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Long-distance fiber link poised to create powerful networks of optical clocks:

An academic-industrial team in Japan has connected three laboratories in a 100-kilometer region with an optical telecommunications fiber network stable enough to remotely interrogate optical atomic clocks. This type of fiber link is poised to expand the use of these extremely precise timekeepers by creating an infrastructure that could be used in a wide range of applications such as communication and navigation systems.

"The laser system used for optical clocks is extremely complex and thus not practical to build at multiple locations," said Tomoya Akatsuka, a member of the research team from telecommunications company Nippon Telegraph and Telephone Corporation (NTT). "With our network scheme, a shared laser would enable an optical clock to operate remote clocks with much simpler laser systems."

In The Optical Society (OSA) journal Optics Express, researchers from NTT, the University of Tokyo, RIKEN, and NTT East Corporation (NTT East), all in Japan, report the new low-noise fiber link.

[...] "Although optical clock networks that simply connect distant clocks have been demonstrated in Europe, our scheme is more challenging because operating remote clocks with the delivered light requires a more stable fiber link," said Akatsuka. "In addition, the country's urban environments tend to contribute more noise to fiber networks in Japan. To cope with that noise, we used a cascaded link that divides a long fiber into shorter spans connected by ultralow-noise laser repeater stations that incorporate planar lightwave circuits (PLCs)."

Optical interferometers fabricated on a small PLC chip were key for enabling a fiber link with extremely low noise. These interferometers were used in laser repeater stations that copy the optical phase of the received light to a repeater laser that is sent to a next station with fiber noise compensation. Applying noise compensation for each short span makes the laser signal less susceptible to noise and thus more stable.

Journal Reference:
Tomoya Akatsuka et al, Optical frequency distribution using laser repeater stations with planar lightwave circuits, Optics Express (2020). DOI: 10.1364/OE.383526

Journal information: Optics Express

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Original Submission