Effect of point-ahead angle on scintillation in laser satellite communication systems equipped with adaptive optics

Abstract

Some ground-based optical terminals used in optical uplinks employ adaptive optics (AO) to pre-correct the outgoing, data-carrying laser beams, which helps reduce scintillation and other, turbulence-induced signal im-pairments. The beacon photons can come from the satellite itself or from a natural guide star. After AO pre-correction, the data-carrying beam is sent at a point-ahead angle (PAA) to hit the moving satellite. How-ever, as this angle approaches or surpasses the isoplanatic angle, the effectiveness of AO in reducing scintillation decreases, becoming entirely ineffective when the PAA is large. This study investigates the effect of varying PAA on the scintillation index and examines the effectiveness of modal correction using Zernike modes. The AO-corrected scintillation index is calculated using the Rytov method combined with Zernike filter functions. Based on this approach, we show how the PAA influences the scintillation index after AO pre-compensation. The results show that the scintillation index increases with increasing PAA and high-altitude C²n scaling factor, whereas it decreases with increasing numbers of AO correction modes and wavelength. Moreover, the effect of PAA on the scintillation index is influenced by the laser beam radius at the transmitter.