Scattering due to random space-charge field in photorefractive crystals

Abstract

The fluctuation of refractive index introduced by the random space-charge field via the Pockels effect is one of the noise sources in photorefractive media. In EO crystals, including photorefractive crystals, charge particles such as ionized dopants and defects are randomly distributed. The space-charge electric field generated by these charge particles induces a fluctuation in dielectric constant (or equivalently index of refraction) from the Pockels effect. Such fluctuation exists even in the presence of hologram recording and two-wave mixing. When the crystal is illuminated by light, scattering occurs in a manner similar to that of Rayleigh scattering. This kind of scattering exists in all photorefractive crystals, where ionized dopants and/or defects are responsible for the formation of index gratings. Such scattering leads to statistical noise in photorefractive devices. In this paper, we consider the scattering from randomly distributed charge particles in photorefractive crystals. The scattering cross section is analyzed in terms of the fluctuation of refractive index induced by the random space-charge field via Pockels effect. We also discuss the noise due to scattering from these noisy index gratings. The signal to noise ratio and the minimum modulation depth for an index grating are discussed. The analysis can be extended to cover all EO media.