Progress on high-precision laser-based underwater frequency transfer

Abundant resources in the ocean are not explored yet. The meaning of ocean exploration is far-reaching for the development of human society. In the future, the construction of underwater observation networks with outstanding performance is the precondition of a variety of scientific experiments, and time-frequency networks like GPS are helpful for the collaborative working of underwater platforms. The success of underwater wireless optical communication expands the dissemination of time-frequency signals over free-space and fiber links to underwater links. Compared with the conventional underwater frequency transfer methods, i.e. sonar, fiber links, and microwave method, the laser-based underwater frequency transfer owns strong competitiveness with the features of high flexibility, high bandwidth, and low latency. Such advantages make the laser-based underwater frequency transfer a promising approach for the construction of future underwater time-frequency networks. The paper introduces the progress on laser-based underwater frequency transfer at the University of Electronic Science and Technology of China, including the property analysis of underwater links, three specific techniques, and the future works that would be conducted. The property analysis of underwater links was conducted from the timing fluctuation attributed to refractive-index perturbation and underwater turbulence introduced power spectral density (PSD) that derived from the Kolmogorov model. The simulation results of Kolmogorov PSD and its modified PSD (von Kármán model) are given and analyzed. Based on the analysis, the experimental demonstrations of frequency transfer over 3 m, 6 m, and 9 m underwater links were conducted. The experimental results show that timing fluctuation is most partly attributed to underwater turbulence. It is necessary for highly stable frequency transfer to suppress timing fluctuation. An electronic phase compensation technique was employed for timing fluctuation suppression. A 100 MHz radio-frequency (RF) signal has been transferred over a 5 m underwater link using this technique. With the help of this technique, the root-mean-square (RMS) timing fluctuation was successfully suppressed from 9.6 ps to 2.1 ps within 5000 s. The noise, limited compensation bandwidth, and residual timing fluctuations of the electronic phase shifters block further improvement of system performance. Consequently, the optical delay line-based optical phase compensation technique was proposed and experimentally demonstrated. A 500 MHz RF signal was transferred over a 5 m underwater link for 5000 s, the RMS timing fluctuation was successfully suppressed from 7.3 ps to 0.162 ps. The experimental results show that the proposed technique could effectively suppress timing fluctuation and lower the system noise floor. Both phase compensation techniques are hard to support multiple-access frequency transfer because compensation configuration is included in the transmitter. A novel scheme for multiple-access frequency transfer was proposed and experimentally demonstrated. A 100 MHz RF signal was transferred over a 3 m underwater link for 5000 s, the RMS timing fluctuation was successfully suppressed from 73.4 ps to 3 ps. With this scheme, the phase of the frequency signal at multiple receivers could be simultaneously locked to the phase of the reference signal at the transmitter. In the future, the optical link of the multiple-access frequency transfer experimental setup would be optimized for further performance improvement. Laser-based underwater frequency transfer experiments with picosecond-level timing fluctuations over hundred meters links would be demonstrated with the help of the single-photon detection technique and a picosecond mode-locked laser at 1064 nm wavelength.