  As light propagates along a waveguide, a fraction of the field can be
reflected by Rayleigh scatterers. In high-quality-factor
whispering-gallery-mode microresonators, this intrinsic backscattering is
primarily caused by either surface or bulk material imperfections. For several
types of microresonator-based experiments and applications, minimal
backscattering in the cavity is of critical importance, and thus, the ability
to suppress backscattering is essential. We demonstrate that the introduction
of an additional scatterer into the near field of a high-quality-factor
microresonator can coherently suppress the amount of backscattering in the
microresonator by more than 30 dB. The method relies on controlling the
scatterer position such that the intrinsic and scatterer-induced
backpropagating fields destructively interfere. This technique is useful in
microresonator applications where backscattering is currently limiting the
performance of devices, such as ring-laser gyroscopes and dual frequency combs,
which both suffer from injection locking. Moreover, these findings are of
interest for integrated photonic circuits in which back reflections could
negatively impact the stability of laser sources or other components.
