Special Report: Optical Patterns
Fig. 12.11. Compact and ultrafast holographic memory with a microlaser array.
In the experimental results shown in Fig. 12.11, memories are recorded in a volume hologram (LiNbO3 crystal, 0.01% Fe doped) that can provide a large storage capacity of around ten billion bits because of the third dimension available in a volume storage medium. The figures on the right-handed side show the images retrieved from the volume hologram by the light from a SELDA. Each laser is separated from the adjacent one by 70 mm, corresponding to an angular separation of about 0.040. As can be seen in the figure, the two independent lasers separated by only 70 mm reconstruct totally different images. In this way, each tiny microlaser can be matched to a separate page, allowing the array to form a selective address generator.
12.4.3 Combined angular and wavelength multiplexing with a two-dimensional MC-SELDA In general, angular multiplexing can be achieved along one direction in a plane formed by both reference and object beam directions because of the degeneracy effect. As can be seen in Fig. 12.12 (a), the Bragg condition is satisfied by waves that have any of the directions that form the surface of a cone (the shaded area in the figure) whose axis is normal to the grating planes [74]. This degeneracy causes cross-talk along the direction perpendicular to the Bragg direction. However, this cross-talk can be greatly reduced by use of different wavelengths along the degenerate directions, as shown in Fig. 12.12(b). Such a concept has been recently demonstrated with five different wavelengths to record a total of 2000 holograms: five different wavelengths with 400 angularly multiplexed images for each wavelength [75].
Fig. 12.12. Breaking degeneracy by use of multiple wavelengths.
The concept can also be implemented in a compact and ultrafast fashion by a 2-D MC-SELDA, as shown in Fig. 12.13. The SELDA has the same wavelength along the angular-multiplexing direction and different wavelengths along the degeneracy direction. In addition, such a combinational multiplexing with a MC-SELDA has an additional angular separation effect along the degenerate direction because each microlaser lies in a different position. In this way, the storage capacity of a volume hologram can be greatly increased in a compact and ultrafast readout system within the total limit imposed by the available number of charge carriers in a medium and the theoretical limit of V/l3 , as is proved by Lee et al. [76] and Psaltis et al. [77].
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