Long-distance telecom-fiber transfer of a radio-frequency reference for radio astronomy

Abstract

Very-long-baseline interferometry (VLBI) for high-resolution astronomical imaging requires phase-stable frequency references at widely separated radio-telescope antennas. For the first time to our knowledge, we have disseminated a suitable radio-frequency (RF) reference for VLBI over a "real-world" telecom optical-fiber link between radio telescopes that are >100 km apart, by means of an innovative phase-conjugation technique. Bidirectional optical amplification is used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length are compensated. This RF-over-fiber approach obviates the need for separate hydrogen masers at each antenna, offering significant advantages for radio-astronomy facilities such as the Square Kilometer Array.

The mid-frequency Square Kilometre Array phase synchronisation system

“…A similar scheme was also implemented using coaxial cable transmission lines for the compact, but high-frequency, Plateau de Bure Interferometer (Guilloteau et al, 1992). In addition, activelystabilised frequency transfer has also been proposed for synchronising what are currently individually-referenced Very Long Baseline Interferometry (VLBI) antennas (He et al, 2018;Clivati et al, 2017;Krehlik et al, 2017;Dierikx et al, 2016) Given the combination of long fibre links, particularly from the increased environmental exposure of overhead cables, and relatively high frequencies of the astronomical observations, the SKA1-MID will therefore employ actively-stabilised frequency transfer technologies to suppress the fibre-optic link noise to maintain phase-coherence across the array . Given these unique challenges and many specific technical requirement of the SKA, a novel, bespoke phase synchronisation system had to be designed specifically for the SKA.…”

“…A similar scheme was also implemented using coaxial cable transmission lines for the compact, but high-frequency, Plateau de Bure Interferometer (Guilloteau et al 1992). In addition, actively stabilised frequency transfer has also been proposed for synchronising what are currently individually referenced Very Long Baseline Interferometry (VLBI) antennas (He et al 2018;Clivati et al 2017;Krehlik et al 2017;Dierikx et al 2016).…”

Ultra-precise timing and synchronization for large-scale scientific instruments

“…Furthermore, ambitious multitelescope array projects [17][18][19][20] require 10 −6 arcsec angular resolution (corresponding to 50-km spatial resolution at 1-light-year distance) for cosmic imaging in order to obtain the surface details of distant astronomical objects, especially to explore exoplanets in the habitable zone. To achieve this goal, the telescope array has to work at terahertz or optical wavelengths [19] spanning over hundreds-of-kilometers distance, and femtosecon/attosecond/-precision synchronization must be ensured among the individual telescopes to perform an accurate phase correlation of the ultrafast detected signals.…”

Common-clock very long baseline interferometry using a coherent optical fiber link

“…Connected-element arrays such as the Atacama Large Millimeter Array [20] already exploit fiber-based frequency distribution on maximum distances of a few tens of kilometers. More recently, also driven by the development of continental-scale fiber networks for metrology applications [7,8], fiber-based time and frequency distribution started to be explored in the context of very long baseline interferometry (VLBI) [21][22][23], in which telescopes are separated by several hundreds of kilometers.…”

“…A straightforward approach to fiber-based frequency dissemination is the encoding of a radio frequency (RF) signal as a modulation of an optical carrier [22,23]. This approach can be used to replace local hydrogen masers at the telescope sites, but it does not improve the frequency stability over that of a local clock up to time scales of several minutes.…”