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

Cecilia Clivati,Alberto Mura,Federico Perini,Roberto Ricci,Roberto Aiello,Mario Siciliani De Cumis,Monia Negusini,Giuseppe Maccaferri,Luigi Santamaria Amato,Valentina Di Sarno,Paolo De Natale,Filippo Levi,Claudio Bortolotti,Pasquale Maddaloni,Davide Calonico,Matteo Stagni,Alberto Tuozzi,Giuseppe Bianco,Mauro Roma

doi:10.1364/optica.393356

Abstract

Among the most powerful techniques for the exploration of the Universe is very long baseline interferometry (VLBI), which is based on the simultaneous observation of radio sources in the sky with arrays of distant ground-based antennas. One of the effects currently limiting its ultimate sensitivity is the phase-instability of the reference clocks adopted at each antenna. This term can be made negligible delivering the same clock signal to multiple telescope sites using optical fibers. We realized such an infrastructure by disseminating a coherent optical frequency signal to two distant radio telescopes using a 1739-km-long fiber. We performed a 24 h geodetic VLBI campaign in which the same clock reference was used at both telescopes and analyzed it using standard VLBI procedures. The results were consistent with the expectations, confirming that the proposed approach is feasible and configures as a novel tool for studying the role of clocks, troposphere, and systematic effects in the ultimate VLBI resolution.

Highlights

The transmission of optical signals with an ultrastable, well-known frequency using optical fibers dates back to the ’90s, when specific techniques were first developed to cancel the effect of mechanical fiber noise on the optical phase [1,2]
This redesigned the experimental practice in fundamental frequency metrology, allowing the comparison of distant atomic clocks at their ultimate accuracy over increasingly longer distances [3,4,5,6,7,8,9], and other unprecedented experiments in high-resolution atomic [10,11,12] and molecular physics [13,14,15], relativistic geodesy [9], quantum communication [16,17], and even seismology, for what concerns the detection of underwater earthquakes in seas and oceans [18]
These are combined into a least squares fit to a global model that takes into account most known effects, such as the coordinates of sources and telescopes, and atmospheric as well as instrumental delays and earth orientation parameters (EOPs)

Summary

INTRODUCTION

The transmission of optical signals with an ultrastable, well-known frequency using optical fibers dates back to the ’90s, when specific techniques were first developed to cancel the effect of mechanical fiber noise on the optical phase [1,2]. The main uncertainty terms in VLBI are due to an incomplete modeling of the random fluctuations of the optical path length in the troposphere, which can account for up to ∼70 ps uncertainty, the source structure (∼35 ps), and instrumental delays such as those due to the antenna structure (∼35 ps) and the clock with its synthesis chain (∼20 ps) [28] In addition to these aspects, statistical noise has to be addressed. This could be feasible when commercial, unmanned optical clocks become available with the same reliability as the present H-masers Another effective strategy is the fiberbased dissemination of a common frequency reference signal to the various telescopes, which allows a full rejection of the oscillator’s noise and a direct link between VLBI sites and Metrology Institutes and specialized laboratories where state-of-the-art atomic clocks are maintained. These experiments demonstrated the feasibility of this approach and open up the possibility of more advanced studies on VLBI limiting effects

DISSEMINATING THE SAME FREQUENCY REFERENCE TO MULTIPLE TELESCOPES

FIBER-BASED COMPARISONS OF REMOTE HYDROGEN MASERS

COMMON-CLOCK VLBI EXPERIMENT

Findings

DISCUSSION