New Fiber-based radio frequency dissemination scheme for branching networks
Schematic diagram of the fiber based radio frequency dissemination scheme for branching networks with the passive phase noise cancellation method. Three remote sites C, D, E are 2, 5 and 10 km away from the local site, respectively.
Time is one of the earliest known fundamental physical quantities in nature. To maintain time synchronization between different places and establish unified time and frequency standards concern all aspects of the human production activities, and show broad application prospects in atomic clock comparison, astronomical observation, and deep space navigation.
In recent years, due to its low loss and high stability, dissemination of time and frequency signals via optical-fiber links has developed considerably. On the other hand, almost all of the conventional schemes have a common "point-to-point" structure, namely there is only one receiving site connected to a single transmit site. Despite its high stability, the limited accessibility greatly limits further applications of the fiber based frequency dissemination technology.
The researchers led by Prof. Lijun Wang, from the Joint institute for Measurement Science (JMI), Tsinghua University, proposed and demonstrated a new fiber-based RF dissemination scheme suitable for a branching network to overcome the main drawback of limited accessing area of conventional fiber-based frequency synchronization schemes. It is reported in Chinese Optics Letters, Vol. 13, No. 6, 2015.
Using the passive phase noise cancelation method, the fiber induced phase fluctuation can be compensated without active feedback-locking loop, and the highly stable reference frequency signal can be delivered to remote sites simultaneously and independently. For 10 km distance dissemination, relative frequency stabilities of 6×10-15/s and 7×10-17/104s are obtained. The scheme can dramatically simplify the set-up and reduce cost. These advantages make it suitable for constructing the frequency synchronization networks for the Square Kilometer Array (SKA) and other large-scale scientific projects..
Associate professor Dr. Bo Wang from the research group thinks that the proposed scheme overcomes the cost and reliability limiting factors of conventional technology, realizes highly stable frequency synchronization for branching networks. Consequently, this new scheme has broad application prospects.
A series of further experiments will be conducted including noise analysis and compensation methods to increase the dissemination stability. In order to verify that the scheme can be really applied in large-scale applications such as SKA, the system reliability and environmental adaptability will also be improved.
新技术打破光纤时频同步的点对点传输局限
图片说明：光纤时频网络化被动同步技术实验示意图，C、D、E三个接收端与发射端分别相距2、5、10 km。
时间是自然界最早被人类所认识到的基本物理量之一，保持异地间的时间同步、建立统一的时频基准关系到人类生产活动的各个方面，并且在原子钟比对、射电天文、深空探测等众多领域具有重要应用。
近年来，基于光纤的时间频率同步具有损耗低、同步精度高等优势，逐渐成为时频同步领域的研究热点。然而，目前已有的光纤时频同步技术大都只适用于点对点的传输模式，这一不足很大程度上限制了其应用范围。
为了克服现有点对点光纤频率传输技术的应用局限性，清华大学王力军教授领导的精密测量联合实验室提出并实验演示了一种新型光纤频率网络化被动同步技术。相关研究成果发表在Chinese Optics Letters 2015年第6期上。
该技术通过在接收端对传输过程中光纤引入的相位抖动进行被动补偿的方式，实现了高精度频率网络化同步。与传统主动补偿方式相比，具有结构简单、多分支相互独立并且易于扩展等优势，在 10 km 的传输距离上，其同步精度可以达到 6×10−15/s 与 7×10−17/104 s，有望应用于平方公里阵列（SKA）等大科学工程。
该课题组王波副研究员认为，该论文提出的新型光纤时频同步技术弥补了传统主动补偿技术设备复杂、成本较高、不适用于多分支网络化同步等缺点，具有广阔的应用前景。
后续还需深入研究光纤频率传输的噪声来源及补偿方法，进一步提高传输稳定度；面向实用需求，提高系统可靠性及环境适应性。