability demands grow.
The KAIST team addressed these limitations by delivering the frequency comb output directly to the radio telescope front end, aiming to improve the fundamental precision of phase alignment by using light based signals from the generation stage and unifying reference generation and phase calibration inside one optical system.
In this scheme, older techniques that struggle at higher frequencies resemble rulers that make phase matching difficult, whereas the new laser based approach acts like an ultra precision ruler that fixes the phase with extremely stable light and prepares the way for intercontinental arrays that operate as coherently as one elaborately controlled telescope.
The researchers validated the concept through test observations at the Korea VLBI Network Yonsei Radio Telescope, where they detected stable interference fringes between telescopes and demonstrated that accurate phase calibration is achievable in real observations rather than only in laboratory tests.
The system has since been installed at the KVN SNU Pyeongchang Radio Telescope, enabling expanded experiments that use multiple sites at once and providing a stronger platform for next generation VLBI campaigns targeting black holes and other compact radio sources.
According to the team, the improved reference scheme is expected to sharpen black hole images by reducing long standing phase delay errors between instruments, which have limited dynamic range and image fidelity in previous high frequency VLBI observations.
Beyond astronomy, the researchers anticipate that the technology can extend to applications that need extremely accurate space time measurements, including intercontinental comparisons of advanced atomic clocks, geodetic studies of Earth using space geodesy techniques, and precise tracking of deep space probes over vast distances.
Professor Jungwon Kim of KAIST explained that the work shows how the limits of electronic signal generation can be surpassed by directly applying optical frequency comb lasers in radio telescopes and that the approach should advance both precision black hole imaging and broader fields such as frequency metrology and time standards.
The project involved co first authors Dr Minji Hyun, now at the Korea Research Institute of Standards and Science, and Dr Changmin Ahn at KAIST, and drew on cooperation with the Korea Astronomy and Space Science Institute, the Korea Research Institute of Standards and Science, and the Max Planck Institute for Radio Astronomy in Germany.
The team reported the results in the journal Light Science and Applications on January 4, 2026 in a paper titled Optical frequency comb integration in radio telescopes: advancing signal generation and phase calibration.
Research Report: Optical frequency comb integration in radio telescopes: advancing signal generation and phase calibration
Related Links
The Korea Advanced Institute of Science and Technology (KAIST)
Understanding Time and Space
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