  The transfer of high-precision optical frequency signals over free-space
links, particularly between ground stations and satellites, will enable
advances in fields ranging from coherent optical communications and satellite
Doppler ranging to tests of General Relativity and fundamental physics. We
present results for the actively stabilized coherent phase transfer of a 193
THz continuous wave optical frequency signal over horizontal free-space links
150 m and 600 m in length. Over the 600 m link we achieved a fractional
frequency stability of 8.9e-18 at one second of integration time, improving to
1.3e-18 at an integration time of 64 s, suitable for transmission of optical
atomic clock signals. The achievable transfer distance is limited by
deep-fading of the transmitted signal due to atmospheric turbulence. We also
estimate the expected additional degradation in stability performance for
frequency transfer to Low Earth Orbit.
