December 2017
Spotlight Summary by Fritz Riehle
Low-loss reciprocal optical terminals for two-way time-frequency transfer
Linking optical clocks through the turbulent air without losing a single cycle of the optical frequency allows for a variety of novel and exciting applications in time metrology, astronomy, geodesy, or defense. The most accurate method for such a link between clocks at remote stations is the Two-Way Time and Frequency Transfer (TWTFT). The two stations simultaneously send pairwise radio frequency signal pulses locked to their clocks via a satellite and determine the time difference between the start and receiving time of the pulse at each location. The difference between these two measurements equals the time difference between the clocks if the signal path is exactly reciprocal. High effort is required to assure reciprocal paths in turbulent air for optical pulses with their much higher frequencies.
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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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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Article Information
Low-loss reciprocal optical terminals for two-way time-frequency transfer
W. C. Swann, L. C. Sinclair, I. Khader, H. Bergeron, J.-D. DeschĂȘnes, and N. R. Newbury
Appl. Opt. 56(34) 9406-9413 (2017) View: Abstract | HTML | PDF