In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
In a holographic storage system with a coaxial arrangement the reference beam and the object beam use a single split aperture. In addition, both beams are modulated by the same SLM. The information is stored in the form of multiplexed Fourier holograms. For a reflection type coaxial holographic storage system it is difficult to couple in the object beam and the reference beams during recording, and also to couple out the reference beam from the reconstructed object beam during reading. As disclosed in EP 1 624 451, in order to increase the selectivity of a so-called collinear arrangement, the reference beam is also pixelated. The object beam and the reference beam are modulated with the same SLM. This arrangement is also known as a so-called split aperture arrangement, as both the object plane and the image plane of the holographic optical system are split into object and reference areas. At the image plane, which is the plane of the detector, the reference pixels form a sharp image and are blocked. A similar transmission type coaxial split aperture system is disclosed in U.S. Pat. No. 6,108,110.
During reading the reference beam is partially diffracted into the image zone. This is depicted in FIG. 1, which shows a model of the diffracted reference beam without any object beam. The data pixels are all off. Part a) of FIG. 1 uses a linear scaling, whereas part b) uses a logarithmical scaling. The reason for this diffraction noise is the low-pass filtering behavior of the optical system. The exit pupil of the optical system causes a sharp cutting of the high frequency components. Because of this sharp cutting, the reference beam is spread out onto the detector surface. This means that a high diffraction efficiency is needed to achieve a sufficient signal-to-noise ratio (SNR) of the image pattern relative to the reference beam diffraction noise. In the conventional setup, almost 0.1% of the energy of the reference beam is diffracted into the image area. At a practically acceptable diffraction efficiency of 10−6 to 10−5, this means that the total energy of the diffraction noise is several times larger (typically 50-500 times larger) than the total energy of the reconstructed data pattern. As can be seen in FIG. 1, the SNR is better in the middle of the image circle and worse close to the outer rim. To achieve low error rates, a high diffraction efficiency is needed, which requires a strong refractive index grating to be recorded in the holographic storage material.
As a consequence, even though the collinear arrangement has a very good selectivity, its SNR is limited. For further details see Horimai et al.: “Collinear Holography”, Appl. Opt. Vol. 44, 2575-2579, and Horimai et al.: “High-density recording storage system by Collinear holography” Proc. SPIE 6187, 618701. As a result of the limited SNR the data density is still limited by the dynamic range of the material, because it is not feasible to multiplex a large number of holograms with a sufficiently large diffraction efficiency.
The main factor of the noise generation is the sharp cutting of the high frequency components. Therefore, increasing the object space numerical aperture, or in other words increasing the radius of the Fourier-plane filter, would not have a sufficient effect if the high frequency components are still cut sharply. Table 1, which shows the total power of the noise within the image area divided by the total power of the reference beam for different radii of the Fourier-plane filter. In the table the radii are given in relation to the Nyquist aperture DN, which is defined as
            D      N        =                  f        ×        λ            pixelsize        ,where f is the focal length of the Fourier objective. The wavelength is measured in the storage material. As can be seen, increasing the filter radius from 0.67×DN by a factor of 1.6 to 1.07×DN means that the noise is reduced to about 1/30th of its original value. For a system with λ=267 nm (the material refractive index is n=1.5), f=7 mm and a pixel size of 12 μm, this means an increase of the diameter from 249 μm to 398 μm. However, the noise is still in the same order of magnitude as the reconstructed object beam, which is not sufficient. Furthermore, this solution has the disadvantage that it requires a better lens system with a larger NA in the object space.
TABLE 1Total power of the noise within the image areadivided by the total power of the reference beam fordifferent radii of the Fourier-plane filter.FP filterradius/DNNoise ratio0.674.58E−040.733.10E−040.802.03E−040.871.16E−040.935.87E−051.002.43E−051.071.29E−051.132.24E−051.204.70E−051.278.59E−05
In Kimura: “Improvement of the optical signal-to-noise ratio in common-path holographic storage by use of a polarization-controlling media structure”, Opt. Let., Vol. 30, 2005, a polarization method for suppressing the noise originating from the reference beam is disclosed. According to this solution the reconstructed object beam and the reference beam are orthogonally polarized. The noise caused by the reference beam is suppressed by a polarization filter. This solution requires a complex holographic storage medium structure with a quarter wave plate on top and on the bottom of the holographic storage material. The efficiency of the polarization-based suppression depends on the numerical aperture. The suppression only works properly for a small NA. However, a high NA in the Fourier plane is required for high data density storage.