One important potential use of volume holograms is in digital data storage; the three dimensional nature of a volume hologram, which refers to the storage of each bit as a hologram extending throughout the volume of the recording medium, renders volume holograms suitable for use in high capacity digital data storage. A group of bits can be encoded and decoded together as a two dimensional array of bits referred to as a page. Various multiplexing methods, such as angular, spatioangular, shift, wavelength, phase-code, and related methods, are used to store multiple pages co-locationally within the same volume or in partially overlapping volumes.
The use of peristrophic or azimuthal multiplexing for recording data page holograms in recording media of moderate thickness is an effective method for increasing the accessible angular bandwidth (see Curtis, et al. Optics Letters, 19, 13, pp 993-994 (1994). With this method, the reconstructed image of all unselected holograms can be rotated completely off the field of view of a detector, while reconstruction of only the selected hologram is imaged onto the detector image plane. In this manner the reconstructed holograms are spatially separated in the image plane. Typically, a CCD or CMOS detector is used.
The rotation, dφ, required for the spatial separation for Fourier plane holograms is approximately given by formula (1) (see Curtis, et al. Optics Letters, 19, 13, 993 (1994))
                              d          ⁢                                          ⁢          ϕ                ≥                              N            ⁢                                                  ⁢                          δ              /              F                                                          sin              ⁢                                                          ⁢                              θ                ref                                      +                          sin              ⁢                                                          ⁢                              θ                obj                                                                        (        1        )            where N is the number of pixels per side of the detector, δ is the pixel pitch, F is the focal length of the lens, and θref and θobj are the incident angles of the reference and object beams, respectively, with respect to a normal to the surface of the recording media. For image plane holograms, the rotation, dφ, required for the spatial separation is substantially smaller and is given by formula (2) (see Curtis, et al. Optics Letters, 19, 13, 993 (1994)):
                              d          ⁢                                          ⁢          ϕ                ≥                              2            ⁢                                                  ⁢                          λ              /              δ                                                          sin              ⁢                                                          ⁢                              θ                ref                                      +                          sin              ⁢                                                          ⁢                              θ                obj                                                                        (        2        )            where λ is the wavelength of the imaging light and δ is the highest spatial frequency in the image.
There are generally three kinds of noise in any holographic data storage system: inter-page and intra-page, and effects due to scattered light from the recording medium. Inter-page noise occurs when light from a data page is diffracted while a different data page is being read out. Intra-page noise is due to light from a pixel contributing to the intensity of another pixel within the same data page that is being read out. Intra-page includes the effects of ordinary diffraction and aberrations, but it can also have other origins such as from ghost reconstructions inherent in speckle multiplexing. The nature and quantity of inter-page and intra-page noise depend on the multiplexing method and the imaging system used to record and reconstruct the data pages.
In angle multiplexing, Bragg mismatch is the mechanism that defines which page is read out at a given position of the reference beam. The angle of incidence of the plane-wave reference beam of a given hologram must therefore deviate by more than the Bragg angle selectivity Δθ from the angle of incidence of reference beams of other holograms multiplexed in the same medium. The quantity Δθ depends on the orientation of the grating vector (i.e., the orientation and spacing of the interference fringes which form the hologram) and the thickness of the holographic medium. Consequently, different pixels within the same data page have different Δθ's because they correspond to slightly different fringe patterns on the hologram. When a different data page is being read out, pixels from other data pages are Bragg mismatched by different amounts. Accordingly, it is impossible to place all pixels of a single data page onto the Bragg null of another data page. This results in inter-page crosstalk noise. (Intra-page noise in the case of angle multiplexing results from diffraction and aberrations.) However, when the reconstructed holograms are spatially separated in the image plane using azimuthal multiplexing, the degree of inter-page crosstalk is reduced from that observed in angle multiplexing. In the undepleted reference beam approximation, which is typically the case when multiplexing many holograms, simultaneous diffraction from a portion of the azimuthal multiplexed holograms can occur. However, this does not impact the signal-to-noise ratio (SNR) of any individual hologram, as the loss in signal is negligible. One approach to reduce cross-talk between recorded images, while keeping the storage density of holographically recorded images constant is to use thicker recording media and record holograms with either angular or planar multiplexing. However, volume scatter increases with thickness and thus the attainable dynamic range of the material is degraded.
An attractive approach to increasing information density is combining angle and azimuthal multiplexing. This combination is effected by rotating the Bragg plane incrementally through about 180° during recording for a particular interbeam angle, defined as the angle between the incident object (signal) and reference beams. This method is particularly effective in achieving the maximum angular bandwidth of the optical system. In this manner, Bragg selectivity from angle multiplexing can be combined with Bragg plane rotation for a particular interbeam angle. This type of combined multiplexing also changes a limiting factor for achieving increased information storage density from that limited by angular bandwidth to that limited by the dynamic range of the recording material. Finally, the combined multiplexing method can be further combined with spatial multiplexing by movement of the recording medium.
Previously described implementations of azimuthal multiplexing, or combination of angle and azimuthal multiplexing employ rotation of the recording material about the optical axis of the object beam, or rotation of the object beam and the reference beam, or rotation of the reference beam that is a part of a co-axial type imaging geometry for the object and reference beams in that the reference beam is concentrically disposed about the object beam. The latter implementation comprises a lens system that includes an inner field for the object beam and an outer annular field for the reference beam. Typically, these embodiments would require that the numerical aperture (NA) of the lens delivering the reference beam be about twice the value necessary for the lens delivering the object beam. These implementations are technologically complex, difficult to produce and consequentially costly and unsuitable for mass production.
Accordingly, the need exists for a compact, technologically simple and inexpensive design of a read/write combined multiplexing device, such as an optical pickup head, that operates to achieve high storage density and is suitable for use in a mass-produced holographic drive.
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