  Exploring the use of single-mode fibers (SMFs) in high precision Doppler
spectrometers has become increasingly attractive since the advent of
diffraction-limited adaptive optics systems on large-aperture telescopes.
Spectrometers fed with these fibers can be made significantly smaller than
typical 'seeing-limited' instruments, greatly reducing cost and overall
complexity. Importantly, classical mode interference and speckle issues
associated with multi-mode fibers, also known as 'modal noise', are mitigated
when using SMFs, which also provide perfect radial and azimuthal image
scrambling. However, these fibers do support multiple polarization modes, an
issue that is generally ignored for larger-core fibers given the large number
of propagation modes. Since diffraction gratings used in most high resolution
astronomical instruments have dispersive properties that are sensitive to
incident polarization changes, any birefringence variations in the fiber can
cause variations in the efficiency profile, degrading illumination stability.
Here we present a cautionary note outlining how the polarization properties of
SMFs can affect the radial velocity measurement precision of high resolution
spectrographs. This work is immediately relevant to the rapidly expanding field
of diffraction-limited, extreme precision RV spectrographs that are currently
being designed and built by a number of groups.
