| High accuracy time transfer in sub-nanosecond or even picosecond is urgently required in basic scientific research,high-tech engineering applications,national defense technology and other fields.Therefore,China started to build a distributed high-precision fiber-optic time transfer system.The key technologies of the distributed high-precision fiber-optic time transfer system are researched in this paper,including the basic technology of optical fiber time transfer and the design of optical fiber time transfer system,to provide technical support for the construction of the distributed high accuracy optical fiber time transfer system in China.The main research work and contributions of this dissertation are as follows:1.Aiming at the key technical problems that affect the core indicators of the distributed highprecision fiber-optic time transfer system,such as timing accuracy,time transfer distance and range,a method to combine the three optical fiber time transfer schemes of the bidirectional same-wavelength time-division multi-access(BSW-TDMA),the bidirectional wavelength-division multiplexing time-division multi-access(BWDMTDMA)and the bidirectional time-division multiplexing time-division multi-access(BTDM-TDMA)is proposed to constitute intra-city,inter-city and inter-provincial threegrade time transfer networks,and the time transfer with stability of 9.2ps@1s is realized on 1085 km the commercial optical fiber link for the first time in China,which makes the time transfer range,accuracy and precision reach the international advanced level.2.Aiming at the key technical problems affecting the time transfer index such as delay and its drift,noise,optical loss of fiber link and distributed time transfer,the accurate measurement technology of fiber link delay based on time-voltage conversion,large-scale high-precision time delay compensation technology based on DDS,noise suppression technology based on carrier recovery time regeneration,bidirectional optical-electricaloptical relay technology and time division multi-access timing technology based on carrier recovery are proposed,and the above problems are solved effectively to lay a technical foundation for the distributed high-precision fiber-optic time transfer system.The measurement stability of the fiber link delay based on time-voltage conversion has reached2.5ps,which is better than the mainstream instrument SR620.In addition,the time delay compensation resolution of time delay compensation technology based on DDS is 0.6ps,which is better than the mainstream instrument DG645.3.For the intra-city distributed high-precision fiber-optic time transfer system,the problem of multi-node and the dispersion deviation caused by bidirectional wavelength inconsistency need to be solved at the same time.The BSW-TDMA optical fiber time transfer scheme based on carrier recovery is proposed.The carrier recovery technology improves the time transfer stability of each node,while the same nominal wavelength of bidirectional laser reduces the dispersion deviation.In order to verify the scheme and explore its limit performance,a local site to 11 remote site timing is realized by experiments with self-developed equipment,13.8ps@1s,2.5ps@40000s and 15.1ps@1s,7.1ps@40000s time transfer stability are obtained on 550 km laboratory fiber link and871.6km field fiber link,respectively.The BSW-TDMA optical fiber time transfer scheme is used for links less than 100 km,and the timing uncertainty is better than 20 ps,which is better than the internationally reported 100 ps uncertainty on 73 km links,and meets the needs of intra-city distributed high-precision fiber-optic time transfer system.4.In order to meet the demand of inter-city distributed high-precision fiber-optic time transfer system,extend the time transfer range and improve the short-term stability,the BWDM-TDMA optical fiber time transfer scheme based on automatic correction of dispersion deviation is proposed.The bidirectional laser with two adjacent channels in the DWDM system inhibits the influence of laser of back scattering and end-face reflection light on signal-to-noise ratio,thus extending the time transfer range and improving the short-term stability.The time transfer stability of 4.9ps@1s,0.8ps@40000s and 9.2ps@1s,5.4ps@40000s are obtained on 800 km laboratory fiber link and 1085 km field fiber link.The BWDM-TDMA optical fiber time transfer scheme is used for links less than 500 km,and the uncertainty of timing can reach 60 ps,which is better than the internationally reported 250 ps uncertainty on 540 km links,and meets the needs of inter-city distributed high-precision fiber-optic time transfer system.5.In order to meet the demand of inter-provincial ultra-high precision fiber-optic time transfer system,under the premise of ensuring short-term stability of time transfer,further extending the range and improving the uncertainty,it is necessary to solve the scientific problem of dispersion deviation caused by long-term drift of bidirectional wavelength in long-range optical fiber time transfer.Therefore,the BTDM-TDMA fiber-optic time transfer scheme based on wavelength tracking is proposed.The dispersion deviation caused by long-term drift of bidirectional wavelength is solved by using laser wavelength tracking technology to make the difference of bidirectional wavelength being small enough and stable for a long time.The method of measuring time interval by event timer module reduces the temperature drift and uncertainty of measurement in link delay measurement.The method of bidirectional time-division multiplexing time comparison is used to ensure that only uplink or downlink light propagates at any time in the link,avoiding the influence of rayleigh backscattering and end-face reflection,and extending the distance of single-stage transmission.The excellent phase preserving ability of the time-keeping circuit based on digital phase locked loop ensures the stability of timedivision bidirectional time comparison.The time transfer stability of 4.7ps@1s,0.4ps@40000s and the uncertainty of 8.4ps are achieved on the 750 km fiber link in the laboratory.If it is on 1000 km optical fiber link,the scheme can obtain the uncertainty of10.9ps according to the analysis,which is better than the internationally reported 112 ps uncertainty on 740 km links,and meets the needs of inter-provincial distributed highprecision fiber-optic time transfer system.Some key technologies studied in this paper have been applied in the construction of the distributed high-precision fiber-optic time transfer system in China.In order to further improve the accuracy of fiber-optic timing,the ultra-high accuracy fiber-optic time transfer technology based on fiber-optic microwave frequency transfer is discussed,and the initial verification is carried out through experiments.The technology of ultra-high precision time transfer based on optical frequency transfer is prospected to obtain higher fiber time transfer precision. |
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