We propose a fiber-based, multi-function time-frequency delivery system assisted by active fiber delay compensation and the wavelength division multiplexing (WDM) technology. Experimentally, three signals with different functions are transferred from the center to the remote end, while a simulated receiving signal is transferred back. All channels are delay stabilized simultaneously.
We propose a fiber-based, multi-function time-frequency delivery system assisted by active fiber delay compensation and the wavelength division multiplexing (WDM) technology. Experimentally, three signals with different functions are transferred from the center to the remote end, while a simulated receiving signal is transferred back. All channels are delay stabilized simultaneously.
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.
In this Letter, we propose a multiple access fiber-optic radio-frequency transfer, time, and phase synchronization scheme using passive optical coupling and symmetric carrier configuration. In the proposed scheme, the RF signal is distributed to distant nodes via a symmetric carrier configuration, which inherently suppresses link length-dependent phase difference. Access nodes are connected to the fiber link via passive optical coupling. Time signals are generated at each node by leveraging transferred frequency signals and synchronized by time-division multiplexing transmission and active delay compensation. Experimentally, a 2 GHz RF signal is transferred to access and terminal nodes over 20-km and 60-km fiber links, respectively, and compensated separately. The measured frequency stability is better than 4×10 −14 and 2×10 −16 at averaging time of 1 s and 10 4 s, respectively. For the synchronized link, the measured time stability is better than 7 ps at 1 s and 4 ps at 10 4 s. The phase consistency of RF signals is better than 10 ps (2 % of the entire signal cycle) when subjected to changes in link length and system restart.
In this paper, we propose and demonstrate a multiple local oscillator (LO) signal phase stabilization technique for long-distance fiber delivery. One of the LO signals, which acts as a reference single, is round-trip transferred between the central station and the remote end, carrying the phase variation arises from perturbations of the fiber link. The wavelength of the optical carrier is adjusted according to the phase variation of the reference LO, to stabilize the delay of the fiber link. Once the delay of the link is stabilized, the phases of other LO signals that transferred through the same fiber link will all be stabilized. Meanwhile, the delay tunable range is in proportion to the fiber length, which means a very long delivery distance can be expected. Experimentally, LOs at frequencies of 2.46 GHz and 8 GHz are transferred through a 30-km fiber link, and significant phase drift compression is observed at both frequencies.
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.
We report a technique for coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is compensated by remote users with passive phase noise correction, rather than at the local site as is conventional. Neither phase discrimination nor active phase tracking is required due to the open-loop design, mitigating some technical problems such as the limited compensation speed and the finite compensation precision as conventional active phase noise cancellation. We theoretically analyze and experimentally demonstrate that the delay-limited residual fiber phase noise after phase noise compensation is a factor of seven higher than the conventional techniques. Using this technique, we demonstrate the transfer laser light through a 145-km-long, lab-based spooled fiber. After being compensated, a relative frequency instability is 1.9 × 10-15at the integration time of 1 s and scales down to the level of 10-18-range at 10,000 s. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is (-0.36 ± 2.6) × 10-18. As the transmitted optical signal remains unaltered until it reaches the remote sites, it can be transmitted simultaneously to multiple remote sites on an arbitrarily complex fiber network, paving a way to develop a multi-node optical frequency dissemination system with post automatic phase noise correction for a number of end-users.
In this paper, we propose a phase-conjugation-based fast radio frequency (RF) phase auto stabilization technique for long-distance fiber delivery. By phase conjugation at the center site, the proposed scheme pre-phase-promotes the RF signal with the shift which is acquired by round-trip transferring another RF whose frequency is half of the one to be sent. Such phase pre-promotion is then used to counteract exactly the following retard induced by one-way delivery. Different from the previous phase-locking-loop-based schemes, the proposed open-loop design avoids the use of any tunable parts and dynamic phase tracking, enabling a fast phase stabilization at the remote site. An end-less compensation capacity can also be achieved. Our design is analyzed by theory. Experimentally, the new scheme is verified by transferring a frequency of 2.42 GHz through a 30-km optical fiber link. Significant phase drift compression is observed. The rapid phase stabilization is verified by introducing sudden time delay change into the link. The recovery time equals to the round-trip time of the link plus the transitional duration of the delay change, which is much shorter than the traditional trial-and-error phase locking loop. Important issues of the system design are discussed.