Research On Ultra-stable Frequency Transfer Over Long-distance Fiber Link

Frequency standard is central in precision measurement and international metrological system.Fundamental physical quantities other than matter quantities can be traced directly or indirectly to the frequency standard.The operation of modern electronic information systems such as precision navigation,radar networking,deep space exploration,high-speed communication systems,and high-precision measurements all require multi-nodes synchronous frequency signals.Optical fiber is an excellent transmission medium,with advantages over microwave channels in terms of loss,interference,damage resistance,and reliability.Building large-scale multi-users frequency synchronization networks based on existing optical fiber communication systems has attracted many researchers.However,there are three serious problems with fiber links over thousands of kilometers:(1)Photonic noise,fiber nonlinearity,dispersion,crosstalk,backscatter,and other transmission impairments accumulate and interact with each other to make signal extraction difficult.(2)The vibrations,stress changes,and temperature changes experienced by the round-trip transmission are not exactly symmetric and equal,so that the round-trip transmission is not ideally able to cancel the phase fluctuations.(3)The frequency instability deteriorates due to transmission loss,round-trip asymmetry,gain competition,and operating point drift.In this paper,we theoretically analyze the noise induced by a long-distance optical fiber link and investigate the optimization of the core device of the system and the comparison of different transmission methods.Multi-node stable radio frequency(RF)dissemination is proposed for long-distance links,yielding outstanding innovative results.The main results of this paper are as follows:1.The relationship between the bandwidth of the PLL and the frequency instability of the stable RF transmission has been proposed in order to address the issue of the effect of noise induced by long-distance fiber links on the transmission performance of the system.Due to the resident chromatic dispersion in optical fiber,simply calcultate the bandwidth of PLL by the transmission delay will worsen asymmetries and eventually decrease the stability.A theoretical analysis to describe the relationship of the PLL bandwidth and the frequency instability is proposed.The optimizaion strategy of PLL bandwidth is presented according to the fiber parameters and an adaptive fourth-order PLL is designed to increase the dissemination stability.In experiment,we transmit a 2.4 GHz frequency signal over a 2500 km optical fiber.The frequency instability of 4.6 × 10-14@1 s and 6.0 × 10-17@10,000 s is demonstrated,which is eligible for extra long-distance atomic clock comparison and continental-scale frequency synchronization network construction.2.Long-distance frequency synchronization through fiber link is attracting more attention in recent years.The repeater becomes increasingly important with the increase of the transmission distance.We analyze the three main relay methods principally including single-span,signal relay,and cascade connection frequency transmission and compare their frequency instability.The proof-ofconcept experiments are also conducted along a 3009.8 km fiber link in the laboratory environment.The system used for comparison is a combination of phase conjugation and phase-locked loop(PLL)designed for long-distance frequency transmission.The best transfer preference can be obtained by using the cascade connection,whose frequency instabilities of 8.8 × 10-14@1 s and 8.4 × 10-17@10,000 s have been measured for phase-stable RF signal.Our study can be useful for the calibration and comparison of ultra-long-distance atomic clocks.3.To meet the demand of flexible access for high-precision synchronization frequency,we demonstrate multi-node stable RF dissemination over a longdistance optical fiber.Stable RF signals can be extracted at any node along the optical fiber,not just at the endpoint.The differential mixing structure(DMS)is employed to avoid the frequency harmonic leakage and enhance the precision.The phase-locked loop(PLL)provides frequency reference for the DMS while improving the signal to noise ratio(SNR)of dissemination signal.We measure the frequency instability of multi-node stable frequency dissemination system(MFDS)at different locations along the 2000 km optical fiber.The measured short-term instability with average time of 1 s are 1.9 × 10-14@500 km,2.8 × 10-14@1000 km,3.5 × 10-14@1500 km,and 3.8 × 10-14@2,000 km respectively.The long-term instability with average time of 10,000 s are basically the same at any position of the optical fiber,which is about(6.24±0.05)× 10-17.The resulting instability is sufficient for the propagation of precision active hydrogen masers.