Patent Publication Number: US-7898924-B2

Title: Apparatus, system, and method for calibrating a holographic storage device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to holographic storage devices and more particularly relates to calibrating holographic storage devices. 
     2. Description of the Related Art 
     Data processing systems are storing ever-larger quantities of data. To meet this increasing demand, storage device manufacturers are increasing the areal densities of magnetic storage devices, optical storage devices, and the like. Unfortunately, limitations such as material limitations, paramagnetic limits, limitations on the wavelengths of available lasers, and the like may restrict future increases in areal densities. 
     Holographic storage may support increased data storage densities, providing a technology to support the increasing demand for storage. However, to be a viable storage technology, holographic storage media must be interchangeable among a plurality of holographic storage devices, including different models of holographic storage devices and devices from different manufacturers. 
     For example, a first holographic storage device must be able to write data to a storage media that can be read by a second holographic storage device. In addition, if the second holographic storage device writes additional data to the storage media, the first holographic storage device must be able read the additional data from the storage media. Similarly, a third holographic storage device must be able to successfully read all data written by both the first and the second holographic storage devices. 
     Unfortunately, a holographic storage device may be sufficiently sensitive to differences between data written by the holographic storage device and data written by other holographic storage devices that interoperability is compromised. For example, a first and second holographic storage device produced by different manufacturers may each be unable to read data written by the other because of read and write channel sensitivities. 
     SUMMARY OF THE INVENTION 
     From the foregoing discussion, there is a need for an apparatus, system, and method that calibrate a holographic storage device. Beneficially, such an apparatus, system, and method would allow interoperability between a plurality of holographic storage devices. 
     The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available holographic storage device calibration methods. Accordingly, the present invention has been developed to provide an apparatus, system, and method for calibrating a holographic storage device that overcome many or all of the above-discussed shortcomings in the art. 
     The apparatus to calibrate a holographic storage device is provided with a plurality of modules configured to functionally execute the steps of reading a factory-stored hologram from a holographic media and calculating a read difference between the read factory-stored hologram and a first holographic pattern. These modules in the described embodiments include a read channel and a calculation module. The apparatus may also include a calibration module and a write channel. 
     The read channel reads a factory-stored hologram from a holographic media. In one embodiment, the factory-stored hologram is precision written on the holographic media. The factory-stored hologram may reside on a read-only portion of the holographic media. Alternatively, the factory-stored hologram may be inserted in the holographic media as the read-only portion. 
     The calculation module calculates a read difference between the read factory-stored hologram and a first holographic pattern. The first holographic pattern digitally describes the factory-stored hologram. In one embodiment, the calculation module calculates the read difference as a plurality of differences between corresponding data elements of the factory-stored hologram and the first holographic pattern. 
     In one embodiment, the calibration module calibrates the read channel with the read difference. The calibration module may calibrate the read channel by detecting a pattern in the read difference and adjusting the read channel to mitigate the pattern. 
     In one embodiment, the write channel writes a second holographic pattern to the holographic media as a second hologram. The second holographic pattern may digitally describe the second hologram. The write channel may only write the second holographic pattern after a read workload is complete and if and only if there is a pending write workload. 
     If the read workload is complete and if and only if there is the pending write workload, the read channel may read the second hologram from the holographic media with the calibrated read channel. In addition, the calculation module may calculate a write difference between the read second hologram and the second holographic pattern. 
     In one embodiment, the calibration module calibrates the write channel with the write difference. The calibration module may calibrate the write channel by detecting a pattern in the write difference and adjusting the write channel to mitigate the pattern. The apparatus calculates the read difference for the read channel using the factory-stored hologram. In addition, the apparatus may calibrate the read channel using the read difference. The apparatus may also calculate the write difference for the write channel and calibrate the write channel. 
     A system of the present invention is also presented to calibrate a holographic storage device. The system may be embodied in a holographic storage device. In particular, the system, in one embodiment, includes a holographic media, a read channel, and a processor. In addition, the system may also include a write channel. 
     The holographic media is configured to store digital data. In addition, the holographic media includes a factory-stored hologram. The read channel reads digital data from a hologram stored in the holographic media. The write channel may write digital data in the form of a hologram to the holographic media. 
     The read channel reads the factory-stored hologram from the holographic media. The processor includes a calculation module and a calibration module. The calculation module calculates a read difference between the read factory-stored hologram and a first holographic pattern. The calibration module calibrates the read channel with the read difference. The system calculates the read difference and may calibrate the read channel using the read difference. In addition, the system may calculate a write difference and calibrate the write channel using the write difference. 
     A method of the present invention is also presented for calibrating a holographic storage device. The method in the disclosed embodiments substantially includes the steps to carry out the functions presented above with respect to the operation of the described apparatus and system. In one embodiment, the method includes reading a factory-stored hologram from a holographic media and calculating a read difference between the read factory-stored hologram and a first holographic pattern. The method also may include writing a second holographic pattern to the holographic media as a second hologram, reading the second hologram, and calculating a write difference. 
     A read channel reads a factory-stored hologram from a holographic media. A calculation module calculates a read difference between the read factory-stored hologram and a first holographic pattern that digitally describes the factory-stored hologram. In a certain embodiment, a calibration module calibrates the read channel with the read difference. 
     In one embodiment, if a read workload is complete and if and only if there is a pending write workload, a write channel writes a second holographic pattern to the holographic media as a second hologram. In addition, the read channel may read the second hologram from the holographic media with the calibrated read channel and the calculation module may calculate a write difference between the read second hologram and the second holographic pattern. The calibration module may further calibrate the write channel with the write difference. The method may calibrate the read and write channels for a holographic storage device. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     The embodiment of the present invention calculates a read difference using a factory-stored hologram. In addition, the present invention may calibrate the read channel with the read difference. The present invention may also calculate a write difference and calibrate a write channel with the write difference. These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram illustrating one embodiment of a read channel reading a factory-stored hologram of the present invention; 
         FIG. 2A  is a schematic block diagram illustrating one embodiment of a write channel of the present invention; 
         FIG. 2B  is a schematic block diagram illustrating one alternate embodiment of a write channel of the present invention; 
         FIG. 3  is a schematic block diagram illustrating one embodiment of a calibrated read channel of the present invention; 
         FIG. 4  is a schematic block diagram illustrating one embodiment of a holographic calibration apparatus of the present invention; 
         FIG. 5  is a schematic flow chart diagram illustrating one embodiment of a holographic calibration method of the present invention; 
         FIG. 6  is a schematic flow chart diagram illustrating one embodiment of a read calibration method of the present invention; 
         FIG. 7  is a schematic flow chart diagram illustrating one embodiment of a write calibration method of the present invention; 
         FIG. 8  is a top view drawing illustrating one embodiment of a holographic media with inserted factory-stored hologram of the present invention; and 
         FIG. 9  is a top view drawing illustrating one embodiment of a holographic media with read-only portion of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Many of the functional units described in this specification have been labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like. 
     Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions, which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. 
     Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
       FIG. 1  is a schematic block diagram illustrating one embodiment of a read channel  100  reading a factory-stored hologram  122  of the present invention. The read channel  100  includes a light source  105 , a first read lens  132 , a holographic media  120 , a second read lens  134 , a detector  130 , a processor  150 , and a memory  140 . The holographic media  120  further includes an outer layer  125 , a data plane  123 , and a substrate  124 . 
     The outer layer  125  may be configured to transmit light within one or more specified ranges of wavelengths. The data plane  123  may be configured to record holographic data as holographic diffractive volume gratings throughout the volume of the data plane  123 . The substrate  124  may be configured to support the data plane  123  and the outer layer. In one embodiment, the substrate transmits light  110 . Alternatively, the substrate  124  may reflect light  110 . 
     In addition, the holographic media  120  further includes a factory-stored hologram  122 . The factory-stored hologram  120  comprises holographic diffractive volume gratings that encode data. In one embodiment, an array of diffractive volume gratings comprises an array of data elements such as pixels. 
     The holographic media  120  rotates about a virtual Z-axis  101 . A virtual X axis  102  is also shown, while a virtual Y axis is perpendicular to the X axis  102  and the Z axis  101 , and so is not shown. In one embodiment, the virtual Y axis extends from the page. The light source  105  produces a beam of light  110 . In one embodiment, the light source  105  is a laser and the light  110  is coherent light. In a certain embodiment, the light  110  is in the range of two hundred to 700 hundred nanometers (200-700 nm). In a particular embodiment, the light  110  is in the range of two hundred to five hundred nanometers (200-500 nm). 
     The first read lens  132  may focus the light  110  on the data plane  123  of the holographic media  120 . In one embodiment, the first read lens  132  and the focus of the light  110  are adjusted by one or more actuators as are well known to those of skill in the art. 
     In the depicted embodiment, the focused light  110  is transmitted through outer layer  125  of the holographic media  120  and is modified by the holographic diffractive volume gratings of the data plane  123 . The second read lens  134  may focus the modified light  110  on the detector  130 . In one embodiment, one or more actuators adjust the position of the second read lens  134  and the focus of the light  110 . The detector  130  receives the modified light  110 . The detector  130  may be an array of light sensitive semiconductor elements such as a charge-coupled detector (CCD) array of semiconductor elements as is well know to those of skill in the art. Alternatively, the detector  130  may be configured as a charge-integrating array (CID). 
     The light  110  diffracts off the diffractive volume gratings of the factory-stored hologram  122  forming a pixilated pattern on the detector  130 . The detector  130  may record the pixilated pattern as a data array. The processor  150  may read the data array from the detector  130  and store the data array in the memory  140  as data  141 . The memory  140  may also store a first holographic pattern  142  as will be described hereafter. The first holographic pattern  142  digitally describes the factory-stored hologram  122 . 
       FIG. 2A  is a schematic block diagram illustrating one embodiment of a write channel  160  of the present invention. The write channel  160  includes elements of the read channel  100  of  FIG. 1 , like numbers referring to like elements. In addition, the write channel  160  includes a beam splitter  111 , a first write lens  135 , a spatial light modulator  114 , a second write lens  137 , and a third write lens  139 . 
     The beam splitter  111  splits the light  110  into a reference beam  112  and a signal beam  113 . The third write lens  139  may focus the reference beam  112  on the data plane  123  of the holographic media  120 . One or more actuators may adjust the position of the third write lens  139  and the focus of the reference beam  112 . The focused reference beam  112  proceeds to the holographic media  120 . The first write lens  135  may focus the signal beam  113  onto the spatial light modulator  114 . One or more actuators may adjust the position of the first write lens  135  and the focus of the signal beam  113 . 
     The spatial light modulator  114  modulates the signal beam  113 . The spatial light modulator  114  is depicted as a reflective spatial light modulator. In one embodiment, the spatial light module is configured as an array micromechanical mirrors that may be positioned in either a reflective position or a non-reflective position. Electronic signals from the processor  150  may dictate the position of each micromechanical mirror. 
     In an alternative embodiment, the spatial light modulator  114  may be configured as a liquid-crystal-on-silicon (LCOS) semiconductor device. Electronic signal from the processor  150  may polarize or un-polarize liquid crystal cells of the semiconductor device. The signal beam  113  may pass through a liquid crystal cell if the cell is un-polarized, reflect off a substrate, and continue to the holographic media  120 . 
     The spatial light modulator  114  encodes the signal beam  113  with a pixilated data array. The second write lens  137  focuses the encoded signal beam  113  on the dataplane  123  of the holographic media  120 . One or more actuators may adjust the second write lens  137  and the focus of the signal beam  113  at the data plane  123 . At the data plane  123  of the holographic media  120 , the signal beam  113  interferes with the reference beam  112 , creating holographic diffractive volume gratings that store the pixilated data array on the holographic media  120 . In the depicted embodiment, the write channel  160  is shown writing a second hologram  121  that will be described hereafter. 
       FIG. 2B  is a schematic block diagram illustrating one alternate embodiment of a write channel  170  with a transmissive spatial light modulator  131 . The write channel  170  includes many of the elements of  FIG. 2A , like numbers referring to like elements. The write channel  170  also includes the transmissive spatial light modulator  131  and a mirror  133 . 
     The beam splitter  111  splits the light  110  into the reference beam  112  and the signal beam  113 . The mirror  133  redirects the reference beam  112  to the holographic media  120 . The third write lens  139  may focus the reference beam  112  on the on the data plane  123  of the holographic media  120 . One or more actuators may adjust the position of the third write lens  139  and the focus of the reference beam  112 . The focused reference beam  112  proceeds to the holographic media  120 . 
     The first write lens  135  may focus the signal beam  113  onto the transmissive spatial light modulator  131 . One or more actuators may adjust the position of the first write lens  135  and the focus of the signal beam  113 . The transmissive spatial light modulator  131  modulates the signal beam  113 . In the depicted embodiment, the transmissive spatial light modulator  131  is a LCOS semiconductor device. The signal beam  113  may pass through a liquid crystal cell if the cell is un-polarized and continue to the holographic media  120 . The second write lens  137  focuses the encoded signal beam  113  on the data plane  123  of the holographic media  120 . One or more actuators may adjust the second write lens  137  and the focus of the signal beam  113  at the data plane  123 . 
     The signal beam  113  interferes with the reference beam  112 , creating holographic diffractive volume gratings that store the pixilated data array on the holographic media  120  as described for  FIG. 2A . In the depicted embodiment, the write channel  170  is shown writing the second hologram  121 . 
       FIG. 3  is a schematic block diagram illustrating one embodiment of a calibrated read channel  180  of the present invention. The calibrated read channel  180  includes the elements of the read channel  100  of  FIG. 1 . The read channel  100  is calibrated using the factory-stored hologram  122  as will be described hereafter. 
     The calibrated read channel  180  is shown reading the second hologram  121  of  FIG. 2 . The memory  140  may store the data of the second hologram  121  as data  141 . The memory is also shown storing a second holographic pattern  143 . The second holographic pattern  143  digitally describes the second hologram  121 . 
       FIG. 4  is a schematic block diagram illustrating one embodiment of a holographic calibration apparatus  200  of the present invention. The description of the apparatus refers to elements of  FIGS. 1-3 , like numbers referring to like elements. The apparatus  200  includes the read channel  100 , the write channel  160 , a calculation module  202 , and a calibration module  204 . In one embodiment, one or more software processes and one or more data sets stored in the memory  140  and executed and/or used by the processor embody the calculation module  202  and calibration module  204 . Although either the reflective write channel  160  or the transmissive write channel  170  may be employed, hereafter for simplicity the reflective write channel  160  will be referred to as the write channel  160 . 
     The read channel  100  reads the factory-stored hologram  122  from the holographic media  120 . In one embodiment, the factory-stored hologram  122  is precision written on the holographic media  120 . The factory-stored hologram  122  may be precision written using a stable reference write channel. The reference write channel may be contained in an environmental chamber set to nominal temperature and humidity. In addition, the reference write channel&#39;s components may be selected to nominal values. The reference write channel may also be regularly calibrated to a specified standard such as the factory-stored hologram  122 . 
     The factory-stored hologram  122  may reside on a read-only portion of the holographic media as will be described hereafter. Alternatively, the factory-stored hologram  122  may be inserted in the holographic media  120  as the read-only portion as will also be described hereafter. 
     The calculation module  202  calculates a read difference between the read factory-stored hologram  122  and the first holographic pattern  142 . In one embodiment, the calculation module  202  calculates the read difference as a plurality of differences between corresponding data elements from the factory-stored hologram  122  that are stored as data  141  in the memory  140  and the first holographic pattern  142  stored in the memory  140 . 
     In a certain embodiment, the calibration module  204  calibrates the read channel  100  with the read difference. The calibration module  204  may calibrate the read channel  100  by detecting a pattern in the read difference and adjusting the read channel  100  to mitigate the pattern. Such read calibration adjustment may include the read power-level of light source  105 , the duration of the read laser light burst, adjusting the first and second read lenses  132 ,  134 , and the sensitivity of detector  130 . 
     In one embodiment, the write channel  160  writes the second holographic pattern  143  to the holographic media  120  as a second hologram  121 . The write channel  160  may only write the second holographic pattern  143  after a read workload is complete and if and only if there is a pending write workload. As used herein, the read workload comprises one or more data elements that are to be read by the read channel  100  from the holographic media  120  while the write workload comprises one or more data elements that are to be written to the holographic media  120  by the write channel  160 . 
     The calibrated read channel  180  may read the second hologram  121  from the holographic media  120 . In addition, the calculation module  202  may calculate a write difference between the read second hologram  121  and the second holographic pattern  143 . In one embodiment, the calibration module  204  calibrates the write channel  160  with the write difference. The calibration module  204  may calibrate the write channel  160  by detecting a pattern in the write difference and adjusting the write channel  160  to mitigate the pattern. The apparatus  200  calculates the read difference for the read channel  100  using the factory-stored hologram  122 . In addition, the apparatus  200  may calibrate the read channel  100  using the read difference. The apparatus  200  may also calculate the write difference for the write channel and calibrate the write channel  160 . Such write calibration adjustments may include the write power-level of light source  105 , the duration of the write laser light burst, and adjusting the write lenses  135 ,  137 ,  139  and the sensitivity of detector  130 . 
     The schematic flow chart diagrams that follow are generally set forth as logical flow chart diagrams. As such, the depicted order and labeled steps are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown. 
       FIG. 5  is a schematic flow chart diagram illustrating one embodiment of a holographic calibration method  300  of the present invention. The method  300  substantially includes the steps to carry out the functions presented above with respect to the operation of the described apparatus  200 , read channels  100 ,  180 , and write channel  160  of  FIGS. 1-4 . The description of the method  300  refers to elements of  FIGS. 1-4 , like numbers referring to like elements. 
     The method  300  begins and the holographic calibration apparatus  200  calibrates  302  the read channel  100  using the factory-stored hologram  122  as will be described hereafter. Calibrating  302  the read channel  100  makes the read channel  100  a calibrated read channel  180 . The calibrated read channel  180  may perform  304  read operations, reading data elements from the holographic media  120 . 
     The processor  150  determines  306  if there is a write workload to write to the holographic media  120 . If the processor  150  determines  306  that there is no write workload, the calibrated read channel  180  continues performing  304  read operations until the holographic media is removed from the holographic storage device. 
     If the processor  150  determines  306  that there is a write workload, the calibration module  204  calibrates  308  the write channel  160  as will be described hereafter. The write channel  160  may then perform  310  write operations, writing data elements to the holographic media  120 , and the method  300  terminates. 
     The method  300  allows holographic media  120  to be used interoperably between a plurality of holographic storage devices by calibrating each holographic storage device to the factory-stored hologram  122 . Because the calibrated read channel  180  is calibrated to the factory-stored hologram  122 , the calibrated read channel  180  may successfully read data-encoded holograms written by write channels  160  that are calibrated using the factory-encoded hologram  122  as will be described hereafter. Similarly, because the write channel  160  is calibrated to the factory-stored hologram  122 , data-encoded holograms written by the write channel  160  may be read by other calibrated read channels  180  that are calibrated with the factory-stored hologram  122 . 
       FIG. 6  is a schematic flow chart diagram illustrating one embodiment of a read calibration method  340  of the present invention. The method  340  substantially includes the steps to carry out the function of step  302  of  FIG. 5 . In addition, the description of the method  340  refers to elements of  FIGS. 1-5 , like numbers referring to like elements. A read channel calibration software process executing on the processor  150  may embody the method  340 . 
     The method  340  begins and in one embodiment, the processor  150  retrieves  342  the first holographic pattern  142  from the memory  140 . The read channel  100  reads  344  the factory-stored hologram  122  from a holographic media  120  and stores the read factory-stored hologram  122  as data  141  in the memory  140 . In one embodiment, the processor  150  directs the read channel  100  to read  344  the factory-stored hologram  122 . 
     The factory-stored hologram  122  may reside on a specified portion of the holographic media  120 . Alternatively, a header may designate the factory-stored hologram  122 . Thus the processor  150  may direct the read channel  100  to read the holographic media  120  until the header of the factory-stored hologram  122  is located. The read channel  100  may then read  344  the factory-stored hologram  122 . 
     The calculation module  202  calculates  346  a read difference between the read factory-stored hologram  122  and the first holographic pattern  142 . In one embodiment, the calculation module  202  may calculate  346  the read difference from errors detected while decoding the data of the read factory-stored hologram  122  encoded with an error correction code (ECC). The ECC may encode the data of the read factory-stored hologram  122  and the first holographic pattern  142  using specified rules of construction such that errors in the data may be detected and corrected. Examples of ECC include but are not limited to Hamming code, Reed-Solomon code, Reed-Muller code, Binary Golay code, convolutional code, and turbo code. The calculation module  202  may calculate  346  the read difference as one or more data errors in the read factory-stored hologram  122  that are identified while decoding the ECC-encoded data. 
     In a certain embodiment, the read difference may be based on a circular redundancy check (CRC) code encoding both the read factory-stored hologram  122  and the first holographic pattern  142 . The data of the read factory-stored hologram  122  and the first holographic pattern  142  may be organized as a plurality of data words. Each data word may be encoded with redundant data using a CRC algorithm to form a unique encoded word as is well known to those of skill in the art. If data bits of the factory-stored hologram  122  are read incorrectly, the error may be detected and/or corrected using a CRC algorithm that decodes the CRC-encoded data. The calculation module  202  may record each error as the read difference. In one embodiment, the calculation module  202  maps the locations of errors with the data of the factory-stored hologram  122  as part of the read difference. 
     Alternatively, the calculation module  202  calculates  346  the read difference as a bit by bit comparison of corresponding bits from the read factory-stored hologram  122  and the first holographic pattern  142 . For example, the calculation module  202  may compare the data of the read factory-stored hologram  122  and the first holographic pattern  142  and map each difference as the read difference. 
     In one embodiment, the calculation module  202  calculates  346  the read difference as the difference between the factory-stored hologram  122  g(x,y) read from the holographic media  120  and a matched filter matched to the impulse response h(x,y)=s*(−x,−y) of the first holographic pattern  142  as shown in eqn.[1], where V(x,y) is the cross-correlation between the factory-stored hologram  122  g(x,y) and the first holographic pattern  142  s(x,y). Eqn.[1] comprises a double integral, meaning that the integration is over the X axis  102  and Y axis directions of the detector  130 . Additionally, ξ is the integration variable along the X axis  102 , η is the integration variable along the Y axis, and * denotes a complex conjugate.
 
 V ( x,y )=∫∫ g (ξ,η) s *(ξ− x, η−y )] dξdη   Eqn. [1]
 
     Mathematically, V(x,y) is a surface varying along the X axis  102  and the Y axis, for each (x,y). There is one value of V(x,y) for each detector element in detector  130 . The range of V(x,y) for each (x,y) is between −1 and +1, where +1 represents the ideal correlation of one hundred (100%). To maximize V(x,y), the following difference surface, Difference(x,y), is defined in Eqn.[2]. As shown, Difference(x,y) is calculated by subtracting the matched filter correlation V(x,y) from unity. Difference(x,y) may be evaluated (a) point-to point, (b) as an arithmetic mean, (c) as a geometric mean, and (d) as a root-mean-square. Difference(x,y) ranges between 0 and +2, and the ideal difference for each value of (x,y) is 0, meaning for a value of 0 that there is no difference between the factory-stored hologram  122  read from the holographic media  120  and the first holographic pattern  142  at that point (x,y). Difference(x,y) may be evaluated point-by-point in read difference calculations, but it may be advantageous to quantify surface Difference(x,y) in terms of a single number, to simply read difference calculations. Such single numbers may be MAX_Difference, which is equal to the maximum value of Difference(x,y). Alternately AM_Difference, the arithmetic mean of the values of Difference(x,y), GM_Difference, the geometric mean of the values of Difference(x,y), or RMS_Difference, the root-mean-square of the values of Difference(x,y) may be used in the read difference calculations.
 
Difference( x,y )=1− V ( x,y )   Eqn.[2]
 
     In an alternate embodiment, the calculation module  202  calculates  346  the read difference for a plurality of read power levels for the light source  105  and durations of the light source read light-burst. In a certain embodiment, the calculation module  202  calculates  348  a read difference for each element of matrix m as shown in eqn. (2), where p is a base read power level, d is a base light source read light-burst duration, Δp is difference of the base read power level, and Δd is a difference of the base light source read light-burst duration. In one embodiment, Δp is in the range of one to fifteen percent (1-15%) of the base read power p. Similarly, Δd may be in the range of one to fifteen percent (1-15%) of the base light source read light-burst duration d. Although for simplicity matrix m of eqn.[3] is shown as a 3×3 matrix, m may be of any dimensions. 
     
       
         
           
             
               
                 
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     The calculation module  202  determines  348  if the read factory-stored hologram  122  and the first holographic pattern  142  agree. In one embodiment, the calculation module  202  determines  348  that the read factory-stored hologram  122  and the first holographic pattern  142  agree if errors detected while decoding an ECC or CRC encoding of the data of the factory-stored hologram  122  are less than a specified threshold. For example, if the specified threshold is ten (10) errors and the calculation module  202  detected twelve (12) errors in the ECC encoded data, the calculation module  202  may determine  348  that the read factory-stored hologram  122  and the first holographic pattern  142  do not agree. In one embodiment, the use of ECC or CRC encoded data in the factory-stored hologram  122  makes storing the first holographic pattern  142  unnecessary. 
     Alternatively, the calculation module  202  may determine  348  that the read factory-stored hologram  122  and the first holographic pattern  142  agree if V(x,y), the cross-correlation between the factory-stored hologram  122  and the first holographic pattern  142  as calculated with eqn.[1], is less than a specified correlation threshold. If the read factory-stored hologram  122  and the first holographic pattern  142  agree, the read channel  100  is already calibrated as a calibrated read channel  180  and the method  340  terminates. 
     If calculation module  202  determines  348  that the read factory-stored hologram  122  and the first holographic pattern  142  do not agree, the calibration module  204  may calibrate  350  the read channel  100  using the read difference. The calibration module  204  may detect a pattern in the read difference. In addition, the calibration module  204  may adjust one or more elements of the read channel  100  to mitigate the pattern. For example, a two-dimensional data array of the factory-stored hologram  122  may be offset by a column from a two-dimensional data array of the first holographic pattern  142 . The calibration module  204  may adjust the timing of reading the factory-stored hologram  122  to mitigate the offset pattern, so that when the read channel  100  reads  344  the factory-stored hologram  122 , the two-dimensional data array of the factory-stored hologram  122  is equivalent to the two-dimensional array of the first holographic pattern  142 . 
     The calibration module  204  may make other read calibration adjustments, including the read power-level of light source  105 , the duration of the read light source light-burst, adjusting the read lenses  132 ,  134 , and the sensitivity of detector  130 . In one example, the calibration module  204  may set the power level of the light source  110  and the duration of the light source light-burst to the values of matrix m of eqn.[3] with the smallest read difference. 
     In an alternate example, the calibration module  204  may detect a pattern in the read difference of data in the center of the data array of the factory-stored hologram  122  being different from data in the center of the data array of the first holographic pattern  142 . The calibration module  204  may adjust the focus of the light source  105  by positioning the read lenses  132 ,  134  to mitigate the pattern in the read difference. 
     After the calibration module  204  calibrates  350  the read channel  100 , the processor  150  may direct the read channel  100  to read  344  the factory-stored hologram  122  and the calculation module  202  again determines  346  if the read factory-stored hologram  122  and the first holographic pattern  142  agree. The method  340  may iteratively calculate  348  the read difference and calibrate  350  the read channel  100  until the read channel  100  is calibrated  350  as a calibrated read channel  180 . 
       FIG. 7  is a schematic flow chart diagram illustrating one embodiment of a write calibration method  380  of the present invention. The method  380  substantially includes the steps to carry out the function of step  308  of  FIG. 5 . In addition, the description of the method  380  refers to elements of  FIGS. 1-5 , like numbers referring to like elements. A write channel calibration software process executing on the processor  150  may embody the method  380 . 
     The method  380  begins and in one embodiment, the write channel  160  writes  382  the second holographic pattern  143  to the holographic media  120  as the second hologram  121 . The second holographic pattern  143  may be the first holographic pattern  142  so that the written second hologram  121  should be equivalent to the factory-stored hologram  122 . 
     In addition, the calibrated read channel  180  may read  384  the second hologram  121  from the holographic media  120 . The calculation module  202  calculates  386  a write difference between the read second hologram  121  and the second holographic pattern  143 . In one embodiment, the calculation module  202  calculates  386  the write difference as the errors detected while decoding ECC-encoded data of the read second hologram  121 . For example, calculation module  202  may decode the ECC-encoded data of the read second hologram  121  and sum the ECC errors as the write difference. Alternatively, the calculation module  202  may map the locations of ECC errors within the data of the read second hologram  121  as the part of the write difference. 
     In a certain embodiment, the write difference may be the errors detected decoding CRC-encoded data of the read second hologram  121 . In one embodiment, the use of ECC or CRC encoded data in the second hologram  121  makes storing the second holographic pattern  143  unnecessary. Alternatively, the calculation module  202  may calculate  386  the write difference as a bit by bit comparison of corresponding bits of the read second hologram  121  and the second holographic pattern  143 . 
     In one embodiment, the calculation module  202  calculates  386  the write difference as the difference between the read second hologram  121  g(x,y) read from the holographic media  120  and a matched filter matched to the impulse response h(x,y)=s*(−x,−y) of the second holographic pattern  143  using eqn.[1]. V(x,y) is the cross-correlation between the second hologram  121  g(x,y) and the second holographic pattern  143  s(x,y). Difference(x,y), eqn.[2], may be used to define the difference between the read second hologram  121  read from the holographic media  120  and the second holographic pattern  143 . 
     In an alternate embodiment, the calculation module  202  calculates  386  the write difference for a plurality of write power levels for the light source  105  and durations of the light source write light-burst. In a certain embodiment, the calculation module  202  calculates  386  a write difference for each element of matrix m as shown in eqn. [2], where p is a base write power level, d is a base light source write light-burst duration, Δp is difference of the base write power level, and Δd is a difference of a base light source write light-burst duration. In one embodiment, Δp is in the range of one to fifteen percent (1-15%) of the base write power p. Similarly, Δd may be in the range of one to fifteen percent (1-15%) of the base light source write light-burst duration d. 
     The calculation module  202  may determine  388  if the read second hologram  121  and the second holographic pattern  143  agree. In one embodiment, the calculation module  202  determines  388  that the read second hologram  121  and the second holographic pattern  143  agree if errors detected while decoding an ECC or CRC encoding of the data of the second hologram  121  are less than a specified threshold. 
     Alternatively, the calculation module  202  may determine  388  that the read second hologram  121  and the second holographic pattern  143  agree if V(x,y), the cross-correlation between the second hologram  121  and the second holographic pattern  143  as calculated with eqn.[1], is less than a specified correlation threshold. If the calculation module  202  determines  388  that the read second hologram  121  and the second holographic pattern  143  agree, the write channel  160  is calibrated and the method  380  terminates. 
     If the calculation module  202  determines  388  that the read second hologram  121  and the second holographic pattern  143  do not agree, the calibration module  204  may calibrate  390  the write channel  160  using the write difference. The calibration module  204  may detect a pattern in the write difference. In addition, the calibration module  204  may adjust one or more elements of the write channel  160  to mitigate the pattern. For example, a data array of the second hologram  121  may be offset by a row from a data array of the second holographic pattern  143 . The calibration module  204  may adjust the alignment of the spatial light modulator  114  to mitigate the pattern. Calibration module  204  may make other write calibration adjustments, including the write power-level of light source  105 , the duration of the write laser light-burst, and adjusting the write lenses  135 ,  137 ,  139 , and the sensitivity of detector  130 . For example, the calibration module  204  may set the write power level of the light source  110  and the write duration of the light source light-burst to the values of matrix m of eqn.[3] with the smallest write difference. 
     After the calibration module  204  calibrates  390  the write channel  160 , the processor  150  may direct the write channel  160  to write  382  the second holographic pattern  143  as another instance of the second hologram  121 . The processor  150  may further direct the calibrated read channel  180  to read  384  the new instance of the second hologram  121  and the calculation module  202  again determines  346  if the read factory-stored hologram  122  and the first holographic pattern  142  agree. The method  380  may iteratively calculate  388  the write difference and calibrate  390  the write channel  160  until the write channel  160  is calibrated. 
       FIG. 8  is a top view drawing illustrating one embodiment of a holographic media  120  with an inserted factory-stored hologram  122  of the present invention. The holographic media  120  may be the holographic media  120  of  FIGS. 1-3 . The holographic media  120  includes the factory-stored hologram  122 . In one embodiment, the factory-stored hologram  122  is configured as a read-only hologram. The factory-stored hologram  122  may be adhered to the holographic media  120 . 
       FIG. 9  is a top view drawing illustrating one embodiment of a holographic media  120  with read-only portion  404  of the present invention. The holographic media  120  may be the holographic media  120  of  FIGS. 1-3 . 
     In one embodiment, the read-only portion  404  maybe configured as one or more tracks of one or more sectors of the holographic media  120 . In addition, the read-only portion  404  may be demarked with one or more header segments. The read-only portion  404  configured on any portion of the holographic media  120 . 
     In one embodiment, the processor  150  is logically prevented from writing within the read-only portion  404 . The factory-stored hologram  122  may be precision written within the read-only portion  404 . 
     The embodiment of the present invention calculates  348  a read difference using a factory-stored hologram  122 . In addition, the present invention may calibrate  350  the read channel  100  with the read difference. The present invention may also calculate  388  a write difference and calibrate  390  the write channel  160  with the write difference. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.