Laser beam fluctuations for long distance propagation in the turbulent atmosphere

TL;DR

This paper investigates laser beam fluctuations during long-distance atmospheric propagation, analyzing beam wandering, partial coherence effects, and the impact of phase diffusers on noise reduction in communication channels.

Contribution

It introduces a photon distribution function approach to model saturated fluctuations and explores how phase diffusers can reduce noise in turbulent atmospheric conditions.

Findings

Size of classical wandering decreases with distance

Quantum wandering increases with distance

Phase diffusers can significantly reduce irradiance noise

Abstract

The method of photon distribution function (PDF) is used to study fluctuations of light beams propagating through a turbulent atmosphere. Our analysis concerns the regime of saturated fluctuations. The focus is on the phenomena of beam wandering and the effect of partial coherence on photon density fluctuations. It is shown that the size of the quasiclassical part of wandering decreases with the propagation distance, while the quantum part increases. We explain this qualitative difference by beam fragmentation, which is accompanied by a loss of correlation between individual parts. The effect of the phase diffuser on the fourth moment (FM) of the irradiance is taken into account. The obtained explicit expression for the FM indicates the possibility of a significant reduction of noise in the communication channel. The diffuser changes the shot noise from delta-correlated (in the spatial domain) to smoothly distributed. The theory developed here is used to estimate the influence of the phase diffuser on light fluctuations.