  We describe the controlled observation of the nonequilibrium Ising-Bloch
transition in a broad area nonlinear optical cavity, namely, a quasi-1D single
longitudinal-mode photorefractive oscilator in a degenerate four-wave mixing
configuration. Our experimental technique allows for the controlled injection
of the domain walls. We use cavity detuning as control parameter and find that
both Ising and Bloch walls can exist for the same detuning values within a
certain interval of detunings, i.e., the Ising-Bloch transition is hysteretic
in our case. A complex Ginzburg-Landau model is used for supporting the
observations.
