Modal Diversity for Robust Free-Space Optical Communications

Free-space communication links are severely affected by atmospheric turbulence, which causes degradation in the transmitted signal. One of the most common solutions to overcome this is to exploit diversity. In this approach, information is sent in parallel over different paths using two or more transmitters that are spatially separated, with each beam therefore experiencing different atmospheric turbulence, lowering the probability of a receive error. In this work we propose and experimentally demonstrate a alternative method of diversity based on spatial modes of light, which we call modal diversity. We remove the need for a physical separation of the transmitters by exploiting the fact that spatial modes of light experience different perturbations, even when traveling along the same path. For this proof-of-principle, we select identical radius modes from the Hermite-Gaussian and Laguerre-Gaussian basis sets and demonstrate an improvement in bit error rate by up to 54% without increasing the total transmit power or receive aperture radius.