  Imperfections in the surface of intracavity elements of an optical ring
resonator can scatter light from one mode into the counterpropagating mode. The
phase-locking of the cavity modes induced by this backscattering is a
well-known example that notoriously afflicts laser gyroscopes and similar
active systems. We experimentally show how backscattering can be circumvented
in a unidirectionally operated ring cavity either by an appropriate choice of
the resonant cavity mode or by active feedback control.
