Effects of turbulence on a combined 1535 nm retro reflective and a low intensity single path 850 nm optical communication link

摘要

Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the 1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical models.