Mitigation of Turbulence-Induced Losses over Terrestrial Laser Links for Quantum and Optical Communications

Simulations and preliminary experimental results for the optimization of a laser link over a long-range (9 km) slant-horizontal terrestrial propagation path between the JPL Mesa Test Range and the roof of Caltech Hall at a distance of 9 km, is described and evaluated. For the simulation, a collimated Gaussian beam was transmitted by a telescope located on the roof of Caltech Hall, propagated through the slant-horizontal line-of-sight (LOS) path to the JPL Mesa Test range where it was received by another telescope, and focused to a point-spread function (PSF) in the imaging plane where it is coupled into a high-performance SMF-28 single-mode optical fiber for detection and further processing. Turbulence along the path was simulated using the Split-Step Beam Propagation Method (SSBPM), which approximates continuous turbulence along the line-of-sight with a large number of discrete phase-screens, with realistic spatial and temporal correlations included in the model. The coupling coefficient was evaluated via analysis and simulation with and without tip-tilt compensation, and the improvement in the coupling coefficient quantified. The simulation model will be used to assess the feasibility of future urban-scale quantum communication links, that will require optimal link efficiency. This model is also applicable to ground-to-space laser links, where the same techniques can be employed to mitigate deep fades at the spacecraft or at the ground receiver due to atmospheric turbulence, thus leading to significant improvements in overall system performance.