Nonvolatile storage in photorefractive crystals

Abstract

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...

Storage of 1000 holograms with use of a dual-wavelength method

“…Most have dealt with Fourier-plane recording, such as the use of spherical readout beams 8 to Bragg-match a larger range of the signal cone or interleaving strips from adjacent holograms. 10 We can also recover all the necessary information by recording in the image plane, without the added complexity of the above methods, if we simply adjust the system parameters according to the resolution of the images that we wish to store.…”

“…Several methods have been developed to address this problem. [1][2][3][4][5][6][7][8][9][10] We use the dual-wavelength method 8 -10 to demonstrate experimentally the longterm storage of 1000 holograms. The motivation for using different wavelengths of light for recording and readout is simple: If a crystal has an absorption spectrum with a substantial variation as a function of wavelength, by recording at a wavelength 1 at which the crystal is highly sensitive and reading out at a second wavelength 2 at which the crystal is relatively insensitive, we can reduce the decay of the gratings caused by the readout illumination.…”

“…Bragg-matched readout occurs when the readout beam is positioned such that the grating vector lies at the intersection of the two k spheres. The condition necessary for achieving Bragg-matched readout must satisfy the following relationships 10 :…”

Shift-multiplexed holographic memory using the two-lambda method

“…8 Here we consider the twolambda method, in which the recording and readout wavelengths are different. 9 The allowable readout time is prolonged if the readout wavelength is selected such that the absorption is lower and hence erasure is reduced. Because the Bragg matching condition at the new wavelength is modified, only a portion of the angular spectrum of the recorded holograms can be Bragg matched with a single plane wave.…”

Real-time spectral imaging in three spatial dimensions

“…The second degeneracy is for pairs ͑k Rp , k Sp ͒ at wavelengths l p fi l r . 10,14 This corresponds to the probe source along a spatial-spectral coupled direction in Fig. 2 …”