  The electrodynamic response of the spin-ladder compound
Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ($x=0, 3, 9$) has been studied from
radiofrequencies up to the infrared. At temperatures below 250 K a pronounced
absorption peak appears around 12 cm$^{-1}$ in Sr$_{14}$Cu$_{24}$O$_{41}$ for
the radiation polarized along the chains/ladders (${\bf E}\parallel {\bf c}$).
In addition a strongly temperature dependent dielectric relaxation is observed
in the kHz - MHz range. We explain this behavior by a charge density wave which
develops in the ladders sub-system and produces a mode pinned at 12 cm$^{-1}$.
With increasing Ca doping the mode shifts up in frequency and eventually
disappears for $x=9$ because the dimensionality of the system crosses over from
one to two dimensions, giving way to the superconducting ground state under
pressure.
