Free-space laser communications with adaptive optics: Atmospheric compensation experiments |
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Authors: | Thomas Weyrauch and Mikhail A Vorontsov |
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Institution: | (1) Intelligent Optics Laboratory, Institute for Systems Research, University of Maryland, A.V. Williams Bldg., Mail Stop 1103, College Park, Maryland 20742, USA;(2) Computational and Information Sciences Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, USA |
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Abstract: | Refractive index inhomogeneities of the turbulent air cause
wave-front distortions of optical waves propagating through the
atmosphere, leading to such effects as beam spreading, beam
wander, and intensity fluctuations (scintillations). These
distortions are responsible for severe signal fading in free-space
optical communications systems and therefore compromise link
reliability. Wave-front distortions can be mitigated, in
principle, with adaptive optics, i.e., real-time wave-front
control, reducing the likeliness of signal fading. However,
adaptive optics technology, currently primarily used in
astronomical imaging, needs to be adapted to the requirements of
free-space optical communication systems and their specific
challenges.In this chapter we discuss a non-conventional adaptive optics
approach that has certain advantages with respect to its
incorporation into free-space optical communication terminals. The
technique does not require wave-front measurements, which are
difficult under the strong scintillation conditions typical for
communication scenarios, but is based on the direct optimization
of a performance quality metric, e.g., the communication signal
strength, with a stochastic parallel gradient descent (SPGD)
algorithm.We describe an experimental adaptive optics system that consists
of a beam-steering and a higher-resolution wave-front correction
unit with a 132-actuator MEMS piston-type deformable mirror
controlled by a VLSI system implementing the SPGD algorithm. The
system optimizes the optical signal that could be coupled into a
single-mode fiber after propagating along a 2.3-km near-horizontal
atmospheric path. We investigate characteristics of the
performance metric under different atmospheric conditions and
evaluate the effect of the adaptive system. Experiments performed
under strong scintillation conditions with beam-steering only as
well as with higher-resolution wave-front control demonstrate the
mitigation of wave-front distortions and the reduction of signal
fading. |
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