Multiple-beam transmission has been proposed to reduce the deleterious effects of atmospheric turbulence on free-space laser communications - especially on uplinks from ground stations to Earth-orbiting satellites. The deleterious effects in question are signal fades and surges associated with scintillation, beam broadening, and beam wander. These effects are caused by amplitude and phase fluctuations that, in turn, are caused by turbulence-induced spatial and temporal fluctuations in the index of refraction of air.
The basic principle of the present multiple-beam-transmission concept is one of incoherent superposition. The beam from the transmitting laser would be split into two or more beams, the optical path lengths of which would differ from each other by more than the laser coherence length. The beams would be transmitted to the receiver from points separated laterally by more than the optical coherence length of the turbulence cells in the atmosphere (for a wavelength of 0.5 µm, this length is about 10 cm). The overlap of the beams in the far field would therefore be characterized by temporal and spatial incoherence among the beams, with consequent stochastic smoothing that would reduce the magnitudes of fades and surges in the signal arriving at the receiver.
The concept was tested in experiments on optical communication between a station at the Table Mountain Facility near NASA's Jet Propulsion Laboratory and a laser-communication apparatus aboard the Japanese ETS-VI satellite. In these experiments, the laser beam from the transmitter on the ground was split, variously, into two or four beams that were directed up to the satellite. Once a signal-tracking loop in the satellite was activated, the laser transmitter aboard the satellite transmitted a downlink signal to the ground station. In spite of some difficulty in deconvolving atmospheric effects from pointing errors and spacecraft vibration, analysis of data from the experiments revealed significant improvements in uplink reception with multiple uplink beams. Downlink signal fluctuations were attributed to pointing jitter, suggesting the need for highly accurate pointing to establish a stable communication link.
This work was done by James Lesh and Keith Wilson of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-20384