Abstract
In this paper, we demonstrate ultrastable radio-frequency (RF) transfer over long-haul optical fiber link. Our stabilized RF transfer technique is based on high-performance dual phase locked loops (dual-PLL) configuration which improves signal to noise ratio (SNR) of the round trip transmitted signals. At the local site, the frequency conversion phase-lock receiver (FCPLR) accomplishes high-quality phase tracking of the RF signal which is transmitted after long distance optical fiber link. The low phase noise signal generated by FCPLR is passively mixed with a local reference signal to realize phase conjugation. Through advisable frequency design, there is no residual RF leakage and nonlinear effect of frequency mixing. Another PLL incorporating a high-quality cleanup oscillator is located at the remote site that favourably improves short-term instability of our transmission system. In our experiment, stabilized 2.4 GHz RF signal transfer over a 1007 km optical fiber link is demonstrated without any electric relay system, and the transmission system achieves a fractional frequency instability of 8.20 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> @1 sand 7.87 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> @10 000 s.
Highlights
In modern astronomy, the interferometric arrays, like Square Kilometre Array (SKA), Atacama Large Millimeter Array (ALMA) and generation Very Large Array, have been expected to tackle many outstanding scientific questions [1]–[3]
The frequency conversion phaselock receiver (FCPLR) accomplishes high-quality phase tracking of the RF signal which is transmitted after long distance optical fiber link
High stable 2.4 GHz RF signal transfer over 1007 km fiber link is demonstrated without any electic relay system
Summary
The interferometric arrays, like Square Kilometre Array (SKA), Atacama Large Millimeter Array (ALMA) and generation Very Large Array (ngVLA), have been expected to tackle many outstanding scientific questions [1]–[3]. These arrays require coherent frequency reference signals at each antenna in the array to achieve phase coherence for performing interferometry and beamforming [4]. As any other transmission medium, optical fiber links display some fluctuations of the propagation delay, which is caused by temperature variations and mechanical vibrations These environmental factors result in phase fluctuations of the transmitted signals. The excess phase noise requires to be detected and subsequently compensated in order to stably transfer the frequency reference signal via long-haul fiber link
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