Fiber-optic round trip time transfer tolerant to OSNR degradation via forward frequency transmission

In this Letter, we propose a fiber-optic round trip time transfer system tolerant to the received optical signal noise ratio (OSNR) degradation through forward frequency transfer. Typically, OSNR degrades with increased transmission distance and noise accumulation in fiber-optic time transfer systems, affecting the received signal-to-noise ratio (SNR) and system stability. The broad bandwidth of time signals limits the effectiveness of filtering to improve the received SNR. The proposed system overcomes these limitations by incorporating a forward-transmitted frequency (FTF) signal and a phase-locked pulse generator (PLPG), enhancing the received SNR and generating high-precision time pulses with minimal jitter. Theoretical simulations confirm the insensitivity to OSNR degradation of the system in short-term stability. Experiments over laboratory fiber links of 320 km, 640 km, and 960 km demonstrate short-term stabilities below 10 ps, with no significant deterioration despite increased transmission distance and OSNR degradation. Given its superior performance and noise resistance, this system holds significant promise for future ground-based fiber-optic time-frequency systems.