Optimized multiemitter beams for free-space optical communications through turbulent atmosphere

TLDR

Less‑than‑perfect spatial coherence in laser beams reduces scintillation in free‑space optical links. The study experimentally demonstrates the scintillation‑reduction benefit of a specific partially coherent beam. The experiment uses a scaled link where a multi‑emitter beam of mutually incoherent Gaussian spots propagates through a Kolmogorov turbulence phase screen. Experiments reveal an optimal beam separation that minimizes scintillation, with a substantial reduction factor that increases with beam number, and results agree with Rytov theory for weak‑to‑intermediate turbulence.

Abstract

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.