  We present the first experimental characterization of the azimuthal Wigner
distribution of a photon. Our protocol fully characterizes the transverse
structure of a photon in conjugate bases of orbital angular momentum (OAM) and
azimuthal angle (ANG). We provide a test of our protocol by characterizing pure
superpositions and incoherent mixtures of OAM modes in a seven-dimensional
space. The time required for performing measurements in our scheme scales only
linearly with the dimension size of the state under investigation. This time
scaling makes our technique suitable for quantum information applications
involving a large number of OAM states.
