Polytopic multiplexing is a new method of overlapping holograms that, when combined with other multiplexing techniques, can increase the capacity of a volume holographic data storage system by more than a factor of 10. This is because the method makes possible the effective utilization of thick media. An experimental demonstration of this technique is also presented.

We demonstrate theoretically and experimentally a new multiplexing method for volume holographic storage using a single reference beam that is composed of multiple plane waves or is a spherical wave. We multiplex the holograms by shifting the recording material or the recording/readout head. The volume properties of the recording medium allow selective readout of holograms stored in successive overlapping locations. High storage densities can be achieved with a relatively simple implementation by use of the new method. Any of these methods or their combinations can be used to multiplex holograms for holographic storage devices.In this Letter we introduce a method for multiplexing holograms by using a reference beam consisting of a spectrum of plane waves (similar to phase code multiplexing). We achieve multiplexing by shifting the recording medium with respect to the signal and reference beams. Alternatively, the two beams can be translated in tandem with respect to the stationary medium.The geometry for shift multiplexing is shown in Fig. 1 for the case of storing Fourier transform holograms. The reference originates from an array of M point sources located in the front focal plane of a Fourier lens and centered around the optical axis z. The lens transforms the field into a fan of M plane waves. The angular separation is uniform, given by Du ഠ d͞F r , where d is the distance between successive point sources and F r is the focal length. Thus, the angle of incidence of the mth component isThe angle of incidence of the central component of the signal with respect to the z axis is denoted by u S . Because the reference consists of M plane waves, we can think of the recording as consisting of M separate holograms recorded simultaneously. On reconstruction, each plane wave in the reference fan reads out not only the hologram that it recorded but also all the holograms recorded by the other plane waves of the reference fan. These reconstructions, or ghosts, produce images that are shifted with respect to the primary reconstruction as a result of the change in readout 0146-9592/95/070782-03$6.00/0

Abstract. An iterative method is introduced for determining the exposure schedule for multiplexing holograms in saturable recording materials, such as photopolymers. This method is designed to share all or part of the available dynamic range of the recording material among the holograms to be multiplexed. Using exposure schedules derived from this method, the authors find that the diffraction efficiency of DuPont's HRF-150 38-and 100-m photopolymer scale is (2.2/M) 2 and (6.5/M) 2 respectively, where M is the number of holograms recorded. Finally, 1000 holograms were multiplexed at a single location in the 100-m thick photopolymer using an exposure schedule derived with this method.

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Multiple digital data pages (480 kbits per page) were holographically recorded and retrieved with low bit-error rates in thick (~250- and ~500-mum) photopolymer media. The photopolymer systems were fabricated with the optical quality and low level of scatter required for digital data storage. We believe that these results represent the first demonstration of holographic storage of high-capacity digital data pages in photopolymer media with the thickness that will be required for such storage densities.

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Page-oriented data storage systems incorporate optical detector arrays [such as complementary metal-oxide semiconductor (CMOS) arrays] in order to read data images. For laboratory demonstrations the detector array is typically pixel matched to the data image [Opt. Lett. 22, 1509 (1997)]. This approach requires exceedingly high-performance optics and mechanics for the simultaneous alignment of each data-bearing pixel image to a detector element to be achieved. Systems intended for commercialization are designed with detector arrays that spatially sample the image at or above the Nyquist rate in order to read poorly aligned and distorted images [S. Redfield, Holographic Data Storage (Springer-Verlag, 2000), pp. 347-349]. However, for data page sizes exceeding a megapixel this approach becomes prohibitive in terms of detector bandwidth, size, power, cost, and processing requirements. We have instead developed a sub-Nyquist oversampling methodology that can recover arbitrarily aligned and distorted megapixel data page images with pixel-matched fidelity by using fewer than double the number of detector pixels. Features required for practicable implementation are described, including fiducials for alignment determination.

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