Patent Application
20080056042
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and follows a hyphen to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components.
One type of storage medium that has been attracting increasing attention because of its large storage capacity uses holographic storage as a specific form of optical data storage. Briefly, a typical technique for storing data holographically uses two coherent light beams and directs them onto a storage medium; in some instances the two beams may originate as a single laser beam that is split by a partially reflective mirror or other optical beamsplitter. One of the coherent beams is a signal beam that is used to encode the data on the storage medium while the other coherent beam is a reference beam. An interference pattern is produced within the storage medium where the beams intersect and stored on the storage medium. The data may subsequently be retrieved by illuminating the storage medium with a beam substantially identical to the reference beam, with the stored interference pattern causing light to be diffracted and reproduce the data beam.
There are two broad classes of materials that have been used to provide the holographic storage medium: photorefractive crystals such as LiNbO3 or BaTiO3, which record the interference pattern by locally changing their refractive index; and photopolymers, which record the inference pattern in the form of induced photochemical changes in a film. The photopolymer typically comprises a dopant chromophore embedded within a polymer matrix, with two gratings being formed when illuminated, one grating corresponding to the chromophores that are attached to the polymer matrix and the other grating corresponding to the chromophores that are not attached to the matrix.
Photopolymer storage media have been increasing in popularity and are believed likely to be the more widely used media for holographic data storage. With such photopolymer media, recording of the interference pattern is followed by a curing step in which the region of the medium written to is exposed to a beam of light to exhaust any remaining photoactive species in the material and thereby fix the image by eliminating the ability to write any additional data to that region. This process is sometimes referred to herein as “bleaching” the region and commonly uses a different wavelength of light than was used in producing the interference pattern, but this is not required and the same wavelength may be used. With some photopolymeric materials, this bleaching may additionally be followed by a brief heating step. An entire sequence of writing data to the storage medium and fixing it with a bleaching step and perhaps also a heating step is referred to herein as a “session.”
The inventor was tasked with investigating how to incorporate this type of photopolymer storage media into a system-level data-storage configuration, and particularly with how to use these media to store as much data as possible. Thus, rather than focusing on the physical and photochemical effects that govern how the data are written and the theoretical limits that might exist to storing data on such media, the inventor was directing attention to how the medium would actually be used within a system-level configuration.
One observation that the inventor made was that, with a certain irony, the very large data-storage capacity of photopolymer media acted to reduce the efficiency with which data were stored. He identified this as a consequence of a combination of the fact that many data-storage sessions involve the writing of significantly less data than the capacity of the storage medium and the fact that each session results in some loss of capacity. In many instances, the number of sessions used to fill a photopolymer storage medium holographically may be large, resulting in a multiplicatively significant loss of capacity.
The loss of capacity that results from each session may be understood with reference to
It should be appreciated that the representation in
Embodiments of the invention accordingly integrate a holographic drive used for writing data to a photopolymer storage medium into a data-storage system that includes sufficient temporary storage to buffer data before writing it to the medium. This reduces the number of times that data are written to the medium, with data being written only a single time to the medium in particular embodiment, and thereby also reducing the unused area of the medium in an embodiment where it acts as a single-session write device. An schematic illustration of one exemplary system that includes such an integration is provided in
The actual manipulation of physical media is generally handled by a robotic system 212 under the control of the host system 200. System 212 can comprise host system 200 generated media-handling instructions communicated to and carried out by a human operator. For example, the robotic system 212 may be provided with instructions from the host system 200 to move particular media identified by the host system 200 from archival locations to read or write stations when data are to be read from or written to the media. Thus, when data are to be written holographically to a photopolymer storage medium 100, the robotic system 212 may be instructed to move the medium 100 for access by a holographic drive 204, also provided under the coordinating control of the host system 200. The holographic drive 204 is instructed to write a collection of data that have been buffered in a temporary storage 208 from multiple separate data packets onto the photopolymer medium 100, thereby limiting the number of write functions performed with that medium 100. The temporary storage 208 may comprise any type of storage device capable of storing amounts of data that correspond to a significant fraction of the storage capacity of the photopolymer medium. Specifically, in different embodiment, the temporary storage 208 is capable of storing greater than 50 gigabytes (“GB”) of data, is capable of storing greater than 100 GB of data, is capable of storing greater than 200 GB of data, is capable of storing greater than 500 GB of data, or is capable of storing greater than 1000 GB of data. In one embodiment, the temporary storage 208 comprises a magnetic disk array. In another embodiment, the temporary storage 208 comprises solid-state storage.
The host system 200 also comprises software elements, shown as being currently located within working memory 270, including an operating system 274 and other code 272, such as a program designed to implement methods of the invention. It will be apparent to those skilled in the art that substantial variations may be used in accordance with specific requirements. For example, customized hardware might also be used and/or particular elements might be implemented in hardware, software (including portable software, such as applets), or both. Further, connection to other computing devices such as network input/output devices may be employed.
Methods of the invention may be more fully understood with reference to
There are numerous different types of criteria that may be imposed at block 312, some of which are described herein for exemplary purposes without intending to limit the scope of the invention. For example, one criterion that may be imposed requires that the collected data have a certain total size that exceeds a predetermined limit, such as 50 GB, 100 GB, 200 GB, 500 GB, or 1000 GB. Imposition of such a criterion advantageously ensures that at least a certain fraction of the photopolymer storage medium is allocated to data storage. For instance, the criterion might specify that the write function be performed when a certain predetermined data size is met to ensure that each photopolymer medium is written to only once. In some embodiments, the writing criteria may be chosen directly to include a temporal requirement, such as by having data written at periodic intervals, say once every day or once every hour depending on the data-storage environment. In still other embodiments, the size and temporal requirements might be combined, such as by triggering a write function whenever the collected data exceed a certain predetermined size, while also triggering a write function of any collected but unwritten data according to a periodic schedule. The criteria imposed at block 312 might also discriminate different types of data, causing certain types of data to be written when a size criterion is satisfied, but causing other types of data to be written according to a periodic schedule. Still other criteria for triggering the write function will be evident to those of skill in the art.
Once the criteria at block 312 have been met, writing the data may begin by organizing the collected data for holographic storage. Because the collected data originate as a plurality of distinct data packets, it may be advantageous to perform such organization and thereby improve either the writing of the data itself or its later retrieval from the storage system. The manner in which the organization is performed may depend on the type of data, the relationships between the different packets of data, and similar characteristics, with the organization being geared to optimizing performance as measured by such features as write rates, read rates, access times, and the like. The organized data are written holographically to the photopolymer storage medium at block 320, a further description of which is provided for one possible configuration in connection with
Depending on the criteria that were applied at block 312 to trigger the write function and/or the actual size of the data written at block 320, it may or may not be possible or desirable for further data to be written to the photopolymer storage medium. If so, as checked at block 332, further data packets are received at block 304, with the method being repeated until no further data are to be written to that storage medium. At that point, the written photopolymer medium may be maintained in an archival location within the data storage system, as indicated at block 336. Such maintenance typically includes recording an inventory of the data that have been recorded so that the host system 200 may issue instructions to retrieve the particular data when desired.
The archived storage medium may then be used in a manner similar to any other form of archived storage medium, including magnetic tape, magnetic disks, or another form of optical data storage, with mechanisms being provided to retrieve the data efficiently. For instance, if a particular piece of data stored on the photopolymer medium is requested at block 340, the host system 200 may identify the particular photopolymer medium using its inventory at block 344. It may issue instructions to the robotic system 212 at block 348 to retrieve the identified storage medium, which may then be illuminated at block 352 with a reference beam to recover the holographically stored data. The retrieved data may then be provided at block 356 to satisfy the request received at block 340.
While the invention is not limited to any particular structure for the holographic drive 204, a general overview is provided in
After expansion by the beam expander 428, the signal beam encounters a spatial light modulator 432, where it is optically modulated in accordance with a pattern provided by an encoder 436 defined by recording data 448. This encoded pattern may be received by the spatial light modulator 432 as an electrical signal, forming a pattern of light and dark dots on a plane to define a dot-pattern representation of the recording data. The modulated beam is then transmitted through a Fourier-transformation lens 440 separated by its focal length f from the spatial light modulator 432 so that the dot-pattern representation is subject to a Fourier transformation and focused onto the photopolymer storage medium 100. At the same time, the reference beam 416 is directed to the photopolymer storage medium 100 where it interferes with the Fourier-transformed beam to generate a holographic representation of the encoded signal. In the illustrated embodiment, the reference beam 416 is directed by an optical routing structure that includes a fixed mirror 424 and a moveable mirror 444, although other structures may be used in different embodiments. The moveable mirror 444 may have two degrees of freedom to permit it to be translated and rotated, and thereby direct the reference beam to different portions of the storage medium 100. The interference pattern is then recorded on the photopolymer medium.
The information stored in this way may be recovered by a reverse process, namely by later irradiating the written storage medium with the reference beam 416. This may be accomplished with the structure of
Thus, having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.
