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Exploring the use of single-mode fibers (SMFs) in high precision Dopplerspectrometers has become increasingly attractive since the advent ofdiffraction-limited adaptive optics systems on large-aperture telescopes.Spectrometers fed with these fibers can be made significantly smaller thantypical 'seeing-limited' instruments, greatly reducing cost and overallcomplexity. Importantly, classical mode interference and speckle issuesassociated with multi-mode fibers, also known as 'modal noise', are mitigatedwhen using SMFs, which also provide perfect radial and azimuthal imagescrambling. However, these fibers do support multiple polarization modes, anissue that is generally ignored for larger-core fibers given the large numberof propagation modes. Since diffraction gratings used in most high resolutionastronomical instruments have dispersive properties that are sensitive toincident polarization changes, any birefringence variations in the fiber cancause variations in the efficiency profile, degrading illumination stability.Here we present a cautionary note outlining how the polarization properties ofSMFs can affect the radial velocity measurement precision of high resolutionspectrographs. This work is immediately relevant to the rapidly expanding fieldof diffraction-limited, extreme precision RV spectrographs that are currentlybeing designed and built by a number of groups.

'Modal-noise' in single-mode fibers: A cautionary note for high precision radial velocity instruments