Measurement of the orbital angular momentum spectrum of partially coherent beams

In this work we implement a Mach-Zehnder interferometer with an image rotator in one of its arms to measure the orbital angular momentum (OAM) spectrum of a partially coherent beam. By measuring the visibility of the interference as a function of the angle of rotation, the OAM distribution can be recovered via a Fourier transform. Theoretical calculations based on the coherent mode decomposition of the cross-spectral density are in excellent agreement with the experimental data.