A method of correcting for the distorting effects of the atmosphere upon a laser beam used in free-space optical communication has been proposed. The original version of the method would be applied to a system in which a laser beam would be transmitted from a ground station to a precisely flat and precisely oriented mirror on a spacecraft in orbit around the Earth, then relayed to a distant spacecraft by reflection from the mirror (see figure). Because propagation of the beam in outer space would be distortionless, the method addresses only the distortions that would arise along the ground-station/near-Earth-spacecraft path.
In the original application, in order to send a sufficiently strong signal to the distant spacecraft, it would be necessary to deliver an approximately diffraction-limited, undistorted laser beam to the orbiting mirror for relay to the distant spacecraft. Therefore, at the ground station, adaptive optics would be used to apply compensatory predistortions to cancel the optical distortions (tilt and scintillation) introduced by the atmosphere. The control signals for the adaptive optics would be generated as explained next.
A small laser aboard the near-Earth spacecraft would transmit a reference beam to the ground station. Because it would propagate along the same path as that of the main communication beam, the reference beam would undergo essentially the same distortions. Thus, measurement of the tilt and scintillation of the reference beam arriving at the ground station would provide the information needed to control the adaptive optics to apply the compensatory predistortions.
To compensate for the predictable aberration caused by orbital motion of the relay mirror, the reference laser would have to be offset from the relay mirror by an angle (as seen from the ground station) of 2v/c radians in the plane of relative motion, where v is the component of orbital velocity orthogonal to the optical path and c is the speed of light. For a typical orbit, the offset distance for this angle would be no more than about 8 m.
The reference beam arriving at the ground station must be bright enough that measurements and corrections can be completed within an interval of about 1 millisecond, which is dictated by the temporal variation of atmospheric optical distortion. This requirement turns out not to be especially severe: for example, it has been estimated that even during the day, the beam from a reference laser with a wavelength of 0.5 µm, a power of 1 mW, and optics with an aperture diameter of about 10 cm, located in a geosynchronous orbit, would be sufficiently bright for measurement through a ground telescope with an aperture diameter of about 1 m and a band-pass optical filter.
This work was done by John Armstrong, Cavour Yeh, and Keith Wilson of Caltech for NASA's Jet Propulsion Laboratory.