Real-time monitoring of beam wander and angle-of-arrival fluctuation under atmospheric turbulence for efficient laser coupling to single-mode optical fibers

Abstract

Atmospheric effects degrade the performance of free-space optical links, which has prompted extensive research over several decades. Most studies have focused on scintillation, which describes the fluctuation in received optical power at the receiver. Meanwhile, the beam wander (BW) and angle-of-arrival (AoA) fluctuations are two factors that directly impact the position and angular movement of the optical beam in free-space. These effects are especially crucial for single-mode fiber-coupled free-space optical (FSO) communication systems to achieve high bandwidth and large-capacity data transmission via well-established fiber-optic communication components. However, despite their significance, detailed studies on BW and AoA fluctuations have not been addressed. In this study, we present a real-time monitoring system capable of simultaneously measuring the BW and AoA fluctuations with high precision using a single commercial camera; the system achieves a positional resolution within 80 mu m (RMSE: 30.2 mu m) and an angular resolution of 80 mu rad (RMSE: 20.6 mu rad). The atmospheric effects were evaluated using a collimated beam at 635 nm wavelength and 2.5 mW output power over a round-trip link of 130 m. The experimental results demonstrate that BW and AoA fluctuations exhibit dispersive probabilistic distributions depending on the strength of optical turbulence expressed with Cn2. Notably, the correlation of the beam centroid and AoA in both x-and y-directions moved with correlation coefficients between-0.72 and + 0.82. This work clarifies the necessity of distinguishing the two atmospheric effects (BW and AoA) and our findings offer valuable insights for optimizing the single-mode fiber-coupled FSO communication systems and high-precision laser-directed systems.