The transmission modes of a fiber introduce an unstable intensity distribut
ion in high-resolution spectra which cannot be eliminated by flat-fielding
because the pattern is sensitive to any small changes in fiber position or
illumination. These instabilities, rather than photon shot noise, limit the
achievable signal-to-noise ratio (S/N). In controlled laboratory experimen
ts at a resolution of similar to 150,000, we found S/N limited to less than
or equal to 500 for a range of fiber perturbations and illumination geomet
ries from simple displacement to simulated seeing variations. We find an em
pirical relation between the number of monochromatic modes and the level of
modal noise for which we offer no rigorous explanation. More important, we
demonstrate that continuous, low-amplitude agitation of the fiber close to
its output completely eliminates the problem.