  Free-space optical (FSO) communications has the potential to revolutionize
wireless communications due to its advantages of inherent security,
high-directionality, high available bandwidth and small physical footprint. The
effects of atmospheric turbulence currently limit the performance of FSO
communications. In this letter, we demonstrate a system capable of
indiscriminately suppressing the atmospheric phase noise encountered by
independent optical signals spread over a range of 7.2 THz (encompassing the
full optical C-Band), by actively phase stabilizing a primary optical signal at
193.1 THz (1552 nm). We show ~30 dB of indiscriminate phase stabilization over
the full range, down to average phase noise at 10 Hz of -39.6 dBc/Hz when using
an acousto-optic modulator (AOM) as a Doppler actuator, and -39.9 dBc/Hz when
using a fiber-stretcher as group-delay actuator to provide the
phase-stabilization system's feedback. We demonstrate that this suppression is
limited by the noise of the independent optical signals, and that the expected
achievable suppression is more than 40 dB greater, reaching around -90 dB/Hz at
10 Hz. We conclude that 40 Tbps ground-to-space FSO transmission would be made
possible with the combination of our stabilization system and other
demonstrated technologies.
