  We present a new cavity-based polarimetric scheme for highly sensitive and
time-resolved measurements of birefringence and dichroism, linear and circular,
that employs rapidly-pulsed single-frequency CW laser sources and extends
current cavity-based spectropolarimetric techniques. We demonstrate how the use
of a CW laser source allows for gains in spectral resolution, signal intensity
and data acquisition rate compared to traditional pulsed-based cavity ring-down
polarimetry (CRDP). We discuss a particular CW-CRDP modality that is different
from intensity-based cavity-enhanced polarimetric schemes as it relies on the
determination of the polarization-rotation frequency during a ring-down event
generated by large intracavity polarization anisotropies. We present the
principles of CW-CRDP and validate the applicability of this technique for
measurement of the non-resonant Faraday effect in solid SiO$_2$ and CeF$_3$ and
gaseous butane. We give a general analysis of the fundamental sensitivity
limits for CRDP techniques and show how the presented frequency-based
methodology alleviates the requirement for high finesse cavities to achieve
high polarimetric sensitivities, and, thus, allows for the extension of
cavity-based polarimetric schemes into different spectral regimes but most
importantly renders the CW-CRDP methodology particularly suitable for robust
portable polarimetric instrumentations.
