Sub-nanosecond optical underwater two-way time transfer using green semiconductor laser

We demonstrate a high-precision optical underwater two-way time transfer based on a green semiconductor laser. With this transfer technique, we constructed an underwater two-way time transfer system and conducted a time transfer experiment on a 6-meter underwater link. Within 20000 seconds, the experimental results show that the total root mean square (RMS) of timing drift was approximately 345 ps. The calculated time stability showed a time deviation of 257 ps at an average time of 1 s, and of 48 ps at an average time of 1000 s. This technique can achieve sub-nanosecond-level underwater time transfer, which means that the approach proposed in this paper can provide important support for underwater sub-nanosecond synchronization systems.