This application makes reference to the following co-pending U.S. patent applications. The first application is U.S. Prov. App. No. 60/855,754, entitled “MONOCULAR HOLOGRAPHIC DATA STORAGE SYSTEM ARCHITECTURE,” filed Nov. 1, 2006. The second application is U.S. Prov. App. No. 60/872,472, entitled “PHASE CONJUGATE READOUT GEOMETRIES FOR HOLOGRAPHIC DATA STORAGE,” filed Dec. 4, 2006. The entire disclosure and contents of the above applications are hereby incorporated by reference.
In compliance with 37 C.F.R. §1.71(g) (1), disclosure is herein made that the inventions described and claimed herein were made pursuant to a Joint Research Agreement as defined in 35 U.S.C. 103 (c) (3), that was in effect on or before the date the inventions were made, and as a result of activities undertaken within the scope of the Joint Research Agreement, by or on the behalf of Hitachi Co., Ltd. and InPhase Technologies, Inc.
1. Field of the Invention
The present invention relates to holographic data storage devices, systems, articles, and methods for recording (storing) and/or reading (recovering) holographic data.
2. Related Art
In holographic data storage, there are many methods used for recording multiplexed pages of data in the same location in the holographic storage medium. Multiplexing many pages of information into the same location is what gives holographic data storage such large possible real bit densities (upwards of 1.6 Tb/in2). The choice of multiplexing geometry is quite involved and has many considerations to take into account: size, cost, complexity, robustness to external environment such as temperature, etc.
For consumer products, size may be a very important factor in producing an inexpensive system. Reducing the number of lens elements in a holographic system may also be important to decreasing size. In addition, smaller spatial light modulators and cameras may be important to size reduction. The size of the lenses used to read and write to the holographic storage medium may also be important in determining the overall height/size of the system.
According to a broad aspect of the present invention, there is provided a holographic data storage devices or systems which utilize a single objective lens (monocular architecture) through which the data beam and reference beam are passed prior to the entering the holographic storage medium and which may combine angle and polytopic multiplexing, as well as utilizing phase conjugate reconstruction and readout of multiple holograms. The monocular architecture in these devices or systems provides more compact recording and reading of data pages in a holographic storage medium. Also according to another broad aspect of the present invention, there are provided methods for carrying out data storage and/or data recovery using such devices or systems. Further according to another broad aspect of the present invention, there are provided articles comprising holographic storage media for recording or for reading recorded data using such devices or systems.
The invention will be described in conjunction with the accompanying drawings, in which:
FIG. 1 is schematic view illustrating data storage by a monocular holographic storage device or system using a moving reference beam lens to create an angularly multiplexed beam;
FIG. 2 is a schematic view illustrating dithering of the reference beam in the device or system of FIG. 1;
FIG. 3 is a rectangular cross-section of an SLM that may be used in the device or system of FIG. 1;
FIG. 4 is a circular cross-section of an alternate SLM that may be used in the device or system of FIG. 1;
FIG. 5 is schematic view illustrating data storage in a monocular holographic storage device and system using a moving objective lens to create an angularly multiplexed beam;
FIG. 6 is a schematic view illustrating dithering reference beam by moving the objective lens in the device or system of FIG. 5;
FIG. 7 is a circular cross-section view of an SLM that may be used in the device and system of FIG. 5;
FIG. 8 is a schematic diagram illustrating data storage by a holographic storage device or system that has minimal overlap of the reference beam and data beam;
FIG. 9 is a schematic diagram illustrating data storage by a holographic storage device or system showing the overlap between a reference beam and a data beam having the same width;
FIG. 10 is a schematic diagram illustrating data storage by a holographic storage device or system showing the overlap between a reference beam and data beam where the reference beam is wider than the data beam;
FIG. 11 is schematic diagram illustrating data storage by a holographic storage device or system in accordance with one embodiment of the present invention showing the overlap regions between a wider reference beam and inner and outer portions of the data beam;
FIG. 12 is an angular space representation of data beams and reference beams where the objective lens is at an extreme position;
FIG. 13 is an angular space representation of data beams and reference beams where showing the grating vectors;
FIG. 14 is an angular space representation of data beams and different reference beam angles from those in FIG. 13 showing the grating vectors;
FIG. 15 is a schematic view of an effect of a corner cube on an incident light beam;
FIG. 16 is a schematic diagram illustrating data recovery by a monocular holographic storage device and system according to one embodiment of the present invention using the corner cube of FIG. 15 in a phase conjugate geometry;
FIG. 17 is a schematic diagram illustrating data recovery by a monocular holographic storage device and system according to one embodiment of the present invention using the corner cube of FIG. 15 in a different phase conjugate geometry;
FIG. 18 is a schematic diagram illustrating data recovery by a monocular holographic storage device and system according to one embodiment of the present invention using a corner cube array in a phase conjugate geometry;
FIG. 19 is a schematic diagram illustrating data recovery by a monocular holographic storage device or system according to one embodiment of the present invention using another phase conjugate geometry;
FIG. 20 is a top plan view of the spatial light modulator (SLM) used in the device or system of FIG. 19;
FIG. 21 is a schematic diagram of an electro-optic (EO) crystal device or system according to one embodiment of the present invention;
FIG. 22 is a schematic diagram illustrating data recovery by a monocular holographic storage device or system according to one embodiment of the present invention using the EO crystal device or system of FIG. 21;
FIG. 23 is a schematic diagram of a diffraction device or system according to one embodiment of the present invention;
FIG. 24 is a schematic diagram illustrating data recovery by a monocular holographic storage device or system according to one embodiment of the present invention using the diffraction device or system of FIG. 23;
FIG. 25 is a schematic diagram illustrating data storage by a monocular holographic storage device or system according to one embodiment of the present invention;
FIG. 26 is a schematic diagram of the same or similar device or system of FIG. 25 but illustrating data recovery;
FIG. 27 is a schematic diagram illustrating data storage by a monocular holographic storage device or system according to one embodiment of the present invention;
FIG. 28 is a schematic diagram of the same or similar device or system of FIG. 27, but illustrating data recovery;
FIG. 29 is a schematic diagram of a monocular holographic storage device or system according to one embodiment of the present invention illustrating both data storage and data recovery;
FIG. 30 is an architectural diagram of an embodiment of a monocular holographic storage device or system showing various components schematically according to one embodiment of the present invention in a record (write) configuration;
FIG. 31 is an architectural diagram of the device or system of 30 , but in a read (recovery) configuration;
FIG. 32 shows a scan of the Bragg selectivity of the hologram compared to standard theoretical Bragg angular selectivity for the device or system of FIG. 30;
FIG. 33 shows a graph of signal to noise ratio (SNR) and relative intensity of 13 holograms multiplexed by moving the reference beam lens of the device or system of FIG. 30;
FIG. 34 shows an image of a recovered data page; and
FIG. 35 shows an SNR map of the recovered data page of FIG. 34.
It is advantageous to define several terms before describing the invention. It should be appreciated that the following definitions are used throughout this application.
Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.
For the purposes of the present invention, the term “coherent light beam” refers to a beam of light including waves with a particular (e.g., constant) phase relationship, such as, for example, a laser beam. A coherent light beam may also be referred to as light in which the phases of all electromagnetic waves at each point on a line normal to the direction of the light beam are identical.
For the purposes of the present invention, the term “corner cube” or “corner reflector” refers to an optical device having the shape of part of a cube with three mutually perpendicular reflecting surfaces. Such an optical device reflects a beam of any incident angle in a parallel direction to the incident beam after reflecting three times inside the corner cube as shown, for example, in FIG. 15.
For the purposes of the present invention, the term “data beam” refers to a beam containing a data signal. For example, a data beam may include beams that have been modulated by a modulator such as a spatial light modulator (SLM), along with a beam generated in response to a reference beam impingent on a holographic storage medium, where the generated beam includes data. The modulation of the data beam may be an amplitude, a phase or some combination of the amplitude and phase. The SLM may be reflective or transmissive. The data beam may be modulated into a binary state or into a plurality of states.
For the purposes of the present invention, the term “data modulated beam” refers to a data beam that has been modulated by a modulator such as a spatial light modulator (SLM). The modulation of the data beam may be an amplitude, a phase or some combination of the amplitude and phase. The SLM may be reflective or transmissive. The data beam may be modulated into a binary state or into a plurality of states.
For the purposes of the present invention, the term “data modulator” refers to any device that is capable of optically representing data in one or two-dimensions from a signal beam.
For the purposes of the present invention, the term “data page” or “page” refers to the conventional meaning of data page as used with respect to holography. For example, a data page may be a page of data (i.e., a two-dimensional assembly of data), one or more pictures, etc., to be recorded or recorded in a holographic storage medium.
For the purposes of the present invention, the term “detector” refers to any type of device capable of detecting something. For example, exemplary detectors may include devices capable of detecting the presence or intensity of light, such as for example, a camera or quad cell, complementary metal-oxide-semiconductor (CMOS) imaging sensors or arrays, charge-coupled device (CCD) arrays, etc.
For the purposes of the present invention, the term “disk” refers to a disk-shaped holographic storage medium.
For the purposes of the present invention, the term “dithering” refers to moving an object, for example, a lens, mirror, reflective layer, etc., back and forth.
For the purposes of the present invention, the terms “holographic grating,” “holograph” or “hologram” (collectively and interchangeably referred to hereafter as “hologram”) are used in the conventional sense of referring to an interference pattern formed when a signal beam and a reference beam interfere with each other. In cases wherein digital data is recorded page-wise, the signal beam may be encoded with a data modulator, e.g., a spatial light modulator, etc.
For the purposes of the present invention, the term “storage medium” refers to any component, material, etc., capable of storing information, such as, for example, a holographic storage medium.
For the purposes of the present invention, the term “holographic storage medium” refers to medium that has a least one component, material, layer, etc., that is capable of recording and storing one or more holograms (e.g., bit-wise, linear array-wise or page-wise) as one or more patterns of varying refractive index imprinted into the medium. Examples of a holographic medium useful herein include, but are not limited to, those described in: U.S. Pat. No. 6,103,454 (Dhar et al.), issued Aug. 15, 2000; U.S. Pat. No. 6,482,551 (Dhar et al.), issued Nov. 19, 2002; U.S. Pat. No. 6,650,447 (Curtis et al.), issued Nov. 18, 2003, U.S. Pat. No. 6,743,552 (Setthachayanon et al.), issued Jun. 1, 2004; U.S. Pat. No. 6,765,061 (Dhar et al.), Jul. 20, 2004; U.S. Pat. No. 6,780,546 (Trentler et al.), issued Aug. 24, 2004; U.S. Patent Application No. 2003/0206320 (Cole et al.) published Nov. 6, 2003; and U.S. Patent Application No. 2004/0027625 (Trentler et al.), published Feb. 12, 2004, the entire contents and disclosures of which are herein incorporated by reference. A holographic storage medium of the present invention may be any type of holographic storage medium including: a transparent holographic storage medium, a holographic storage medium including a plurality of components or layers such as a reflective layer, a holographic storage medium including a reflective layer and a polarizing layer so reflection may be controlled with polarization, a holographic storage medium including a variable beam transmission layer that may be pass, absorb, reflect, be transparent to, etc., light beams, grating layers for reflecting light beams, substrates, substrates with servo markings, etc.
For the purposes of the present invention, the term “upper surface” refers to the surface of the holographic storage medium that acts as an interface between the air and the holographic storage medium.
For the purposes of the present invention, the term “holographic recording” refers to the act of recording a hologram in a holographic storage medium. The holographic recording may provide bit-wise storage (i.e., recording of one bit of data), may provide storage of a 1-dimensional linear array of data (i.e., a 1×N array, where N is the number linear data bits), or may provide 2-dimensional storage of a page of data.
For the purposes of the present invention, the term “multiplexing” refers to recording, storing, etc., a plurality of holograms in the same volume or nearly the same volume of the holographic storage medium by varying a recording parameter(s) including, but not limited to, angle, wavelength, phase code, shift, correlation, peristrophic, etc., including combinations of parameters, e.g. angle-polytopic multiplexing For example, angle multiplexing involves varying the angle of the plane wave or nearly plane wave of the reference beam during recording to store a plurality of holograms in the same volume. The multiplexed holograms that are recorded, stored, etc., may be read, retrieved, reconstructed, recovered, etc., by using the same recording parameter(s) used to record, store, etc., the respective holograms.
For the purposes of the present invention, the term “light source” refers to a source of electromagnetic radiation having a single wavelength or multiple wavelengths. The light source may be from a laser, one or more light emitting diodes (LEDs), etc.
For the purposes of the present invention, the term “mode” refers to a wavelength of light generated by a light source.
For the purposes of the present invention, the term “single mode” refers to a single wavelength of light generated by a light source. For example, a single mode laser produces a single dominant wavelength.
For the purposes of the present invention, the term “multi-mode” refers to multiple wavelengths of light generated by the light source. For example, a multi-mode laser produces multiple wavelengths of light with significant power.
For the purposes of the present invention, the term “optical steering subsystem” refers to any device or combination of devices capable of directing light in a particular direction. Exemplary optical steering subsystems may include a mirror (e.g., a galvo mirror), a combination of mirrors, lenses, and/or other devices, etc.
For the purposes of the present invention, the term “partially reflective surface” refers to any surface of an object capable of reflecting a portion of light while allowing another portion to pass through the surface.
For the purposes of the present invention, the term “plane wave” refers to a constant-frequency wave whose wavefronts (surfaces of constant phase) are substantially or nearly parallel planes of constant amplitude and normal to the direction of the wave and exist in a localized region of space. Exemplary plane waves may include collimated light such as those associated with laser beams for laser pointers, etc.
For the purposes of the present invention, the term “processor” refers to a device capable of, for example, executing instructions, implementing logic, calculating and storing values, etc. Exemplary processors may include application specific integrated circuits (ASIC), central processing units, microprocessors, such as, for example, microprocessors commercially available from Intel and AMD, etc.
For the purposes of the present invention, the term “reading data” refers to retrieving, recovering, or reconstructing holographic data stored in a holographic storage medium.
For the purposes of the present invention, the term “recording data” refers to storing or writing holographic data into a holographic storage medium.
For the purposes of the present invention, the term “recording light” refers to a light source used to record information, data, etc., into a holographic storage medium.
For the purposes of the present invention, the term “phase conjugate” when referring to a light beam refers to a light beam which is an exact or very close replica of a second light beam, but propagating exactly or very closely in the reverse direction of the second light beam.
For the purposes of the present invention, the term “phase conjugate optical system” refers to any device that causes a reference beam (also referred to as a “reconstruction beam” when used for data recovery) of a holographic storage device or system to be reflected (directed) back along the path of the reference (reconstruction) beam in the opposition direction. Examples of phase conjugate optical systems may include a corner cube, a corner cube array, a controlled electro-optic (EO) crystal, a controlled blazed grating, a holographic grating, surface relief structure, and the combination of a variable layer and a grating (whether a holographic grating or surface relief structure) as shown in, for example, FIGS. 28 and 29, etc.
For the purposes of the present invention, the term “recovered beam” refers to a beam generated by the reference (reconstruction) beam which is provided by the phase conjugate optical system. The phase conjugate of the reference (reconstruction) beam will reconstruct the phase conjugate of the data beam which propagates backwards along the original data beam's optical path to be recovered as a data page by a detector (e.g., camera). The recovered beam is formed by the phase conjugate reference (reconstruction) beam diffracting from a hologram of a data page stored in the holographic storage medium. For example, with angle multiplexed holograms, for a given angle a certain data page will be Bragg matched and the phase conjugate reference (reconstruction) beam will diffract and form the recovered beam. Since the phase conjugate reference (reconstruction) beam is used at that correct angle and wavelength (Bragg condition), the desired data page will be reconstructed as a phase conjugate beam which propagates back to where the data beam originated from. The phase conjugate nature allows the recovered beam to undo aberrations that may have been introduced during recording of the holograms and to form a higher quality data page at the detector. This happens if the hologram and reference (reconstruction) beam are within tolerance of being phase conjugate of the original reference beam and relative location to similar optics. For some optical designs, these tolerances may be multiple waves of aberration in the phase conjugate reference (reconstruction) plane wave and many tens of microns in relative position of the hologram and optical system. The reference (reconstruction) beam may also Bragg match out a traditional hologram, but will propagate out of the optical system (i.e., not back to the detector/SLM).
For the purposes of the present invention, the term “reference beam” refers to a beam of light not modulated with data. Exemplary reference beams include non-data bearing laser beams used while recording data to, or reading data from a holographic storage medium. In some embodiments, the reference beam may refer to the original reference beam used to record the hologram, to a reconstruction beam when used to recover data from the holographic storage medium, or to the phase conjugate of the original reference (reconstruction) beam.
For the purposes of the present invention, the term “refractive index profile” refers to a three-dimensional (X, Y, Z) mapping of the refractive index pattern recorded in a holographic storage medium.
For the purposes of the present invention, the term “dynamic range” or “M#” of a material refers to a conventional measure of how many holograms at a particular diffraction efficiency may be multiplexed at a given location in the material (e.g., recording material layer, holographic storage medium, etc.) and is related to the materials index change, material thickness, wavelength of light, optical geometry, etc.
For the purposes of the present invention, the term “spatial light modulator” (SLM) refers to a device that stores information on a light beam by, for example, modulating the spatial intensity and/or phase profile of the light beam.
For the purposes of the present invention, the term “spatial light intensity” refers to a light intensity distribution or pattern of varying light intensity within a given volume of space.
For the purposes of the present invention, the term “book” or “stack” refers to a group of angular multiplexed holograms that span a particular angular range. A book is a group of angular multiplexed holograms that may be all in one location in the holographic storage medium or slightly shifted from one another or shifted from another group of holograms. The term book refers to both traditional books and composite books.
For the purposes of the present invention, the term “short stack” refers to sub-group of holograms within the address range of a book. For example, a book may be considered as a set of addresses that contain angles 1 - 500 . This angular range may be further separated into “short stacks” so that short stack # 1 contains angles 1 - 100 , short stack # 2 contains angles 101 - 200 , etc.
For the purposes of the present invention, the term “composite book” refers to a book where at least some of the short stacks of the book do not occupy the same spatial location. In fact, it may be useful to “smear” out any optically induced distortions by placing short stacks in different spatial locations. In a composite book, the spatial locations of the short stacks may partially overlap one another, but differ enough spatially to mitigate any non-ideal media buildup due to multiple recordings in the same location.
For the purposes of the present invention, the term “beam block” refers to any device capable of absorbing light, such as, for example, an incident light beam.
For the purpose of the present invention, the term “waveplate” refers to any device that may be used to change the polarization of light. A waveplate is also sometimes referred to as a retarder and the terms may be used interchangeably herein. Exemplary waveplates, include a λ/4 waveplate (QWP) that may be used, for example, to cause a ¼ wavelength phase shift in a light beam that may result in changing linearly polarized light to circular and vice versa. Further, for example, a light beam twice passing through a λ/4 waveplate may undergo a 90 degree rotation in the linear polarization of the light.
For the purpose of the present invention, the term “device” may refer to an apparatus, a mechanism, equipment, machine, etc.
For the purpose of the present invention, the term “holographic storage device or system” refers to a device or system which may record (store) holographic data, which may read (recover) holographic data, or which may record (store) and read (recover) holographic data.
In holographic data storage, there are many methods which may be used for recording multiplexed pages of data in the same location in the holographic storage medium. See, for example, U.S. Pat. No. 6,721,076 (King et al.), issued Apr. 13, 2004 (angle multiplexing), the entire disclosure and contents of which is hereby incorporated by reference. Among these methods, angle-polytopic multiplexing using a phase conjugate geometry for readout of data may be used to achieve higher capacity and faster transfer rates at the same time. See, for example, U.S. Pat. No. 7,092,133 (Anderson et al.), issued Aug. 15, 2006; Ken Anderson, et al, “High Speed Holographic Data Storage at 500 Gb/in2,” SMPTE Motion Imaging Journal, May/June 2006 pp 200-20, the entire contents and disclosure of the foregoing documents being hereby incorporated by reference. In many multiplexing techniques such as angle-polytopic multiplexing, two beam paths may be used: an object (data) beam path and a reference beam path. But as the lenses shrink, the working distance for these optics becomes very small and it may be difficult to relay these beams into the holographic storage medium. Both the data beam path lens and the reference beam path lens may be competing for space as the lenses get smaller, the focal lengths shrink, etc., as the various components get as close as possible to the holographic storage medium.
One way of addressing this working distance problem in holographic storage architectures is to use a single lens for the two recording beams, namely the data beam and the reference beam. This allows the single lens to be parallel to the holographic storage medium which significantly improves the working distance problem, thus allowing the lenses to shrink in size even further. There have been many holographic storage architectures proposed for incorporating both the data (object) and reference beams through a single lens that focuses the combination onto or into the holographic storage medium. Most of these systems use correlation multiplexing (a complex reference beam) or shift multiplexing (a spherical reference beam). See, for example, U.S. Pat. No. 6,909,529 (Curtis), issued Jun. 21, 2005 (correlation multiplexing), Yukiko Nagasaka, et al., “Multiplexing Method with Non-Coaxial Spherical Waves for Holographic Data Storage” ISOM 2006, Th-I-28 (shift multiplexing), and U.S. Pat. No. 6,995,882 (Horimai), issued Feb. 7, 2006 (shift multiplexing). Correlation multiplexing and shift multiplexing may have some disadvantages in transfer rate, scatter, noise, and environmental effects (such as holographic storage medium expansion with temperature). Some of these methods also use reference beams that are not plane waves and/or not suitable for angle multiplexing.
Previously, single lens holography might use a multiplexing method that relies on the holographic storage medium to move between pages, but is often too slow to achieve reasonable transfer rates. In addition, the achievable numerical aperture (NA) that the data beam may use is limited and therefore the achievable density is limited to lower values. Also, while temperature compensation and interchange may be demonstrated with angle multiplexed holograms, it is much more difficult to achieve and still has not been achieved for multiplexing methods that rely on shifts between holograms to multiplexed holograms.
For these reasons, angle multiplexing has the advantage and benefit of being able to utilize fast mechanisms such as mechanically rotating mirrors or shifting lenses to accomplish fast page-to-page writes and reads. But prior attempts to use angle multiplexing with transmission of both beams through one objective lens do not use larger two-dimensional pages that may be required for high density or fast transfer rate which also require high numerical aperture (NA) optics. Combining angle and polytopic multiplexing with a single lens design and also utilizing phase conjugate reconstruction of multiple holograms for readout of the recorded data is, therefore, not obvious.
The present invention provides a novel and unobvious way of angle multiplexing, or combined angle and polytopic multiplexing for increased density storage, that provides the advantages and benefits of a single lens design while still retaining the advantages and benefits of traditional angle multiplexing, or angle-polytopic multiplexing. The present invention uses techniques that may be used to accomplish angle multiplexing or angle-polytopic multiplexing in a very simple architecture using smaller optics and simpler and faster mechanical mechanisms to accomplish the multiplexing. This architecture utilizes a single objective lens closer to the holographic storage medium to focus both the data beam and the reference beam into the holographic storage medium, e.g., a focal length of the lens in the range of from about 1 to about 7 mm, for example from about 1 to about 4 mm, and a working distance from the lens to the surface of the holographic storage medium in the range of from about 500 to about 3000 microns. It is thus possible to combine the benefits of angle multiplexing with the simplicity of single lens architectures. This architectural technique with phase conjugate reconstruction for data recovery is referred to hereafter as “monocular architecture.” By using phase conjugation, a higher numerical lens may be used and therefore much higher storage density may be achieved. In monocular architecture, the reference beam shares part of the objective lens (also referred to interchangeably as the “object lens” or “storage lens”) with the data beam. The incident angle of a focused reference beam through the objective lens is related to the distance, h, of the focused beam from the optical axis of the objective lens.
In monocular architecture, the distance h may be changed in several different ways and therefore produce a change in angle of the reference beam inside the holographic storage medium. One method that is used to change h is to produce h with a reference beam lens that dithers back and forth in direction in the plane of the desired angle change, as shown, for example, in the device or system of FIGS. 1 and 2, described in greater detail below. This dithering may be done very fast with, for example, a flexure mount, such as on a DVD or CD objective. This changes the location of the focus while the optical axis remains fixed. A second way that this angle may be changed is by changing h by leaving the focus point fixed while dithering the optical axis of the objective lens (see, for example, FIGS. 5 and 6, described in greater detail below). It is not obvious that this may be done because dithering the objective lens also changes the angle of the data beam. But it was discovered that this is not a problem and may actually provide some advantage and benefit from the perspective that it mitigates data beam correlational noise buildup since the data beam is constantly moving. It may also possible to use a galvo mirror or a Microelectromechanical Systems (MEMs) mirror to change the angle of the reference beam incident to the reference beam lens. This would form a standard four focal length with two lenses (4F) relay system.
According to one embodiment of the present invention, there is provided a holographic storage device or system comprising:
According to another embodiment of the present invention, there is provided a holographic storage device or system comprising:
According to another embodiment of the present invention, there is provided a method comprising the following steps:
According to another embodiment of the present invention, there is provided a holographic storage device or system comprising:
According to another embodiment of the present invention, there is provided a method comprising the following steps:
According to another embodiment of the present invention, there is provided a holographic storage device or system comprising:
According to another embodiment of the present invention, there is provided a method comprising the following steps:
According to another embodiment of the present invention, there is provided an article comprising holographic storage medium comprising:
According to another embodiment of the present invention, there is provided an article comprising holographic storage medium comprising:
For example, FIGS. 1 and 2 show a monocular holographic storage device or system 102 according to one embodiment of the present invention (illustrating data storage but which may also be used for data recovery). Device or system 102 includes a reference beam 104 , a data beam represented by an inner pixel wave front 106 (hereafter referred to as inner data beam portion 106 ) and an outer pixel wave front 107 (hereafter referred to as outer data beam portion 107 ), an objective lens 108 (which may also be referred to interchangeably as an “object lens” or “storage lens”) and a holographic storage medium 110 . Between objective lens 108 and holographic storage medium 110 is an air gap 114 . A reference beam lens 122 focuses reference beam 104 on the back focal plane of objective lens 108 . Reference beam lens 122 has an optical axis 124 . Reference beam lens 122 is moved in a direction shown by two-headed arrow 126 parallel to upper surface 128 of holographic storage medium 110 . Inner data beam portion 106 and outer data beam portion 107 are angled by objective lens 108 to form, respectively, an angled inner data beam 132 portion and an angled outer data beam portion 134 that are relayed into holographic storage medium 110 as plane waves and overlap in generally diamond-shaped region 136 . Holographic storage system 102 also includes an SLM 142 , a camera 144 , a polarizing beam splitter (PBS) 146 , and a polytopic filter coating 148 on PBS 146 . SLM 142 and PBS 146 are shown in FIG. 1 as being positioned between reference beam lens 122 and objective lens 108 . Holographic storage medium 110 includes a lower substrate 152 , a recording material 154 and an upper substrate 156 . Objective lens 108 takes the Fourier transform of data beam portions 106 and 107 off of SLM 142 . While polytopic filter coating 148 is shown in FIG. 1 as being on PBS 146 , coating 148 may also be on or part of the objective lens 108 , or on camera 144 and/or SLM 142 . See also U.S. Prov. App. No. 60/907,445, entitled “NON-FT PLANE POLYTOPIC FILTERS,” filed Apr. 2, 2007, the entire disclosure and contents of which is hereby incorporated by reference, for suitable materials for coating 148 or for polytopic filtering without using standard relay lenses and apertures. By moving reference beam lens 122 as shown by arrow 126 , reference beam 104 is dithered to form a dithered reference beam 162 which becomes angled dithered reference beam 164 (shown by dashed lines) after passing through objective lens 108 and may be used for multiplexing storage (and recovery) of data. Angled dithered beam 164 is relayed into holographic storage medium 110 as a plane wave and overlaps and interferes with angled data beam portions 132 and 134 in larger region 166 (which includes overlap region 136 ) to form holograms (e.g., data pages) which are recorded in recording material 154 of holographic storage medium 110 . Angled inner data beam portion 132 has an angle of incidence 174 on holographic storage medium 110 . Angled dithered reference beam 164 has an angle of incidence 184 on holographic storage medium 110 . Optical axis 124 of reference beam lens 122 is a distance 188 from optical axis 178 of objective lens 108 (i.e., distance 188 corresponds to the distance, h, described above). Arrow 196 shows direction of a light beam which is incident on PBS 146 and which illuminates the entire SLM 142 and generates the data beam which includes portions represented by 106 and 107 .
The monocular holographic storage device or system shown in FIGS. 1 and 2 allows minimizing the size of the holographic optical head by allowing the data beam and the reference beam to share the same objective lens. In the device or system of FIGS. 1 and 2, the reference beam is generated by focusing the reference beam onto the same plane as the SLM but slightly offset in position from the SLM pixels. The focused reference beam is turned into a plane wave at the holographic storage medium by the larger objective lens. By dithering the objective lens in one dimension with a similar mechanism to a DVD lens actuator, the positional shift in focus is turned into an angular change at the holographic storage medium. Using a high numerical aperture (NA) objective lens (e.g., a numerical aperture of at least about 0.85 with a focal length of 4 mm), a lens shift in the range of about 1 mm may create up to about a 25 degree angular change in the reference beam. By using a very high numerical aperture, the numerical aperture (angles) used by the data beam may be kept very high (i.e., many pixels) which may be needed to get to higher densities and transfer rates. The size of the reference beam in the holographic storage medium may be determined by the numerical aperture of the dithering lens and may be easily modified to give different beam sizes. An additional benefit of this technique is that a Bragg degenerate correction may be easily generated by a slight lens offset into or out of the page.
This monocular architecture may significantly simplify the holographic storage device or system layout or configuration, but may require that the objective lens be able to produce a high quality plane wave at the outside edge of the objective lens for good phase conjugation. In addition, there may not be complete overlap between the reference and data beam and this may cause some degradation in the signal to noise ratio (SNR) of the holograms. The size of the reference beam may need to be optimized for best overlap and minimal waste of the holographic storage medium. The size is determined by the NA of reference beam at its focal point (as related to reference beam lens).
FIG. 3 shows a rectangular cross-section of SLM 302 that may be used in the device or system of FIG. 1. SLM 302 includes an absorbing or non transmitting (e.g., mirrored) surface 308 with a channel 304 in surface 308 for the reference beam focus to pass through, with 306 indicating the part of SLM 302 used for displaying data. FIG. 4 shows a circular cross-section of an alternate SLM 402 where only the portion that may adequately pass through the objective lens is used (i.e., a circular field of view). SLM 402 includes an absorbing or non transmitting (e.g., mirrored) surface 408 with a channel 404 in surface 408 for the reference beam focus to pass through, with 406 indicating the part of SLM 402 used for displaying data.
A second monocular method may be accomplished by leaving the reference beam focus at the same location and dithering the objective lens as shown in FIGS. 5 and 6. This produces the same angular deviation of the reference beam as the previous architecture shown in, for example, the embodiment of FIGS. 1 and 2. But in the case of the embodiment of FIGS. 5 and 6, the data beam angles also change as a function of the objective lens position. The overall result is the same or similar, and thus puts the multiplexing movement completely into the objective lens. This movement would be the identical or similar to the movement required in a CD or DVD objective lens. Because the recovered holograms may be shifted on the detector, clear over-sampled detection may be required.
FIGS. 5 and 6 show a monocular holographic storage device or system 502 according to one embodiment of the present invention (illustrating data storage but which may also be used for data recovery). Device or system 502 includes a reference beam 504 , a data beam represented by an inner pixel wave front 506 (hereafter referred to as inner data beam portion 506 ) and an outer pixel wave front 507 (hereafter referred to as outer data beam portion 507 ), an objective lens 508 and a holographic storage medium 510 . Between objective lens 508 and holographic storage medium 510 is an air gap 514 . A reference beam lens 522 focuses reference beam 504 on the back focal plane of objective lens 508 . Reference beam lens 522 has an optical axis 524 . Objective lens 508 is moved, as shown by two-headed arrow 526 , in a direction parallel to upper surface 528 of holographic storage medium 510 . Inner data beam portion 506 and outer data beam portion 507 are angled by objective lens 108 to form, respectively, an angled inner data beam 532 portion and an angled outer data beam portion 534 that are relayed into holographic storage medium 510 as plane waves and overlap in generally diamond-shaped region 536 . Holographic storage system 502 also includes an SLM 542 , a camera 544 , a polarizing beam splitter (PBS) 546 , and a polytopic filter coating 548 on PBS 542 . SLM 542 and PBS 546 are shown in FIG. 1 as being positioned between reference beam lens 522 and objective lens 508 . Holographic storage medium 510 includes a lower substrate 552 , a recording material 554 and an upper substrate 556 . Objective lens 508 takes the Fourier transform of data beam portions 506 and 507 off of SLM 542 . While polytopic filter coating 548 is shown in FIG. 5 as being on PBS 546 , coating 543 may also be on or part of the objective lens 508 , or on camera 544 and/or SLM 542 . See also U.S. Prov. App. No. 60/907,445, entitled “NON-FT PLANE POLYTOPIC FILTERS,” filed Apr. 2, 2007, the entire disclosure and contents of which is hereby incorporated by reference, for suitable materials for coating 548 or for polytopic filtering without using standard relay lenses and apertures. By moving objective lens 508 as shown by arrow 526 , reference beam 504 is dithered to form a dithered reference beam 562 which becomes angled dithered reference beam 564 (shown by dashed lines) after passing through objective lens 508 and is used for multiplexing storage (and recovery) of data. Angled dithered reference beam 564 is relayed into holographic storage medium 510 as a plane wave and overlaps and interferes with angled data beam portions 532 and 534 in larger region 566 (which includes overlap region 536 ). Angled dithered reference beam 564 has an angle of incidence 574 on holographic storage medium 510 . Optical axis 524 of reference beam lens 522 is a distance 588 from optical axis 578 of objective lens 508 (i.e., the distance 588 corresponds to the distance, h, described above). Arrow 596 shows the direction of a light beam which is incident on PBS 546 and which illuminates the entire SLM 542 and generates the data beam which includes portions represented by 506 and 507 .
While the embodiment shown in FIGS. 5 and 6 further simplifies the layout or configuration of the holographic device or system, it also adds slightly more to the complexity of the objective lens. In this case, it may require the objective lens to be designed such that there is a minimum amount of position sensitivity in the objective lens such that the reconstructed image (data) does not become aberrated by propagating through a different part of the lens than it was recorded from. This effect is only a secondary effect, because, for the most part, there will only be a minimal shift necessary that comes from any offset between the reference beam and the data beam that comes from shrinkage or thermal expansion.
FIG. 7 is circular cross-section of an SLM 702 which may be used in the device or system of FIG. 5 where only the portion that may adequately pass through the objective lens is used (i.e., a circular field of view). SLM 702 includes an absorbing or non transmitting (e.g., mirrored) surface 708 with a channel 704 in surface 706 for the reference beam focus to pass through, with 706 indicating the part of SLM 702 used for displaying data.
The overlap between the data and reference beams may be important to the performance of a holographic data storage system. Improper overlap may result in loss of diffraction efficiency and a broadening of Bragg selectivity and therefore a loss in density/capacity. FIG. 8 illustrates this beam overlap when the data beam (external pixel) and reference beam size are identical. FIG. 8 shows part of a monocular holographic storage system 802 (illustrating data storage) which includes reference beam 804 , data beam 806 (represented as the outer most pixel wave front on the SLM 842 ), an objective lens 808 and a holographic storage medium 810 . Between objective lens 808 and holographic storage medium 810 is an air gap 814 . Reference beam 804 and data beam 806 are relayed into holographic storage medium 810 as plane waves 830 and 832 and overlap and interfere in region 834 to form holograms (e.g. data pages) which are recorded in recording material 854 of holographic storage medium 810 . Holographic storage system 802 also includes an SLM 842 and a polarizing beam splitter (PBS) 846 . Holographic storage medium 810 includes a lower substrate 852 , a recording material 854 and an upper substrate 856 . Objective lens 808 has an optical axis 878 . Objective lens 808 takes the Fourier transform of data beam 806 off of SLM 842 . A polytopic filter may be in objective lens 808 , on camera 844 and/or SLM 842 , or on PBS 846 .
In FIG. 8, the reference beam may be too small to completely overlap the farthest pixel that makes up the data beam. This pixel is used because it represents the worst case overlap between a data beam pixel and the reference beam. Data beam pixels closer to the reference beam will have better overlap. Better beam overlap may be achieved by making the reference beam wider, but this may affect many system parameters such as SLM size, reference beam location, lens performance, etc. To optimize the reference beam size and improve the resulting effects, it is easiest to start with complete beam overlap and work backwards from there.
For example, FIG. 9 schematically illustrates data storage by a holographic storage device or system having smaller data and reference beam overlap similar to FIG. 8. FIG. 9 shows a reference beam 904 , a data beam 906 and a holographic storage medium 910 . Reference beam 904 and data beam 906 are shown as having the same diameter. Holographic storage medium 910 includes a lower substrate 922 , a recording material 924 , an upper substrate 926 , and upper surface 928 . Reference beam 904 and data beam 906 overlap and interfere with each other in a recording region 932 in recording material 924 . Reference beam 904 has an optical axis 934 and data beam 906 has an optical axis 936 .
By contrast, FIG. 10 illustrates data storage by a holographic storage device or system having greater beam overlap with a broader (wider) reference beam. FIG. 10 shows a reference beam 1004 , a data beam 1006 and a holographic storage medium 1010 . Reference beam 1004 has a larger diameter than data beam 906 . (Factors affecting the diameter of reference beam 1004 may include the numerical aperture of the reference lens, the focal length of the objective lens or beam divergence in the reference beam path, etc.) Holographic storage medium 1010 includes a lower substrate 1022 , a recording material 1024 , an upper substrate 1026 , and an upper surface 1028 . Reference beam 1004 and data beam 1006 overlap and interfere with each other in an overlap recording region 1032 in recording material 1024 . Reference beam 1004 has an optical axis 1034 and data beam 1006 has an optical axis 1036 .
In FIGS. 9 and 10, D is the distance between the optical axes of the reference beam and data beam at the upper surface of the holographic recording medium, R is the distance from the optical axis of the reference beam to the edge of the reference beam in a plane parallel at the upper surface of the recording material, and a. is the distance from the top of the recording material to the plane where the optical axes of the reference beam and data beam intersect. As can be seen by comparing FIG. 10 to FIG. 9, because reference beam 1004 is wider than reference beam 904 , overlap recording region 1032 is larger than overlap recording region 932 . Also, as can be seen in FIG. 10, reference beam 1004 and data beam 1006 additionally overlap in non-recording overlap regions 1042 and 1044 in lower substrate 1022 and upper substrate 1026 , respectively.
Once the required radius of the reference beam is known, it is important to determine how this larger size affects the size of the SLM and the position of the reference beam with respect to the SLM. The major effect comes from the fact that the reference beam size may need to be taken into account when determining the angular bandwidth (how fast it expands) of the reference beam as the focus position is always at the SLM plane for the reference beam to be a plane wave. This effect may be illustrated in FIG. 11.
FIG. 11 illustrates data storage by a monocular holographic storage device or system 1102 according to one embodiment of the present invention. Device or system 1102 includes a data beam represented by an inner pixel wave front 1104 (hereafter referred to as inner data beam portion 1104 ) and an outer pixel wave front 1106 (hereafter referred to as outer data beam portion 1106 ), an objective lens 1108 , a holographic storage medium 1110 , and a wide reference beam 1112 . Between objective lens 1108 and holographic storage medium 1110 is an air gap 1114 . Holographic storage medium includes an upper surface 1128 . Inner data beam portion 1104 and outer data beam portion 1106 are angled by objective lens 1108 to form, respectively, angled inner data beam portion 1132 and angled outer data beam portion 1134 which are relayed into holographic storage medium 1110 as plane waves and overlap in generally diamond-shaped region 1136 . Wide reference beam 1112 is angled by objective lens 1108 to form an angled wide reference beam 1138 which is relayed as a plane wave into holographic storage medium 1110 and overlaps and interferes with angled data beam portions 1132 and 1134 in generally X-shaped region 1140 . Holographic storage device or system 1102 also includes an SLM 1142 and a polarizing beam splitter (PBS) 1146 . Holographic storage medium 1110 includes a lower substrate 1152 , a recording material 1154 and an upper substrate 1156 . Objective lens 1108 takes the Fourier transform of data beam portions 1104 and 1106 off of SLM 1142 . A polytopic filter may be in objective lens 1108 , on camera 1144 and/or SLM 1142 , or on PBS 1146 . Double-headed arrow 1172 shows the width of objective lens 1108 . Double-headed arrow 1174 shows the distance in a plane parallel to upper surface 1128 from the edge of objective lens 1102 to the point where wide reference beam 1112 enters objective lens 1110 . Objective lens 1102 has an optical axis 1178 .
In one embodiment of the present invention the width of the objective lens (e.g., lens 1108 in FIG. 11) may be about 5 mm, for example, in the range of from about 1 to about 20 mm. There may be a tradeoff in reference beam size and angular multiplexing range and data beam size (number of pixels). This is due to the limited bandwidth of the objective lens even for very high NA lenses such as a numerical aperture of about 0.65 or higher, for example, NA lenses which have a numerical aperture of about 0.85 or higher which are attractive in allowing very large data pages (e.g. data pages sizes larger than 256×256 pixels, for example 1200×600 pixels) and large angle sweeps for the reference beam. In addition, in addition, these very high NA lenses are have the same NA as used in Blu-raym disk products (described below) which may allow for greater compatibility between systems.
FIGS. 12, 13 and 14 illustrate how the angles of the data beam and the reference beam at the holographic storage medium change with monocular dithering of the objective lens or the reference beam lens upstream of the objective lens, or changing the incident angle into the reference beam lens by changing, for example, the mirror tilt. FIG. 12 is an angular space representation of data beams and reference beams where the objective lens is at an extreme position. Reference beam angles are shown in region 1212 and data beam angles are shown in region 1214 relative to holographic storage medium surface 1218 . Region 1222 represents a dead space. Angle 1232 represents the minimum angle of the data beam relative to axis 1240 normal to the plane of holographic storage medium surface 1218 .
FIG. 13 is an angular space representation of data beams and reference beams showing the grating vectors. A reference beam vector is represented by arrow 1312 with various data beam grating angles being shown in region 1314 relative to holographic storage medium surface 1318 . The holographic grating vectors are represented by arrows 1320 between the reference beam 1312 and all the plane wave components (one for each pixel) in data beam 1314 . The axis 1340 normal to the plane of holographic storage medium surface 1318 is also shown.
FIG. 14 is an angular space representation of data beams and different reference beam angles from those in FIG. 13 showing the grating vectors. A reference beam vector is represented by arrow 1412 with various data beam grating angles being shown in region 1414 relative to holographic storage medium surface 1418 . The holographic grating vectors are represented by arrows 1420 between the reference beam 1412 and all the plane wave components (one for each pixel) in data beam 1414 . It should be noted that these spectrum of grating vectors 1420 are different from those shown in FIG. 13 for a different reference angle location. Thus the Bragg selectivity will separate these two holograms. The reference beam angular range is indicated by angular sweep 1424 .
In some embodiments of the present invention, in addition to dithering the lenses (i.e., reference beam lens, objective lens or both), the reference beam lens or objective lens may be moved in a direction parallel to the optical axis of the objective lens (commonly called the “focus direction”), thereby generating a diverging or converging reference beam at the holographic storage medium that may be used to compensate for page focus, magnification, shift, other system or medium changes, etc.
In other embodiments, the reference beam angle at the upper surface of the holographic storage medium may be changed to compensate for tilt of the holographic storage medium in a radial or tangential direction. The reference beam and/or objective lens movement may be in a direction parallel to holographic storage medium and in a direction orthogonal to the optical axis of the objective lens and the multiplexing direction. Also, the reference beam or objective lens may be moved in one direction and the holographic storage medium may be moved in a different direction. The change in the angle of the reference beam parallel or in a plane perpendicular to the optical axis of the objective lens may be used to compensate for tilting of the holographic storage medium, tilts in the holographic storage medium, or tilt errors in the holographic drive device relative to the holographic storage medium.
In yet other embodiments of the present invention, changing the angle and the wavelength of the reference beam may be used to compensate for temperature changes. See U.S. Pat. No. 6,348,983 (Curtis et al.), issued Feb. 19, 2002 and in an article by Alan Hoskins, et al., “Temperature Compensation Strategy for Holographic Storage,” ODS 2006, Apr. 23-36, 2006, the entire contents and disclosure of the foregoing patent and article being hereby incorporated by reference.
In one embodiment of the present invention, there is also a provided one or more methods for reading (recovering) data from a monocular holographic storage device or system where data is stored, as described above.
For consumer products, size may be a very important factor in the market, as well as in producing an inexpensive data storage and/or recovery device or system. Therefore, in order to achieve compact optics, it may be desirable that most of the holographic drive components and electronics be kept together on the same side of the holographic storage medium, such as by using phase conjugate readout geometries for data recovery. Phase conjugate readout geometries may involve using galvo mirrors on the back side of the holographic storage medium to fold the reference (reconstruction) beam back for data recovery. See, for example, U.S. Published Application No. 2006/0279823 (Riley et al.), published Dec. 14, 2006, the entire contents and disclosure of which is hereby incorporated by reference. Because this galvo mirror rotates, it may retro-reflect (phase conjugate) the different plane waves used to angularly multiplex the holograms.
Embodiments of the holographic storage device or system of the present invention may use phase conjugate geometries involving galvo mirrors on the back side of the holographic storage medium to fold the reference beam back for data recovery, such as those described in U.S. Published Application No. 2006/0279823 (Riley et al.), published Dec. 14, 2006. But these phase conjugate geometries involving phase conjugator galvo mirrors may prevent or make it difficult to achieve compact optics. For this reason, some embodiments of the present invention relate to other phase conjugate readout geometries to make the optics of the device or system more compact.
Embodiments of monocular holographic storage devices or systems of the present invention involving phase conjugate geometries for data recovery that may provide more compact optics are shown and described below in FIGS. 16-29. The objective lens in these various embodiments may be moved (e.g. dithered) in the various ways described above in, for example, the data storage devices or systems of FIGS. 1-7, to change the angle of the reference beam entering the holographic storage medium. Also, although not shown in FIGS. 16-29 as described below, each of these devices or systems may include a reference beam lens that may be moved (e.g., dithered) to change the angle of the reference beam entering the holographic storage medium. Also, for simplicity of illustration, in the devices or systems of FIGS. 16-28 many conventional data storage device/system and data recovery device/system features such as spatial light modulators, beam splitters, detector arrays, reference beam generating systems, data beam generating systems, cure/erase systems, additional lenses, additional mirrors, laser sources, collimators, etc. are not shown but may be part of these devices or systems, as shown in the device or system of, for example, FIG. 29. In addition, the reference beam angle may be changed by changing the incident angle of the reference beam into the reference beam lens by rotating or translating a mirror or MEMs reflector, etc.
In one embodiment of the holographic storage device or system of the present invention using a phase conjugate geometry for data recovery, a corner cube may be used for phase conjugation. A corner cube is an optical device that has the shaped of part of a cube with three mutually-perpendicular reflecting surfaces. Such optical devices have the ability to reflect (direct) a beam of any incident angle in a direction parallel to the incident beam after reflecting three times inside the corner cube, as illustrated in FIG. 15. FIG. 15 shows an incident beam 1512 that reflects off three walls 1514 , 1516 and 1518 of a corner cube 1520 before exiting corner cube 1520 as a reflected (directed) beam 1522 that is parallel to incident beam 1512 . Corner cube 1520 has a center point 1532 and an optical axis 1534 .
FIG. 16 shows a monocular holographic storage device or system 1602 according to one embodiment of the present invention using a corner cube (such as that of FIG. 15) in a phase conjugate geometry illustrating data recovery (but where device or system 1602 may also be used to store the data that is recovered). Holographic storage device or system 1602 includes a reference beam 1612 (also referred to as a reconstruction beam when used for data recovery), a representative portion of recovered beam 1614 (the recovered beam in its entirety being able to reconstruct all of the data stored), an objective lens 1616 , a holographic storage medium 1618 , a reflective layer 1620 and a corner cube 1622 . There is an air gap 1632 between objective lens 1616 and holographic storage medium 1618 . Holographic storage medium 16 l 8 includes a lower substrate 1634 , a recording material 1636 , an upper substrate 1638 and an upper surface 1640 . Holographic storage device or system 1602 allows a data page 1652 stored in recording material 1636 to be recovered (read) as recovered beam 1614 . Corner cube 1622 has a center 1662 and an optical axis 1664 . Although only a single data page 1652 is illustrated as being recovered in FIG. 16, holographic storage device or system 1602 of FIG. 16 may be used to recover all of the data pages stored in recording material 1636 of holographic storage medium 1618 .
In the holographic storage device or system of FIG. 16, a mirror or reflective layer (reflective layer) 1620 may be placed on the back side of the storage medium, inside the storage medium, or under the back side of the storage medium in the holographic drive device to reflect (direct) reference (reconstruction) beam 1612 to the front side of holographic storage medium 1618 and towards corner cube 1622 placed on the front side of medium 1618 . In this phase conjugate geometry, readout is performed with the reference (reconstruction) beam reflected (directed) by reflective layer 1620 and corner cube 1622 . The pivot point of reference (reconstruction) beam 1612 in changing the reference (reconstruction) beam angle for angular multiplexing is set on the center of corner cube 1622 . By making the reference (reconstruction) beam directed from reflective layer 1620 illuminate the center of corner cube 1622 , the reference (reconstruction) beam is reflected (directed) back in the opposite direction without any displacement. That is to say, this optical layout or configuration shown in FIG. 16 may achieve phase conjugation. FIG. 16 illustrates a single pixel reconstruction, but may be extended to page-wise storage and readout of multiple bits at a time. Holographic data storage devices or systems that record and recover 1.3-1.4 million bits at a time (such as those described in the documents cited above) may be adapted or modified to provide a phase conjugate readout geometry using a corner cube such as 1622 shown in FIG. 16.
FIG. 17 shows a monocular holographic storage device or system 1702 according to one embodiment of the present invention using a corner cube (such as that of FIG. 15) in a different phase conjugate geometry illustrating data recovery (but where device or system 1702 may also be used to store the data that is recovered). Holographic storage device or system 1702 includes a reference beam 1712 (also referred to as a reconstruction beam when used for data recovery), a representative portion of recovered beam 1714 , an objective lens 1716 , a holographic storage medium 1718 , a reflective layer 1720 and a corner cube 1722 . There is an air gap 1732 between objective lens 1716 and holographic storage medium 1718 . Holographic storage medium 1718 includes a lower substrate 1734 , a recording material 1736 , an upper substrate 1738 and an upper surface 1740 . Holographic storage device or system 1702 allows a data page 1752 stored in recording material 1736 to be recovered (read) as recovered beam 1714 . Corner cube 1722 has a center 1762 . The movement of corner cube 1722 is shown by double-headed arrow 1772 and ghost lines 1774 . Although only a single data pagel 752 is illustrated as being recovered in FIG. 16, the holographic storage device or system of FIG. 17 may be used to recover all of the data pages stored in recording material 1736 of holographic storage medium 1718 .
As in the device or system of FIG. 16, readout may be performed by the device or system of FIG. 17 with the reference (reconstruction) beam reflected (directed) by a mirror or reflective layer and corner cube 1762 . But the pivot point of the reference (reconstruction) beam when changing the reference (reconstruction) beam angle for angular multiplexing is not always on the center of corner cube 1762 . In this case, the angle of the reference (reconstruction) beam reflected (directed) by corner cube 1762 does not change but its position does move with the change of reference beam angle. In order to avoid such movement of the reference (reconstruction) beam, the position of corner cube 1762 may be controlled (as illustrated by movement to the position illustrated by ghost lines 1774 ) so that the reference (reconstruction) beam reflected (directed) by the mirror or reflective layer always illuminates the center of corner cube 1762 .
FIG. 18 shows a monocular holographic storage device or system 1802 according to one embodiment of the present invention using a corner cube array in a phase conjugate geometry illustrating data recovery (but where device or system 1802 may also be used to store the data that is recovered). Holographic storage device or system 1802 includes a reference beam 1812 (also referred to as a reconstruction beam when used for data recovery), a representative portion of recovered beam 1814 , an objective lens 1816 , a holographic storage medium 1818 , a reflective layer 1820 and a corner cube array 1822 . There is an air gap 1832 between objective lens 1816 and holographic storage medium 1818 . Holographic storage medium 1818 includes a lower substrate 1834 , a recording material 1836 , an upper substrate 1838 and an upper surface 1840 . Holographic storage device or system 1802 allows a data page 1852 stored in recording material 1836 to be recovered (read) as recovered beam 1814 . For example, corner cube array 1822 may include three corner cubes 1854 , 1856 and 1858 , each having a respective center 1862 , 1864 and 1866 . Although only a single data page 1852 is illustrated being recovered in FIG. 18, holographic storage device or system 1802 of FIG. 18 may be used to recover all of the data pages stored in recording material 1836 of holographic storage medium 1818 . Additional smaller corner cubes may also be used in arrays such as 1822 illustrated in FIG. 18. A corner cube array such as 1822 with a plurality of smaller corner cubes reduces the size of the corner cube system versus one large corner cube.
As in the device or system of FIG. 17, readout is performed in the device or system of FIG. 18 with the reference (reconstruction) beam reflected (directed) by a mirror or reflective layer and corner cube array 1822 . Again, the pivot point of the reference (reconstruction) beam in changing the reference (reconstruction) beam angle for angular multiplexing is not always on the center of corner cube array 1822 . The amount of beam shift due to reflection (direction) of the reference (reconstruction) beam by the corner cube is also proportional to the size of corner cube. Because corner cube array 1822 comprises a plurality of smaller cube arrays (e.g., 1854 , 1856 and 1858 ), each of which may be smaller than the reconstructed beam size, it may be possible to suppress this beam shift of reference (reconstruction) beam so that phase conjugation may be achieved. In addition, this beam shift may be suppressed by a micro corner cube array such 1822 without moving the array (as may be required with a larger corner cube such as 1762 illustrated in FIG. 17).
In the devices or systems of FIGS. 16, 17 and 18 , optical components may also be placed on the same side of holographic storage medium as the objective lens. According to this layout or configuration of the optical components, it may be possible to downsize the height of holographic drive device because most of the optics and electronics may be on the same side of the holographic storage medium. Alternatively, the foregoing phase conjugation optical components may be placed on the opposite side of the medium. In this alternate configuration, there is no need to place a mirror or reflective layer on the back side of the medium, as illustrated in FIGS. 16, 17 and 18 .
FIG. 19 shows a monocular holographic storage device or system 1902 according to one embodiment of the present invention using another phase conjugate geometry illustrating data recovery (but where device or system 1902 may also be used to store the data that is recovered). Holographic storage device or system 1902 includes a reference beam 1912 (also referred to as a reconstruction beam when used for data recovery), a representative portion of recovered beam 1914 , an objective lens 1916 , a holographic storage medium 1918 , a reflective layer 1920 , an SLM 1922 and a mirror 1924 . There is an air gap 1932 between objective lens 1916 and holographic storage medium 1918 . Holographic storage medium 1918 includes a lower substrate 1934 , a recording material 1936 , an upper substrate 1938 and an upper surface 1940 . Holographic storage device or system 1902 allows a data page 1952 stored in recording material 1936 to be recovered (read) as recovered beam 1914 . Reflective layer 1920 is mounted on lower substrate 1934 . Mirror 1924 is mounted on SLM 1922 above transparent space 1926 , with SLM 1922 having another transparent space to allow reference (reconstruction) beam 1912 to pass through. Objective lens 1916 has an optical axis 1962 . Although only a single data page 1952 is illustrated as being recovered in FIG. 19, holographic storage device or system 1902 of FIG. 19 may be used to recover all of the data pages stored in recording material 1936 of holographic storage medium 1918 . Other layers may also be included in holographic storage medium 1918 , including absorption layers, polarization layers or variable beam transmission layers that may limit recording of holograms to the transmission of holograms during record and eliminate or reduce the effect of reflections from reflective layer 1920 . Reflective layer 1920 also only needs to be behind recording layer 1936 .
FIG. 20 is top plan view of SLM 1922 represented in FIG. 19 that may be used in device or system 1902 . SLM 1902 includes data pixel portion 2012 represented as a generally H-shaped area. Transparent space 1928 used to introduce the reference (reconstruction) beam 1912 is represented on one side of SLM 1922 as a rectangular-shaped area. Mirror 1924 is represented as a smaller filled (black) rectangular-shaped area which is above and adjacent transparent space 1926 represented on the opposite side of SLM 1922 as a rectangular-shaped area.
During data recovery with device or system 1902 of FIG. 19, reference (reconstruction) beam 1912 is focused on the same plane as SLM 1922 through transparent space 1928 provided in SLM 1922 . After passing through transparent space 1928 , reference (reconstruction) beam 1912 passes through objective lens 1916 , is angled (incident) onto holographic storage medium 1918 and then passes through objective lens 1916 again after reflection (direction) by reflective layer 1920 on the back side of medium 1918 . Reflected (directed) reference (reconstruction) beam 1912 is focused back by objective lens 1916 onto the same plane as SLM 1922 as shown in FIG. 19 towards transparent space 1926 . Mirror 1924 is placed over transparent space 1926 at this focus plane. Reference (reconstruction) beam 1912 is then reflected (directed) back by mirror 1924 and travels along the same path but in the opposite direction. Readout of data page 1952 from recording material 1936 of holographic storage medium 1918 is performed with this reference (reconstruction) beam 1912 reflected (directed) from mirror 1924 and then from reflective layer 1920 as a phase conjugate wave to provide recovered beam 1914 .
In another embodiment of the holographic storage device or system of the present invention, an electro-optic crystal (EO crystal) may be used as phase conjugate optical device or system. An optical beam traveling through an EO crystal is deflected (directed) by voltage application to the EO crystal because injected electrons induce gradation of the refractive index inside the EO crystal. By controlling the applied voltage to the EO crystal, even if the reference (reconstruction) beam angle changes during the recovery process, the incident angle of each reference (reconstruction) beam entering the EO crystal should always be perpendicular to a mirror mounted at one end of the EO crystal, as illustrated in FIG. 21 below. Use of an EO crystal is similar in concept as galvo mirror but using an EO crystal, or alternatively an acoustic optical (AO) cell, may provide more compact optical components than a galvo mirror.
FIG. 21 shows an EO crystal device or system 2102 useful in an embodiment of a holographic storage device or system of the present invention. EO crystal system 2102 includes a mirror 2112 placed on an end