  We demonstrate experimentally that a broad area laser-like optical oscillator
(a nondegenerate photorefractive oscillator) with structured injected signal
displays two-phase patterns. The technique (G. J. de Valc\'arcel and K.
Staliunas, Phys. Rev. Lett. (105), 054101 (2010)) consists in spatially
modulating the injection, so that its phase alternates periodically between two
opposite values, i.e. differing by pi

Optics;Pattern Formation and Solitons