This application claims the benefit, under 35 U.S.C. §119, of European Patent Application No. 06123229.4 on 31 Oct. 2006. 
     FIELD OF THE INVENTION 
     The present invention is directed to a method for generating a security mark on an optical data carrier, a data carrier, comprising a security mark, a method for reading out the security mark stored on the data carrier and a device for reading out the security mark stored on the data carrier. 
     BACKGROUND OF THE INVENTION 
     The term security mark is to denote a bit sequence, which is for example used for providing an authentication key or decrypting key on a data carrier. Authentication and encryption are commonly used on data carriers in order to prevent unauthorized copying of information, which is stored on the data carrier and protected by copyright law. The authentication key is used for establishing or confirming that the data carrier contains a legally sold copy and has not been copied in breach of copyright protection. The readout of data from the data carrier is prevented if the authentication key is inexistent or incorrect. Alternatively, the data may be encrypted, such that a decrypting key is necessary for obtaining the stored information from the read out bit sequences. 
     The security mark may only protect unauthorized copying and reproduction of data stored on a data carrier if the security mark may not be copied using standard disk drives. Conventional disk drives such as Compact Disks (CD), Digital Versatile Disks (DVD) or BluRay Disks (BD) use a focused laser beam for storing data on the disk. Thereby, a series of microscopic indentations (“pits”, with the gaps between them referred to as “lands”) are formed in the recording area of the disk. The laser beam is directed onto a reflective surface of the disk to read the pattern of pits and lands. The pattern of changing intensity of the reflected beam is converted into binary data. 
     The international patent application WO 2005/048256 discloses using a hologram, from which a key or authentication mark can be derived. A hologram is an advanced form of photographic recording that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store, retrieve, and process information. To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction pattern on the film, which is called the hologram or the interference pattern. Therefore two coherent light beams, an object beam and a reference beam, are necessary for recording a hologram. A conventional disk drive does not have the means for recording a hologram. The technical equipment and know how for copying a holographic recording is complex and expensive. Furthermore, the analysis and reproduction of the diffraction pattern on the film is a formidable task. Therefore, unauthorized copying and reproduction of the security mark is impeded. 
     However, the security mark stored as a holographic recording may not be readout using a standard disk drive. According to the international patent application WO 2005/048256 a unique spatial modulation filter must be applied at the level of the analysis of the security mark. Therefore, the holographic security mark according to the state of the art necessitates a difficult and expensive modification of the disk drive in order to be readable. The data carriers having these holographic security marks are not compatible with existing disk drives. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an improved data carrier, comprising a security mark, which may not be copied using standard disk drive. The security mark should be readable using disk drives, which do not require an ample and expensive modification in relation to conventional disk drives. Furthermore, it is object to provide a method for generating the improved optical data carrier having the security mark, a method for reading out the security mark and a device for reading out the security mark. 
     The object is solved by the method for generating a security mark on an optical data carrier and the respective optical data carrier according to the appended claims. 
     According to the present invention, a method for generating a security mark on an optical data carrier is provided. The method comprises the steps of directing a first and second light beam onto a hologram area of the optical data carrier. The first and second light beams are coherent light beams. Consequently, the first and second light beams form an interference pattern, if they are superimposed on each other. The first light beam is directed onto the hologram area along a first direction. Preferably, the first direction is perpendicular relative to the surface of the optical data carrier. Simultaneously, the second light beam is directed onto the hologram area along a second direction. The second direction is tilted by a predefined tilt angle with respect to the first direction. The first light beam and the second light beam are either directed to the same side or to opposite sides of the optical data carrier. 
     The first and second light beams form an interference pattern, which is recorded in the hologram area. A conventional disk drive may not record an interference pattern, since it does not have the means for generating two coherent light beams, which may be simultaneously directed to a hologram area. Therefore, the security mark on an optical data carrier generated in the aforementioned way is safe from unauthorized copying. Furthermore, the reproduction of the security mark may be accomplished without expensively modifying existing disk drives. The normal photodetector provided for readout of the data stored on the optical data carrier is preferably used for reading the data stored in the hologram area. The additional security mark is preferably stored in a hologram area, which is an integral part of the optical data carrier. However, the security marks may likewise be recorded in a separate holographic data carrier, which is affixed to the optical data carrier. An example of such a holographic data carrier is a self-adhesive polymer tape. This material is available at very low cost under the trademark name Tesa Scribos. Of course, other holographic materials may also be used. The holograms stored in the hologram area may represent a visible image of a brand or logo, so that a consumer can identify the product. 
     Preferably, the tilt angle u s  is greater than or equal to −5° and smaller than or equal to 5°, i.e. −5°≦u s ≦5°. Restricting the size of the tilt angle in this way has the advantage that a reproduction of the recording in the hologram area may be accomplished using the normal photodetector provided for readout of the data stored on the optical data carrier. The local shift between the reconstructed images on the detector area is determined by the change in the tilt angle. 
     According to a preferred embodiment of the present invention, the first and/or second light beams are focused onto the hologram area. Thereby, the recording area may be appropriately restricted. A greater amount of data may be stored in the security mark. A focused readout beam needs to be used in order to reproduce the stored security data. Since conventional disk drives provide focused laser beams, no further modification of the disk drive would be needed in terms of the laser beam. However, alternatively a defocused first or second light beam could be used. In this case, the disk drive must be modified in order to generate a corresponding defocused readout beam. The focusing system of the readout device preferably compares the focus position of the data layer with the one necessary to read out the hologram. This is advantageously used as an additional security feature. 
     Preferably, a single laser source is used for generating the first and second light beams. A beam splitter is used for splitting laser light emitted from the laser source into the first and second light beams. Due to the long coherence length of laser light, the first and second light beams interfere when they reach the hologram area. 
     A plurality of identical security marks are preferably generated in the hologram area by shifting said first and second light beam in a third direction, e.g. perpendicular to the first and second direction. Consequently, the plurality of identical marks forms a line of overlapping identical security marks in the hologram area. The tilt angle is preferably perpendicular to the line. In this case, a reconstructed light beam does not change, if the readout light beam is shifted along the line of identical security marks. The readout of the security mark is insensitive to a shift of the readout light beam in the direction of the line of identical security marks. No additional guiding track for positioning of the readout light beam needs to be provided. Thereby, the additional cost for producing the security mark may be lessened. If the tilt angle is parallel or diagonal to the line, a guiding track needs to be provided. 
     Favorably, the hologram area is arranged on the optical data carrier in such a way that the line of overlapping identical security marks is perpendicular to a scanning direction for reading the information stored in the hologram area. Conventionally, the scanning direction is perpendicular to a radial direction of a circular disk. The security marks may be positioned between the centre of the circular disk and a data storage area. In this configuration, the lines are aligned along a radius of the circular disk. 
     The present invention is directed to an optical data carrier, which comprises a security mark. The security mark is preferably generated using the method for generating a security mark on an optical data carrier according to the present invention. However, the optical data carrier of the present invention may also be generated using other methods. The same security marks may be produced using a master hologram and the known process of contact replication, as described by Inphase et al in Optics Letter 2006, p. 1050. 
     The present invention also relates to a method for reading out the security mark stored on the optical data carrier according to the present invention. The inventive method comprises the step of directing a readout light beam onto the security mark along the first direction. A reconstructed light beam along the second direction is generated by the security mark, which is directed onto a detection area. An information bit corresponding to said tilt angle is detected by determining an intensity distribution of the reconstructed light beam on the detection area. The recorded security mark forms different diffraction patterns for the readout light beam depending on the tilt angle used for recording. Therefore, the reconstructed light beam, which is generated by the diffraction of the readout light beam by the security mark, forms different patterns on the detection area depending on the tilt angle. The pattern of the reconstructed light beam is detected in order to determine the information bit stored in the security mark. Information is preferably encoded in the sign of the tilt angle. However, if a position sensitive photo detector is used, it is also possible to encode information in the value of the tilt angle. 
     Preferably, for detecting the intensity distribution of the reconstructed light beam the detection area is divided into a plurality of adjacent photo detector areas. The detected light intensity of the adjacent photo detector areas is compared with each other. If, for example, the detection area is split in only two adjacent photo detector areas, then it is possible to determine which photo detector area has received more light by comparing the light intensity detected by the respective photo detector areas. 
     A readout device for reading out the security mark stored on the data carrier according to the present invention comprises a light source adapted to direct a readout light beam onto the security mark along the first and/or second direction. The readout light beam interacts with the security mark and generates a reconstructed light beam. A collimator is adapted to direct the reconstructed light beam onto a photo detector. The photo detector is adapted to detect an information bit corresponding to the tilt angle by analyzing the intensity distribution of the reconstructed light beam on the photo detector. 
     Preferably, a readout device for reading out the security mark stored on the data carrier according to the present invention comprises a detector having a plurality of photo detectors. The detection area is divided into a plurality of adjacent photo detector areas. The readout device comprises a comparator adapted to compare the detected light intensity of the adjacent photo detector areas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention is described hereinafter with reference to the accompanied drawings. The preferred embodiments may not be construed as limiting the scope of the present invention, which is defined by the appended claims. They are only meant to exemplify the present invention. 
         FIG. 1  shows a top view of a schematic depiction of a data carrier having a region with security marks according to the preferred embodiment of the present invention, 
         FIG. 2  shows a top view of a close up of the region of security marks of the data carrier of  FIG. 1 , 
         FIG. 3  shows a top view of a close up of a line of security marks of  FIG. 2 , 
         FIG. 4  shows a lateral view of the data carrier of  FIG. 1 , 
         FIG. 5  shows a device for generating a security mark on an optical data carrier, 
         FIG. 6  shows a readout device for reading out the security mark stored on the data carrier of  FIG. 1 , 
         FIG. 7  shows a detection area of the readout device of  FIG. 6 , 
         FIG. 8  shows the detection area of  FIG. 7 , wherein the detection area is illuminated by first readout light beams, 
         FIG. 9  shows the detection area of  FIG. 7 , wherein the detection area is illuminated by second readout light beams, and 
         FIG. 10  shows a graph of normalized total signal intensity on the detection area of  FIG. 7  versus a shift of the incident readout light beam. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a top view of a schematic depiction of a data carrier having a region with security marks according to the preferred embodiment of the present invention. The data carrier has the shape of a disk  1 . A hologram area  10  is provided on the disk  1  in a region close to the centre of the disk  1 . The hologram area  10  is either an integral part of the disk  1 , or it is an independent holographic storage medium fixed to the disk  1 . The hologram area  10  forms an annulus, i.e. a ring-shaped geometric figure. The disk  1  comprises a region  20  for data storage. The data storage region  20  also has an annular shape, which encloses the hologram area  10 . 
       FIG. 2  shows a top view of a close up of the hologram area  10  of the data carrier of  FIG. 1 . The hologram area  10  is used for storing security marks on the data carrier. The security marks are formed as a plurality of stripes  30 , which are arranged essentially in parallel to each other. More precisely, each stripe is collinear with a straight line projecting from the centre of the circular data carrier in  FIG. 1  to the circumference of the circular data carrier. Adjacent stripes are approximately parallel to each other. Though the stripes  30  are drawn as separate stripes  30 , they can likewise overlap. 
     In  FIG. 3  four stripes  30  of the hologram area  10   FIG. 2  are depicted. The stripes  30  consist of a plurality of approximately circular holograms  40 , which are superimposed upon each other. As a consequence, the reconstructed object beam does not change when the readout beam is shifted along the stripe. The readout of the security mark is insensitive to a shift of the readout light beam in the direction of the stripe of identical holograms  40 . Therefore, no additional guiding track for positioning of the readout light beam needs to be provided. 
     Each stripe  30  in  FIG. 3  represents a single bit of information. The information bit is read out by illuminating the stripe  30  with a readout light beam. Through diffraction by the holograms  40  a reconstructed object beam is generated. The pattern of the reconstructed object beam remains virtually unchanged if the readout light beam is shifted in the lengthwise direction of each stripe  30 . Therefore, the radial position of the readout light beam with respect to the stripes  30  in the hologram area  10  is uncritical. Preferably, the light beam for readout is focused in the plane of the hologram area  10 . The focus position can be controlled via the conventional focus servo providing that the disk is sufficiently reflective in the region of the hologram area  10 . 
       FIG. 4  shows a cross section of the data carrier of  FIG. 1 . The centre of the circular data carrier is depicted by the vertical line  60  through the center hole of the data carrier. The surface of the data carrier is essentially flat except for the annular hologram area  10 , which sticks out from the surface of the data carrier. In this case a readout apparatus for the data carrier  1  needs to be capable of focusing a readout light beam. onto the surface of the data carrier. Of course, the hologram area  10  may likewise be integrated within the data carrier, as indicated by the dashed lines. 
       FIG. 5  shows a device for generating a security mark on an optical data carrier. The optical data carrier  1  and the hologram area of  FIG. 1  are designated by reference signs  1  and  10 , respectively. A laser source  3  is shown in  FIG. 5 . The laser light emitted from the source  3  is collimated by a collimator lens  4 . It is directed through a beam splitter  5 , which outputs a first collimated laser beam  7  and a second collimated laser beam  6 . The first laser beam  7  is directed along a first direction and focused onto the hologram area  10  by a first objective lens  8   a . The second laser beam  6  is directed along a second direction through the second objective lens  8   b  onto the hologram area  10 . Two fixed mirrors  11   a ,  11   b  and an adjustable mirror  12  are provided in  FIG. 5  in order to redirect the second laser beam  6 . The second laser beam  6  reaches the hologram area  10  from the opposite side than the first laser beam  7 . Of course, it is likewise possible that both laser beams  6 ,  7  impinge on the hologram area  10  from the same side, e.g. from the side of the first laser beam  7 . Said second direction of the second laser beam  6  is slightly tilted by a tilt angle with respect to the direction of the first laser beam  7 . The tilt angle can be adjusted by the adjustable mirror  12 , which directs the second laser beam  6  through the second objective lens  8   b.    
     The first and second laser beams  7  and  6  overlap within the hologram area  10  and create an interference pattern within the material. The interference pattern depends on the tilt angle, which is preferably fixed between −5° and 5°. The maximum angle depends on the type of optical data carrier and the objective lens used for playback. For example, for a Compact Disk a larger angle is needed than for a BluRay disk. The hologram area  10  in  FIG. 5  is shown in a cross sectional view similar to the depiction in  FIG. 4 . The interference pattern introduced into the holographic material corresponds to one of the holograms  40  in  FIG. 3 . The second laser beam  6  in  FIG. 5  is tilted in a direction perpendicular to the lengthwise direction of one of the stripes  30  in  FIGS. 2 and 3 . The tilt angle lies in a plane perpendicular to the radial direction of the circular data carrier  1  in  FIG. 1 . Therefore, the optical properties of the resulting interference pattern do not change in the lengthwise direction of the stripes  30 . However, the shift selectivity in a direction perpendicular to the length of the stripes  30  is high. An information bit of 0 or 1 is encoded by the positive or negative shift angle of the second laser beam  6  and the resulting interference pattern. Several overlapping holograms  40  are written for each data bit. Furthermore, the radius of the two focused laser beams  6  and  7  within the hologram area  10  has the same magnitude. 
       FIG. 6  shows readout device for reading out the security mark stored on the data carrier of  FIG. 1 . Identical reference signs in  FIGS. 5 and 6  denote similar objects in both Figures. In particular the laser source  3 , the collimator lens  4 , and the collimated laser beam  7  are arranged essentially in the same way as in  FIG. 5 . The laser beam  7  is directed through a beam splitter  13  and an objective lens  8  onto the hologram area  10 . A security mark is stored in the hologram area  10  in  FIG. 6 . A reflective layer  18  and a substrate  19  are positioned below the hologram area  10 . Furthermore, the readout device comprises a detector  15  for detecting the light reflected from the reflective layer  18 . A focus lens  14  focuses the reflected light onto the detector  15 . A comparator  20  analyzes the intensity distribution detected by the detector  15  for determining the tilt angle of a reconstructed signal beam  17 . 
     Three different laser beams  7 ,  16  and  17  are shown in  FIG. 6 . All three beams reach the detector through the detector lens  14 . The first beam  7  is a beam, which passes through the interference pattern of the hologram without being diffracted. The reflection reaches the beam splitter  13  and is reflected from there to the detector  15 . Part of the laser beam  7  is diffracted by the interference pattern stored in the hologram area  10  and generates the signal beam  17 . This signal beam  17  is directed through the objective lens  8 , the beam splitter  13  and the focus lens  14  onto the detector  15 . As the signal beam  17  is slightly tilted, the focus of the signal beam  17  is situated off the centre on the detector  15 . In addition, there is a second signal beam  16 , which is created by the reflected, phase-conjugated readout beam  7 . This phase-conjugated signal beam  16  propagates first to the reflective layer  18 , before it is focused onto the detector  15 . Due to the reflection losses at the mirror layer  18  the phase-conjugated signal beam  16  is much weaker than the direct signal beam  17 . As a consequence, the light pattern on the detector surface of the detector  15  is formed by the superimposition of three distinct beams  7 ,  16  and  17 . 
       FIG. 7  shows the detection area of the detector  15  of  FIG. 6 . The detection area is formed of four distinct detector elements a, b, c and d, which are positioned in a chequered arrangement. 
       FIGS. 8 and 9  each show the detection area of  FIG. 7 . The detection area is illuminated by the different light beams  7 ,  16 ,  17  coming from the hologram area  10  of  FIG. 6 . Light beams  7 ,  16  and  17  correspond to the circular spots  7   a/b ,  16   a/b  and  17   a/b , respectively. The undiffracted laser beam  7  forms a circular spot  7   a/b  in the centre of the detection area in both  FIGS. 8 and 9 . The position of the spot resulting from the signal beam  17  is shifted to the right in  FIG. 8  (spot  17   a ) and to the left in  FIG. 9  ( 17   b ). The spot resulting from the phase-conjugated signal beam  16  is shifted to the left in  FIG. 8  (spot  16   a ) and to the right in  FIG. 9  (spot  16   b ). The spots  7   a  and  7   b  of the undiffracted laser beam  7  have the greatest intensity of all the spots. Therefore the intensity of the spots  17   a / 17   b  is larger than the intensity of the spots  16   a/b . Therefore, the light intensity distribution in  FIG. 8  is shifted to the right, whereas the light intensity distribution in  FIG. 9  is shifted to the left. The position of the spots  16   a/b  and  17   a/b  depends on the tilt angle used during the production of the interference pattern in the hologram area  10 . Therefore, the information bit corresponding to the shift angle may be determined by finding out whether the intensity distribution is shifted to the left or right on the detection are. Let Ia, Ib, Ic and Id represent the total light intensity in the respective detector areas a, b, c and d. Then, the sign of S=(Ia+Ic−Ib−Id)/(Ia+Ib+Ic+Id) is representative of the stored information bit. The signal S is similar to a tangential push-pull signal. If a position sensitive photo detector is used, it is also possible to encode more than one information bit. In addition to the sign of the tilt angle also the value of the tilt angle can be detected. 
     The total normalized diffracted signal as a function of the readout beam shift is given by the following equation: 
     
       
         
           
             
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     In this equation, the variables δx, δy represent the shift of the incident light beam  7  in  FIG. 6  with respect to the recorded interference pattern in the hologram area. δy represents a shift of the incident light beam in a radial direction of the disk in  FIG. 2 . δx represents a shift in a scanning direction of the incident laser beam, which is perpendicular to the radial direction. The above formula applies for a single, non overlapping hologram recorded using the setup of  FIG. 5 . L x  and L y  designate the lateral extension of the hologram. L is the thickness of the hologram. n is the refractive index of the holographic material. R 1  is the radius of the focused incident laser beam  7  in  FIG. 5 . R 2  is the radius of the focused laser beam  6  in  FIG. 5 ; U s  represents the tilt angle between the incident laser beams  7  and  6  in  FIG. 5 . λ is the wavelength of the light emitted by the laser source. If both incident waves  6  and  7  have the same radius (R 1 =R 2 ), then the resulting total normalized diffracted signal is independent of a signal shift in the direction δy. 
       FIG. 10  shows the total normalized diffracted signal I(δx,δy) in relation to the shift δx. Since the signal is normalized, the maximum signal intensity is set equal to one. The curve in  FIG. 10  has been calculated for the following values, which are realistic for a disk using a blue laser beam.
 
R 1 =R 2 =R=110 μm
 
L=10 μm
 
L x =L y =70 μm
 
n=1.6
 
u s =3°
 
     The total normalized diffracted signal represents a distribution, which has a singular peak at δx=0. The total signal intensity decreases continually in the positive and negative direction δx. At around ±5 μm the total signal intensity has reached less than ⅕ of the peak. The shift selectivity is roughly about 10 μm. Therefore, adjacent interference patterns are distinguishable if they are recorded at a distance of 10 μm. If the security markers are recorded in a distance r=20 mm from the centre of the disk in  FIG. 2 , then the circumference of the security marker region is equal to 2πr and the total number of bits is equal to 2πr/10 μm=12500 raw data bits. 
     The main advantages of the security mark recorded according to the preferred embodiment are the following. The additional security mark is compatible with existing disk families such as Compact Disk (CD), Digital Versatile Disk (DVD) and BluRay disk (BD). The readout device of  FIG. 6  corresponds largely to existing readout devices. A comparator  20  for determining the light intensity distribution on the detector area has to be added to existing readout devices in order to detect the stored information bit. However, the standard photodetector for data readout can be used for reading the security marks. The costs for adapting the existing readout devices are low. The production of the respective data carriers having the security mark is also very low. 
     The above preferred embodiment is not intended to restrict the scope of protection to be conferred to the present application. The preferred embodiment is only meant to exemplify a convenient way for implementing the invention. The invention is defined by the appended claims.
A device for producing a security mark on an optical data carrier includes directing means for directing a first and a second light beam onto a hologram area of the optical data carrier, wherein the first and second light beams are coherent light beams. The directing means are adapted to direct the first light beam onto the hologram area along a first direction, and the second light beam onto the hologram area along a second direction. Tilting means are provided for tilting the second direction by a predefined tilt angle with respect to the first direction.