  We designed and built a new Four-Stokes-Parameter spectral line Polarimeter
(FSPPol) for the Caltech Submillimeter Observatory (CSO). The simple design of
FSPPol does not include any mirrors or optical components to redirect or
re-image the radiation beam and simply transmits the beam to the receiver
through its retarder plates. FSPPol is currently optimized for observation in
the 200-260 GHz range and measures all four Stokes parameters, I, Q, U, and V.
The very low level of instrument polarization makes it possible to obtain
reliable measurements of the Goldreich-Kylafis effect in molecular spectral
lines. Accordingly, we measured a polarization fraction of a few percent in the
spectral line wings of ^{12}\mathrm{CO} (J=2\rightarrow1) in Orion KL/IRc2,
which is consistent with previous observations. We also used FSPPol to study
the Zeeman effect in the N=2\rightarrow1 transition of CN in DR21(OH) for the
first time. At this point we cannot report a Zeeman detection, but more
observations are ongoing.
