Measurement of the phase structure of elliptically polarized undulator radiation

This study theoretically and experimentally investigates the phase structure of radiation emitted from an elliptically polarized undulator. Analytic expressions for the emitted electromagnetic fields are fully derived and the radiation's phase structure is found to change according to polarization. When the polarization is circular, a helical structure is observed; however, when the polarization changes from circular to elliptical, a phase structure comprising several orbital angular momentum modes is observed. Herein, phase gradients of the undulator's radiation are measured using a double-slit interferometer. A sampling moiré method is used to accurately extract the phase difference on the transverse plane from the observed interference fringe. The measured phase gradients of the first and second harmonics reveal a similar change to the calculated results. However, under circular polarization, the change exhibited by the third harmonic is smaller than the calculated value. This phase gradient reduction is due to the split in phase singularities and is attributed to both the fluctuation in the undulator's peak magnetic fields and the radiation emitted from the entrance and exit of those magnetic fields. (paper)