  Optical fiber modal noise is a limiting factor for high precision
spectroscopy signal-to-noise in the near-infrared and visible. Unabated,
especially when using highly coherent light sources for wavelength calibration,
modal noise can induce radial velocity (RV) errors that hinder the discovery of
low-mass (and potentially Earth-like) planets. Previous research in this field
has found sufficient modal noise mitigation through the use of an integrating
sphere, but this requires extremely bright light sources, a luxury not
necessarily afforded by the next generation of high-resolution optical
spectrographs. Otherwise, mechanical agitation, which "mixes" the fiber's modal
patterns and allows the noise to be averaged over minutes-long exposures,
provides some noise reduction but the exact mechanism behind improvement in
signal-to-noise and RV drift has not been fully explored or optimized by the
community. Therefore, we have filled out the parameter space of modal noise
agitation techniques in order to better understand agitation's contribution to
mitigating modal noise and to discover a better method for agitating fibers. We
find that modal noise is best suppressed by the quasi-chaotic motion of two
high-amplitude agitators oscillating with varying phase for fibers with large
core diameters and low azimuthal symmetry. This work has subsequently
influenced the design of a fiber agitator, to be installed with the EXtreme
PREcision Spectrograph, that we estimate will reduce modal-noise-induced RV
error to less than 3.2 cm/s.
