We describe optical disks that store data holographically in three dimensions by using either angle multiplexing or wavelength multiplexing. Data are stored and retrieved in parallel blocks or pages, and each page consists of approximately 106 bits. The storage capacity of such disks is derived as a function of disk thickness, pixel size, page size, and scanning parameters. The optimum storage density is approximately 120 bits/pLm 2 .

An optical network is described that is capable of recognizing at standard video rates the identity of faces for which it has been trained. The faces are presented under a wide variety of conditions to the system and the classification performance is measured. The system is trained by gradually adaptingphotorefrac tive holograms.

We propose and demonstrate a nonvolatile holographic recording system for storing two-dimensional images.The readout light in this system is not absorbed by the holographic medium, and the data are preformatted or postformatted so that lines from different holograms are interleaved to satisfy the Bragg-matching condition.Holographic storage in photorefractive crystals'`3 has a high storage density owing to three-dimensional storage and a high readout rate owing to parallel retrieval. The practical development of such memories has been impeded by a lack of materials that have all the suitable properties. Specifically, the most difficult requirement to satisfy is the simultaneous need for materials that are maximally sensitive to light in order to increase the recording speed and the need for a nonvolatile memory that is not affected by illumination to light during readout. Thermal' or electrical 4 5 fixing are among solutions that were demonstrated to address this problem. Thermal fixing is the most commonly used method now, but it is difficult to design practical systems with this approach. Moreover, it is incompatible with a reprogrammable memory. Electrical fixing is in principle more compatible with a practical reprogrammable memory; however, it is still not well developed. Another approach is the use of periodic copying. 6 -9 With this approach the contents of the hologram are refreshed, preventing decay owing to readout. We can also use materials such as strontium barium niobate, whose absorption varies significantly as the polarization is rotated.10 Holograms can then be recorded with one polarization and read out with an orthogonally polarized beam that is less absorbing. This introduces a write/erase asymmetry that can delay significantly the erasure of the holograms.In this Letter we describe a nonvolatile holographic memory that employs different wavelengths in the recording and readout phases. A dual-wavelength scheme was used previously by McRuer et al." to implement nonvolatile optical interconnections with a photorefractive crystal. Pauliat et al.' 2 used the dual-wavelength method to construct a deflector. Kfilich1 3 devised a dual-wavelength storage scheme in which spherical waves are used to construct holograms. In his scheme he could read out a single hologram at a different wavelength by changing the sphericity of the reconstructing wave. In the dualwavelength memory that we describe, plane-wave references are used instead, and multiple holograms can be stored and recalled. The recording wavelength Al is selected near the peak of the trapinduced absorption spectrum of the material. The readout wavelength A 2 is chosen in a region where absorption is as small as possible. Typically the recording wavelength is shorter than the readout wavelength. The large drop in absorption away from the absorption band yields a very large write/erase asymmetry. For example, we recorded holograms at Al = 488 nm and exposed the hologram to A 2 = 632.8 nm light at a non-Bragg-matched angle for approximately 20...

We describe the rotational alignment sensitivity of three-dimensional holographic disks. It is shown that the reconstructed image always rotates by the angle by which the disk rotates; however, the center and the radius of rotation change as the recording geometry changes. A comparison among image plane, Fourier plane, and Fresnel holograms is given, and an optimum configuration (in terms of alignment sensitivity) in which the radius of rotation is zero is derived. We present experimental results and also discuss how the rotation alignment sensitivity affects the storage density and the readout -recording speed of the three-dimensional disk. A brief summary of other sources of misalignment is given.

We describe a photorefractive 3-D optical disk for data storage. The information is stored at several recording locations on the disk as angle-multiplexed holograms. Design parameters that affect alignment sensitivity and diffraction efficiency are considered.