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. A straightforward example of an SLM is an amplitude SLM, where the pixels with the value ‘0’ block the light, and the pixels with the value ‘1’ transmit or reflect it. In a simplified view, this means that the amplitude SLM has black and white pixels. Supposing an equal probability for the black pixels and the white pixels, about 50% of the object beam power is blocked. The blocked light is wasted and decreases a writing data rate. If, in addition, low white rate codes are applied, which is often the case in the field of holographic storage, the light loss for the object beam is even more than 50%. For example, using a 20% white rate code the light loss is about 80%.
The above problem is overcome by using a phase SLM for imprinting the information into the object beam. In this case a phase shift of ‘0’ and ‘π’ of the pixels corresponds to information bits ‘0’ and ‘1’ (or vice versa) of the input data array, respectively. The phase shift of ‘π’ corresponds to an optical path difference of λ/2, where λ is the wavelength of the object and the reference beam. Of course, it is likewise possible to apply further intermediate phase shifts. As no light is blocked, there is no light loss when a phase SLM is used for imprinting of the information onto the object beam. However, as the detector array can only detect the light intensity, the phase distribution of the reconstructed object beam has to be converted into an intensity distribution before the light impinges on the detector array.
WO 2004/112045 discloses a holographic storage system, in which a phase contrast filter is placed in the readout beam path to convert the phase modulation into an amplitude modulation, which is detected by an array detector.
In WO 02/49018 and WO 02/03145 a phase SLM is provided for imprinting the information onto the object beam. For converting a reconstructed phase distribution into an intensity distribution, the interference of a reflected reference beam with the read-out beam is used. On the detector surface the reference beam reflected from the surface of the holographic storage medium is a plane wave with a constant phase, while the read-out beam is a binary phase modulated beam. The interference of these two beams is a binary intensity distribution. This solution requires that the interfering beams have similar amplitudes. If the amplitudes are significantly different, the visibility of the intensity distribution of the interference pattern is very low. In case of multiplexing, however, the diffraction efficiency for the read-out beam is only 10−4 to 10−6, i.e. the intensity difference between the interfering beams is 4-6 orders of magnitude. This means that the solution is not applicable to holographic storage systems using highly multiplexed holograms, as the visibility and the signal-to-noise ratio are very low.