| Time synchronization technology is widely used in deep space exploration,navigation and positioning,phased array radar and other fields.With the in-depth development of atomic clocks and optical clocks,the time-frequency transmission system based on satellite can no longer meet the needs of high-precision occasions.Optical fiber is another promising time-frequency transmission medium,which has the advantages of wide coverage,anti-electromagnetic interference and low loss.However,optical fiber is easily affected by external environmental factors such as vibration and temperature change,which leads to the time delay drift of optical signal in the transmission process of optical fiber links.In order to synchronize the time signals of the two places,two-way comparison can be carried out between the stations to dynamically calibrate the signals and improve the stability of the time transmission system.Compared with intensity modulation,the phase modulator eliminates the DC drift effect of the traditional Mach-Zehnder modulator.In recent years,the phase modulation method has provided a new scheme for optical fiber transmission time signal.The second pulse time signal can be directly encoded into the binary phase shift keying format by a phase modulator and loaded onto the optical carrier.However,the difficulty lies in signal demodulation.Phase information cannot be directly detected by the photodetector,and it is usually first converted into intensity changes through coherent demodulation.In this paper,theoretical and experimental researches on modulation method,demodulation method and phase compensation in optical fiber time synchronization transmission are carried out.The influence of fiber delay jitter and fiber link loss on system transmission is discussed.Two traditional time synchronization technologies,two-way comparison and round-trip,are analyzed.Finally,a time transmission scheme based on phase modulation and Michelson interferometer is proposed.The main research contents and results of this paper are as follows:1.A two-way time transmission scheme based on phase modulation and Michelson interferometer is proposed and demonstrated.The binary phase shift keying modulation format is adopted to load the 1 PPS time signal directly onto the optical carrier through a phase modulator.A Michelson interferometer is used to convert the phase information into intensity information.The key part of signal demodulation is the Michelson interferometer composed of optical coupler,adjustable optical attenuator,two Faraday mirrors and a phase modulator.2.Design the optical phase compensation module and feedback control algorithm.In order to compensate the phase difference between the two arms of the interferometer,a low bandwidth phase modulator is placed on one of the arms as a phase shifter.The demodulated optical carrier passes through the photodetector and Bias Tee to generate DC and high-frequency pulse signals,which are respectively used for feedback input and time bidirectional comparison.An optical phase feedback control algorithm is designed to compensate the phase-shifting noise of interferometer in time and keep the phase difference between two arms stable.3.Build and test the back-to-back optical fiber time signal modulation and demodulation system,and the signal transmission stability is 1.32×10-11@1s,2.31×10-11@1000s.Experiments show that the Michelson interferometer can effectively recover the pulse signal.On this basis,the laboratory's 1556 km optical fiber link platform is used to carry out long-distance two-way optical fiber time synchronization experiments.The system stability is 3.55×10-11@1s,5.62×10-11@1000s. |
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