Technique to improve image fidelity in photorefractive media

Most photorefractive crystals display a measure of imperfection, such as striations, curved surfaces, or other phase-disturbing properties, which distort transmitted images and degrade the fidelity of holographic storage. We consider strontium barium niobate crystals with striations perpendicular to the c-axis. In such crystals, extraordinary polarized light experiences the strongest photorefractive effect but scatters off the striae, degrading the image fidelity. Conventional four-wave mixing, using counterpropagating pump beams, does not completely alleviate the problem, as the pump beams are also scattered by the striae. This difficulty can be overcome by using two true phase-conjugate pump beams. We describe hologram storage in striated strontium barium niobate, using a true conjugate of the reference beam generated by internal self-pumping in a second photorefractive crystal. The conjugate of the reference beam generates a conjugate of the signal beam, which then reproduces the stored image with high fidelity. This technique enables the use of imperfect crystals in photorefractive applications. We explore uses of this technique for holographic data storage.