| With the development of science and technology,optical clock is gradually replacing atomic clock as a new generation of accurate time and frequency standards.The precision of the existing clock reference source is difficult to meet the ultra-high precision time-frequency synchronization requirements of the future communication fields such as 5G,Internet of Things,quantum communication and so on.Therefore,the method about how to improve the stability of time-frequency transmission system has become a hot topic in current research in various countries.Compared to the traditional two-way satellite time-frequency transmission scheme,optical fiber-based microwave direct intensity modulation transmission has the characteristic of low loss,high stability,and is not susceptible to electromagnetic interference,so it can greatly improve the transmission accuracy.The fiber dissemination can be affected by external environmental factors,so the received signals's phase stability will be deteriorating.Which means it is necessary to design a frequency-stabilized transmission system to detect and pre-compensate the phase jitter introduced during transmission.The subject of this thesis is the long-distance fiber-based time-frequency transmission system,a frequency transmission scheme based on combing passive phase conjugation with PLL module is proposed.This scheme can increase the transmission distance supported by the system.The major work can be divided into following parts.1)On the base of introducing the theory which are related,the significance and research background of time-frequency transmission are expounded.Then this thesis compares traditional satellite-based time synchronization schemes with fiber-based time-frequency transmission schemes,and analyzes the advantages and disadvantages of various programs.At the same time,Allan variance is introduced to quantify the accuracy of the time-frequency transmission systems and the factors that affect the stability of system transmission are enumerated.2)A phase compensation system is proposed and demonstrated.The frequency transmission system proposed in this thesis combines active compensation and passive compensation to compensate the phase jitter introduced by the signal during the fiber transmission in real time.The active compensation uses phase lock loop module whose principle is introduced in detail in this thesis.We design and optimize the structure and parameters of each module of phase-locked loop for long-distance fiber link time-frequency transmission system.The active compensation and passive compensation both have the advantage of a large compensation range.However,the former utilizes the excellent short-stability characteristics of the constant temperature voltage controlled crystal oscillator,so it can improve the system's short-term performance.In contrast,passive compensation does not introduce additional active electrical components,so it has an advantage in long-term stability performance.In this thesis,passive compensation is applied at the local end of the transmission system,so the most of the phase jitter introduced during fiber optic transmission is offset,which means this system enables long-distance transmission while ensuring good short-term and long-term stability.3)To measure the performance of the system proposed in this thesis,the transmission stability of the system over short-distance situations such as back-to-back and 10km fiber links is detected.The 1007km fiber link system is built by debugging the bidirectional erbium-doped fiber amplifier in real time at the laboratory.In the mean time,we analyze the existing measurement methods and optimize the system according to the measurement process.Finally,the transmission and synchronization of the 2.4GHz frequency signal on the 1007km single-mode optical fiber link is realized and the system's fractional frequency-transfer stability is 8.2×10-14/S and 7.88×10-17/104s. |
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