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Photonics

Microwave Frequency Dissemination over a 212 km Cascaded Urban Fiber Link with Stability at the 10−18 Level

Honglei Quan, Wenxiang Xue, Wenyu Zhao, Xing Yan, Haifeng Jiang, Wenge Guo, Shougang Zhang

Published Jun 6, 2022• 8 citations
Publisher

Abstract

To synchronize standard frequency signals between long-distance laboratories, we carried out a frequency dissemination experiment over a 212 km cascaded urban fiber link. This cascaded link was composed of two 106 km fiber links, in which the fiber noise was compensated by two microwave frequency dissemination systems. The two adjacent frequency dissemination systems used different frequency transmitted signals, preventing the influence of signal crosstalk between the received signal of the previous stage and the transmitted signal of the second stage caused by microwave signal leakage. The frequency dissemination over the cascaded link showed a dissemination fractional frequency instability of 6.2 × 10−15 at 1 s and 6.4 × 10−18 at 40,000 s, which is better than the transfer stability over the same 212 km single-stage link.

DOI
10.3390/photonics9060399

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To synchronize standard frequency signals between long-distance laboratories, we carried out a frequency dissemination experiment over a 212 km cascaded urban fiber link. This cascaded link was composed of two 106 km fiber links, in which the fiber noise was compensated by two microwave frequency dissemination systems. The two adjacent frequency dissemination systems used different frequency transmitted signals, preventing the influence of signal crosstalk between the received signal of the previous stage and the transmitted signal of the second stage caused by microwave signal leakage. The frequency dissemination over the cascaded link showed a dissemination fractional frequency instability of 6.2 × 10−15 at 1 s and 6.4 × 10−18 at 40,000 s, which is better than the transfer stability over the same 212 km single-stage link.
Comparing and synchronizing atomic clocks between distant laboratories with ultra-stable frequency transfer are essential procedures in many fields of fundamental and applied science. Existing conventional methods for frequency transfer based on satellite links, however, are insufficient for the requirements of many applications. In order to achieve high-precision microwave frequency transfer over a thousand kilometers of fiber and to construct a fiber-based microwave transfer network, we propose a cascaded system for microwave frequency transfer consisting of three 100-km single-span spooled fiber links using an improved electronic phase compensation scheme. The transfer instability measured for the microwave signal reaches 1.1 × 10−14 at 1 s and 6.8 × 10−18 at 105 s, which agrees with the root-sum-square of each span contribution. It is feasible to extend the length of the fiber-based microwave frequency transfer up to 1200 km using 4 stages of our cascaded system, which is still sufficient to transfer modern cold atom microwave frequency standards. Moreover, the transfer instability of 9.0 × 10−15 at 1 s and 9.0 × 10−18 at 105 s for a 100-MHz signal is achieved. The residual phase noise power spectral density of the 300-km cascaded link measured at 100-MHz is also obtained. The rejection frequency bandwidth of the cascaded link is limited by the propagation delay of one single-span link.
Abstract: we report the first demonstration of a long-distance ultra stable frequency dissemination in the microwave range. A 9.15 GHz signal is transferred through a 86-km urban optical link with a fractional frequency instability of 1.3×10-15 at 1 s integration time and below 10-18 at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radiofrequency links and compares well with recently demonstrated full optical links.
We have demonstrated a microwave transfer over a 204-km noisy urban fiber link by a cascaded system with 2 stages, which connected 10-GHz and 1-GHz transfer systems in series. A diurnal phase-noise cancellation ratio of 45 dB was obtained by use of an electronic phase-noise compensation system. Additionally, the stabilities reached 6 x 10(-14) at 1 s and 5 x 10(-17) at one-half day, which agreed with the root-sum-square of those of the 10-GHz and 1-GHz transfers. We verified for the first time that the transfer stability degrades only square root N times in a cascaded system with N stages.
We demonstrate the transmission of a microwave frequency signal at 10 GHz over a 112-km urban fiber link based on a novel simple-architecture electronic phase compensation system. The key element of the system is the low noise frequency divider by 4 to differentiate the frequency of the forward signal from that of the backward one, thus suppressing the effect of Brillouin backscattering and parasitic reflection along the link. In terms of overlapping Allan deviation, the frequency transfer instability of 4.2 × 10−15 at 1-s integration time and 1.6 × 10−18 at one-day integration time was achieved. In addition, its sensitivity to the polarization mode dispersion in fiber is analyzed by comparing the results with and without laser polarization scrambling. Generally, with simplicity and robustness, the system can offer great potentials in constructing cascaded frequency transfer system and facilitate the building of fiber-based microwave transfer network.
Experiments of transmission of sub-Hz cavity-stabilized 1542 nm laser frequency using a pair of 43 km dark fibers in urban environment are reported on successively 86 km and 172 km, with fractional frequency instability in the 10-19 range. A new approach is then introduced consisting in using part of an optical telecommunications network carrying simultaneously data traffic using a DWDM scheme to multiplex the metrological signal. This method is experimentally implemented using 22 km of fiber linking Université Paris 13 to its internet access point without degradation of the link instability. We finally present a project of large scale link between Paris and the German border using RENATER network which could constitute the first step of the building of a European optical network for ultrastable frequency dissemination and comparison.
We transferred the frequency of an ultrastable laser over 86 km of urban fiber. The link is composed of two cascaded 43 km fibers connecting two laboratories, Laboratoire National de Métrologie et d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) and Laboratoire de Physique des Lasers (LPL), in the Paris area. In an effort to realistically demonstrate a link of 172 km without using spooled fiber extensions, we implemented a recirculation loop to double the length of the urban fiber link. The link is fed with a 1542 nm cavity-stabilized fiber laser having a sub-Hz linewidth. The fiber-induced phase noise is measured and cancelled with an all fiber-based interferometer using commercial off-the-shelf pigtailed telecommunication components. The compensated link shows an Allan deviation of a few 10−16 at one second and a few 10−19 at 10,000 seconds.