400-Gbit/s QPSK free-space optical communication link based on four-fold multiplexing of Hermite–Gaussian or Laguerre–Gaussian modes by varying both modal indices

We experimentally demonstrate the four-fold multiplexing of Hermite-Gaussian (HG_mn) or Laguerre-Gaussian (LG_lp) modes to achieve a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical communication link. In this experiment, both modal indices for the HG and LG modes are simultaneously utilized to achieve a larger potential orthogonal modal space. Moreover, we also investigate and compare the effects of aperture size, lateral displacement, and rotation on the system. We measure the degree of performance degradation due to the following: (1) a receiver aperture with a limited size causes power loss and crosstalk for both HG and LG modes; (2) a lateral misalignment between the transmitter and receiver which leads to crosstalk for LG modes, but might not affect HG_0n or HG_m0 modes due to axial symmetry; and (3) a rotational misalignment between the transmitter and receiver which causes crosstalk for the HG modes, but does not tend to affect the LG modes due to azimuthal symmetry.