  We propose and experimentally demonstrate novel types of composite sequences
of half-wave and quarter-wave polarization retarders, either permitting
operation at ultra-broad spectral bandwidth or narrow bandwidth. The retarders
are composed of stacked standard half-wave retarders and quarter-wave retarders
of equal thickness. To our knowledge, these home-built devices outperform all
commercially available compound retarders, made of several birefringent
materials.
