Optical fiber links have proven their powerful ability to transfer the ultrastable clock signals over a national and continental metrological optical network. In this Letter, we propose and demonstrate a new scheme that time and frequency signals are modulated on the same laser wavelength with phase and intensity modulation by an independent electro-optic modulator, respectively. Different wavelengths are used in two opposite directions to prevent Rayleigh scattering. The noise changes caused by the fluctuations of the transfer links are actively canceled at the local site by optical delay lines. Radio frequency with 1 GHz and time signal with one pulse per second (1 PPS) are transmitted over 110 km fiber spools. The experimental results demonstrate that frequency instabilities of 1.7 E − 14 at 1 s and 5.9 E − 17 at 10 4 s and time interval transfer of 1 PPS signal with sub-ps stability after 1000 s are obtained. This novel scheme offers advantages with respect to reduce the channel in the fiber network, and can keep the time and frequency signal free from interference with each other.
Optical fiber links have proven their powerful ability to transfer the ultrastable clock signals over a national and continental metrological optical network. In this Letter, we propose and demonstrate a new scheme that time and frequency signals are modulated on the same laser wavelength with phase and intensity modulation by an independent electro-optic modulator, respectively. Different wavelengths are used in two opposite directions to prevent Rayleigh scattering. The noise changes caused by the fluctuations of the transfer links are actively canceled at the local site by optical delay lines. Radio frequency with 1 GHz and time signal with one pulse per second (1 PPS) are transmitted over 110 km fiber spools. The experimental results demonstrate that frequency instabilities of 1.7 E − 14 at 1 s and 5.9 E − 17 at 10 4 s and time interval transfer of 1 PPS signal with sub-ps stability after 1000 s are obtained. This novel scheme offers advantages with respect to reduce the channel in the fiber network, and can keep the time and frequency signal free from interference with each other.
We propose a novel scheme that uses only a single passive phase compensation device to achieve stable optical and radio frequency joint transfer. The phase noises of optical and radio frequency can be simultaneously compensated by passively embedding their phase information on the two optical carrier sidebands generated by an electro-optical modulator without using the phase discrimination and active servo controller. As a result, this scheme has many advantages, such as high spectral purity, short settling time and infinite compensation accuracy. We experimentally demonstrate the joint transfer of optical and 1 GHz RF over 120 km fiber spools. The optical frequency stability achieves 6.9 × 10 −17 at 1 s and 7.03 × 10 −19 at 10000 s, while the 1 GHz RF is 6.47 × 10 −13 at 1 s and 3.96 × 10 −16 at 10000 s.
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.
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.
In this paper, we present a remote time-base-free technique for a coherent optical frequency transfer system via fiber.At the remote site, the thermal noise of the optical components is corrected along with the link phase noise caused by environmental effects.In this system, a 1 × 2 acousto-optic modulator (AOM) is applied at the remote site, with the first light being used to eliminate the noise of the remote time base and interface with remote users while the zeroth light is used to establish an active noise canceling loop.With this technique, a 10 MHz commercial oscillator, used as a time base at the remote site, does not contribute to the noise of the transferred signal.An experimental system is constructed using a 150 km fiber spool to validate the proposed technique.After compensation, the overlapping Allan deviation of the transfer link is 7.42 × 10 -15 at 1 s integration time and scales down to 1.07 × 10 -18 at 10,000 s integration time.The uncertainty of the transmitted optical frequency is on the order of a few 10 -19 .This significantly reduces the time-base requirements and costs for multi-user applications without compromising transfer accuracy.Meanwhile, these results show great potential for transferring ultra-stable optical frequency signals to remote sites, especially for point-to-multi-users.
In this Letter, we propose and demonstrate a multi-access fiber-optic radio frequency dissemination with passive phase noise cancelation. A forward phase-conjugated signal is generated at the local site by frequency mixing between the standard signal and the round-trip probe signal. A stable frequency signal is achieved by frequency mixing the tapped forward phase-conjugated signal and the backward probe signal at an arbitrary point along the fiber link. Different wavelengths for forward and backward directions are employed to efficiently suppress the effect of backscattering. At the same time, the increase of bidirectional asymmetry with the increase of user ends is avoided by employing the same wavelengths for all user ends. A multi-access frequency transfer over a 45 km fiber link based on the proposed scheme is demonstrated. The radio frequency signals with relative frequency stabilities of 10−17/20,000 s level are reproduced at two points of 5 and 40 km far from the local site along the fiber link, respectively.
In this Letter, we propose a fiber-based stable radio frequency transfer system by a hybrid frequency modulation scheme. Creatively, two radio frequency signals are combined and simultaneously transferred by only one laser diode. One frequency component is used to detect the phase fluctuation, and the other one is the derivative compensated signal providing a stable frequency for the remote end. A proper ratio of the frequencies of the components is well maintained by parameter m to avoid interference between them. Experimentally, a stable 200 MHz signal is transferred over 100 km optical fiber with the help of a 1 GHz detecting signal, and fractional instability of 2×10(-17) at 10(5) s is achieved.