Free-space laser communications with adaptive optics: Atmospheric compensation experiments

Refractive index inhomogeneities of the turbulent air causewave-front distortions of optical waves propagating through theatmosphere, leading to such effects as beam spreading, beamwander, and intensity fluctuations (scintillations). Thesedistortions are responsible for severe signal fading in free-spaceoptical communications systems and therefore compromise linkreliability. Wave-front distortions can be mitigated, inprinciple, with adaptive optics, i.e., real-time wave-frontcontrol, reducing the likeliness of signal fading. However,adaptive optics technology, currently primarily used inastronomical imaging, needs to be adapted to the requirements offree-space optical communication systems and their specificchallenges.In this chapter we discuss a non-conventional adaptive opticsapproach that has certain advantages with respect to itsincorporation into free-space optical communication terminals. Thetechnique does not require wave-front measurements, which aredifficult under the strong scintillation conditions typical forcommunication scenarios, but is based on the direct optimizationof a performance quality metric, e.g., the communication signalstrength, with a stochastic parallel gradient descent (SPGD)algorithm.We describe an experimental adaptive optics system that consistsof a beam-steering and a higher-resolution wave-front correctionunit with a 132-actuator MEMS piston-type deformable mirrorcontrolled by a VLSI system implementing the SPGD algorithm. Thesystem optimizes the optical signal that could be coupled into asingle-mode fiber after propagating along a 2.3-km near-horizontalatmospheric path. We investigate characteristics of theperformance metric under different atmospheric conditions andevaluate the effect of the adaptive system. Experiments performedunder strong scintillation conditions with beam-steering only aswell as with higher-resolution wave-front control demonstrate themitigation of wave-front distortions and the reduction of signalfading.