Michelson interferometer based OSNR monitoring and optimization method for phase-modulated fiber-optic time transfer system

In long-haul fiber-optic time and frequency (T/F) transfer systems, the accumulation of amplified spontaneous emission (ASE) from amplifiers degrades the optical signal-to-noise ratio (OSNR) of the transmitted signal, thereby affecting the performance of the system. Therefore, we propose a scheme for OSNR monitoring and optimization in fiber-optic time transfer systems. The OSNR monitor exploits the difference in coherence between the signal and the ASE. It employs a Michelson delay line interferometer at the receiver, commonly used to demodulate the phase-modulated time signal (one pulse per second). By adjusting amplifier gains, we achieve optimal OSNR, and the Michelson interferometer continuously monitors OSNR changes to ensure reliable system operation. This scheme was verified over a 480 km laboratory fiber link containing five high-isolation bi-directional erbium-doped fiber amplifiers (HI-BiEDFAs). Experimental results show an SNR improvement of over 7 dB compared to the initial gain settings. The time deviation of the round-trip time transfer system is 18.84 ps at 1 s and 0.90 ps at 10,000 s, showing more than a twofold improvement in short-term stability over the traditional method of maintaining constant output power of HI-BiEDFAs. This scheme offers both pulses demodulation and OSNR monitoring without any additional modules, and the flexible utilization of monitoring data, enabling state awareness across the network—a key technological vision for future applications.