We demonstrate an optical detection and decoding strategy to increase the information rate and spectral efficiency of free-space laser communication links affected by turbulence by means of dense orbital angular momentum (OAM) modulation. Using three candidate receiver architectures–based on a Shack-Hartmann sensor, a Mode Sorter, and a complex conjugate projection scheme as a base case–we demonstrate an algorithmic classification system based on the received OAM spectra produced by these architectures. This classification scheme allows low-error-rate data transmission in turbulence using 16-OAM, 32-OAM, and 64-OAM symbol constellations, with OAM states between −20 and 20. We evaluate and compare their performance under weak to strong atmospheric turbulence conditions using an accuracy metric and confusion matrices.
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