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

Abstract: 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.

摘要：本文描述并评估了针对喷气推进实验室（JPL）Mesa测试场与加州理工学院大楼屋顶之间9公里长倾斜-水平地面传播路径上激光链路优化的仿真及初步实验结果。在仿真中，准直高斯光束由加州理工学院大楼屋顶的望远镜发射，经倾斜-水平视距（LOS）路径传播至JPL Mesa测试场，由另一台望远镜接收，并在成像平面聚焦为点扩散函数（PSF），随后耦合至高性能SMF-28单模光纤以进行探测和后续处理。路径中的湍流通过分步光束传播法（SSBPM）进行仿真，该方法利用大量离散相位屏近似视距上的连续湍流，并在模型中纳入了真实的空间和时间相关性。通过分析与仿真评估了有无倾斜补偿时的耦合系数，并量化了耦合系数的改善效果。该仿真模型将用于评估未来城市级量子通信链路的可行性——此类链路需具备最优链路效率；同时，该模型也适用于地空激光链路，其中相同技术可用于缓解大气湍流导致的航天器或地面接收器处的深度衰落，从而显著提升整体系统性能。