  We experimentally demonstrate a broadband and an ultra-broadband spectral
bandwidth polarization rotators. Both polarization rotators have modular
design, that is, they are comprised of an array of half-wave plates rotated to
a given angle. We show that the broadband and ultra-broadband performance of
the polarization rotators is due to the adiabatic nature of the light
polarization evolution. In this paper we experimentally investigate the
performance of broadband and ultra-broadband polarization rotators comprising
of ten multi-order half-wave plates or ten commercial achromatic half-wave
plates, respectively. The half-wave plates in the arrays are rotated gradually
with respect to each other starting from an initial alignment between the fast
polarization axis of the first one and the incoming linearly polarized light,
to the desired polarization rotation angle.
