image: Figure 1 | Schematic of bias-free optical frequency transfer. The system transmits an ultra-stable optical signal through noisy field fiber and restores it with relay stations using digital phase recording, bias-free compensation, and optical filtering. This approach enables stable transfer over 2067 km with fractional frequency instability of 2.9 x 10^-21 at one day.
Credit: Jian-wei Pan et al.
The dissemination of ultra-stable optical frequencies over long-haul fiber networks is essential for next-generation precision metrology. It supports optical clock comparisons, tests of fundamental physics, relativistic geodesy, and distributed quantum networks. However, achieving extreme stability in real-world fiber networks remains difficult because long-distance links suffer from strong environmental noise, accumulated phase fluctuations, and frequent loss of lock.
Conventional optical phase-locked loop systems are also limited by residual compensation errors caused by bidirectional frequency asymmetry. In practical fiber links, frequency shifting is required to suppress parasitic reflections, but this introduces a systematic bias that limits transfer accuracy. At the same time, urban field fibers can exhibit very high noise levels, making stable long-term operation especially challenging.
In a new paper published in Light: Science & Applications, a team of scientists led by Professor Hai-Feng Jiang and Professor Qiang Zhang from the University of Science and Technology of China developed a scalable solution for robust optical frequency transfer in noisy field environments. Their system combines bias-free noise compensation based on digital radio-frequency phase recording, multifunctional relay stations, and intermediate optical noise purification.
The team demonstrated the system over a 2067 km field-deployed fiber network built on standard telecommunications infrastructure. Even under extreme noise levels of about 5000 rad^2/Hz/km at 1 Hz, the link maintained stable operation and achieved a fractional frequency instability of 2.9 x 10^-21 at one day. The system also remained continuously locked for more than four days.
A key feature of the work is the use of digital phase recording with time-to-digital converters, which enables real-time correction of compensation bias and provides a very large phase tracking range. In addition, Hertz-level optical filtering at intermediate nodes suppresses noise accumulation in cascaded links. These results show that high-performance optical frequency transfer can be achieved over very long distances using existing fiber infrastructure.
This work establishes a robust foundation for future intracontinental and potentially global optical frequency networks, with important applications in optical clocks, precision timing, geodesy, and quantum technologies.
Journal
Light: Science & Applications
Article Title
10−21-Level optical frequency dissemination over 2067 km of noise-loaded field-deployed fiber network