Benchmarking Laguerre-Gaussian mode decoding in atmospheric turbulence: phase-only versus ideal complex-field approaches

Abstract

Orbital Angular Momentum (OAM)- based Free Space Optical (FSO) communication offers a potential solution for achieving high spectral efficiency. OAM multiplexing and demultiplexing are key components of these systems. The core of OAM demultiplexing is OAM mode decoding, especially in the presence of atmospheric turbulence. However, OAM beams are distorted by atmospheric turbulence, severely impairing decoding efficiency and limiting the usefulness of the technology. Existing phase-only decoding approaches, such as spiral phase plate (SPP) decoders, have limited capacity to recover OAM modes in the presence of significant turbulence. In this work, we use a non-unitary ideal complex-field-based method to propose a theoretical decoding benchmark and compare it with a realistic phase-only approach. The OAM spectrum reconstruction and modal purity are analysed in weak-to-strong turbulence regimes. The study quantifies the performance gap between practical phase-only decoding and idealised upper-bound complex-field reconstruction under atmospheric turbulence. Findings show that the SPP-based phase-only decoding exhibits a gradual loss of modal purity of the target mode as turbulence severity increases, whereas the theoretical approach performs better and sets an upper-bound performance benchmark.