Multi-dimensional QAM equivalent constellation using coherently coupled orbital angular momentum (OAM) modes in optical communication.

In this work, a 1550 nm fiber-to-free-space optical communication link is successfully demonstrated employing the superposition of two coherently coupled orbital angular momentum (CCOAM) states. Information is encoded onto both the amplitude and phase of the CCOAM beams and is mapped to a three-dimensional (3D) constellation space using quadrature amplitude modulation (QAM) equivalent architecture. The 3D QAM constellation is based on a higher-order Poincare sphere equivalent for OAM states, and multiple spherical constellations are demonstrated for 64- and 128-QAM, providing a 6X and 7X increase in spectral efficiency by fully exploiting the available 3D space. The experimental results are presented showing a bit error rate (BER) below the forward error correction (FEC) limit. Multiple experimental parameters which could contribute to constellation distortions are also discussed.