| High-precision time and frequency synchronization technology is widely used in areas,such as precision navigation,time-frequency measurement and cutting-edge fundamental researches.At present,with the continuous improvement of the accuracy and stability of atomic clocks,the traditional time and frequency transfer method through satellite links is unable to meet the requirements of high-performance atomic clock in remote synchronization.Since optical fiber links have advantages of low loss and anti-jamming capacity,the time and frequency transfer stability can reach sub-femtosecond or even attosecond level,which meets the urgent needs of remote comparison and synchronization of high-performance atomic clocks in the future.It has also played important role in national science facilities and other international science projects,such as the next generation of X-ray free electron laser,as well as the square kilometer array telescope(SKA).As the length of transfer path increases,optical phase jitter is introduced by fiber links inevitably.Therefore,it is necessary to carry out precise delay compensation.Time and frequency transfer technology through fiber usually adopts round-trip discrimination scheme.By obtaining the phase error between the femtosecond laser pulses transmitted back and forth from remote site and local microwave reference,the phase noise of optical fiber links can be compensated in a way of electrical phase conjugation compensation or optical path active control.Compared with electrical phase conjugation compensation,the optical path active control can realize time synchronization,which has more important application value.To solve the above problems,we demonstrate a remote synchronization system based on femtosecond laser pulses in long fiber links.In this system,an optical-microwave phase-locked loop(PLL)scheme is designed and implemented for high precision phase discrimination and real-time locking between femtosecond laser pulses and microwave signals,based on the second-order PLL scheme.In the optical-microwave PLL,the phase errors between optical pulses and microwave signals are detected by fiber-loop optical-microwave phase detectors(FLOM-PDs),which avoids the intense phase noise caused by direct photoelectric detection,therefore improves the accuracy of phase detection.In addition,an active optical path control module using fiber stretchers and a motorized optical delay line are made to jointly achieve delay compensation.Finally,the1100 m dispersion compensation fiber link is used for the 10 km single-mode fiber link dispersion compensation,and a 0.8 nm fiber band-pass filter is used for further suppress the pulses broadening as well.The result shows that,under the 11 km optical fiber transmission link,the time deviations are 1.55 fs/s and 0.12 fs/1000s,respectively.Correspondingly,the frequency stabilities are 3.06×10-15/s and 1.99×10-19/1000s,respectively.In the end,after out-loop test,the fractional frequency instability between local and remote 7.5 GHz microwave oscillators in terms of Allan deviation reaches 2.3×10-14/s and 7.67×10-17/400s. |
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