Claire Gu

Cross-talk-limited storage capacity of volume holographic memory

We consider the storage density limited by the cross talk between stored pages during readout. We review some earlier work done in this area and present new theoretical work that characterizes the storage capacity limitations due to the cross talk. The results show that because of the presence of degeneracy noise, storage capacity does not benefit from expanding the reference points from one dimension to two dimensions. An optimum configuration that fully utilizes storage capacity and completely eliminates degeneracy noise is given for an angularly multiplexed volume holographic memory system.

Cross talk in wavelength-multiplexed holographic memories

The cross talk between wavelength-multiplexed holograms is analyzed. The signal-to-noise ratio (SNR) is calculated for a recording schedule that places the center of each image at the null (in wavelength space) of the adjacent hologram. An asymptotic closed-form expression for the minimum SNR is derived in a general reflection geometry. The reflection geometry with counterpropagating signal and reference beams is shown to have the best SNR.

Correlation patterns and cross-talk noise in volume holographic optical correlators

We analyze the correlation pattern and the cross-talk noise of optical correlators, using multiplexed volume holograms as matched filters. Our results show that sidelobes of the correlation patterns are suppressed in such an optical system; therefore the signal-to-noise ratio in pattern recognition is significantly higher than that in a planar holographic correlator system. We consider both angle- and wavelength-multiplexed holograms and compare the results for various thicknesses of volume holographic media. Our results show that the degree of sidelobe suppression depends on the thickness of the holographic medium and the angle between the object beam and the reference beam during recording. When the correlation patterns obtained with the transmission holograms in angle multiplexing are compared with the reflection holograms in wavelength multiplexing, it is seen that the sidelobes decrease much faster with the increase of thickness with angle multiplexing than with wavelength multiplexing. In addition, we demonstrate the effect of sidelobe suppression, and our experimental result is in good qualitative agreement with theoretical predictions.

Dynamics of grating formation in photovoltaic media

The Kukhtarev equations are solved taking into account the photovoltaic effect and different boundary conditions. In the case of open circuit, the voltage across the crystal is found to vary with a time scale similar to the photorefractive time constant. This effect explains the dynamic behavior observed experimentally.

Statistical analysis of cross-talk noise and storage capacity in volume holographic memory

We present a statistical analysis of cross-talk noise and consider the cross-talk-limited storage capacity of volume holographic memory. Random-walk approximation is employed to estimate the intrapage interpixel cross-talk noise and the signal-to-noise ratio. We obtain simple expressions of the storage capacity in terms of number of bits per volume for a given signal-to-noise ratio or bit-error rate. The results indicate that the storage density is reduced from the ultimate density of lambda(-3) by a factor related to the signal-to-noise ratio.

Form birefringence dispersion in periodic layered media.

We analyze the form birefringence dispersion of periodic dielectric layered media. Using Bloch wave analysis, we investigate the shape of the normal surface and its dependence on wavelength. Our higher-order uniaxial approximation gives an explicit expression for the additional dispersion of the form birefringence that is due to the finite layer thickness.

Diffraction properties of fixed gratings in photorefractive media

The diffraction properties of fixed volume gratings, including the effect of additional photorefractive energy coupling between the incident wave and the diffracted wave, are considered. Both transmission gratings and reflection gratings are treated. Analytical solutions for phase gratings are derived, and numerical solutions for absorption gratings are also obtained. The results are presented and discussed.

2-D shift-invariant volume holographic correlator

Abstract We describe a shift-invariant optical correlator with a volume hologram as a matched filter. Shift-invariance is achieved by the combination of degeneracy in one direction and image shifting in the perpendicular direction. The amount of shift-invariance along the degeneracy direction is evaluated and various methods to achieve image shifting are proposed. Both theoretical and experimental results are presented and discussed.

Scattering due to randomly distributed charge particles in photorefractive crystals

Scattering due to noisy gratings produced by randomly distributed charge particles in photorefractive crystals is considered. The scattering cross section is analyzed in terms of the fluctuation of the refractive index introduced by the random space-charge field through the Pockels effect. We also consider the noise in photorefractive gratings due to such scattering. A signal-to-noise ratio of 1 leads to a minimum modulation depth for an index grating.

Phase‐locked sustainment of photorefractive holograms using phase conjugation

A method for sustaining multiply exposed photorefractive holograms, in a phase-locked fashion, by using a pair of phase-conjugating mirrors is described. It is shown that a steady state exists where the overall diffraction efficiency is independent of the number of holographic exposures and the final holograms are exactly in phase with the initial ones. Both analytical and experimental results are presented.