Remote Optical Clocks Comparisons with Long-Haul Fiber Links

Paul-Éric Pottie

Christian Lisdat, Gesine Grosche, Nicolas Quintin +32 more

Clara Zyskind

Dong-Jie Wang, Xiang Zhang, Jie Liu +8 more

We demonstrate a novel polarization control system based on a gradient descent algorithm, applied to a 450-km optical frequency transfer link. The power of the out-loop beat note is retrieved by controlling the polarization state of the transferred signal, with a recovery time of 24 ms, thereby ensuring the long-term evaluation of the fiber link. As a result, data utilization is enhanced from 70% to 99% over a continuous measurement period of ∼12 h. A fractional transfer instability of 7.2 × 10−20 is achieved at an integration time of 10000 s. This work lays the foundation for the comparison of a remote optical clock system via a long-haul optical fiber link.

Anna Tampellini, Cecilia Clivati, Filippo Levi +2 more

Two-way frequency transfer on optical fibers is a powerful technique for the comparison of distant clocks over long and ultra-long hauls. In contrast to traditional Doppler noise cancellation, it is capable of sustaining higher link attenuation, mitigating the need of optical amplification and regeneration and thus reducing the setup complexity. We investigate the ultimate limitations of the two-way approach on a 300 km multiplexed fiber haul, considering fully independent setups and acquisition systems at the two link ends. We derive a theoretical model to predict the performance deterioration due to a bad synchronisation of the measurements, which is confirmed by experimental results. This study demonstrates that two-way optical frequency transfer is a reliable and performing technique, capable of sustaining remote clocks comparisons at the resolution, and is relevant for the development of a fiber network of continental scale for frequency metrology in Europe.