3 D Optical Storage

3-D OPTICAL DATA STORAGE engine room * *ABSTRACT 3D optic entropy remembering is the term disposed to any form of optic nurture entrepot in which information can be recorded and/or hire with lead dimensional resolution (as opposed to the two dimensional resolution afforded, for example, by CD). Current opthalmic entropy terminus media, such(prenominal) as the CD and DVD store information as a serial publication of reflective marks on an internal surface of a dish aerial.In beau monde to show upgrowth storage capacity, it is possible for discs to hold two or even more than of these entropy layers, scarce their number is poorly limited since the addressing laser interacts with every layer that it passes through on the way to and from the address layer. These interactions ca design preventive that limits the technology to approximately 10 layers. 3D optic entropy storage manners tease this issue by using addressing methods where only the specifically addres sed voxel (volumetric pixel) interacts substantially with the addressing easygoing.This necessarily involves nonlinear info reading and piece methods, in particular non linear optics. 3D ocular information storage is cogitate to (and competes with) holographic info storage. Traditional examples of holographic storage do non address in the trio dimension, and be thus non 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 distributively activated e. g. electrically) is as well as closely related. This innovation has the potential to provide terabyte- head mass storage on DVD- sized disks. information arranging and readback ar achieved by cerebrateing lasers within the medium. However, because of the volumetric nature of the entropy structure, the laser light must(prenominal) work through other information targets before it reaches the point where reading or preserve is desired. Therefore, some variant of nonlinearity is required to ensure that these other 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 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 no nlinear 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 be 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. Schematic representation 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 first two layers and only interacts with the third, since here the light is at a full(prenominal) intensity. As an example, a prototypical 3D optical data storage system whitethorn use a disk that looks untold like a transparent DVD. The disc contains many layers of information, each at a differe nt depth in the media and each consisting of a DVD-like helical 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 off-key on it causes a photochemical change in the media.As the disc spins and the read/write learning ability moves along a radius, the layer is written just as a DVD-R is written. The depth of the focus whitethorn then be changed and another 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 similar procedure is used except this time or else of causing a photochemical change in the media the laser causes fluorescence. This is achieved e. g. by using a demoralise laser power or a different laser wavelength.The intensity or wavelength of the fluorescence is differ ent 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 noted that the size of individual chromophore molecules or photoactive color centers is much diminutiveer than the size of the laser focus (which is determined by the diffraction limit). The light therefore addresses a considerable 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. 2. HistoryThe origins of the field date back to the 1950s, when Yehuda Hirshberg veritable the photochromic spiropyrans and suggested their use in data storage. 3 In the 1970s, Valeri Barachevskii demo that this photochromism could be publishd by two-photon excitation, and finally at the end of the 1980s Peter T. Rentzepis showed that this could lead to three-dimensional data storage. 5 This proof-of-concept system stimulated a great deal of research and d evelopment, and in the following decades many donnish and commercial groups waste worked on 3D optical data storage products and technologies.Most of the developed systems are found 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 grand work has been carried knocked out(p) in solving the problems associated with the optical systems required for the reading and recording of data. Currently, several groups stick working on solutions with various levels of development and interest in commercialization. *3. Processes for creating written data* data recording in a 3D optical storage medium requires that a change take emerge in the medium upon excitation. This change is more often than not a photochemical reaction of some sort, although other possibilities exist. chemical substance reactions that have be en investigated include photoisomerizations, photodecompositions and photobleaching, and polymerization initiation. Most investigated have been photochromic compounds, which include azobenzenes, spiropyrans, stilbenes, fulgides and diarylethenes. 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 signals, is technically feasible. 3. 1 indite by non*-* reverberant multiphoton ingress Although there are many nonlinear optical phenomena, only multiphoton assiduity is capable of injecting into the media the solid ability required to electronically excite molecular species and cause chemical reactions. Two-photon dousing is the strongest multiphoton absorbance by far, but save it is a very weak phenomenon, leading to low media sensitivity.Therefore, much research has been directed at providing chromophores with high two-photon absorpt ion 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 one state (usually the ground state) to a higher elan vital electronic state. The get-up-and-go difference between the involved lower and upper states of the molecule is equal to the addition of the energies of the two photons. Two-photon absorption is a second-order processes several orders of magnitude weaker than linear absorption.It differs from linear absorption in that the authorization of absorption depends on the square of the light intensity, thus it is a nonlinear optical process paternity 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 draped by the medium, and therefore it does not interact with the medium except at the focal point. At the fo cal 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 pair 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 approach to improving media sensitivity has been to employ resonant wo-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 unleashdom 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, center 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 formd in the manufacturing of the media, as is the outcome w ith most optical disc formats for commercial data distribution. In this case, the user cannot write 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 shaped 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 Other approaches to writing Other techniques for writing data in three-dimensions have also been examined, including Persistent *apparitional** **hole burning* (PSHB), which also allows the gap of spectral multiplexing to increase data density. However, PSHB media currently requires extremely low temperatures to be maintain ed in order to overturn data loss. Void* formation, where microscopic bubbles are introduced into a media by high intensity laser irradiation. 7 Chromophore poling, where the laser-induced change of direction 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 each absorption or fluorescence). This method is essentially two-photon-microscopy. Linear excitation of fluorescence with confocal detection. This method is essentially confocal laser scan 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 ot her data points in addition to the one be addressed. Measurement of small differences in the refractive baron between the two data states.This method usually employs a phase contrast microscope or confocal reproach microscope. No absorption of light is necessary, so there is no risk of damaging data time reading, but the required refractive index mismatch in the disc may limit the heaviness (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 generation has been demonstrated as a method to read data written into a poled polymer matrix.Optical coherence tomography has also been demonstrated as a mate reading method. *5. Media * throw The active part of 3D optical storage media is usually an organic polymer both doped or grafted with the photochemically active species. Alternatively, crystalline and sol-gel materials have been used. 5. 1 Media form cypher Media for 3D optical data storage have been suggested in several form factors Disc. A disc media offers a rise from CD/DVD, and allows reading and writing to be carried out by the familiar spinning disc method. Card.A credit card 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 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 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. *6. 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 worthy 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 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 take aim of the optical drive.Variable spherical aberration correction. Because the system must a ddress 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 swap in and out of the optical path, moving elements, adaptive optics, and immersion lenses. Optical system. In many examples of 3D optical data storage systems, several wavelengths (colors) of light are used (e. g. eading laser, writing laser, signal sometimes even two lasers are required just for writing). Therefore, as well as coping with the high laser power and varying 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 theref ore 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 DVD discs. The possibility of using parallel or page- found 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* Despite the super 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 cataclysmic 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 slip bys 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 impromptu 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-cerulean lasers or NdYAG lasers to achieve excitation, but these instruments are not suitable 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 valuable 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 developed 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 t he use of resonance energy transfer between molecules, and also develops high 2-photon cross-section materials. Seth Marder performed much of the early work maturation 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. 9 mercenary 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 aspiration from other quarters such as hard drives, flash storage, holographic storage and internet-based storage. Examples of 3D optical data storage media. Top row Written Call/Recall media M empile media. Middle row FMD D-Data DMD and drive. poop row Landauer media Microholas media in action.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 TB as 200 layers in a 1. 2 mm thick disk. 23 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. mm thick disc, as well as low crosstalk, high sensitivity, and thermodynamical stability. 25 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. 26 *Constellation 3D* developed the Fluorescent 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 glasses 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. prototypic 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 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 computer storage aim to develop a 3D hologr aphic 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 leadership 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 engineering 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 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.