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IEEE International Conference on High Performance Switching and Routing (HPSR)

Exploring High-Precision Transmission of Optical Reference Frequencies via Wavelength Division Multiplexing

Kota Nishiyama, Mayuri Nakagawa, Kohjun Koshiji, Masahiro Marshall Nakagawa, Hiromitsu Imai, Kenichi Hitachi, Tomoya Akatsuka, Katsuya Oguri, Rie Hayashi

Published May 20, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1109/hpsr64165.2025.11038852

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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.
Optical clocks can be used as the absolute frequency reference due to their high accuracy and stability. In a precise optical clock transmission system, the instability loss of the link is usually evaluated by beating the remote optical signal with the reference clock, which is not suitable for real frequency distribution applications. Therefore, it is necessary to assess the performance directly at the remote site for the optical frequency transfer, because the two sites of the link are usually not co-located. In this paper, we proposed a comb-based remote residual instability evaluation scheme. Two coherent optical combs with different wavelengths were extracted from a frequency stable comb and transmitted after multiplexing. The residual instability was evaluated directly at the remote site by measuring the phase fluctuation difference between the two combs. We achieved 8.61 × 10−19 at 40,000 s over a 10 km fiber link, reaching the instability of optical clocks. These results revealed that our scheme can evaluate optical clock frequency transmission directly at the remote site, which made the method truly practical.
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.
Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
Abstract Optical clocks with unprecedented accuracy of 10−18 promise innovations in many research areas. Their applications rely to a large extent on the ability of precisely converting the frequency from one optical clock to another, or particularly to the frequencies in the fiber telecom band for long-distance transmission. This report demonstrates a low-noise, high-precision optical frequency divider, which realizes accurate optical frequency conversion and enables precise measurement of optical frequency ratios. By measuring against the frequency ratio between the fundamental and the second harmonic of a 1064-nm laser instead of a second copy of the same system, we demonstrate that the optical frequency divider has a fractional frequency division instability of 6 × 10−19 at 1 s and a fractional frequency division uncertainty of 1.4 × 10−21. The remarkable numbers can support frequency division of the best optical clocks in the world without frequency-conversion-caused degradation of their performance.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.
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.