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Optical Fiber Communication Conference

Optical clock distribution over stable fiber links in noisy environments

Tomoya Akatsuka, Hiromitsu Imai, Masao Takamoto, Ichiro Ushijima, Takashi Goh, Toshikazu Hashimoto, Hidetoshi Katori, Katsuya Oguri, Tetsuomi Sogawa

Published Mar 1, 2023• 0 citations
Publisher

Abstract

We demonstrate a 190-km-long optical fiber link that achieves high-stability frequency transfer in noisy environments. A four-stage cascaded link with ultralow-noise laser repeater stations connects distant optical lattice clocks without deteriorating their stabilities.
DOI
10.23919/ofc49934.2023.10116777

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We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at Riken and the University of Tokyo. We use a transfer laser at 1397 nm which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1×1017for an averaging time τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with stability of 10 (τ/s) by synchronously operating the two distant clocks.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.