Inverse design of a spatial demultiplexer for free-space optical communications: direct optimization over turbulence statistics

TL;DR

This paper introduces a novel inverse design method for a spatial demultiplexer in free-space optical communications, optimizing directly over turbulence statistics to improve coupling efficiency under atmospheric turbulence.

Contribution

It presents a direct optimization approach for designing a compact optical system that enhances turbulence resilience without relying on specific modal bases.

Findings

Optimized system outperforms direct fiber coupling.

Approaches ideal modal projection performance.

Maintains robustness across various turbulence conditions.

Abstract

Atmospheric turbulence severely limits the coupling of received optical wavefronts into single-mode fibers in satellite-to-ground free-space optical links. Spatial demultiplexing receivers address this challenge by distributing the incoming field across a bundle of single-mode fibers whose outputs are recombined coherently, relaxing the requirements on wavefront correction. In this work, we investigate the design of such receivers from two complementary angles. We first compare the power coupling statistics achieved by several modal bases and show that the spatial support of the modes matters far more than the specific choice of basis, questioning the relevance of mode-selective approaches for this application. We then present the inverse design of a compact two-plane refractive system optimized directly over an ensemble of turbulence realizations using stochastic gradient descent, with no constraint imposed on the input modal decomposition. The optimized system significantly improves over direct coupling into the fiber bundle, approaches the performance of an ideal modal projection, and remains competitive across a broad range of turbulence conditions.