A Clock Distribution and Synchronization Scheme Over Optical Links for Large-Scale Physics Experiments

In large-scale physics experiments, there is a trend to implement clock distribution and synchronization over multi-Gigabit serial links on optical networks. The accuracy of clock synchronization in such a fashion is mainly determined by two aspects: one is the stability of the clock distribution over the links, and the other is the employed mechanism of clock synchronization. To achieve a high synchronization accuracy in a range of tens of picoseconds, this article proposes a new optical fiber link and synchronization scheme based on optical circulators and serial transceivers embedded in field-programmable gate arrays (FPGAs). Benefiting from the single wavelength fiber connection, high-precision time-to-digital converters (TDCs) and phase interpolator (PI) sub-block in state-of-the-art Serializer-Deserializer (SerDes) transceivers on FPGAs, the offset of distributed clocks can be measured on-line and precisely compensated. The time synchronization performance is evaluated on a prototype system with three levels of structure. Through multiple power or reset cycles and long-term operation tests, the clock synchronization accuracy over a 5-km fiber connection is measured to be lower than 15 ps, and the recovered clocks at nodes at different levels have independent jitter, all lower than 4.2 ps. In addition, the achieved high performance is also proved to be insensitive to operating temperature and the connection distance.