Patent Application
20120250120
The invention relates generally to holographic devices, and more particularly to multi-wavelength holographic methods and systems.
Generally, 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 conventional volume holographic storage, or 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. This results in a material modification, for example a chemical reaction, thereby, changing or modulating 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.
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 or four layers of data storage that may be achieved in DVD and Blu-ray Disk formats by complex layer-by-layer production process, holographic disks may have multiple layers of data storage that are formed by optical beams in an otherwise homogeneous recording medium, providing data storage capacities that may be measured in terabytes (TB). Thus, in bit-wise holographic storage, high capacities of storage medium are achieved by filling the volume of the medium with layers of micro-holograms (Bragg reflectors) that represent ones and zeros of a data channel. The number of layers in a holographic disk is defined by interlayer spacing, which is restricted by signal cross talk, as the readout laser beam has to pass through the layer above and below the one it is addressing. The closer together the layers are, the more undesired signal reflected from the adjacent layers will be collected by a detector. Multiplexing techniques, such as angle multiplexing or wavelength multiplexing, utilize recording of multiple holograms into the same physical volume in the disc. This can be leveraged to increase storage capacity and potentially the data transfer rate, however, the approach puts additional burden on the material performance: the achievable refractive index change must be higher to support overlapping holograms with comparable diffraction efficiency.
To satisfy growing demands of data storage market and to ensure multi-generation technology roadmap, there is a need for holographic systems and methods for increasing the capacity of micro-holographic media.
In accordance with an embodiment of the invention, a holographic system for recording and reading information is provided. The system includes at least one laser for providing a laser beam. The system also includes a subsystem configured for multi-wavelength operation of said holographic system and recording micro-holograms at different wavelengths in substantially non-overlapping volumes of a holographic medium.
In accordance with an embodiment of the invention, an optical holographic medium is provided. The holographic recording medium includes multiple recording layers. The recording layers further include multiple micro-holograms recorded with multiple wavelengths in an interleaved manner in the optical holographic medium.
In accordance with an embodiment of the invention, a holographic system for processing information is provided. The system includes at least one laser for providing a laser beam. The system further includes a subsystem configured for multi-wavelength operation for retrieving data from a preformatted holographic medium and recording by modifying micro-holograms of the preformatted holographic medium.
In accordance with an embodiment of the invention, a method for processing information in a holographic medium is provided. The method includes directing multiple laser beams with multiple wavelengths. The method also includes providing a subsystem configured to switch between wavelengths and modulate the one or more laser beams. Finally, the method further includes recording adjacent micro-holograms in the holographic medium at different wavelengths.
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:
As discussed in detail below, embodiments of the invention are directed towards multi-wavelength holographic methods and systems for increased data storage capacity in storage mediums. As used herein, the term ‘index profile grating’ or ‘gratings’ refers to micro-holograms located in the holographic storage medium or disk.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Further, the term ‘processing’ may refer to reading or recording or rewriting or retrieving of data from a holographic data storage system. Any examples of operating parameters are not exclusive of other parameters of the disclosed embodiments.
To achieve a high data storage density in the data storage medium 12, the system 10 includes a subsystem 24 that is configured for multi-wavelength operation, thereby recording adjacent micro-holograms in the optical data storage medium 12 at different wavelengths. The subsystem 24 includes a series of optical elements (not shown) for projecting the laser beam onto the optical data storage medium 12. A reflected beam is picked up from the optical data storage medium 12 by one or more optical detectors (not shown). In one embodiment, the one or more detectors include in-phase and quadrature phase (I/Q) detectors for simultaneously reading of multiple layers of the optical data storage medium 12. In another embodiment, the pick up head device 22 may include any number of different elements designed to generate excitation beams, focus the beams on the optical data storage medium 12, and detect the reflection beam coming back from the optical data storage medium 12. The pick up head devices 22 are controlled through a coupling 26 to an optical drive electronics package 28. The optical drive electronics package 28 may include units such 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 storage medium 12.
The location of the pick up head device 22 over the optical data storage medium 12 is controlled by a focus and tracking servo 30 which has a mechanical actuator 32 configured to move the pick up head device 22 in axial and radial directions in relation to the optical data storage medium 12. The optical drive electronics package 28 and the tracking servo 30 are controlled by a processor 34. The processor 34 is responsive to the data detected by the pick-up head 22 and is capable of sending a location signal and coordinating the movement of the one or more pick-up heads 22. In some embodiments in accordance with the present techniques, the processor 34 may be capable of determining the position of the pick up head device 22, based on sampling information that may be received by the pick up head device 22 and fed back to the processor 34.
The subsystem 24 uses the multi-wavelength approach to record or retrieve micro-holograms using multiple wavelengths in an interleaved manner in the optical data storage medium 12. More particularly, the subsystem 24 records or retrieves micro-holograms using multiple different wavelengths in adjacent layers of the optical data storage medium 12. In another embodiment, the subsystem 24 records or retrieves micro-holograms using multiple different wavelengths in adjacent tracks of the optical data storage medium 12. Further, in one embodiment, the subsystem 24 is configured for recording or reading adjacent micro-holograms with multiple different wavelengths in a single track of any layer of the optical data storage medium 12.
Furthermore, the processor 34 also controls a motor controller 36, which provides power to a spindle motor 38. The spindle motor 38 is coupled to a spindle 40 that controls the rotational speed of the optical data storage medium 12. It should be noted that embodiments of the invention are not limited to any particular processor for performing the processing tasks of the invention. The term “processor,” as that term is used herein, is intended to denote any machine capable of performing the calculations, or computations, necessary to perform the tasks of the invention. The term “processor” is intended to denote any machine that is capable of accepting a structured input and of processing the input in accordance with prescribed rules to produce an output. It should also be noted that the processor may be equipped with a combination of hardware and software for performing the tasks of the invention, as will be understood by those skilled in the art. It should be understood that it is within the scope of this invention that bit wise holographic recording and retrieval system can be used either in single integrated system or individually.
In one embodiment, a holographic system for processing information is provided. Such a holographic system, in a non-limiting manner, may include a holographic disc player/recorder that is used by a user. The system includes at least one laser for providing a laser beam. The system further includes a subsystem configured for multi-wavelength operation for retrieving data from a preformatted holographic medium and recording by modifying micro-holograms of the preformatted holographic medium. The subsystem is configured to record data using a single-wavelength high power laser source. Further, the subsystem retrieves data using one or more low power laser sources with tunable wavelength functionality.
It is to be noted that the distance between adjacent data layers or tracks in a holographic medium defines the eventual data capacity of the medium. This data capacity is limited by cross-talk between the signals (reflections) generated by the different layers or tracks. When the layers or tracks are too tightly spaced, the readout beam focused on the layer or track that is being read is partially reflected from the adjacent layers or tracks, and these secondary reflected beams partially reach the detector and interfere with the signal from the main read layer or track, thereby, increasing error rate. When the adjacent layers or tracks are recorded using a slightly different wavelength than the main layer or track, the reflections from the adjacent layers or tracks that occur upon the readout of the main layer or track are suppressed due to the wavelength mismatch that exists between the readout and recording light for the adjacent layers or tracks. This reduces the interlayer cross-talk and allows closer positioning of layers or tracks to increase usable data capacity of the holographic medium.
High NA is necessary to achieve small micro-hologram size and high capacity of the storage medium. Higher NA optics allows creating smaller micro-holograms, which can be placed closer together both laterally and in depth, resulting in higher data density. A non-limiting example includes a system operating at NA=0.85 that may allow to achieve about 1 Terabyte of data in a CD sized disk medium by accommodating about 40 or 50 layers of micro-holograms. In this case, the capacity may still be limited by the minimum interlayer spacing, which is dictated by the reflected signal interlayer cross-talk. However, the wavelength detuning curve for a high-NA system is substantially broader than in a low-NA system, as illustrated in
Furthermore, if the readout is done with the beam of a wavelength different from the recording beam (for example at 435 nm versus 405 nm recording in
In one embodiment, a pre-recorded medium with multiple wavelengths (as described above) carries different part of content on different sub-sets of layers with common wavelength. A non-limiting example includes a holographic recording medium having one or more layers that carry basic content (such as a movie) recorded at a first wavelength and remaining layers having an extended content recorded or readout by a holographic system based on multi-wavelength functionality. The extended content may be stored on the satellite-wavelengths layers (such as extra scenes, 3D content, alternate view angles). In a non-limiting example, the basic content is read on a simple single-wavelength player device and extended content is accessible by devices with multi-wavelength capability. This approach offers backward compatibility of media to early-generation devices used at lower capacity, and offers a route to multi-generation media with significant generation increase of capacity per medium via increase of hardware complexity.
In another embodiment a multi-wavelength recording is performed within the same layer, for example interleaved adjacent tracks are recorded at different wavelengths. This can improve inter-track cross-talk and result in better BER and higher useful capacity. Furthermore, in one embodiment, the optical holographic recording medium includes a preformatted holographic medium used for a single sided recording with a single-wavelength high power laser source of a holographic device at a user end. The holographic device retrieves or tracks data using a low power laser source with multiple wavelength functionality.
Furthermore, it is to be noted that in the present invention, the holographic medium that is used for multi-wavelength recording includes a material that demonstrates comparable sensitivity at varying intensity of laser beams with different wavelengths. In a non-limiting example, one such material of the holographic medium for multi-wavelength recording includes a composition of Polyvinylcinnamate (PVCm) and a Bis(1-ethynyl-4-(phenylethynyl)benzene)bis(triphenylphosphine)Pd (II) (Pd-PE2). The Organic formulae of Pd-PE2 is given by
In a non-limiting example, the material (Pd-PE2) is prepared by adding 4-ethynyl diphenyl phenyl acetylene (0.202 grams, 0.001 mole), Bis-(tributylphosphine)palladium dichloride (0.350 grams, 0.0005 moles), and copper (I) iodide (0.010 grams) to a 250 mL round bottom flask. This mixture is stirred in 25 mL of diethylamine at room temperature under nitrogen for 18 hrs. The diethyl amine is then removed under vacuum and the residue is dissolved in methylene chloride (5 mL) and purified by silica gel chromatography (hexanes-ethyl acetate, 10:1) to provide 0.130 grams of pale yellow solid (Pd-PE2). The proton nuclear magnetic resonance (NMR) spectroscopy details of the material are as follows:1H NMR 0.97, t (18H), 1.45 m (12H), 1.63 m (12H), 2.05 m (12H), 7.10 m (6H), 7.30-7.45 m (8H), 7.93 m (4H). Further during the experiment, the Ultraviolet-visible spectroscopy absorption data for Pd-PE2 is given by the following table:
wherein, λmax [wavelength at which the dye molecule Pd-PE2 has the maximum absorption] and εmax [symbol stands for extinction coefficient of the dye molecule Pd-PE2] were measured in methylene chloride for Pd-PE2. The extinction coefficient at 405 nm is calculated using concentrated solutions (˜10−2-10−3M).
Further, thin film samples of Pd-PE2/PVCm is made by preparing a solution of Polyvinylcinnamate (1 g), 4.0 wt % (0.04M) of Pd-PE2 using dichloroethane/methylene chloride solvent mixture (15 g, 2:8 v/v) as solvent. The solution is filtered using 0.45 μm filter, poured onto a glass rim (5 cm diameter) on a glass plate setup and dried on a hot plate maintained at about 45° C. for 5 hours and at about 55° C. overnight. After drying on a hot plate, the films are removed from the glass plates and vacuum dried at 60° C. for 6 hours. This prepared film of holographic medium demonstrates comparable sensitivity at varying intensity of laser beams with different wavelengths as shown in
A non-limiting example of an experimental result 170 of measured sensitivity derived from micro-hologram recording performed on the prepared holographic film (PVC/Pd-PE2 (2% doped)) is illustrated in
In another embodiment, a single layer of the holographic medium employs two two-symbol alphabets, {0, microhologram at wavelength 1} (“alphabet P”) and {0, microhologram at wavelength 2} (“alphabet Q”) to minimize the cross-talk. It is to be noted that adjacent symbols are chosen from different alphabets to minimize cross talk. A detector similar to the detector 406 (as shown in
It should be understood that even though some of the discussed examples refer to two different wavelength, the current embodiments can be broadened for the case of more than two wavelengths.
Advantageously, the present method and system enables increased data storage in a holographic data storage medium. The present method and system also enables easy and rapid processing of information. The present invention further enables the retrieval of information rapidly by minimizing the cross talk between adjacent microholograms in adjacent layers or tracks. Another advantage of the present method is the ability to use the same disc media in a multi-generation technology by adding capacity via added multi-wavelength capability to more advanced generation of the product, while leaving the basic single-wavelength structure the same and backward compatible with early generations. Yet another application opportunity may be in pre-recorded content distribution, where core content could be recorded to be accessible with all single-wavelength devices, while extended content carried by the alternate wavelength(s) recording would only be accessible by advanced devices.
Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
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.
