3D optical data storage
Encyclopedia
3D optical data storage is the term given to any form of optical data storage
in which information can be recorded and/or read with three dimensional
resolution
(as opposed to the two dimensional resolution afforded, for example, by CD).
This innovation has the potential to provide petabyte
-level mass storage
on DVD
-sized disks. Data recording and readback are achieved by focusing lasers within the medium. However, because of the volumetric nature of the data structure, the laser light must travel through other data points before it reaches the point where reading or recording is desired. Therefore, some kind of nonlinearity is required to ensure that these other data points do not interfere with the addressing of the desired point.
No commercial product based on 3D optical data storage has yet arrived on the mass market, although several companies are actively developing the technology and claim that it may become available "soon".
media, such as the CD and DVD
store data as a series of reflective marks on an internal surface of a disc. In order to increase storage capacity, it is possible for discs to hold two or even more of these data layers, but their number is severely limited since the addressing laser interacts with every layer that it passes through on the way to and from the addressed layer. These interactions cause noise that limits the technology to approximately 10 layers. 3D
optical data storage methods circumvent this issue by using addressing methods where only the specifically addressed voxel
(volumetric pixel) interacts substantially with the addressing light. This necessarily involves nonlinear data reading and writing methods, in particular nonlinear optics
.
3D optical data storage is related to (and competes with) holographic data storage
. Traditional examples of holographic storage do not address in the third dimension, and are therefore not strictly "3D", but more recently 3D holographic storage has been realized by the use of microholograms. Layer-selection
multilayer technology (where a multilayer disc has layers that can be individually activated e.g. electrically) is also closely related.
As an example, a prototypical 3D optical data storage system may use a disk that looks much like a transparent DVD. The disc contains many layers of information, each at a different depth in the media and each consisting of a DVD-like spiral track. In order to record information on the disc a laser
is brought to a focus
at a particular depth in the media that corresponds to a particular information layer. When the laser is turned on it causes a photochemical change in the media. As the disc spins and the read/write head moves along a radius, the layer is written just as a DVD-R is written. The depth of the focus may then be changed and another entirely different layer of information written. The distance between layers may be 5 to 100 micrometer
s, allowing >100 layers of information to be stored on a single disc.
In order to read the data back (in this example), a similar procedure is used except this time instead of causing a photochemical change in the media the laser causes fluorescence
. This is achieved e.g. by using a lower laser power or a different laser wavelength. The intensity or wavelength of the fluorescence is different depending on whether the media has been written at that point, and so by measuring the emitted light the data is read.
The size of individual chromophore molecules or photoactive color centers is much smaller than the size of the laser focus (which is determined by the diffraction limit). The light therefore addresses a large number (possibly even 109) of molecules at any one time, so the medium acts as a homogeneous mass rather than a matrix structured by the positions of chromophores.
, and polymerization
initiation. Most investigated have been photochromic compounds, which include azobenzene
s, spiropyrans, stilbene
s, fulgides and diarylethene
s. If the photochemical change is reversible, then rewritable data storage may be achieved, at least in principle. Also, multilevel recording
, where data is written in "grayscale
" rather than as "on" and "off" signals, is technically feasible.
is the strongest multiphoton absorbance by far, but still it is a very weak phenomenon, leading to low media sensitivity. Therefore, much research has been directed at providing chromophores with high two-photon absorption cross-sections.
Writing by 2-photon absorption can be achieved by focusing the writing laser on the point where the photochemical writing process is required. The wavelength of the writing laser is chosen such that it is not linearly absorbed by the medium, and therefore it does not interact with the medium except at the focal point. At the focal point 2-photon absorption becomes significant, because it is a nonlinear process dependent on the square of the laser fluence
.
Writing by 2-photon absorption can also be achieved by the action of two lasers in coincidence. This method is typically used to achieve the parallel writing of information at once. One laser passes through the media, defining a line or plane. The second laser is then directed at the points on that line or plane that writing is desired. The coincidence of the lasers at these points excited 2-photon absorption, leading to writing photochemistry.
, focused beams of light are used to record submicrometre-sized holograms in a photorefractive material, usually by the use of collinear beams. The writing process may use the same kinds of media that are used in other types of holographic data storage
, and may use 2-photon processes to form the holograms.
format. Data may be written by a nonlinear optical method, but in this case the use of very high power lasers is acceptable so media sensitivity becomes less of an issue.
The fabrication of discs containing data molded or printed into their 3D structure has also been demonstrated. For example, a disc containing data in 3D may be constructed by sandwiching together a large number of wafer-thin discs, each of which is molded or printed with a single layer of information. The resulting ROM disc can then be read using a 3D reading method.
polymer
either doped or grafted with the photochemically active species. Alternatively, crystalline and sol-gel materials have been used.
- the molding
of a disk in one piece - is a possibility for some systems. A more complex method of media manufacturing is for the media to be constructed layer by layer. This is required if the data is to be physically created during manufacture. However, layer-by-layer construction need not mean the sandwiching of many layers together. Another alternative is to create the medium in a form analogous to a roll of adhesive tape.
Data storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
in which information can be recorded and/or read with three dimensional
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
(as opposed to the two dimensional resolution afforded, for example, by CD).
This innovation has the potential to provide petabyte
Petabyte
A petabyte is a unit of information equal to one quadrillion bytes, or 1000 terabytes. The unit symbol for the petabyte is PB...
-level mass storage
Mass storage
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. Devices and/or systems that have been described as mass storage include tape libraries, RAID systems, hard disk drives, magnetic tape drives, optical disc drives, magneto-optical...
on DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
-sized disks. Data recording and readback are achieved by focusing lasers within the medium. However, because of the volumetric nature of the data structure, the laser light must travel through other data points before it reaches the point where reading or recording is desired. Therefore, some kind of nonlinearity is required to ensure that these other data points do not interfere with the addressing of the desired point.
No commercial product based on 3D optical data storage has yet arrived on the mass market, although several companies are actively developing the technology and claim that it may become available "soon".
Overview
Current optical data storageData storage device
thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....
media, such as the CD and DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
store data as a series of reflective marks on an internal surface of a disc. In order to increase storage capacity, it is possible for discs to hold two or even more of these data layers, but their number is severely limited since the addressing laser interacts with every layer that it passes through on the way to and from the addressed layer. These interactions cause noise that limits the technology to approximately 10 layers. 3D
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
optical data storage methods circumvent this issue by using addressing methods where only the specifically addressed voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...
(volumetric pixel) interacts substantially with the addressing light. This necessarily involves nonlinear data reading and writing methods, in particular nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
.
3D optical data storage is related to (and competes with) holographic data storage
Holographic data storage
Holographic data storage is a potential technology in the area of high-capacity data storage currently dominated by magnetic and conventional optical data storage. Magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface...
. Traditional examples of holographic storage do not address in the third dimension, and are therefore not strictly "3D", but more recently 3D holographic storage has been realized by the use of microholograms. Layer-selection
LS-R
LS-R, or the Layer-Selection-Type Recordable Optical Disk, is the term coined by Hitachi in 2003 for a next-generation optical disc technology which allows much larger data storage densities than DVD, HD DVD or Blu-ray Disc, by allowing the use of a large number of data layers in a single disc...
multilayer technology (where a multilayer disc has layers that can be individually activated e.g. electrically) is also closely related.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is brought to a focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
at a particular depth in the media that corresponds to a particular information layer. When the laser is turned on it causes a photochemical change in the media. As the disc spins and the read/write head moves along a radius, the layer is written just as a DVD-R is written. The depth of the focus may then be changed and another entirely different layer of information written. The distance between layers may be 5 to 100 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s, allowing >100 layers of information to be stored on a single disc.
In order to read the data back (in this example), a similar procedure is used except this time instead of causing a photochemical change in the media the laser causes fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
. This is achieved e.g. by using a lower laser power or a different laser wavelength. The intensity or wavelength of the fluorescence is different depending on whether the media has been written at that point, and so by measuring the emitted light the data is read.
The size of individual chromophore molecules or photoactive color centers is much smaller than the size of the laser focus (which is determined by the diffraction limit). The light therefore addresses a large number (possibly even 109) of molecules at any one time, so the medium acts as a homogeneous mass rather than a matrix structured by the positions of chromophores.
History
The origins of the field date back to the 1950s, when Yehuda Hirshberg developed the photochromic spiropyrans and suggested their use in data storage. In the 1970s, Valeri Barachevskii demonstrated that this photochromism could be produced by two-photon excitation, and finally at the end of the 1980s Peter M. Rentzepis showed that this could lead to three-dimensional data storage. This proof-of-concept system stimulated a great deal of research and development, and in the following decades many academic and commercial groups have worked on 3D optical data storage products and technologies. Most of the developed systems are based to some extent on the original ideas of Rentzepis. A wide range of physical phenomena for data reading and recording have been investigated, large numbers of chemical systems for the medium have been developed and evaluated, and extensive work has been carried out in solving the problems associated with the optical systems required for the reading and recording of data. Currently, several groups remain working on solutions with various levels of development and interest in commercialization (see below).Processes for creating written data
Data recording in a 3D optical storage medium requires that a change take place in the medium upon excitation. This change is generally a photochemical reaction of some sort, although other possibilities exist. Chemical reactions that have been investigated include photoisomerizations, photodecompositions and photobleachingPhotobleaching
Photobleaching is the photochemical destruction of a fluorophore. In microscopy, photobleaching may complicate the observation of fluorescent molecules, since they will eventually be destroyed by the light exposure necessary to stimulate them into fluorescing...
, and polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
initiation. Most investigated have been photochromic compounds, which include azobenzene
Azobenzene
Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the best known example of an azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of molecules that share the core azobenzene structure, with different chemical...
s, spiropyrans, stilbene
Stilbene
-Stilbene, is a diarylethene, i.e., a hydrocarbon consisting of a trans ethene double bond substituted with a phenyl group on both carbon atoms of the double bond. The name stilbene is derived from the Greek word stilbos, which means shining....
s, fulgides and diarylethene
Diarylethene
In chemistry, diarylethene is the general name of a class of compounds that have aromatic groups bonded to each end of a carbon-carbon double bond...
s. If the photochemical change is reversible, then rewritable data storage may be achieved, at least in principle. Also, multilevel recording
MultiLevel Recording
MultiLevel Recording was a technology originally developed by Optex Corporation and promoted by Calimetrics to increase the storage capacity of optical discs. It failed to establish itself on the market...
, where data is written in "grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
" rather than as "on" and "off" signals, is technically feasible.
Writing by nonresonant multiphoton absorption
Although there are many nonlinear optical phenomena, only multiphoton absorption is capable of injecting into the media the significant energy required to electronically excite molecular species and cause chemical reactions. Two-photon absorptionTwo-photon absorption
Two-photon absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state to a higher energy electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the...
is the strongest multiphoton absorbance by far, but still it is a very weak phenomenon, leading to low media sensitivity. Therefore, much research has been directed at providing chromophores with high two-photon absorption cross-sections.
Writing by 2-photon absorption can be achieved by focusing the writing laser on the point where the photochemical writing process is required. The wavelength of the writing laser is chosen such that it is not linearly absorbed by the medium, and therefore it does not interact with the medium except at the focal point. At the focal point 2-photon absorption becomes significant, because it is a nonlinear process dependent on the square of the laser fluence
Fluence
In physics, fluence is the flux integrated over time. For particles, it is defined as the total number of particles that intersect a unit area in a specific time interval of interest, and has units of m–2...
.
Writing by 2-photon absorption can also be achieved by the action of two lasers in coincidence. This method is typically used to achieve the parallel writing of information at once. One laser passes through the media, defining a line or plane. The second laser is then directed at the points on that line or plane that writing is desired. The coincidence of the lasers at these points excited 2-photon absorption, leading to writing photochemistry.
Writing by sequential multiphoton absorption
Another approach to improving media sensitivity has been to employ resonant two-photon absorption (also known as "1+1" or "sequential" 2-photon absorbance). Nonresonant two-photon absorption (as is generally used) is weak since in order for excitation to take place, the two exciting photons must arrive at the chromophore at almost exactly the same time. This is because the chromophore is unable to interact with a single photon alone. However, if the chromophore has an energy level corresponding to the (weak) absorption of one photon then this may be used as a stepping stone, allowing more freedom in the arrival time of photons and therefore a much higher sensitivity. However, this approach results in a loss of nonlinearity compared to nonresonant 2-photon absorbance (since each 1-photon absorption step is essentially linear), and therefore risks compromising the 3D resolution of the system.Microholography
In microholographyHolography
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
, focused beams of light are used to record submicrometre-sized holograms in a photorefractive material, usually by the use of collinear beams. The writing process may use the same kinds of media that are used in other types of holographic data storage
Holographic data storage
Holographic data storage is a potential technology in the area of high-capacity data storage currently dominated by magnetic and conventional optical data storage. Magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface...
, and may use 2-photon processes to form the holograms.
Data recording during manufacturing
Data may also be created in the manufacturing of the media, as is the case with most optical disc formats for commercial data distribution. In this case, the user can not write to the disc - it is a ROMRead-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
format. Data may be written by a nonlinear optical method, but in this case the use of very high power lasers is acceptable so media sensitivity becomes less of an issue.
The fabrication of discs containing data molded or printed into their 3D structure has also been demonstrated. For example, a disc containing data in 3D may be constructed by sandwiching together a large number of wafer-thin discs, each of which is molded or printed with a single layer of information. The resulting ROM disc can then be read using a 3D reading method.
Other approaches to writing
Other techniques for writing data in three-dimensions have also been examined, including:- Persistent spectral hole burningSpectral hole burningSpectral hole burning is the frequency selective bleaching of the absorption spectrum of a material, which leads to an increased transmission at the selected frequency....
(PSHB), which also allows the possibility of spectral multiplexingMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
to increase data density. However, PSHB media currently requires extremely low temperatures to be maintained in order to avoid data loss. - Void formation, where microscopic bubbles are introduced into a media by high intensity laser irradiation.
- Chromophore poling, where the laser-induced reorientation of chromophores in the media structure leads to readable changes.
Processes for reading data
The reading of data from 3D optical memories has been carried out in many different ways. While some of these rely on the nonlinearity of the light-matter interaction to obtain 3D resolution, others use methods that spatially filter the media's linear response. Reading methods include:- Two photon absorption (resulting in either absorption or fluorescence). This method is essentially two-photon microscopy.
- Linear excitation of fluorescence with confocal detection. This method is essentially confocal laser scanning microscopyConfocal laser scanning microscopyConfocal laser scanning microscopy is a technique for obtaining high-resolution optical images with depth selectivity. The key feature of confocal microscopy is its ability to acquire in-focus images from selected depths, a process known as optical sectioning...
. It offers excitation with much lower laser powers than does two-photon absorbance, but has some potential problems because the addressing light interacts with many other data points in addition to the one being addressed. - Measurement of small differences in the refractive index between the two data states. This method usually employs a phase contrast microscope or confocal reflection microscopeMicroscopeA microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
. No absorption of light is necessary, so there is no risk of damaging data while reading, but the required refractive indexRefractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
mismatch in the disc may limit the thickness (i.e. number of data layers) that the media can reach due to the accumulated random wavefront errors that destroy the focused spot quality. - Second harmonic generationSecond harmonic generationAn optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
has been demonstrated as a method to read data written into a poled polymer matrix. - Optical coherence tomographyOptical coherence tomographyOptical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media . Optical coherence tomography is an interferometric technique, typically employing near-infrared light...
has also been demonstrated as a parallel reading method.
Media design
The active part of 3D optical storage media is usually an organicOrganic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
either doped or grafted with the photochemically active species. Alternatively, crystalline and sol-gel materials have been used.
Media form factor
Media for 3D optical data storage have been suggested in several form factors:- Disc. A disc media offers a progression from CD/DVD, and allows reading and writing to be carried out by the familiar spinning disc method.
- Card. A credit cardCredit cardA credit card is a small plastic card issued to users as a system of payment. It allows its holder to buy goods and services based on the holder's promise to pay for these goods and services...
form factor media is attractive from the point of view of portability and convenience, but would be of a lower capacity than a disc. - Crystal, Cube or Sphere. Several science fictionScience fictionScience fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
writers have suggested small solids that store massive amounts of information, and at least in principle this could be achieved with 3D optical data storage.
Media manufacturing
The simplest method of manufacturingManufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
- the molding
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
of a disk in one piece - is a possibility for some systems. A more complex method of media manufacturing is for the media to be constructed layer by layer. This is required if the data is to be physically created during manufacture. However, layer-by-layer construction need not mean the sandwiching of many layers together. Another alternative is to create the medium in a form analogous to a roll of adhesive tape.
Drive design
A drive designed to read and write to 3D optical data storage media may have a lot in common with CD/DVD drives, particularly if the form factor and data structure of the media is similar to that of CD or DVD. However, there are a number of notable differences that must be taken into account when designing such a drive, including:- Laser. Particularly when 2-photon absorption is utilized, high-powered lasers may be required that can be bulky, difficult to cool, and pose safety concerns. Existing optical drives utilize continuous waveContinuous waveA continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
diode lasers operating at 780 nm, 658 nm, or 405 nm. 3D optical storage drives may require solid-state laserSolid-state laserA solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
s or pulsed lasers, and several examples use wavelengths easily available by these technologies, such as 532 nm (green). These larger lasers can be difficult to integrate into the read/write head of the optical drive. - Variable spherical aberration correction. Because the system must address different depths in the medium, and at different depths the spherical aberrationSpherical aberrationthumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
induced in the wavefrontWavefrontIn physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
is different, a method is required to dynamically account for these differences. Many possible methods exist that include optical elements that swap in and out of the optical path, moving elements, adaptive opticsAdaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
, and immersion lenses. - Optical system. In many examples of 3D optical data storage systems, several wavelengths (colors) of light are used (e.g. reading laser, writing laser, signal; sometimes even two lasers are required just for writing). Therefore, as well as coping with the high laser power and variable spherical aberration, the optical system must combine and separate these different colors of light as required.
- Detection. In DVD drives, the signal produced from the disc is a reflection of the addressing laser beam, and is therefore very intense. For 3D optical storage however, the signal must be generated within the tiny volume that is addressed, and therefore it is much weaker than the laser light. In addition, fluorescence is radiated in all directions from the addressed point, so special light collection optics must be used to maximize the signal.
- Data tracking. Once they are identified along the z-axis, individual layers of DVD-like data may be accessed and tracked in similar ways to DVDs. The possibility of using parallel or page-based addressing has also been demonstrated. This allows much faster data transfer rates, but requires the additional complexity of spatial light modulatorSpatial light modulatorA spatial light modulator is an object that imposes some form of spatially-varying modulation on a beam of light. A simple example is an overhead projector transparency. Usually when the phrase SLM is used, it means that the transparency can be controlled by a computer. In the 1980s, large SLMs...
s, signal imaging, more powerful lasers, and more complex data handling.
Development issues
Despite the highly attractive nature of 3D optical data storage, the development of commercial products has taken a significant length of time. This results from limited financial backing in the field, as well as technical issues, including:- Destructive reading. Since both the reading and the writing of data are carried out with laser beams, there is a potential for the reading process to cause a small amount of writing. In this case, the repeated reading of data may eventually serve to erase it (this also happens in phase change materials used in some DVDs). This issue has been addressed by many approaches, such as the use of different absorption bands for each process (reading and writing), or the use of a reading method that does not involve the absorption of energy.
- Thermodynamic stability. Many chemical reactions that appear not to take place in fact happen very slowly. In addition, many reactions that appear to have happened can slowly reverse themselves. Since most 3D media are based on chemical reactions, there is therefore a risk that either the unwritten points will slowly become written or that the written points will slowly revert to being unwritten. This issue is particularly serious for the spiropyrans, but extensive research was conducted to find more stable chromophores for 3D memories.
- Media sensitivity. 2-photon absorption is a weak phenomenon, and therefore high power lasers are usually required to produce it. Researchers typically use Ti-sapphire laserTi-sapphire laserTi:sapphire lasers are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses...
s or Nd:YAG laserNd:YAG laserNd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
s to achieve excitation, but these instruments are not suitable for use in consumer products.
Academic development
Much of the development of 3D optical data storage has been carried out in universities. The groups that have provided valuable input include:- Peter T. Rentzepis was the originator of this field, and has recently developed materials free from destructive readout.
- Watt W. WebbWatt W. WebbWatt W. Webb is known for his co-invention of Multiphoton microscopy in 1990.- Biography :Professor Watt W...
codeveloped the two-photon microscope in Bell LabsBell LabsBell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, and showed 3D recording on photorefractive media. - Masahiro Irie developed the diaryletheneDiaryletheneIn chemistry, diarylethene is the general name of a class of compounds that have aromatic groups bonded to each end of a carbon-carbon double bond...
family of photochromic materials. - Yoshimasa Kawata, Satoshi Kawata and Zouheir Sekkat have developed and worked on several optical data manipulation systems, in particular involving poled polymer systems.
- Kevin C Belfield is developing photochemical systems for 3D optical data storage by the use of resonance energy transfer between molecules, and also develops high 2-photon cross-section materials.
- Seth Marder performed much of the early work developing logical approaches to the molecular design of high 2-photon cross-section chromophores.
- Tom Milster has made many contributions to the theory of 3D optical data storage.
- Robert McLeod has examined the use of microholograms for 3D optical data storage.
- Min Gu has examined confocal readout and methods for its enhancement.
Commercial development
In addition to the academic research, several companies have been set up to commercialize 3D optical data storage and some large corporations have also shown an interest in the technology. However, it is not yet clear whether the technology will succeed in the market in the presence of competition from other quarters such as hard drives, flash storage, and holographic storage.- Call/Recall was founded in 1987 on the basis of Peter Rentzepis' research. Using 2-photon recording (at 25 Mbit/s with 6.5 ps, 7 nJ, 532 nm pulses), 1-photon readout (with 635 nm), and a high NA (1.0) immersion lens, they have stored 1 TBTerabyteThe terabyte is a multiple of the unit byte for digital information. The prefix tera means 1012 in the International System of Units , and therefore 1 terabyte is , or 1 trillion bytes, or 1000 gigabytes. 1 terabyte in binary prefixes is 0.9095 tebibytes, or 931.32 gibibytes...
as 200 layers in a 1.2 mm thick disk. They aim to improve capacity to >5 TB and data rates to up to 250 Mbit/s within a year, by developing new materials as well as high-powered pulsed blue laser diodes. - Mempile are developing a commercial system with the name TeraDisc. In March 2007, they demonstrated the recording and readback of 100 layers of information on a 0.6 mm thick disc, as well as low crosstalk, high sensitivity, and thermodynamic stability. They intend to release a red-laser 0.6-1.0 TB consumer product in 2010, and have a roadmap to a 5 TB blue-laser product.
- Constellation 3DConstellation 3DConstellation 3D was a company developing a new optical medium, the Fluorescent Multilayer Disc and Card .The company was shut down following a failure to receive a committed investment by a Swiss based investor and the unavailability of other financial sources...
developed the Fluorescent Multilayer DiscFluorescent Multilayer DiscFluorescent Multilayer Disc was an optical disc format developed by Constellation 3D that uses fluorescent, rather than reflective materials to store data. Reflective disc formats have a practical limitation of about two layers, primarily due to interference, scatter, and inter-layer cross talk...
at the end of the 1990s, which was a ROM disk, manufactured layer by layer. The company failed in 2002, but the intellectual property (IPIntellectual propertyIntellectual property is a term referring to a number of distinct types of creations of the mind for which a set of exclusive rights are recognized—and the corresponding fields of law...
) was acquired by D-Data Inc., who are attempting to introduce it as the Digital Multilayer DiskDigital Multilayer DiskDigital Multilayer Disk is an optical disc format developed by D Data Inc. It is based on the 3D optical data storage technology developed for the Fluorescent Multilayer Disc by the defunct company Constellation 3D. DMDs can store between 22 and 32 GB of binary information...
(DMD). - Storex Technologies has been set up to develop 3D media based on fluorescent photosensitive glasses and glass-ceramic materials. The technology derives from the patents of the RomaniaRomaniaRomania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...
n scientist Eugen PavelEugen PavelDr. Eugen Pavel is a Romanian scientist and the inventor of the Hyper CD-ROM, a 3D optical data storage medium with a claimed initial capacity of 10 TB and with a theoretical capacity of 1 PB on a single disc. It is considered by some to be the next revolution in computer storage.Dr. Pavel...
, who is also the founder and CEO of the company. At ODS2010 conference were presented results regarding readout by two non-fluorescence methods of a Petabyte Optical Disc. - Landauer inc. are developing a media based on resonant 2-photon absorption in a sapphireSapphireSapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
single crystal substrate. In May 2007, they showed the recording of 20 layers of data using 2 nJ of laser energy (405 nm) for each mark. The reading rate is limited to 10 Mbit/s because of the fluorescence lifetime. - Colossal Storage aim to develop a 3D holographic optical storage technology based on photon induced electric field polingPhoton induced electric field polingIn physics, photon induced electric field poling is a phenomenon whereby a pattern of local electric field orientations can be encoded in a suitable ferroelectric material, such as perovskite...
using a far UVUltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
laser to obtain large improvements over current data capacity and transfer rates, but as yet they have not presented any experimental research or feasibility study. - Microholas operates out of the University of Berlin, under the leadership of Prof Susanna Orlic, and has achieved the recording of up to 75 layers of microholographic data, separated by 4.5 micrometres, and suggesting a data density of 10 GB per layer.
- 3DCD Technology Pty. Ltd. is a university spin-off set up to develop 3D optical storage technology based on materials identified by Daniel Day and Min Gu.
- Several large technology companies such as FujiFuji-People:* Mr. Fuji, ring name of wrestling and manager Harry Fujiwara* Keiko Fuji, a Japanese singer of the 1960s and 1970s, and mother of Hikaru Utada* Sumiko Fuji, a Japanese actressFictional characters* Fuji , a character in the Stormwatch series...
, RicohRicohor Ricoh, is a Japanese company that was established in 1936 on February 6th, as , a company in the RIKEN zaibatsu. Its headquarters is located in Ricoh Building in Chūō, Tokyo....
and Matsushita have applied for patents on 2-photon-responsive materials for applications including 3D optical data storage, however they have not given any indication that they are developing full data storage solutions.