  Are we alone? In our quest to find life beyond Earth, we use our own planet
to develop and verify new methods and techniques to remotely detect life. Our
Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes
spectropolarimeter to be deployed in the field and in space, looks for signals
of life on Earth by sensing the linear and circular polarization states of
reflected light. Examples of these biosignatures are linear polarization
resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump,
as well as circular polarization resulting from the homochirality of biotic
molecules. LSDpol is optimized for sensing circular polarization. To this end,
LSDpol employs a spatial light modulator in the entrance slit of the
spectrograph, a liquid-crystal quarter-wave retarder where the fast axis
rotates as a function of slit position. The original design of LSDpol
implemented a dual-beam spectropolarimeter by combining a quarter-wave plate
with a polarization grating. Unfortunately, this design causes significant
linear-to-circular cross-talk. In addition, it revealed spurious polarization
modulation effects. Here, we present numerical simulations that illustrate how
Fresnel diffraction effects can create these spurious modulations. We verified
the simulations with accurate polarization state measurements in the lab using
100% linearly and circularly polarized light.
