A robust fiber-based frequency synchronization system immune to strong temperature fluctuation

(a) Artist's impression of the full Square Kilometre Array (SKA) at night (image provided by SKA Organization). (b) Scheme diagram of the robust fiber-based frequency synchronization system immune to strong temperature fluctuation for SKA designed by JMI in Tsinghua University. (c) Production of the frequency synchronization system.

Square Kilometre Array (SKA) is an international large-scale scientific engineering project that involves multiple countries, aiming at building the largest and most sensitive radio telescope in the world. SKA will be constructed in the areas in Australia and South Africa. The total collecting area of all the SKA antennas combined will be over one square kilometre, about the size of 140 football fields. A key prerequisite for the telescope array to work is to keep thousands of telescopes in the array highly synchronized in time and frequency.

The fiber network structure for SKA frequency synchronization is complicated and the environment condition is tough. The fiber network is thousands of kilometers long. The SKA sites are mostly located deep in the deserts, where there is always strong temperature fluctuation, together with occasionally bad weather conditions such as heavy rain and hail. In some parts, overhead fiber links are used instead of buried ones, which makes them confronted with severer temperature fluctuation, vibration and mechanical stress, and raises great difficulties in frequency transfer. So far there has not been any previous fiber-based frequency synchronization network at such a large scale, high precision and reliability in the world

With the rapid development of the technology on fiber-based frequency synchronization and dissemination in recent years, various related designs and applications keep showing up. The Joint Institute for Measurement Science (JMI) led by Prof. Lijun Wang in Tsinghua University have participated in the Signal and Data Transport consortium of SKA since 2013, providing fiber-based solutions for reference frequency dissemination among each telescope.

Regarding the specific requirements of SKA, the JMI team proposed an innovative design which uses an asymmetrical phase noise compensation method. By introducing a small frequency offset intentionally, the non-linear effect and signal leakage in the system can be suppressed efficiently, so that the system can be immune to strong ambient environment disturbances. This work has been published in Chinese Optics Letters, Volume 16, No. 1, 2018 (Xi Zhu, et al., Robust fiber-based frequency synchronization system immune to strong temperature fluctuation).

"Currently this frequency dissemination system can fully meet SKA requirements in all aspects such as synchronization stability and environmental adaptability," says the corresponding author Dr. Bo Wang. "And this system has the advantages of simple structure, easy operation and expansion, which make it very suitable for large-scale utilization in SKA. It also gives a good reference to other applications with similar demands."

The design was selected as the frequency dissemination scheme for SKA-Australia and will go into the phase of procurement and construction in 2018.

适用于剧烈温度变化环境的光纤频率同步系统

(a)夜晚的SKA效果图（由SKA组织提供）；(b)清华团队设计的适用于SKA复杂使用环境的高精度频率分发同步系统原理图；(c)该频率分发同步系统外观。

Square Kilometre Array (SKA)是一项由多个国家参与的国际大科学工程，旨在建设世界最大、最灵敏的分布式综合孔径望远镜。SKA将建在澳大利亚和南非，有效接收面积约为1平方公里，相当于140个足球场的面积。数千面望远镜之间保持高精度的时频同步是SKA正常工作的前提。

SKA光纤网络长度达几千公里，台址地处沙漠深处，温度变化剧烈，时有暴雨冰雹等恶劣天气，还需要使用裸露于空气中的架空光缆，给高精度的时频传输带来巨大的挑战。目前国际上还没有类似的大规模，高可靠性，高精度的光纤频率同步网络。

基于光纤的频率传输与同步技术近些年来发展迅速，并不断涌现出新的方案和应用。清华大学王力军教授领导的精密测量联合实验室在该领域有丰富的研究经验和成果，2013年该团队正式加入SKA信号与数据传输工作包联盟，并负责提供SKA阵列望远镜单元间的参考频率同步解决方案。

针对SKA的应用需求，该课题组创新性地提出了一种不对称的相位噪声补偿方案，通过人为设置频率差，显著提高对系统中非线性效应和非理想器件泄漏的抑制效率，使系统具有抵抗剧烈环境温度变化的能力。相关工作发表在Chinese Optics Letters 2018年第1期上 (Xi Zhu,, et al., Robust fiber-based frequency synchronization system immune to strong temperature fluctuation)

该论文通讯作者王波博士介绍道：“目前这套频率分发同步系统，无论从同步精度还是环境适应性方面，均超额满足SKA的使用需求，并且结构简单、易操作、易扩容，十分适合SKA的大规模应用，对于其他有类似使用需求的工程应用也有借鉴意义。”

目前该高精度频率分发同步系统被选为SKA-澳大利亚阵列望远镜的频率同步方案，2018年将进入采购和工程建设阶段。