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International Conference on Telecommunications and Signal Processing (TSP)

Joint stable optical frequency and precise time transfer over 406 km of shared fiber lines — Study

Josef Vojtěch, Ondřej Havliš, Martin Šlapák, Pavel Škoda, Vladimír Smotlacha, Radek Velc, Petr Münster, Jan Kundrát, Michal Altmann, Jan Radil, Lada Altmannová, Rudolf Vohnout, Michal Hažlinský, Tomáš Horváth, Radan Slavı́k, Martin Čížek, Lenka Pravdová, Šimon Řeřucha, Jan Hrabina, Ondřej Čı́p

Published Jul 1, 2017• 6 citations
Publisher

Abstract

We present a preliminary study of bidirectional ultra-stable optical frequency and precise time transmission over 406 km long path in the telecom-grade fiber optic infrastructure of dense wavelength division multiplexing. The main challenge in this application is the need of bidirectional amplification to compensate for signal loss. Directional non-reciprocities of the time transfer are evaluated together with their uncertainties. Solutions are proposed to significantly limit the final uncertainty.

DOI
10.1109/tsp.2017.8076076

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We present a preliminary study of bidirectional ultra-stable optical frequency and precise time transmission over 406 km long path in the telecom-grade fiber optic infrastructure of dense wavelength division multiplexing. The main challenge in this application is the need of bidirectional amplification to compensate for signal loss. Directional non-reciprocities of the time transfer are evaluated together with their uncertainties. Solutions are proposed to significantly limit the final uncertainty.
SummaryPrecise and stable coherent optical frequency transfer over long-haul fiber link is required by diverse scientific applications, such as large-scale optical clock networks, global positioning and timing, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. To meet the needs of the above applications, active compensation schemes for cancelling fiber-induced phase noise has developed considerably over the past ten years. However, optical signal severely reduces across distant fiber-optic networks, which degrades optical beat signal and feedback control.Here, we adopt active phase compensation and bidirectional optical frequency locking to achieve ultra-stable optical carrier transfer. The phase noise of the fiber link is effectively suppressed, meanwhile, the ultra-stable optical frequency signal is regenerated accurately at the remote site.Preliminary experiment over 100 km fiber link indicates that the additional frequency stability exhibits 1.9×10-16and 2.5×10-17at an averaging time of 1 s and 100 s, respectively. This approach has a capacity of extending to the parallel transfer of microwave and optical frequency, and free-space optical frequency transfer.
We transfer the frequency of an ultra-stable laser over a cascaded optical link comprising two compensated links of 150 km and a repeater station. Each link passes through two important nodes of the telecommunication network and includes 114 km of Internet fiber simultaneously carrying data traffic, through a dense wavelength division multiplexing scheme. The metrological signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station is working independently without any remote control. The phase noise on the two links is compensated with the usual round-trip technique. The 300-km multiplexed cascaded link shows an Allan deviation of 3×10-15at one second and 7×10-20at 20 hours. This work paves the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network.
Transmitting an ultra-high precision frequency of an optical clock via wavelength division multiplexing (WDM) is a key technology for economically realizing innovative applications enabled by significantly accurate and stable time and frequency. In this study, we investigated an optical frequency transmission method using a commercial WDM system, employing our highly stable optical frequency repeater systems with a fiber noise compensation (FNC) function. For the 20-km WDM transmission, we achieved the frequency instability of 4 × 10–15at an averaging time of 1 second (s) and reaches ~ 1 × 10–18at 34,000 s. This frequency stability enables optical clock reference frequencies to be transmitted over long distances, demonstrating the potential of our WDM link to enable the widespread deployment of the optical clock frequency at the frontier of precision.
Precise frequency comparisons are becoming more urgent given the recent rapid progress in next-generation frequency standards. This paper describes a new type of bidirectional optical amplifier that overcomes the fiber loss limits that have prevented accurate frequency comparisons between widely separated places; such comparison is realized by bidirectionally transmitting wavelength-division-multiplexed (WDM) signals along a single fiber. The proposed optical amplifier has an optical isolator in each two-way channel divided by wavelength filters to suppress the optical reflection that causes amplification instability. The additional insertion optical loss due to this method is about 1.5 dB. The optical gain greater than 30 dB is obtained for both signals with good optical isolation of 65 dB. A radio-frequency reference signal can directly be sent by simple intensity modulation and direct detection (IM-DD) devices in the 1550-nm region widely used in telecommunication networks. Phase comparisons of the received signals and the frequency standards at each terminal are used for frequency comparisons. The amplifier is tested in the field using two hydrogen masers. A 120-km fiber with loss of 52.5 dB is used to connect the National Metrology Institute of Japan (NMIJ) to the University of Tokyo. Because of this loss, an amplifier is needed to realize sufficient receiving power. The frequency stability of the system with a 10-MHz direct transmission is evaluated by returning the optical signal from a halfway point [55 km from the NMIJ] where the amplifier is installed. The result is 2.6 × 10-16(Allan deviation) with the averaging time of 7 × 104s. The laboratory result is 8.7 × 10-17(¿ = 4 × 104s). The amplifier's long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons.
Summary form only given. The transfer of ultra-stable frequencies between distant laboratories is a fundamental issue for a wide range of high-sensitivity experiments in advanced metrology and fundamental physics. Since a few years, several experiments of optical frequency transfer were reported over dedicated fiber, with a record distance of 920 km [1] recently demonstrated in Germany. The challenge is now to build a wide frequency dissemination network connecting laboratories and research agencies in Europe.For that purpose we exploit the existing fine meshing of the Internet fiber network, connecting laboratories via the National Research Networks (NRENs). In that scheme, the ultra-stable frequency signal is copropagating simultaneously with the Internet traffic in the same fiber using a dedicated channel in a dense wavelength division multiplexing (DWDM) approach. Optical add-drop multiplexers enable to insert and extract the ultra-stable frequency signal in and from the network.We have demonstrated an ultra-stable optical link over 540 km, including 470 km of public fiber network of the French NREN (RENATER) simultaneously carrying Internet data traffic [2]. The signal bypasses four amplification stages and two Data Center Facilities using multiplexers and custom-made bidirectional erbium-doped fiber amplifiers. One specific point of this experiment is that with Internet fibers, we have a very limited control on the attenuation and noise. For instance the Erbium fiber doped bidirectional amplifiers gain is drastically limited to avoid lasing oscillations due to parasitic reflections due to connectors, splices and more fundamental Rayleigh backscattering. Nevertheless the results are comparable with those obtained with dedicated fibers. Figure 1 shows the measured fractional frequency stability, we obtain 6×10-15 at 1 s and below 10-18 after 10000 second integration time. Such optical link will enable a broad range of high-sensitivity measurements, including the search for fundamental constants variation and gravitational mapping. This concept of fiber link has been very recently used to implement a giant fiber gyroscope [3]
We demonstrate the ultra stable transfer an optical frequency over 300 km of installed optical fibers. This ultrastable optical link uses an optical telecommunication network simultaneously carrying Internet data and goes through two Data Center Facilities using multiplexers and bidirectional erbium-doped fiber amplifiers. We have obtained an instability of 3×10-15at 1 s which scales down to around 7×10-20after about 20 hours. These results are promising and represent an intermediate step for the future development of continental-scale frequency transfer.