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abstract:
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It is well known that Gaussian modes of paraxial light
can possess orbital angular momentum (OAM) as a result
of a exp( −ilφ) azimuthal angular dependence[1]. Recent
experiments on the VISA free-electron laser operating as
self-amplified spontaneous amplifier (SASE) at λ=840 nm
have produced transverse electromagnetic radiation inten-
sity patterns that suggest the presence of one or more OAM
modes. Though the intensity profiles from VISA are com-
pelling, 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 mod-
els, and simulations. Possible methods to generate OAM
modes in situ are discussed.
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