  Using harmonic emission from circularly polarized undulator is a procedure
that is normally employed on synchrotron beamlines in order to extend the
covered spectral range. A similar capability is likewise beneficial for next
generation free-electron lasers. In this paper, we perform a first quantitative
experimental analysis of the angular distribution of free-electron laser
harmonic emission from helical undulators. Experimental results are compared to
those obtained by means of a theoretical model based on the paraxial solution
of Maxwell's equations.
