Sync mark correction for holographic data pages

The presented invention relates to a method for reading data from a data page from an optical data storage medium, e.g. a holographic storage medium and to an apparatus for performing this method. At least one missing or wrong positioned sync mark on a read data page is identified and its corrected position is estimated. The estimated corrected sync mark position is used for further processing.

This application claims the benefit, under 35 U.S.C. §119 of EP Patent Application 07122928.0 filed Dec. 11, 2007.

FIELD OF THE INVENTION

The present invention relates to a method for reading data from a data page for optical data storage in an optical storage system, e.g. a holographic storage system, including the correction of missing or wrong positioned sync marks, and to an apparatus for reading from an optical storage medium performing this method.

The invention is described below using a holographic storage system as an example. It is apparent to a person skilled in the art that the invention is applicable within other optical storage systems.

BACKGROUND OF THE INVENTION

In holographic data storage digital data are stored by encoding 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 (SLM) 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 recoding. 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 single or few two-dimensional 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 intensity variations, i.e. a two-dimensional binary array or an array of grey values, which code multiple bits. Data pages consisting of patterns showing different phases can also be used. This allows achieving increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the SLM and detected with a detector array.

Data pages include synchronization marks, also referred to as sync marks, to determine the exact scaling factor from the SLM to the detector and to correct image distortion. Sync marks usually consist of a specific bit pattern, which is known and can be identified clearly by the reading apparatus. For any holographic data storage system the correct sync mark detection is essential for a successful demodulation procedure. As the scaling factor and the image distortion can vary locally, sync marks are usually distributed over the entire data page. If the sync mark detection fails in a part of the data page then in most cases the demodulation will also fail in this region. Due to defects in the holographic material or distortions such as detector noise the correct detection of a local sync mark may fail.

SUMMARY OF THE INVENTION

It is an object of the invention to propose a method for detecting such failures and/or correcting wrong positioned or missing sync marks. Readout is improved by using estimated sync marks.

According to the invention, the method for reading data from a data page from an optical data storage medium has the steps of:identifying at least one missing or wrong positioned sync mark on a read data page,estimating a corrected sync mark position, andusing the estimated corrected sync mark position for further processing.

According to this method, wrong positioned or missing sync marks can be identified even if a number of sync mark detections failed. Nevertheless, it is assumed that most of the sync marks are detected correctly to provide a basis for the calculation of the estimated corrected sync mark position. Using the proposed sync mark correction allows to determine image distortion and locally varying scaling factors on a data page reliably. The method is numerically simple and efficient. It can be implemented independent from the underlying modulation scheme of a data page. After the replacement the possibility to demodulate the data correctly in the region of the formerly wrong positioned sync mark increases. The bit error rate is reduced.

Advantageously, the method is used for reading from a holographic data storage medium. In a holographic data storage system, data are stored using two-dimensional data pages. In this case, it is of particular importance to determine image distortion and locally varying scaling factors previous to demodulation. To perform this determination reliably, correct sync mark positions are needed. Increasing the accuracy of the sync mark detection increases the reliability of the readout process.

Advantageously, further data processing according to the invention includes data detection in the region of the missing or wrong positioned sync mark using the estimated sync mark position. Data is read out reliably essentially independent from wrong positioned or missing sync marks.

Favourably, wrong positioned or missing sync marks are corrected by estimating their correct position by interpolating sync marks from corresponding rows and/or columns. Sync mark positions are estimated, e.g. by the intersection of two regression curves of the corresponding row and column. This leads to an accurate estimation of the corrected sync mark position.

Advantageously, the corrected sync mark position is estimated by interpolating sync marks from a set of sync marks nearby. Within a defined area around a wrong positioned or missing sync mark, other sync marks are used to determine the correct position of the wrong positioned or missing sync mark. Consequently, local differences of the optical path and of the optical elements are considered during estimation of the corrected sync mark position.

Favourably, a measure for the deviation of a sync mark position from its expected position is calculated to detect wrong positioned or missing sync marks. The measure is calculated, for example, using regression curves of the corresponding row or column, or the measure is calculated using other sync marks within a defined area around an estimated sync mark position. The mathematical measure is calculated, for example, using quadratic filtering of the read out sync marks or using the statistical variance of the sync mark positions. The mathematical measure can be adapted to an algorithm which fits best to the system configuration. The invention is flexible and adjustable to the basic conditions.

Favourably, a data area around a wrong positioned sync mark is shifted according to a sync mark deviation. This improves read out accuracy.

According to a further aspect of the invention an apparatus for reading a data storage media uses a method according to the invention for detecting missing or wrong positioned sync marks and correcting the sync mark positions.

Favourably, in a holographic storage medium the sync marks have a light intensity distribution and a spatial frequency distribution similar to the light intensity and spatial frequency distribution of the data blocks. A similar light intensity and spatial frequency distribution over the whole data page leads to a more uniform utilization of the holographic material resulting in a higher storage capacity. Using the method for sync mark detection according to the invention, the light intensity of the sync marks can be reduced and adapted to the light intensity distribution of the whole data page. The sync mark detection errors resulting from the lower light intensity of the sync marks are corrected by the invention.

For better understanding the invention shall now be explained in more detail in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1schematically depicts a part of a data page1including two rows5and three columns4of sync marks2. A sync mark2is composed of e.g. a 5×5 pixel pattern. Different sync marks2can be used and distributed over the data page1to put additional information into the sync marks2. The sync mark detection is realized e.g. by searching for local maxima in the correlation of the read out data page and the sync mark pattern. The distance between two adjacent sync mark columns4is dx. The distance between two adjacent sync mark rows5is dy. The offset from the upper left corner of the data page1to the first sync mark position is ox and oy, respectively. Data blocks3are indicated schematically by dots.

FIG. 2shows an example of a part of a data page1as a combination of digital patterns. A pattern like this is imprinted on an SLM during writing. Sync marks2have a specific shape and can be retrieved from the figure. This data page1contains four sync marks2in a row5and four sync marks in a column4. In this example, sync marks2are equally spaced. The positions of the sync marks2on a data page1and the pattern of the sync marks2are also known in advance by the reading apparatus. The white lines are for clarification only and divide the data page1into subpages, each one containing 4×4 blocks. One of the blocks is a sync block2.

FIG. 3schematically shows a part of a data page1, including sync marks2arranged in rows5and columns4. The dotted lines indicate a row5and a column4of sync marks. They are not the same as the lines indicated inFIG. 2showing the subpages. There is one wrong positioned sync mark6indicated. The correct detection of the sync mark failed. Sync marks in row5and column4are assumed to be detected correctly. The expected position of sync mark6is at the intersection point of the interpolation curves of sync marks in row5and column4. The distance of a sync mark6from the expected position for this sync mark can be expressed by a mathematical measure, e.g. the variance. If this measure exceeds a certain limit, it is assumed that the sync mark detection failed. In this case, the corrected position of the sync mark is estimated to be at the intersection point of the interpolation curves of the correctly detected sync marks. The sync mark which was identified to be detected at a wrong position is shifted to this intersection point.FIG. 4shows the replacement of the sync mark7.

FIG. 5shows an example of the detected data pattern depicted inFIG. 2. Sync marks2can be identified in the picture as bright areas. Generally, sync marks2have a higher ratio between ‘on’ and ‘off’ pixels compared to a data block3. Nevertheless, on a detected data page, they cannot be identified by eye. The data page1shown inFIG. 5uses sync marks2with an even higher light intensity compared to the data blocks3for demonstration purposes only.

The distance between two adjacent sync marks2is known in advance. Therefore, a grid9can be established on a data page1. Between four neighbouring grid points9, a sync mark2has to be detected. In case a part of the data page8is not detected correctly, also the sync mark detection within this area fails. This can be caused e.g. due to a bad signal-to-noise ratio (SNR), local defects in the holographic material or local defects of the detector or the optical apparatus. Without using the method according to the invention, in the area11between four neighbouring grid points9the sync mark7is not readable. The sync mark is determined at a data block6which looks most similar to a sync pattern. All four subpages10using the erroneous sync mark6as a boundary show significant read errors. In contrast, using a corrected sync mark7according to the invention allows reading all data within the four surrounding subpages10except for the data in the defect region8. A significantly reduced error rate arises.

FIG. 6shows a data page1using sync marks2with a light intensity and a spatial frequency distribution similar to the light intensity and spatial frequency distribution of the whole data page1. The sync marks2can not be identified by eye. The SNR on the sync mark2is worse compared to the SNR of the sync marks2shown inFIG. 5.

InFIG. 7an apparatus20for reading and/or recording a holographic storage medium29is shown schematically. A source of coherent light, e.g. a laser diode21, emits a light beam22, which is collimated by a collimating lens23. The light beam22is then divided into two separate light beams26,27. In the example the division of the light beam22is achieved using a first beam splitter24. However, it is likewise possible to use other optical components for this purpose. A spatial light modulator (SLM)25modulates one of the two beams, the so called “object beam”26, to imprint a two-dimensional data pattern. Both the object beam26and the further beam, the so called “reference beam”27, are focused into a holographic storage medium29, e.g. a holographic disk or card, by an objective lens28. At the intersection of the object beam26and the reference beam27an interference pattern appears, which is recorded in a photo-sensitive layer of the holographic storage medium29.

The stored data are retrieved from the holographic storage medium29by illuminating a recorded hologram with the reference beam27only. The reference beam27is diffracted by the hologram structure and produces a copy of the original object beam26, the reconstructed object beam30. This reconstructed object beam30is collimated by the objective lens28and directed onto a two-dimensional array detector32, e.g. a CCD-array, by a second beam splitter31. The array detector32allows to reconstruct the recorded data.