JP5319068B2 - Brightness profile correction of data pages in holographic storage - Google Patents

Description

  The present invention relates to a method for recording a data page on a holographic recording medium, and more particularly to a method for correcting a sensitivity change of a holographic material of a holographic recording medium.

  In holographic data storage, user data is typically modulated into a two-dimensional data page consisting of pixels with binary values of 0 or 1. In order to record data, a spatial light modulator (SLM) converts user data fringes into intensity modulation of the target beam. This target beam is superimposed on the reference beam, thereby creating interference fringes. This fringe is recorded on a holographic storage medium, such as a holographic disk or card. During the reading process, the reference beam (usually called the probe beam) is diffracted by the interference fringes. This creates a reconstructed target beam that retains the stored information. The data page is reproduced by projecting the reconstructed target beam onto a photosensitive detector.

  In the optimal case, the reconstructed data page has a uniform luminance profile where all regions show the same pixel contrast. However, this is not the usual case for several reasons. One reason is the non-uniform sensitivity of the material due to, for example, the chemical properties of the holographic polymer used. For example, in the case of shift multiplexing, a region that has already been irradiated by another hologram has a different sensitivity than a region that has not yet been irradiated. The difference in sensitivity depends on the sensitivity curve of the material. This effect causes some of the holographic interference fringes to exhibit higher contrast than others. This results in uneven contrast of the reproduced data page. Contrast within a region is understood as the difference between the average luminance of data pixels in the “0” state and the average luminance of data pixels in the “1” state in this region. High contrast means that the difference is large.

  For reliable pixel detection, it should be ensured that the low contrast part of the reconstructed data page is bright enough for pixel detection, while the high contrast part should exceed the dynamic range of the detector is not. Furthermore, since the bright part shows a higher brightness than that required for detection, a portion of the available beam energy is consumed.

  It is an object of the present invention to provide a method for recording a data page on a holographic recording medium, which can correct the sensitivity change of the holographic material of the holographic recording medium.

According to the present invention, this object is
Modulating a light beam using a spatial light modulator to generate page data having data to be recorded;
In order to reduce the contrast change of the reconstructed data page, it is achieved by a method of recording a data page on a holographic recording medium comprising the step of applying a correction profile to the data page during recording.

  For this purpose, the holographic storage system involves a spatial light modulator that modulates the light beam according to the data recorded on the holographic recording medium, in order to reduce the contrast change of the reconstructed data page. A contrast change corrector that applies a correction profile to the data page during recording.

  The present invention aims to correct the sensitivity change of the holographic material during the recording process. By modulating the brightness of the SLM data page pixels according to the sensitivity change, a more uniform contrast over the entire data page is achieved during the reading process.

  The relative position of the areas with different sensitivities depends on the temporal and spatial history of the already written hologram. This history is often referred to as a “write schedule” or “write plan”. Thus, the luminance profile applied to the data page is calculated individually for each new hologram. The required profile is preferably calculated from the irradiation history and the material sensitivity curve. The irradiation history can be obtained by storing the position used during the writing time or by using a protocol with information about the used position stored in a storage medium or elsewhere. The sensitivity curve of the holographic material can be predicted in advance by simulation, calculation or measurement, or directly measured by driving with writing and reading processes. The calculated profile is then applied to the data page by the SLM. For this purpose, the SLM not only switches between the “0” and “1” states, but also enables gray values. By writing the hologram in this way, the contrast change in the reproduced data page is greatly suppressed. This results in a low bit error rate. As an alternative, an additional SLM that enables gray values is used in combination with a binary SLM to generate data pages. Further SLMs have lower pixels than binary SLMs because the contrast changes on a large scale compared to the pixels of the normal data page. Furthermore, further SLM pixels can be adapted to a specific iterative correction profile caused by a standard exposure history. For example, the pixels are striped.

  Apart from correcting the effects caused by the non-uniform sensitivity of the material, the present invention also makes it possible to correct other effects that result in regions with different pixel contrasts in the reproduced data page. Illustrative of these effects is a non-uniform beam intensity profile, such as a Gaussian distribution, or an effect caused by an optical device, because a simple optical device results in a non-uniform image. In single drive, these effects are static for each hologram, while they vary between different drives. The method according to the invention makes it possible to correct such tolerances. However, in this case, the correction profile is preferably obtained by sequentially writing and reading one or more test holograms, or optical, to project a test data page onto a detector with or without media. It is determined by using all or part of the device. A specific optical device can also be used. The correction profile is then adjusted so that a uniform contrast is achieved. The determined correction profile is static and applied to all further holograms. For this purpose, simple optical elements, such as specifically adapted filters, are also used. Of course, both determining a static correction profile and calculating a dynamic correction profile are possible as well. The profiles are then combined and applied during recording.

  For a better understanding, the present invention is explained in more detail in the following description with reference to the drawings. It is to be understood that the invention is not limited to this exemplary embodiment and that specific features are preferably combined and / or improved without departing from the scope of the invention.

  FIG. 1 shows a vertical section of the material of the holographic recording medium 1. According to the irradiation history, the sensitivity of the material changes in the volume of the holographic material. Two different cases are shown in which the history of the recorded hologram causes a non-uniform sensitivity distribution within the holographic material. In FIG. 1 a), the actual hologram is recorded by the recording beam 2 between two or more sensitivity areas resulting from the already recorded hologram 3 shown in fine line areas. Arrow 4 indicates a region with a high sensitivity of s = 1, while arrow 5 indicates a region with a low sensitivity of s = 0.5. The sensitivity value is an arbitrary value used only for the purpose of illustration. In FIG. 1 b) a series of already recorded holograms 3 results in a sensitivity step in the area indicated by the arrow 6. The sensitivity step is again an arbitrary value and s = 1, s = 0.75, s = 0.5, and s = 0.25.

  Using the beam propagation method, the effect of the above sensitivity distribution on the contrast in the region of the data page was simulated. The resulting contrast distributions in the reconstructed data page for the two illustrations of FIGS. 1a) and 1b) are shown in FIGS. 2a) and 2b), respectively. Obviously, some of the reconstructed data pages show low contrast. In FIG. 2 a), the contrast is s = 1 in the region indicated by “A”, while the contrast is s = 0.5 in the region indicated by “B”. In FIG. 2b), the contrast is s = 1 in the region indicated by “C”, while the contrast is s = 0.2 in the region indicated by “D”.

  In FIG. 3a), bit errors due to the non-uniform state of FIG. 1b) are shown. Bits detected in error are shown in white. FIG. 3b) shows the bit error in the uniform case.

  The method according to the invention is shown schematically in FIG. Before recording a data page as a hologram on a holographic recording medium, a correction profile is determined (10). The correction profile takes into account static and / or position-dependent contrast changes. A data page is then generated by modulating the light beam using a spatial light modulator (11). The determined correction profile is then applied to the data page (12). This is done by the spatial light modulator during the generation of the data page or by using a further spatial light modulator or filter. Finally, a data page is recorded on the holographic recording medium (13).

  In holographic data storage, digital data is stored by recording interference fringes created by the superposition of two coherent laser beams. In FIG. 5, an exemplary holographic storage system 20 according to the present invention is depicted. Of course, the present invention can be implemented in other types of holographic storage systems as well. A coherent light, for example a laser diode light source 21 emits a light beam 22 which is collimated by a collimator lens 23. The light beam 22 is then split into two individual light beams 2, 26. Illustratively, the splitting of the light beam 22 is accomplished using a beam splitter 24. However, it is equally possible to use other optical components for this purpose. A spatial light modulator (SLM) 25 modulates one of the two beams, the so-called “target beam”, ie the recording beam 2, and applies a data page, ie a two-dimensional data fringe. Both the recording beam 2 and a further beam 26 called “reference beam” are focused by the objective lens 27 onto the holographic recording medium 1, for example a holographic disk. Interference fringes appear at the intersection of the recording beam 2 and the reference beam 26 and are recorded on the photosensitive layer of the holographic recording medium 1. During recording, the spatial light modulator 25 also applies a correction profile to the recording beam 2. As an alternative, the correction profile can be applied to the reference beam 26 or to both the two beams 2, 26 individually or after their superposition.

  The stored data is read from the holographic recording medium 1 by irradiating a hologram recorded using only the reference beam 26. The reference beam 26 is diffracted by the hologram structure and produces a copy of the original recording beam 2, ie a reconstructed recording beam 29. This reconstructed recording beam 29 is collimated by an objective lens 27 and directed by a further beam splitter 28 to a two-dimensional array detector 30, for example a CCD array. The array detector 30 makes it possible to reconstruct the recording data.

2 shows a vertical cross section of a material of a holographic recording medium having a non-uniform sensitivity distribution. Fig. 4 shows the resulting contrast distribution in a reproduced data page. FIG. 1b shows the bit error rate due to the non-uniform state of FIG. 1 schematically shows a method according to the invention. 1 represents a holographic storage system according to the invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Holographic recording medium 2 Recording beam 3 Already recorded hologram 4 Arrow 5 Arrow 6 Arrow 10 Correction profile determination 11 Data page generation 12 Correction profile application 13 Data page recording 20 Holographic storage system 21 Light source 22 Light beam 23 collimator lens 24 beam splitter 25 spatial light modulator 26 reference beam 28 beam splitter 29 reconstructed recording beam 30 two-dimensional array detector

Claims (5)

A method of recording a data page having a plurality of pixels on a holographic recording medium,
Determining a correction profile for the data page to be recorded , the correction profile being influenced by the recorded hologram on the sensitivity of the material of the holographic recording medium and the recording of the data page to be recorded Taking into account the temporal and spatial history of other holograms recorded at the position ;
To generate a data page with data to be the recording, the step of modulating a light beam using the spatial light modulator,
Applying a pre-Symbol correction profile data page to be the records with a further spatial light modulator,
Recording the data page as a hologram on the holographic recording medium;
Including a method. Wherein the correction profile, by one or more test holograms continuously written and read, and is determined by adjusting the correction profile, Method according to claim 1. The method according to claim 1 , wherein the correction profile takes into account at least one of the profiles of the light beam and contrast non-uniformities due to optical arrangement. Wherein the correction profile, the applied by modulating the luminance of the pixels of the data page to be recorded, the method according to any one of claims 1 to 3. An apparatus for writing a data page having a plurality of pixels to a holographic recording medium,
A spatial light modulator that modulates the light beam to generate the data page to be recorded,
The compensation profile, and a further spatial light modulator to be applied to the data page to be recorded,
Wherein the correction profile, the temporal and spatial history of other holograms recorded in the recording position of the data page to be recorded, and the influence of a recorded hologram on the sensitivity of the material of the holographic recording medium Determined from the device.

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