Frequency dissemination with free-space optical links

Optical clock technology is very certain to be integrated in the next high precision time standards. The demand for higher accuracy in time synchronization makes the use of free-space optical links necessary for frequency dissemination, especially if mobile partners are involved. These links are the most suitable solution in order to disseminate high frequencies in between satellites, ground stations, aircrafts, and any other mobile vehicles. However, setting up an optical link requires good knowledge of the atmospheric channel as well as sophisticated engineering technology. The available facilities at DLR Institute of Communications and Navigation, Optical Communications Group, offer the opportunity of performing frequency dissemination measurements by using its aeronautic terminals and optical ground station. These facilities were already successfully used for measurements of the atmospheric channel, investigation of modulation formats and data communication applying optical downlinks from satellite, aircraft, and high-altitude platforms. For free-space optical links, atmospheric turbulence is one of the main impairments to deal with since it causes degradation of spatial and temporal coherence of the propagating wave. The application of adaptive optics is proposed here to correct for these impairments and thus to achieve stable frequency transfer. In this paper, based on the former and current field trials of free-space optical communication links, we present possible link scenarios in which the channel behavior for frequency transfer can be investigated and name the main challenges to overcome.