3 D Optical Storage

3-D OPTICAL DATA STORAGE TECHNOLOGY * *ABSTRACT 3D opthalmic information shop is the term assumption to any inning of visual selective information retention in which information croup be indicateed and/or read with three dimensional annunciation (as debate to the two dimensional resolution afforded, for example, by CD). Current optical selective information remembering media, such as the CD and DVD interject info as a series of reflective marks on an home(a) surface of a disk.In order to increase transshipment center dexterity, it is realizable for discs to hold two or purge more than than of these selective information classs, further their number is severely modified since the addressing laser moves with all(prenominal) layer that it passes by means of on the way to and from the address layer. These interactions ca routine ruffle that limits the engine room to thoroughly-nigh 10 layers. 3D optical information storeho custom manners circumvent this hack by employ addressing methods where only the specifically addressed voxel (volumetric pixel) interacts substantially with the addressing blowzy.This needs assumes nonanalogue information learning and musical composition methods, in particular non linear optics. 3D optical info terminal is cerebrate to (and competes with) holographic selective information memory board. Traditional examples of holographic repositing do non address in the third dimension, and atomic number 18 therefore non strictly 3D, except more recently 3D holographic storage has been realized by the use of microholograms. Layer-selection multilayer technology (where a multilayer disc has layers that offer be respectively activated e. g. electrically) is in like manner closely related. This innovation has the potential to provide terabyte-level troop storage on DVD- surfaced disks. selective information arrangement and read impale are get by means ofd by pore lasers within the sensiti ve. However, because of the volumetric nature of the information structure, the laser light essential travel through divers(prenominal) data points before it r individuallyes the point where narration or recording is desired. Therefore, roughly kind of nonlinearity is required to ensure that these former(a) data points do not interfere with the addressing of the desired point. 1. Overview Current optical data storage 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 f 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 retort by using addressing methods where only the specifical ly addressed voxel (volumetric pixel) interacts substantially with the addressing light. This necessarily involves nonlinear data reading and piece of 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 too closely related. Schematic delegacy of a cross section through a 3D optical storage disc (yellow) along a data track (orange marks). Four data layers are seen, with the laser currently addressing the third from the top.The laser passes through the number 1 two layers and only interacts with the third, since here the light is at a naughty intensity. As an example, a prototypical 3D optical data storage system whitethorn use a disk that looks very practically kindred a transparent DVD. The disc contains more 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 transfer in the media.As the disc spins and the read/ compose head moves along a radius, the layer is indite just as a DVD-R is written. The depth of the focus whitethorn because be falsifyd and different entirely different layer of information written. The distance between layers may be 5 to 100 micrometers, allowing 100 layers of information to be stored on a single disc. In order to read the data back (in this example), a alike(p) procedure is used except this time instead of create a photochemical change in the media the laser causes fluorescence. This is achieved e. g. by using a inflict laser business office 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. It should be remark that the size of individual chromophore molecules or photoactive color centers is much down in the m come inher 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 unmatchable time, so the medium acts as a homogeneous mass rather than a matrix structured by the positions of chromophores. 2. HistoryThe origins of the field date back to the 1950s, when Yehuda Hirshberg demonstrable the photochromic spiropyrans and suggested their use in data storage. 3 In the 1970s, Valeri Barachevskii demonstrated that this photochromism could be pr oduced by two-photon ardor, and finally at the end of the 1980s Peter T. Rentzepis showed that this could headliner to three-dimensional data storage. 5 This proof-of-concept system stimulated a great switch of research and increase, and in the following decades some(prenominal) academic and commercial groups sport 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 keep been investigated, large numbers of chemical systems for the medium have been developed and evaluated, and spacious work has been carried out in solving the problems associated with the optical systems required for the reading and recording of data. Currently, several(prenominal)(prenominal)(prenominal) groups remain working on solutions with various levels of education and interest in commercialization. *3. Processes for creating written data* data recording in a 3D optical storage medium requires that a change con 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 let in photoisomerizations, photodecompositions and photobleaching, and polymerization initiation. Most investigated have been photochromic compounds, which include azobenzenes, spiropyrans, stilbenes, fulgides and diaryle whereforees. If the photochemical change is reversible, then rewritable data storage may be achieved, at least n principle. Also, multilevel recording, where data is written in grayscale rather than as on and off contracts, is technically feasible. 3. 1 makeup by non*-*resonant multiphoton denseness Although there are many nonlinear optical phenomena, only multiphoton immersion is capable of injecting into the media the significant energy required to electronically turn on molecular species and cause che mical reactions. Two-photon submerging is the strongest multiphoton absorbance by far, but still it is a very weak phenomenon, leading to low media aesthesia.Therefore, much research has been enjoin at providing chromophores with amply two-photon submerging cross-sections. Two photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from mavin state (usually the ground state) to a high energy electronic state. The energy difference between the involved demean and upper states of the molecule is equal to the sum of the energies of the two photons. Two-photon absorption is a second-order surgeryes several orders of magnitude weaker than linear absorption.It differs from linear absorption in that the say-so of absorption depends on the square of the light intensity, thus it is a nonlinear optical process Writing by 2-photon absorption can be achieved by focusing the writing laser on the point whe re 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 match 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. 3. 2 Writing by sequential multiphoton absorption another(prenominal) approach to improving media sensitivity has been to employ resonant wo-photon absorption (also known as 1+1 or seq uential 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 mustiness bewilder 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 stilldom 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. 3. 3 Microholography In microholography, 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. . 4 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 cannot bring out to the disc it is a ROM 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 wrought 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. 3. 5 early(a) approaches to writing Other techniques for writing data i n three-dimensions have also been examined, including Persistent * ghostlike** **hole burning* (PSHB), which also allows the possible action of spectral multiplexing to increase data density. However, PSHB media currently requires extremely low temperatures to be kept up(p) in order to avoid data loss. Void* formation, where microscopic bubbles are introduced into a media by high intensity laser irradiation. 7 Chromophore poling, where the laser-induced reorientation of chromophores in the media structure leads to readable changes. *4. 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 medias linear response.Reading methods include Two photon absorption (resulting in either absorption or fluorescence). This method is essentially two-photon-microscopy. Linear excitation o f fluorescence with confocal detection. This method is essentially confocal laser scanning microscopy. 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 addendum 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 reflectiveness microscope. No absorption of light is necessary, so there is no risk of damage data while reading, but the required refractive index match 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 repose the focused spot quality. Second harmonic generation has been demonstrated as a method to read data written into a poled polymer matrix. optic coherence tomography has also been demonstrated as a parallel readi ng method. *5. Media *design The active part of 3D optical storage media is usually an organic polymer either doped or grafted with the photochemically active species. Alternatively, diaphanous and sol-gel materials have been used. 5. 1 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 waggle 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(prenominal) science fiction 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. 5. 2 Media manufacturing The simplest method of manufacturing the molding of a disk in one piece is a possibility for some systems. A more multifo rm 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 equal to a roll of adhesive tape. *6. Drive design* A tantalize 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 guiding light differences that must be taken into account when designing such a drive, including Laser.Particularly when 2-photon absorption is gived, high-powered lasers may be required that can be bulky, difficult to cool, and pose safety concerns. Existing optical drives utilize continuous wave diode lasers operating at 780 nm, 658 nm, or 405 nm. 3D optical storage drives may require solid-state lasers 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 craziness correction. Because the system must address different depths in the medium, and at different depths the spherical aberration induced in the wavefront is different, a method is required to dynamically account for these differences. Many possible methods exist that include optical elements that switch over in and out of the optical path, moving elements, adaptive optics, and immersion lenses. visual system. In many examples of 3D optical data storage systems, several wavelengths (colors) of light are used (e. g. eading laser, writing laser, note 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 particular(a) 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 DVD discs. 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 modulators, signal imaging, more powerful lasers, and more complex data handling. *7.Development issues* in spite of 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 pecuniary backing in the field, as well as technical issues, including negative 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 i s therefore a risk that either the unwritten points forget 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 lasers or NdYAG lasers to achieve excitation, but these instruments are not capable for use in consumer products. *8. Academic development* Much of the development of 3D optical data storage has been carried out in universities.The groups that have provided invaluable input include Peter T. Rentzepis was the originator of this field, and has recently developed materials free from destructive readout. *Watt W. Webb* co developed the two-photon microscope in Bell Labs, and showed 3D recording on photorefractive media. Masahiro Irie de veloped the diarylethene family of photochromic materials. 13 Yoshimasa Kawata, *Satoshi Kawata* and Zouheir Sekkat have developed and worked on several optical data manipulation systems, in particular involving poled polymer systems. 14 Kevin C Belfield is growth 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 scheme 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. 9 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 ever come to market in the presence of competition from other quarters such as hard drives, flash storage, holographic storage and internet-based storage. Examples of 3D optical data storage media. Top row indite Call/Recall media Mempile media. Middle row FMD D-Data DMD and drive. Bottom row Landauer media Microholas media in action.Call/Recall was founded in 1987 on the basis of Peter Rentzepis research. development 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 TB as 200 layers in a 1. 2 mm thick disk. 23 They withdraw 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 recordi ng and readback of 100 layers of information on a 0. mm thick disc, as well as low crosstalk, high sensitivity, and thermodynamic stability. 25 They intend to deform a red-laser 0. 6-1. 0 TB consumer product in 2010, and have a roadmap to a 5 TB blue-laser product. 26 *Constellation 3D* developed the light Multilayer Disc at the end of the 1990s, which was a ROM disk, manufactured layer by layer. The company failed in 2002, but the intellectual property (IP) was acquired by D-Data Inc. who are attempting to introduce it as the Digital Multilayer Disk (DMD).Storex Technologies has been set up to develop 3D media based on fluorescent photosensitive furnish and glass-ceramic materials. The technology derives from the patents of the Romanian scientist Eugen Pavel, who is also the founder and CEO of the company. First results, 40 nm marks recorded into 3D virtual layers separated by 700 nm, were presented in October 2009 at the ISOM2009 conference. Landauer inc. are developing a media based on resonant 2-photon absorption in a sapphire single crystallisation 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 poling using a far UV 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 leading of Prof Susanna Orlic, and has achieved the recording of up to 75 layers of microholographic data, separated by 4. micrometres, and suggesting a data density of 10 GB per layer. 33 3DCD Technology Pty. Ltd. is a university bear set up to develop 3D optical storage technology based on materials identified by Daniel Day and Min Gu. some(prenomi nal) large technology companies such as Fuji, Ricoh 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.