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.