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.
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。
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)