A new observation on optical phase conjugation using barium titanate: Dark hole effect

New experimental observations on a dark hole obtained at the center of self-pumped phase conjugated image are presented. We focus on beam fanning and the relative position of the back focal plane of a Fourier transforming lens to the crystal (Sanders' barium titanate) used. A collimated argon-ion laser beam (514.5nm) with Gaussian profile illuminates a clear circular aperture as an input object and is focused into the crystal of 5mm × 4.8mm × 4.2mm (c axis) by the lens as shown in Fig. 1. When the center of the crystal is located behind the back focal plane of the lens, the optical phase conjugated image contains a dark hole at its center as shown in Fig. 2(a). As the center of the crystal moves away from the back focal plane, the size of the dark hole tends to decrease as shown in Fig. 2(b). No dark hole was obtained when the crystal was further moved backward as shown in Fig. 2(c). It is speculated that low spatial frequency components of the resulting Fourier pattern are suppressed by beam fanning inside the crystal and that the cutoff frequency decreases as the crystal moves backward.