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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Dissemination of Optical Frequency for Future Telecom Networks and Applications

Lakshmi Rajagopal, Andrew Lord, Mike Gilson, Yeshpal Singh

Published May 15, 2023• 0 citations
Publisher

Abstract

Optical atomic lattice clocks are a precise source for time with frequency accuracy to a few parts in 10-18. The optical frequency from these clocks needs to be disseminated over a fiber to make them fit for different applications. The paper investigates a methodology for disseminating optical frequency over a fiber by sending the operative frequency as the beat frequency of a heterodyne signal. The work details results obtained using this technique for frequency transfer over several kilometers of fiber in the lab and real environment.

DOI
10.1109/eftf/ifcs57587.2023.10272108

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Optical atomic lattice clocks are a precise source for time with frequency accuracy to a few parts in 10-18. The optical frequency from these clocks needs to be disseminated over a fiber to make them fit for different applications. The paper investigates a methodology for disseminating optical frequency over a fiber by sending the operative frequency as the beat frequency of a heterodyne signal. The work details results obtained using this technique for frequency transfer over several kilometers of fiber in the lab and real environment.
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The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.
We transferred the frequency of an ultrastable laser over a 108-km-long urban fiber link comprising 22 km of an optical communications network fiber simultaneously carrying Internet data traffic. The metrological signal and the digital data signal were transferred over two different frequency channels in a dense wavelength-division multiplexing scheme. The metrological signal was inserted in and extracted from the communication network using bidirectional off-the-shelf optical add-drop multiplexers. The link-induced phase noise was measured and canceled with a round-trip technique using an all-fiber-based interferometer. The compensated link showed an Allan deviation of a few 10(-16) at 1 s and below 10(-19) at 10,000 s. This work paves the way to a wide dissemination of ultrastable optical clock signals between distant laboratories via the Internet.