A Coherent Fiber Link for Very Long Baseline Interferometry

We realize a coherent fiber link for application in Very Long Baseline Interferometry (VLBI) for radioastronomy and geodesy. A 550 km optical fiber connects the Italian National Metrological Institute (INRIM) to the main radiotelescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. As a transfer oscillator we use an ultrastable laser, frequency-referenced to the primary standard; at the radiotelescope, a RF signal is generated from the laser by using an optical frequency comb. This scheme is now used for the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7e-16 accuracy. The fiber link is never limiting the experiment and is robust enough to sustain radioastronomical campaigns. This experiment sets the basis for a replacement of the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common-clock VLBI, based on a network of telescopes connected by fiber links.