Objective Time and frequency synchronization technology based on optical fiber transmission method has good synchronization performance, which can meet application requirements such as clock comparison, very long baseline interference, and radar networking. However, some coherent detection applications put forward new requirements for fiber-based time and frequency synchronization technology. Phase synchronization must also be ensured based on frequency synchronization, and low phase noise transmission is required simultaneously. Phase synchronization means that no matter what changes the system experiences, the phase difference between remote sites can remain a constant value, ensuring the phase coherence of the frequency signal. But low phase noise transmission may bring uncertainty to phase synchronization. Therefore, this paper has studied a consistent fiber-optic phase synchronization system with phase noise purification function.

Methods The transmitted frequency signal is input into an electrical phase-locked loop (EPLL) at the remote site. The EPLL contains an oven-controlled crystal oscillator (OCXO) with ultra-low phase noise. After completing phase lock, the phase noise of the output frequency signal will be the same as the OCXO at frequency offsets higher than the phase-locking bandwidth. Therefore, the deteriorated far-end phase noise of the frequency signal due to fiber transmission can be purified by the EPLL, realizing low phase noise transmission. Through the “round-trip delay stabilization” method, the round-trip phase difference can remain constant, while the one-way phase difference may change as long as system status changes. EPLL and the phase compensation process can bring phase uncertainties to the one-way phase difference through theoretical analysis. According to the relationship between the round-trip time delay of the time pulse and the phase difference of the frequency signal, the change value of the round-trip delay can be controlled to the even periods of the frequency signal, thereby eliminating the half-period phase uncertainty caused by phase compensation process. The effect of the one-way phase difference introduced by EPLL needs experimental measurements to evaluate. Experiments on phase noise purification and phase consistency are conducted.

Results and Discussions First, the far-end phase noise of the 1-GHz frequency signal has deteriorated significantly after 26-km fiber link transmission, such as 21.7 dB (from -145.0 to -123.3 dBc·Hz ^-1) at 100 kHz frequency offset; after EPLL purification, the far-end phase noise is purified, decreasing 17.0 dB (from -123.3 to -140.3 dBc·Hz ^-1) at 100 kHz frequency offset (Fig. 2). The results show that the phase noise of the frequency signal can be effectively purified by setting the EPLL at the remote end. Then, by measuring the additional phase shift introduced by the EPLL under multiple shutdowns and restarts of the EPLL, the mean values of the one-way phase difference are between 0.698 and 0.824 rad, with an inconsistency of 0.126 rad, corresponding to 2% of one full cycle (Fig. 3). So, according to theoretical analysis, EPLL will bring ~0.063 rad uncertainty to the one-way phase difference, which is 3 dB lower than its inconsistency. Finally, when the system experiences restart operations and fiber link changing, the phase synchronization process based on optical compensation can be performed as expected. After each closed loop, a relatively consistent phase difference can be achieved with an inconsistency less than 0.068 rad, corresponding to ~1% of one full cycle (Fig. 4 and Fig. 5). The inconsistency is determined by the coupling of various asymmetry effects, including the use of EPLL and the operating temperature drift of devices.

Conclusions A consistent fiber-optic phase synchronization system with phase noise purification function is studied in this paper to meet the application requirements in the field of coherent detection. After 26-km fiber link transmission, the phase noise of 1-GHz frequency signal deteriorates but can be compensated by the EPLL. At the frequency offset of 100 kHz, phase noise is increased by 21.7 dB after optical fiber transmission, from -145.0 to -123.3 dBc·Hz ^-1; and then decreased by 17.0 dB after EPLL purification, from -123.3 to -140.3 dBc·Hz ^-1. Experimental measurements show that the EPLL introduces additional phase shift while purifying phase noise, with a phase inconsistency of 0.126 rad. The round-trip delay control of the time pulse eliminates the π rad (half of one full cycle) phase uncertainty during the remote transmission of the frequency signal. The phase uncertainty introduced by the additional phase shift of EPLL is reduced by 3 dB (half of 0.126 rad). Experiments results show that when the system experiences shutdown and restart and removing the 1-km fiber link, the mean values of the one-way phase difference have an inconsistency less than 0.068 rad, corresponding to ~1% of one full cycle. Therefore, a consistent phase difference is achieved based on low phase noise.