To verify the performance of undulators and beam lines with respect to polarization measurements using the BESSY Soft X-Ray Polarimeter are performed. In this versatile multi-axis diffractometer the first optical component is a free standing transmission multilayer acting as polarizer/phase retarder. The second optical component is a reflecting multilayer acting as analyzer for linear polarization. The detector is a polarization insensitive photo diode. The angles of incidence of both multilayers (θP and θA) are set to be near the Bragg condition. The analyzer layer thickness is near the Brewster condition. Atomic resonances in either the spacer or the absorber material are exploited to enhance the performance, e.g. a Cr/Sc multilayer is used near the Sc 2p edge at 398 eV or near the Cr 2p edge at 574 eV depending on its layer spacing.
Rotating independently (angle α and β) the two optical components around the optical axis a self calibrating polarization measurement is performed (F.Schäfers et al., Appl.Optics 38, 4074 (1999)). As an example the determination of the normalized Stokes parameters at the UE52 - SGM beam line as functions of the longitudinal position of the four magnetic rows (shift) of the undulator are shown. Circularly polarized radiation is detected for parallel shift and linearly polarized radiation at varying azimuthal angle is detected for anti-parallel shift in perfect agreement with magnetic field calculations and beam line settings. Note that the soft X-ray radiation is completely polarized.
The polarimeter is also used for a variety of investigations including magneto-optics and multilayer characterizations.