During the last few years, synchrotron radiation centers around the world have
experienced a dramatic demand to provide radiation of variable polarization. The recent
interest in the polarized nature of synchrotron radiation is spurred by applications such
as magnetic circular dichroism (MCD) and circular intensity differential scattering
(CIDS), which probe the magnetic electronic properties and chiral structure of solids and
Considering the achievable flux and limited degree of circular polarization, Pcirc ,
intrinsic to crossed-field undulators, combined with the potential problems the operation
of this device might impose on other users of the electron storage ring, the Synchrotron
Radiation Center opted for a rather unconventional way of generating circular-polarized
light. The use of a multiple-reflection polarizer is proposed as a highly effective tool
by utilizing the phase difference between the s- and p-components of a multiple reflected
The Quadruple Reflection l/4 Phase Shifter is being developed by physicists Hartmut
Höchst, Peter Bulicke and Rajesh Patel of the Synchrotron Radiation Center, and
mechanical engineer Fred Middleton of the Physical Sciences Laboratory. The two-year-long
project is expected to be completed by the end of this year.
The quadruple reflector uses a linearly polarized beam of light from a bending magnet
or undulator beam line of the Aladdin storage ring. It converts the linearly polarized
light into elliptical or a circular polarization by a series of oblique angle reflections
from mirrors, which mix together and polarization components with a variable phase
Either the polarization alone, or a combination of polarization and intensity, can be
optimized by selecting particular reflection angles, which is entirely under computer
control. The concept of the quadruple reflector can be used over a wide energy range of 8
- 120 eV.
Two pairs of reflectors are used to keep the beam directed in the horizontal plane.
After entering the apparatus, light reflects from the first reflector to a parallel
second. Since the exiting light has the same angle of incidence, it leaves horizontally
and parallel to the incoming light and to the instrument. The light then impinges on a
second pair of parallel reflectors which are a mirror image of the first pair. Light
reflects from the third reflector to the fourth, which is on the axis of rotation of the
complete reflector housing assembly. Light then exits the apparatus.
Middleton, who designed the mechanical realization device, said that one requirement
was to maintain a fixed angle in the rotators while the housing is rotated. This is an
important factor since every reflector angle corresponds to a different polarization of
light. To keep the light on track, the two pairs of reflectors are kept parallel to within
less than a thousandth of a degree, and they counter-rotate equally to within less than a
thousandth of a degree.
Optical encoding and computer control are used to maintain the reflector positions. As
the housing drive is rotated, the reflector drive is simultaneously and equally
counter-rotated to prevent relative motion between the internal reflector drive and the
reflector housing. Relative motion would result in an angular change in the position of
The entire assembly is rotated to determine whether the light will be left or right
polarized For each energy point in a scan, the polarization can be flipped automatically
to measure a difference (dichroism) spectrum.