  We report experimental observation of the conversion of a phase-invariant
nonlinear system into a phase-locked one via the mechanism of rocking [G. J. de
Valcarcel and K. Staliunas, Phys. Rev. E 67, 026604 (2003)]. This conversion
results in that vortices of the phase-invariant system are being replaced by
phase patterns such as domain walls. The experiment is carried out on a
photorefractive oscillator in two-wave mixing configuration.A model for the
experimental device is given that reproduces the observed behavior.
