Beam wander effects on the scintillation index of a focused beam

Abstract

First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) scintillation component of an untracked focused beam projected along a horizontal path will become significantly smaller as the size of the transmitter aperture increases. At the same time, the radial component near the diffractive edge of the beam is predicted to increase without bound. The results of recent computer simulations are at odds with this behavior, and we believe that this discrepancy is due to the fact that Rytov theory does not correctly account for the effects of beam wander. We present a theoretical structure that accurately describes far-field irradiance fluctuations caused by uncorrected tilt jitter. This new theory is validated by demonstrating excellent agreement between the predicted scintillation index and computer code results for both tracked and untracked beams. For many applications of practical interest, such as free-space optical communications, a good understanding of the time-average Strehl ratio is also essential; simulation results for this parameter are presented and shown to be in good agreement with theory.