  We present measurements of how multimode fiber focal-ratio degradation (FRD)
and throughput vary with levels of fiber surface polish from 60 to 0.5 micron
grit. Measurements used full-beam and laser injection methods at wavelengths
between 0.4 and 0.8 microns on 17 meter lengths of Polymicro FBP 300 and 400
micron core fiber. Full-beam injection probed input focal-ratios between f/3
and f/13.5, while laser injection allowed us to isolate FRD at discrete
injection angles up to 17 degrees (f/1.6 marginal ray). We find (1) FRD effects
decrease as grit size decreases, with the largest gains in beam quality
occurring at grit sizes above 5 microns; (2) total throughput increases as grit
size decreases, reaching 90% at 790 nm with the finest polishing levels; (3)
total throughput is higher at redder wavelengths for coarser polishing grit,
indicating surface-scattering as the primary source of loss. We also quantify
the angular dependence of FRD as a function of polishing level. Our results
indicate that a commonly adopted micro-bending model for FRD is a poor
descriptor of the observed phenomenon.
