Abstract:
A system and method of operating a dual-beam detection system of a holographic data storage disc, including: impinging a data beam on a data layer of the holographic data storage disc; impinging a tracking beam on a tracking element of the holographic data storage disc; detecting a reflection of the tracking beam from the tracking element; and coordinating position of the data beam relative to the tracking beam.

Description:
BACKGROUND 
       [0001]    The present techniques relate generally to holographic data storage techniques. More specifically, the techniques relate to methods and systems for dual-beam recording and reading on holographic data storage media or discs. 
         [0002]    As computing power has advanced, computing technology has entered new application areas, such as consumer video, data archiving, document storage, imaging, and movie production, among others. These applications have provided a continuing push to develop data storage techniques that have increased storage capacity. Further, increases in storage capacity have both enabled and promoted the development of technologies that have gone far beyond the initial expectations of the developers, such as gaming, among others. 
         [0003]    The progressively higher storage capacities for optical storage systems provide a good example of the developments in data storage technologies. The compact disk, or CD, format, developed in the early 1980s, has a capacity of around 650-700 MB of data, or around 74-80 min. of a two channel audio program. In comparison, the digital versatile disc (DVD) format, developed in the early 1990s, has a capacity of around 4.7 GB (single layer) or 8.5 GB (dual layer). The higher storage capacity of the DVD is sufficient to store full-length feature films at older video resolutions (for example, PAL at about 720 (h)×576 (v) pixels, or NTSC at about 720 (h)×480 (v) pixels). 
         [0004]    However, as higher resolution video formats, such as high-definition television (HDTV) (at about 1920 (h)×1080 (v) pixels for 1080 p), have become popular, storage formats capable of holding full-length feature films recorded at these resolutions have become desirable. This has prompted the development of high-capacity recording formats, such as the Blu-ray Disc™ format, which is capable of holding about 25 GB in a single-layer disk, or 50 GB in a dual-layer disk. As resolution of video displays, and other technologies, continue to develop, storage media with ever-higher capacities will become more important. One developing storage technology that may better achieve future capacity requirements in the storage industry is based on holographic storage. 
         [0005]    Holographic storage is the storage of data in the form of holograms, which are images of three dimensional interference patterns created by the intersection of two beams of light in a photosensitive storage medium. Both page-based holographic techniques and bit-wise holographic techniques have been pursued. In page-based holographic data storage, a signal beam which contains digitally encoded data is superposed on a reference beam within the volume of the storage medium resulting in a chemical reaction which, for example, changes or modulates the refractive index of the medium within the volume. This modulation serves to record both the intensity and phase information from the signal. Each bit is therefore generally stored as a part of the interference pattern. The hologram can later be retrieved by exposing the storage medium to the reference beam alone, which interacts with the stored holographic data to generate a reconstructed signal beam proportional to the initial signal beam used to store the holographic image. 
         [0006]    In bit-wise holography or micro-holographic data storage, every bit is written as a micro-hologram, or Bragg reflection grating, typically generated by two counter-propagating focused recording beams. The data is then retrieved by using a read beam to reflect off the micro-hologram to reconstruct the recording beam. Accordingly, micro-holographic data storage is more similar to current technologies than page-wise holographic storage. However, in contrast to the two layers of data storage that may be used in DVD and Blu-ray Disk™ formats, holographic disks may have 50 or 100 layers of data storage, providing data storage capacities that may be measured in terabytes (TB). Further, as for page-based holographic data storage, each micro-hologram contains phase information from the signal. 
         [0007]    Although holographic storage systems may provide much higher storage capacities than prior optical systems, they may be vulnerable to poor tracking control due to the presence of multiple layers of data. Accordingly, techniques that improve tracking control of the disc may be advantageous. 
       BRIEF DESCRIPTION 
       [0008]    An aspect of the invention relates to a method of operating a dual-beam detection system for a holographic data storage disc, including: passing a data beam through a first set of optics to a data layer of the holographic data storage disc; passing a tracking beam through a second set of optics to the holographic data storage disc; detecting a reflection of the tracking beam; and synchronizing positioning of the first set of optics with the second set of optics. 
         [0009]    An aspect of the invention relates to a a method of operating a dual-beam detection system of a holographic data storage disc, including: impinging a data beam on a data layer of the holographic data storage disc; impinging a tracking beam on a tracking element of the holographic data storage disc; detecting a reflection of the tracking beam from the tracking element; and coordinating position of the data beam relative to the tracking beam. 
         [0010]    An aspect of the invention includes a dual-beam detection system of a holographic data storage disc. The system includes a first optical excitation device configured to provide a data beam at a first wavelength to impinge on data layers of the holographic data storage disc; a second optical excitation device configured to provide a tracking beam at a second wavelength to impinge on a servo plane of the holographic data storage disc; and an optical assembly configured to coordinate a position of the data beam with respect to the tracking beam. 
     
    
     
       DRAWINGS 
         [0011]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0012]      FIG. 1  is a schematic diagram of an optical disc reader in accordance with embodiments of the present technique; 
           [0013]      FIG. 2  is a top view of an optical disc in accordance with embodiments of the present technique; 
           [0014]      FIGS. 3 and 3A  are a schematic diagram of a detection head for multilayered optical data storage media; 
           [0015]      FIG. 4  is a schematic diagram of a detection head for multilayered optical data storage media in accordance with an embodiment of the present techniques; 
           [0016]      FIG. 5  is a schematic diagram of a detection head for multilayered optical data storage media in accordance with an embodiment of the present techniques; 
           [0017]      FIG. 6  is a simplified schematic of a detection head for multilayered optical data storage media in accordance with an embodiment of the present techniques; and 
           [0018]      FIGS. 7 and 7A  are a schematic diagram of the detection head of  FIGS. 3 and 3A  employing synchronized actuators as discussed with respect to  FIG. 4  in accordance with an embodiment of the present techniques. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present techniques are directed to coinciding data layers and a tracking layer in holographic data storage systems. Single-bit holographic data storage records data in a plurality of virtual data layers. Initial recording of these virtual layers of micro-gratings benefits from the recording beams to be precisely positioned with respect to a reference point in the medium and to be generally independent of the possible variations due to disk wobble, vibrations, etc. An approach to link the position of the writing and reading beam to the same volume in the bulk is to use surface relief features, such as grooves similar to those in CD-R and DVD disks. A tracking beam (usually of a different wavelength than the data beam) focused on the grooved layer can generate focusing and tracking error signals employable to lock the position of the objective and the beam on the disk via a feedback servo loop. For a discussion of various aspects of holographic data storage, see U.S. Pat. No. 7,388,695, incorporated herein by reference in its entirety. 
         [0020]    Turning now to the drawings,  FIG. 1  is an optical reader system  10  that may be used to read data from optical storage discs  12 . The data stored on the optical data disc  12  is read by a series of optical elements  14 , which project a read beam  16  onto the optical data disc  12 . A reflected beam  18  is picked up from the optical data disc  12  by the optical elements  14 . The optical elements  14  may comprise any number of different elements designed to generate excitation beams, focus those beams on the optical data disc  12 , and detect the reflection  18  coming back from the optical data disc  12 . The optical elements  14  are controlled through a coupling  20  to an optical drive electronics package  22 . The optical drive electronics package  22  may include such units as power supplies for one or more laser systems, detection electronics to detect an electronic signal from the detector, analog-to-digital converters to convert the detected signal into a digital signal, and other units such as a bit predictor to predict when the detector signal is actually registering a bit value stored on the optical data disc  12 . 
         [0021]    The location of the optical elements  14  over the optical data disc  12  is controlled by a tracking servo  24  which has a mechanical actuator  26  configured to move the optical elements back and forth over the surface of the optical data disc  12 . The optical drive electronics  22  and the tracking servo  24  are controlled by a processor  28 . In some embodiments in accordance with the present techniques, the processor  28  may be capable of determining the position of the optical elements  14 , based on sampling information which may be received by the optical elements  14  and fed back to the processor  28 . The position of the optical elements  14  may be determined to enhance and/or amplify the reflection  18  or to reduce interferences of the reflection  18 . In some embodiments, the tracking servo  24  or the optical drive electronics  22  may be capable of determining the position of the optical elements  14  based on sampling information received by the optical elements  14 . 
         [0022]    The processor  28  also controls a motor controller  30  which provides the power  32  to a spindle motor  34 . The spindle motor  34  is coupled to a spindle  36  that controls the rotational speed of the optical data disc  12 . As the optical elements  14  are moved from the outside edge of the optical data disc  12  closer to the spindle  36 , the rotational speed of the optical data disc may be increased by the processor  28 . This may be performed to keep the data rate of the data from the optical data disc  12  essentially the same when the optical elements  14  are at the outer edge as when the optical elements are at the inner edge. The maximum rotational speed of the disc may be about 500 revolutions per minute (rpm), 1000 rpm, 1500 rpm, 3000 rpm, 5000 rpm, 10,000 rpm, or higher. 
         [0023]    The processor  28  is connected to random access memory or RAM  38  and read only memory or ROM  40 . The ROM  40  contains the programs that allow the processor  28  to control the tracking servo  24 , optical drive electronics  22 , and motor controller  30 . Further, the ROM  40  also contains programs that allow the processor  28  to analyze data from the optical drive electronics  22 , which has been stored in the RAM  38 , among others. As discussed in further detail herein, such analysis of the data stored in the RAM  38  may include, for example, demodulation, decoding or other functions necessary to convert the information from the optical data disc  12  into a data stream that may be used by other units. 
         [0024]    If the optical reader system  10  is a commercial unit, such as a consumer electronic device, it may have controls to allow the processor  28  to be accessed and controlled by a user. Such controls may take the form of panel controls  42 , such as keyboards, program selection switches and the like. Further, control of the processor  28  may be performed by a remote receiver  44 . The remote receiver  44  may be configured to receive a control signal  46  from a remote control  48 . The control signal  46  may take the form of an infrared beam, an acoustic signal, or a radio signal, among others. 
         [0025]    After the processor  28  has analyzed the data stored in the RAM  38  to generate a data stream, the data stream may be provided by the processor  28  to other units. For example, the data may be provided as a digital data stream through a network interface  50  to external digital units, such as computers or other devices located on an external network. Alternatively, the processor  28  may provide the digital data stream to a consumer electronics digital interface  52 , such as a high-definition multi-media interface (HDMI), or other high-speed interfaces, such as a USB port, among others. The processor  28  may also have other connected interface units such as a digital-to-analog signal processor  54 . The digital-to-analog signal processor  54  may allow the processor  28  to provide an analog signal for output to other types of devices, such as to an analog input signal on a television or to an audio signal input to an amplification system. 
         [0026]    The reader  10  may be used to read an optical data disc  12  containing data as shown in  FIG. 2 . Generally, the optical data disc  12  is a flat, round disc with one or more data storage layers embedded in a transparent protective coating. The protective coating may be a transparent plastic, such as polycarbonate, polyacrylate, and the like. In the case of a holographic medium, the material of the disk may be functional that actively changes in response to recording light to produce a data mark hologram. The data layers may include any number of surfaces that may reflect light, such as the micro-holograms used for bit-wise holographic data storage or a reflective surface with pits and lands. The optical disk  12  is mounted on the spindle  36  (see  FIG. 1 ) with spindle hole  56  so that the disk may be rotated around its axis. On each layer, the data may be generally written in a sequential spiraling track  58  from the outer edge of the disc  12  to an inner limit, although circular tracks, or other configurations, may be used. 
         [0027]      FIGS. 3 and 3A  depict an exemplary dual-beam detection head system  60 . A light source  62  emits a read beam  64  at a first wavelength which passes through a polarizing beam splitter  66  and depth selecting optics  68 . The read beam  64  is reflected off a dichroic mirror  70  and directed through the quarter wave plate  72  and the lens  74  to a micro-hologram  76  in the disc  12 . The reflected data beam  78  from the micro-hologram  76  is passed back through the lens  78 , quarter wave plate  72 , dichroic mirror  70 , and depth selecting optics  68 . The reflected beam  78  is then passed through the polarizing beam splitter  66 , collecting optics  80  and detector  82  where the data of the micro-hologram  76  is read. 
         [0028]    Further, a light source  84  emits a tracking beam  86  at a second wavelength which passes through a beam splitter  88  and depth selecting optics  90 . The tracking beam  86  passes through the dichroic mirror  70 , quarter wave plate  72 , and the lens  74  to the disc  12 . In the illustrated embodiment, the tracking beam  86  reflects off the disc  12  (e.g., near or at the bottom the disc), which may have a reflective layer, tracks, grooves, and the like. The reflected tracking beam  92  passes through the lens  74 , quarter wave plate  72 , dichroic mirror  70 , collecting optics  90 , beam splitter  88 , and collecting optics  94  to a detector  96 . 
         [0029]    In volumetric storage media with a grooved reference plane used for tracking beam positions, one grooved tracking layer is generally sufficient to ensure the positioning of the beam in the medium volume. However, to be able to record multiple layers, the recording and tracking beam focal spots should be separated from each other in depth. When focused on the grooved layer, the tracking beam produces tracking and focusing error signals that facilitate maintaining a repeatable position of the beam with respect to the disk and surface and the track that is being read, generally unaffected by the disk runout. The recording/readout beam should be focused on the virtual data layer in the bulk of the recording medium. To reduce deviations of the reading/writing beam from the track, a favorable scheme would utilize the same objective lens for both tracking and recording/readout beams. This would, in turn, have at least one of the beams to be uncollimated. 
         [0030]    However, unfortunately, the relative position of the two focal spots may change when the medium (disc) wobbles around its original position if the objective lens is the only moving element. In other words, the working distance between the lens and medium for a beam focused at a certain depth (layer) is generally independent of the disc position only for a collimated beam. In summary, a focusing servo with a single lens used with a collimated and an uncollimated beam may not ensure that the relative focal spot positions are fixed with respect to each other when a random (unrepeatable) axial runout and/or tilt are present. Different approaches to separating the beam spots in depth may be beneficial. 
         [0031]    Using grooved-patterned surface to control focusing and tracking of the objective lens (axial and radial actuator movement), a beneficial design accomodates the objective lens that would separate positions of the focal spots in depth to focus the tracking beam (e.g. red) on the grooved surface, and the recording/readout beam (e.g. green/blue) in the bulk of the medium (disc) on a virtual data layer. With a single-element objective lens, only one collimated beam can typically be used while the other one should be divergent/convergent to focus at a different depth, unless this element is highly dispersive due to the material property or by design. In a more general case, both tracking and data beams may be either convergent or divergent with different divergence cone angles. 
         [0032]    Positioning of the read/write beam on a desired data layer and track can be achieved by locking the tracking beam on the groove at the surface (or a special servo-plane) of the disk, while the position of the read/write beam is fixed relative to the tracking beam, and thus to the disk. In order to deterministically write and read data in the volume of the medium when the disk is rotating and wobble and runout occur, the servo system should keep the tracking beam focal spot on the track of the grooved layer, and read/write beams fixed with respect to the tracking beam. This involves axial and radial movements of the optical pickup element (lenses) to follow stochastic changes of the disk position. For a collimated beam, this implies that the distance between the pickup lens and the disk is constant, i.e. the pickup lens will follow the disk movement. When a divergent or convergent beam is focused with the same objective lens, the distance between the focused spot and the lens varies as the lens is moved around to follow the disk wobble. 
         [0033]    In one implementation, if the data beam is collimated and the uncollimated beam is used for focusing, the servo loop will keep the focused spot of the tracking beam on the grooved tracking layer of the medium by moving the lens to null the focus error signal (FES). However the distance between the disk and the lens will also change because the conjugate plane of the objective lens is at the finite distance from the lens. This may result in the spot from the collimated data beam to shift with respect to the material of the disk. In another implementation, the tracking beam is collimated so that the servo loop will keep the tracking beam spot on the tracking layer and the distance between the lens and the disk fixed. At the same time, the depth of the data beam spot will vary as the distance between the objective lens and the rest of stationary optics changes. 
         [0034]    The present techniques utilize a scheme that may facilitate positioning of the recording beam in the bulk medium at a fixed depth with reduced axial runout. As discussed below, one embodiment utilizes two synchronized actuators to carry two optics elements. Another embodiment employs two different lenses for the tracking and data beams mounted on the same actuator driven by tracking/focusing error signals. Yet another embodiment uses segmented optics and Fresnel-type optics to introduce dispersion into the system and produce different effective focal length of the objective at wavelengths of data and tracking beams. The elements described in the realization may also carry a function of aberration correction for both beams, which could be static or adaptive. Preliminary optical systems modeling shows it is relatively easily realizable for two wavelength system (e.g., 532 nm data and 670 nm tracking beams), i.e., two-color master-slave tracking in single-bit holographic/3D media. 
         [0035]      FIG. 4  depicts a dual-beam detection system  110  having synchronized actuators  112  and  114  for a first lens  116  and a second lens  118 . A data beam  120  passes through the second lens  118 , a dichroic beam splitter  122 , and the first lens  116  to a data layer ( 126 ) in the disc  12 . A tracking beam  124  passes through the beam splitter  122  and first lens  116  to a tracking grooved layer in the disc  12 . Of course, additional optics may be included in the system  110 . The data beam  120  and tracking beam  124  are typically of different wavelengths. In the illustrated embodiment, the pair of lenss  116  and  118  may be synchronized in motion with the disc  12 . In this example, both beams  120  and  124  can be used originally collimated. The first lens  116  is the objective lens shared by the beams  120  and  124 . 
         [0036]    The tracking beam  120  is focused on and reflected off the tracking grooved layer of the disk. Focusing and tracking error signals may be generated using reflected tracking beam from the grooved surface and fed into the servo that adjusts the position of the first lens  116  to compensate the wobble of the disk  12 . The data beam  124 , in order to be collected at a different depth in the disk  12  (closer to the lens  116  in this example) passes through a second lens  118 , the dichroic beam splitter  122 , and enters the first lens  116  with convergent rays. One of the beams (in this example, the data beam  124 ) enters through both lenses  116  and  118 , while the other beam (e.g., the tracking beam  120 ) enters the system between the two lenses  116  and  118  (via a dichroic beam splitter  122 , etc.) and typically only passes through the objective lens  116 . Thus, advantageously, the focal spots of the two beams  120  and  124  lie at different depths. However, as the disk  12  rotates and wobbles, the depths of the data beam focus spot may wary with respect to that of the reference beam. This will result in a deviation (in depth or laterally) of the focused data beam  124  from the micro-hologram  76  in a data layer  126  that is being read. This deviation can be compensated by a movement of the second lens  118  to follow (with a proper scaling) the movement of the first lens  116 . 
         [0037]    In view of the foregoing, the synchronized movement of optics containing uncollimated beams “decouples” the motion of the disc. Both the first and the second lenses  116  and  118  may function as aberration compensating optics for the tracking beam  120  and data beam  124 . The second lens  118  as well as possible additional adaptive optics elements may function also as a working depths selector to address different data layers  126  in the disk  12 . Although only the beam depth compensation was used here as an example, a similar runout compensation in the radial and/or tangential directions may be implemented to compensate the corresponding deviations between the data beam and the tracking beam focus positions. 
         [0038]    In another embodiment,  FIG. 5  depicts a dual-beam detection system  140  having two lenses  142  and  144  integrated into a single actuator  146 . The system  140  facilitates collimated operation for both the tracking beam  148  and the data beam  150 . In this instance, the pair of discrete lenses or lens assemblies  142  and  144  may be designed respectively for wavelengths/depths of the tracking beam  148  and data (read/write) beam  150 , and which, again, the lenses  142  and  144  are mounted on a common actuator  146 . In the illustrated embodiment, the tracking beam  148  passes through lens  142  to a guide groove on the disc  12 . The data beam  150  passes through the lens assembly  144  to a data layer  126  in the disc  12 . The lens assembly  144  used to focus the data beam  150  may be designed to have an adjustable focus length, as indicated by reference numeral  152 , so that different data layers  126  can be accessed. Of course, additional optics may generally be included that, for example, statically or dynamically compensate aberrations. As the disk  12  rotates and undesirably wobbles, the actuator  146  adjusts the position of both tracking and data optics ( 142  and  144 ) in the same way to accurately follow the reference grooves that facilitates that the data layers and bits are correctly accessed with the data beam  150 . Additional disk tilt detection and feedback can be applied to the moving part of the actuator. 
         [0039]    In yet another embodiment,  FIG. 6  depicts a dual-beam detection system  160  having a dispersive element  162 . In this example, the dispersive element  162  (e.g., a dye-doped plate with dye distribution profile) is configured to change the focal length of a beam at one wavelength without significantly affecting another beam at a different wavelength. The single-element  162  may exhibit significant dispersion either due to structural design such as Fresnel phase plate, or a dispersive element such as non-uniformly distributed dye or liquid crystal transparent to one of the beams  164  or  166 , but resonantly interacting with the other. In the illustrated embodiment, the tracking beam  164  passes through the dispersive element  162  and lens  168  to a tracking or guide element on the disc  12 . The data beam  166  reflects from a beam splitter  170 , passes through the dispersive element  162 , and lens  168  to data layers  126  on the disc  12 . An actuator  172  facilitates positioning of the system  160 . 
         [0040]    In sum, the dispersive element  162  may provide for a highly different refractive index for the tracking beam  164  (e.g., red wavelength) versus the data beam  166  (e.g., green or blue wavelength). Indeed, the element  162  may provide for high chromatic separation. The described dispersive property may be incorporated into the lens  168 . Moreover, the dispersive properties of the dispersive element may be tunable, such as via an electro-chromic effect. Lastly, this example of  FIG. 6  may also include additional optics and actuators similar to those, for example, mentioned with respect to  FIG. 4 . Such additional optics may facilitate the selecting of different data layers and compensating for the residual runout difference between the data beam and the tracking beam, for example. 
         [0041]      FIGS. 7 and 7A  depicts the detection head of  FIGS. 3 and 3A  employing synchronized actuators as discussed with respect to  FIG. 4 . A dual-beam detection system  180  having synchronized actuators  182  and  184  is illustrated. A block diagram of a control scheme is also depicted. In this example, the detector  96  that reads the reflected tracking beam  92  feeds a signal to a controller  186  for tracking error, focusing error, and tilt error. The controller  186  provides a control signal an objective actuator driver  188  and also to a depth and tilt correction signal generator  190 . The objective actuator driver  188  controls the actuator  182 , and the depth and tilt correction signal generator  190  controls the actuator  184 . The shared objective lens  74  may incorporate dispersive beam separation as described with respect to  FIG. 6 . 
         [0042]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.