Hybrid Fiber-Optic Time-Frequency Transfer for Enhanced Long-Term Frequency Stability

Coherent Doppler noise cancellation is a cornerstone technique for the high-fidelity dissemination of optical frequency references. However, conventional implementations exhibit an inherent asymmetry by employing offset optical frequencies at the remote end to separate parasitic reflections from the return signal in a round-trip configuration. This asymmetry induces a residual phase error that accumulates with variations in the link propagation delay, fundamentally limiting the long-term frequency stability. We present a hybrid time-frequency transfer scheme that simultaneously disseminates a timing signal to probe the link delay and actively correct this asymmetry-induced error. Experimentally, we validate the method over a 300 km fiber spool subjected to a 7 °C temperature change, inducing a round trip link delay variation of 92.2 ns. With real-time monitoring and active compensation, the optical frequency instability is reduced from 2.1 × 10-19to 1.3 × 10-20at 30000 s averaging—an order-of-magnitude improvement. Simultaneously, the time transfer achieves 1.2 ps stability over the same period. This approach enables simultaneous high-precision time and frequency dissemination over long-haul fiber links with enhanced environmental immunity, establishing a practical pathway toward next-generation phase-stabilized fiber networks.