  Structured light offers wider bandwidths and higher security for
communication. However, propagation through complex random media, such as the
Earth's atmosphere, typically induces intermodal crosstalk. We show numerically
and experimentally that coupling of photonic orbital angular momentum (OAM)
modes is governed by a universal function of a single parameter -- the ratio
between the random medium's and the beam's transverse correlation lengths, even
in the regime of pronounced intensity fluctuations.
