Outdoor atmospheric optical two-way time transfer with serial time code

We demonstrated an optical two-way time transfer scheme in the outdoor free-space link using a simple complex programmable logic device-based serial time coder/decoder. With this scheme, we have transferred a 100 Hz signal with time information over a 120-m outdoor atmospheric link. The time drift, time deviation, and frequency instability are all measured to estimate the quality of the transferred time signal during the transfer process. Within 11 h, the experimental result shows that the total root-mean-square time drift is about 81 ps, with the time deviation of 70 ps at 1-s averaging time and down to 10 ps above 100-s averaging time. The calculation shows that the fractional frequency instability of the transmission link is on the order of 1.4 × 10^-10 at 1 s and of 3.0 × 10^-15 at 10 000 s. The time deviation and frequency instability for the optical two-way time transfer are superior to those of the Global Positioning System (GPS)-based time transfer method, which implies the technique proposed in this paper is able to be directly used in high-precision time transfer over atmospheric links in a short distance.