W. C. Swann and his colleagues have set up two terminals for optical TWTFT and demonstrated how they achieved a reciprocal path for both beam directions of the transmitted short-infrared pulses over distances of up to 12 km. Each terminal comprises beam-expanding optics and a short-pulsed laser that is used to generate the outgoing pulses and to detect the incoming pulses by heterodyne detection. A beacon laser beam, superimposed on the outgoing pulsed beam in the first terminal, is used to detect the influence of atmospheric turbulences via the beam deflection observed by a quadrant detector in the second terminal. The corresponding error signal in the second terminal is used to stabilize the two transverse directions of the overlayed pulses and the second beacon beam emitted by this terminal. A corresponding scheme stabilizes the counterpropagating beams. The authors show that with proper apertures in such moderately priced terminals with low insertion loss, the two counterpropagating pulses and their beacon beams fill nearly the same spatial mode volume, thereby ensuring an optimal beam reciprocal path.
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