| In the recent years,due to the continuous development of science and technology,the stability of atomic clock has been greatly improved.The stability of the existing hydrogen clock has been able to reach the order of10-16 at an averaging time of one day.In many time and frequency applications,not only the high stability frequency reference are needed,but the high-precision frequency transfer technology are also needed.The existing transmission technology based on the traditional satellite link can only reach the stability of 10-15 at an averaging time of one day,which can't meet the needs of the next generation of frequency reference transmission.Compared with the traditional satellite link,the optical fiber link has the advantages of low loss,low dispersion and anti-interference.Therefore,high-precision frequency transmission using optical fiber link has been extensively studied.Because the fiber link is subject to temperature variations and mechanical perturbations,passing the reference frequency signal through the actual fiber link introduces additional phase noise,which affects the frequency stability of the signal at the receiver,and requires real-time phase compensation to compensate for the jitter.In order to expand the application of fiber frequency transmission,such as to meet the distributed radar,array antenna applications,we need point to multipoint frequency transfer system.The existing multipoint transfer schemes include both active and passive schemes.In the active scheme,the phase noise is detected by the round-trip delay of the signal,the phase noise is compensated by the real-time phase-detector and according to the algorithm to driven the brake to compensate the link noise of each node,which is of high complexity.Recently,passive compensation technology based on mixer structure has been extensively studied.This technique detect the phase noise of the link at each user node by passive mixing without complex control algorithms,and has fast compensation speed and unlimited dynamic.However,there are some problems,such as backscattering in the system and asymmetry with the increment of the users.In order to achieve high-performance and low-cost solution,we need to solve the backscattering,symmetry and system complexity and other issues,the existing method basically failed to take all these aspects into account.Based on this,this paper presents a distributed fiber frequency transfer scheme based on passive compensation technology.In this scheme,the effect of back-scattered noise on the short-term stability of frequency transfer is avoided effectively by WDM.Meanwhile,the number of used wavelengths does not increase with the number of intermediate nodes,which effectively reduces the wavelength asymmetry for the impact of long-term stability,and the cost of using multiple wavelength channels.The main work of the paper includes:?1?Introduce and analyze the noise existing in the fiber frequency transmission link and its influence on the frequency stability.The advantages and disadvantages of the existing distributed frequency transfer schemes are analyzed.?2?A distributed frequency transfer system based on passive phase noise compensation is designed and analyzed.?3?This paper introduces the testing principle of the system and the corresponding circuit module.On this basis,the distributed frequency transfer along 45km optical fiber link is completed.In this paper,two user nodes are mounted on the main link,and the frequency stability can reach 4.2×10-14/s and 3.7×10-17/20000 s at the user node of 5 km.the frequency stability can reach 4.4×10-14/s and 6.2×10-17/20000s at the user node of 40km. |
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