Characterization of orbital angular momentum modes in FEL radiation

It is well known that Gaussian modes of paraxial light can possess orbital angular momentum (OAM) as a result of a exp(-ilphi) azimuthal angular dependence[1]. Recent experiments on the VISA free-electron laser operating as self-amplified spontaneous amplifier (SASE) at lambda=840 nm have produced transverse electromagnetic radiation intensity patterns that suggest the presence of one or more OAM modes. Though the intensity profiles from VISA are compelling, work remains to discern the phase structure since OAM modes are revealed by a characteristic helical phase dependence, with a pathological singularity on axis. Here we discuss current efforts to characterize the mode content of the VISA FEL through experiments, theoretical models, and simulations. Possible methods to generate OAM modes in situ are discussed.