  We theoretically propose a type of tunable polarization retarder, which is
composed of sequences of half-wave and quarter-wave polarization retarders,
allowing operation at broad spectral bandwidth. The constituent retarders are
composed of stacked standard half-wave retarders and quarter-wave retarders
rotated at designated angles relative to their fast-polarization axes. The
proposed composite retarder can be tuned to an arbitrary value of the
retardance by varying the middle retarder alone, while maintaining its
broadband spectral bandwidth intact.
