ADAPT OAM: adaptive distance-aware passive transmission for orbital angular momentum–based free-space optical communication

Orbital angular momentum (OAM) mode free-space optical (FSO) communication provides a high-capacity solution for wireless urban networks in the future. Yet, its use in actual scenarios is assailed by four main challenges: (1) divergence and structural aberration of OAM beams over different link lengths, (2) distance-dependent beam spread-receiver aperture mismatch, (3) absence of power-efficient adaptive hardware for conditioning beams over link lengths, and (4) decoding failure due to atmospheric turbulence-induced mode crosstalk and ring deformation. To address these challenges, we present ADAPT OAM, A Passive Optical Architecture for Distance-Adaptive OAM-FSO Communication. ADAPT-OAM introduces a distance-aware passive optical pre-conditioning framework that reshapes OAM beams prior to propagation, eliminating the need for adaptive optics or machine learning–based correction. In the transmitter, a new rotatable Holographic Phase Plate (HPP) with several lens regions is employed to pre-curve OAM beams for transmission distances of 200 m to 1700 m. These lens regions regulate beam divergence and control the radial intensity distribution of OAM modes during propagation and a special Achromatic Doublet Lens provides accurate collimation with very low chromatic and spherical aberration. The Graded Index (GRIN) lens at the receiver carries out spatial refocusing of arriving beams that have possible residual deformation during atmospheric propagation, thus realigning the beam for correct mode separation. It is preceded by a fork diffraction grating, which space-resolves OAM modes, and a photodetector array that passively demultiplexes them into digital symbols without moving parts, feedback control, or machine learning. Simulation-based evaluation demonstrates that ADAPT OAM attains a Bit Error Rate (BER) less than 10⁻⁶, Mode Purity Index (MPI) greater than 94% even under turbulence, and high Received Power Efficiency (RPE) over tested distances. It has a low Scintillation Index (SI) and continually reconstructs beams with a Normalized Root Mean Square Error (NRMSE) of less than 0.08. In comparison with seven recent state-of-the-art OAM-based FSO systems, ADAPT OAM performs better on all major criteria, providing a reconfigurable, energy-efficient, and scalable FSO platform designed for smart urban connectivity.