Two-way time transfer method using optical communication links based on single photon counting

We are presenting a new method for optical Two-Way Time Transfer based on the Single Photon Counting approach. The method can be used in parallel with standard fiber or free-air media bi-directional optical communication link without causing any interference using any type of optical transmitters including standard Small Form-Factor Pluggable laser modules. Additionally, as only an average signal of -100 dBm or less is required, it can also be performed on very long distances of over 500 km within optical fibers without any amplification, but in the latter case limited to cases when the data communication is not necessary due to high attenuation. The method was tested either with simulated and real data sets measured in our laboratory and it has proven its reliability and precision in the order of 1 ps or less. Thanks to the photon counting approach the systematic errors are minimized to ps level. The aim of our further research is to test the method on real communication data links, either in optical fiber or free-air media.