Abstract
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.
Highlights
In recent years, the transfer of optical frequencies via phasestabilized telecom fibers has proved to be the most performing technique to compare distant frequency standards, allowing uncertainties below the mid 10−19 level [1,2,3,4] and enabling remote comparisons of optical clocks over thousands of kilometers at their intrinsic level of accuracy [5, 6].The frequency dissemination over fiber relies on the socalled Doppler noise cancellation scheme, where an ultrastable laser at telecom wavelength is delivered to a remote laboratory and partly reflected back
This study demonstrates that twoway optical frequency transfer is a reliable and performing technique, capable of sustaining remote clocks comparisons at the 10−19 resolution, and is relevant for the development of a fiber network of continental scale for frequency metrology in Europe
The power spectral density (PSD) of the phase-difference between the signals transmitted through the two fibers was 20 dB lower than the PSD of the signal transmitted through each of them in the Fourier frequency range between 0.1 Hz and 100 Hz
Summary
The transfer of optical frequencies via phasestabilized telecom fibers has proved to be the most performing technique to compare distant frequency standards, allowing uncertainties below the mid 10−19 level [1,2,3,4] and enabling remote comparisons of optical clocks over thousands of kilometers at their intrinsic level of accuracy [5, 6].The frequency dissemination over fiber relies on the socalled Doppler noise cancellation scheme, where an ultrastable laser at telecom wavelength is delivered to a remote laboratory and partly reflected back. The round-trip signal is compared to the original one in order to detect and compensate the additional phase noise due to environmental-induced fiber length variations. The simplest in terms of implementation is using a cascade of bidirectional erbium doped fiber amplifiers (EDFAs). In most implementations the link attenuation cannot be completely compensated, resulting in sub-μW power on the round-trip laser signal and, as a consequence, weak beatnote amplitude. Cascaded links where the optical carrier is regenerated at intermediate shelters [9] and optical injections [10] may be valid alternatives. In these cases, the setup complexity is increased
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