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At 09:00 AM on May 19, 2025, the PhD thesis defense for Tsinghua University doctoral student Dai Hongfei was held in the conference room on the first floor of Tsinghua’s Welding Building. The defense committee comprised Prof. Rong Zhu from Tsinghua University (defense committee chair), Researcher Renfu Yang from the Beijing Academy of Quantum Information Sciences, Prof. Yanhui Wang from Peking University, and Associate Profs. Yanling Feng and Bo Wang from Tsinghua University (Dai’s supervisor).

Dai’s doctoral dissertation was titled “Research on the Frequency Tampering Risks and Monitoring Methods of Fiber-Optic Time and Frequency Synchronization Systems.”
The dissertation explored the frequency tampering risks and monitoring methods of fiber-optic time and frequency synchronization systems, a topic of both scientific significance and practical value. The innovative achievements of the dissertation are as follows:
(1) A frequency tampering method based on frequency lenses was proposed, exposing the fundamental security risks in fiber-optic frequency synchronization technology. Using this method, a frequency tampering module suitable for fiber-optic time and frequency synchronization systems was developed and experimentally verified on a 200-kilometer fiber-optic frequency synchronization system.
(2) A vibration sensing fusion method based on the idle parameters of fiber-optic time synchronization systems was developed. This method enables simultaneous time synchronization and vibration sensing without additional sensing resources. Under multi-user link conditions, it achieved good time synchronization performance and highly sensitive vibration detection.
(3) A vibration sensing fusion method that multiplexes frequency synchronization optical carriers was proposed. Tailored to radio array requirements, it achieved high-performance frequency standard transmission in one-to-many scenarios. The receiver can detect abnormal vibrations at multiple points along the link, offering a new technical approach for risk monitoring in radio array time and frequency synchronization networks.

After an anonymous vote, the defense committee unanimously approved the dissertation and recommended that Dai Hongfei be awarded the Doctor of Engineering degree.