A coherent fiber link for very long baseline interferometry.

Area of Science:

• Physics
• Astronomy
• Metrology

Background:

• Very Long Baseline Interferometry (VLBI) requires highly stable and accurate frequency references for radio astronomy and geodesy.
• Current VLBI systems often rely on local hydrogen masers, which can limit performance.
• Disseminating precise time standards over long distances is crucial for synchronizing telescopes.

Purpose of the Study:

• To establish a coherent fiber link for disseminating primary clock stability and accuracy.
• To enable traceability of local masers to the SI second using an optical frequency comb.
• To explore the potential of replacing hydrogen masers with optically-synthesized RF signals in VLBI.

Main Methods:

• A 550-km optical fiber link was established between the Italian National Metrological Institute (INRIM) and a radio telescope.
• An ultrastable laser, referenced to a primary Cs fountain clock, served as a transfer oscillator.
• An optical frequency comb was used at the radio telescope to generate an RF signal from the laser.

Main Results:

• The fiber link achieved traceability of the local maser to the SI second with an accuracy of 1.7 × 10⁻¹⁶.
• The fiber link performance did not limit the experiment and proved robust for radio astronomical campaigns.
• The system successfully demonstrated the generation of an RF signal from a laser via an optical frequency comb.

Conclusions:

• The developed coherent fiber link is a viable solution for precise time dissemination in VLBI.
• Optically-synthesized RF signals can potentially replace local hydrogen masers, improving VLBI resolution.
• This technology paves the way for common-clock VLBI networks, enhancing observational capabilities.