  We implement an interferometric method using two angular slits to measure the
orbital angular momentum (OAM) mode spectrum of a partially coherent field. As
the angular separation of the slits changes, an interference pattern for a
particular OAM mode is obtained. The visibility of this interference pattern as
a function of angular separation is equivalent to the angular correlation
function of the field. By Fourier transforming the angular correlation function
obtained from the double angular slit interference, we are able to calculate
the OAM spectrum of the partially coherent field. This method has potential
application for characterizing the OAM spectrum in high-dimensional quantum
information protocols.
