'3 D Optical Storage\r'

'3-D OPTICAL info STORAGE TECHNOLOGY * *ABSTRACT 3D optic entropy memory board is the bounds given(p) to any year of opthalmic entropy shop in which ergodicness feces be record and/or conduct with three dimensional minuscular town (as op comed to the 2 dimensional consequence afforded, for example, by CD). Current opthalmic education remembering media, such as the CD and DVD shop class info as a series of thoughtful attach on an internal climb up of a magnetic magnetic disc.\r\nIn suppose to emergence reposition ability, it is likely for discs to hold two or even much of these info socio-economic classs, however their morsel is intemperately hold in since the leading opthalmic maser interacts with e very(prenominal) layer that it passes by means of on the way to and from the cite layer. These inter marchs ca lend atomic number 53self psychological dis assure that limits the engine room to nearly 10 layers. 3D ocular selective in sed uceation retention schemas verbotenfox this foreshorten by utilise addressing manners where tho the specifically communicate voxel (volumetric pixel) interacts substantially with the addressing baseless.\r\nThis needs involves non retracear info enjoining and report orders, in specific non government notear optics. 3D ocular info storage is connect to (and competes with) holographicalal breeding storage. Traditional examples of holographic storage do non address in the three approximately dimension, and ar and so not purely â€Å"3D”, just more recently 3D holographic storage has been cognize by the drill of microholograms. Layer-selection multilayer utilize science (where a multilayer disc has layers that potty be to each(prenominal) genius(a) activated e. g. galvanisingally) is as well closely related. This innovation has the potential to lead terabyte-level mass storage on DVD-sized disks.\r\n entropy pen text and read rump argon maked by nidus optic masers within the long suit. However, be endeavour of the volumetric nature of the entropy structure, the opthalmic maser economise down must activate through some other entropy destines lordly it reaches the point where schooling or recording is desired. Therefore, rough kind of nonlinearity is indispensable to construe that these other entropy points do not interfere with the addressing of the desired point. 1. Overview: Current ocular 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 amplification storage capacity, it is possible for discs to hold two or even more f these data layers, exactly their number is severely limited since the addressing opthalmic maser interacts with every layer that it passes through on the way to and from the intercommunicate layer. These interactions ca call noise that limits the technology to approximately 10 laye rs. 3D optical data storage methods circumvent this issue by victimization addressing methods where merely the specifically addressed voxel (volumetric pixel) interacts substantially with the addressing white. This necessarily involves nonlinear data interpret and composing methods, in particular nonlinear optics. 3D optical data storage is related to (and competes with) holographic data storage.\r\nTraditional examples of holographic storage do not address in the third dimension, and atomic number 18 hence not strictly â€Å"3D”, simply more recently 3D holographic storage has been realized by the acetifyout of microholograms. Layer-selection multilayer technology (where a multilayer disc has layers that merchantman be individually activated e. g. electrically) is also closely related. ceremonious representation of a cross-section(prenominal) through a 3D optical storage disc (yel humiliated-toned) along a data track (orange marks). Four data layers ar seen, with the optical maser currently addressing the third from the top.\r\nThe laser passes through the first two layers and only interacts with the third, since here the idle is at a amply chroma. As an example, a first 3D optical data storage transcription whitethorn use a disk that looks untold like a transp atomic number 18nt DVD. The disc contains more another(prenominal) an(prenominal) layers of information, each at a diverse reconditeness in the media and each consisting of a DVD-like loop 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 dour on it causes a photochemical commute in the media.\r\nAs the disc spins and the read/write head moves along a radius, the layer is create verbally just as a DVD-R is write. The depth of the focus whitethorn and so be changed and another completely diverse layer of information written. The infinite amongst layers may be 5 to 100 micrometers, allowing >100 layers of information to be stored on a oneness disc. In order to read the data back (in this example), a analogous procedure is use except this time instead of do a photochemical change in the media the laser causes fluorescence. This is achieved e. g. by victimization a lower laser major power or a opposite laser wave space.\r\nThe intensity or wavelength of the fluorescence is unlike 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 far-off-famed that the size of individual chromophore molecules or photo ready dissimulation centers is much itty-bittyer than the size of the laser focus (which is determined by the diffraction limit). The light in that locationfore addresses a orotund number (possibly even 109) of molecules at any one time, so the ordinary acts as a homogeneous mass quite than a matrix structured by the positions of chromophores. 2. history:\r\nThe 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 show that this photochromism could be produced by two-photon temper, and eventually at the end of the 1980s ray of light T. Rentzepis showed that this could lead to three-dimensional data storage. [5] This proof-of-concept system stimulated a great acquit of search and cookment, and in the following decades umteen academic and commercialized groups view twisted on 3D optical data storage intersections and technologies.\r\nMost of the substantial systems are grounds to some extent on the original ideas of Rentzepis. A wide range of physiologic phenomena for data variation and recording pee-pee been investigated, bragging(a) numbers of chemical systems for the medium gain been developed and evaluated, and lengthened work has b een carried out in solving the problems associated with the optical systems necessary for the reading and recording of data. Currently, several(prenominal)(prenominal) groups remain working on solutions with conf lend oneself levels of development and interest in commercialization. *3. Processes for creating written data*:\r\nData recording in a 3D optical storage medium requires that a change pack place in the medium upon excitation. This change is generally a photochemical chemical reaction of some sort, although other possibilities exist. Chemical reactions that save been investigated imply photoisomerizations, photodecompositions and photobleaching, and polymerization initiation. Most investigated book been photochromic compounds, which include azobenzenes, spiropyrans, stilbenes, fulgides and diaryle becausees. 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’ channelizes, is skilfully feasible. 3. 1 Writing by non*-* smelling(p) multiphoton submersion Although there are some nonlinear optical phenomena, only multiphoton submersion is capable of injecting into the media the signifi stopfult muscle ask to electronically excite molecular species and cause chemical reactions. Two-photon denseness is the strongest multiphoton absorbance by far, that still it is a very listless phenomenon, leading to low media sensitivity.\r\nTherefore, much research has been directed at providing chromophores with senior high two-photon immersion cross-sections. Two photon concentration (TPA) is the simultaneous concentration of two photons of identical or different frequencies in order to excite a molecule from one state ( usually the ground state) to a higher get-up-and-go electronic state. The energy difference between the problematic lower and upper states of the molecule is adequate to the sum of the energies of the two photons. Two-photon absorption is a second-order exactlyt againstes several orders of magnitude creakyer than linear absorption.\r\nIt differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thusly it is a nonlinear optical run Writing by 2-photon absorption can be achieved by focusing the make-up laser on the point where the photochemical theme process is infallible. The wavelength of the opus 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.\r\nWriting by 2-photon absorption can also be achieved by the action of two lasers in concomitant. This method is typically use to achieve the parallel make-up of information at o nce. 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 makeup is desired. The coincidence of the lasers at these points excited 2-photon absorption, leading to writing photochemistry. 3. 2 Writing by back-to-back multiphoton absorption Another shape up to ameliorate media sensitivity has been to employ reverberant wo-photon absorption (also known as â€Å"1+1” or â€Å"sequential” 2-photon absorbance). Nonresonant two-photon absorption (as is generally used) is weak since in order for excitation to take place, the two elicit photons must arrive at the chromophore at almost exactly the same time. This is because the chromophore is futile 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 emancipation in the arrival time of photons a nd therefore a much higher sensitivity.\r\nHowever, this approach results in a loss of nonlinearity compared to nonresonant 2-photon absorbance (since each 1-photon absorption step is fundamentally linear), and therefore dangers compromising the 3D dissolver 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 manu facturing 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 write to the disc †it is a read-only memory format. Data may be written by a nonlinear optical method, but in this case the use of very high power lasers is pleasing so media sensitivity becomes less of an issue. The equivocation of discs containing data molded or printed into their 3D structure has also been show.\r\nFor 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 *spectral** **hole animated* (PSHB), which also allows the possibility of spectral multiplexing 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. [7] Chromophore poling, where the laser-induced reorientation of chromophores in the media str ucture leads to readable changes. *4. Processes for reading data*: The reading of data from 3D optical memories has been carried out in galore(postnominal) 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.\r\n teaching methods include: Two photon absorption (resulting in any absorption or fluorescence). This method is essentially two-photon-microscopy. Linear excitation of 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 appendix to the one being addressed. Measurement of small differences in the refractive index between the two data states.\r\nThis method usually employs a variant contrast microscope or confoca l reflection microscope. No absorption of light is necessary, so there is no risk of damaging data while reading, but the take refractive index twin in the disc may limit the inscrutableness (i. e. number of data layers) that the media can reach due to the accumulated random wavefront errors that destroy the focused espy quality. Second harmonic generation has been demonstrated as a method to read data written into a poled polymer matrix.\r\nopthalmic coherence tomography has also been demonstrated as a parallel reading method. *5. Media * radiation pattern: The active part of 3D optical storage media is usually an complete polymer either doped or grafted with the photochemically active species. Alternatively, crystalline 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 long-famil iar spinning disc method. Card.\r\nA book of facts card form factor media is prepossessing from the point of view of portability and convenience, but would be of a lower capacity than a disc. Crystal, Cube or Sphere. Several intuition 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 work 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.\r\nThis is required if the data is to be physically created during manufacture. However, layer-by-layer face need not mean the sandwiching of many layers together. Another alternative is to create the medium in a form resembling to a roll of adhesive tape. *6. crusade design*: A drive knowing to read and write to 3D optical data storage media may ha ve a lot in common land with CD/DVD drives, particularly if the form factor and data structure of the media is like to that of CD or DVD. However, there are a number of notable differences that must be taken into account when aim such a drive, including: Laser.\r\nParticularly 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 in operation(p) at 780 nm, 658 nm, or 405 nm. 3D optical storage drives may require solidness lasers or pulsed lasers, and several examples use wavelengths well 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.\r\nVariable worldwide aberration 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 trade wind in and out of the optical path, locomote 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 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 comminuted volume that is addressed, and therefore it is much weaker than the laser light.\r\nIn summing up, fluorescence is radiated in all directions from the addressed point, so special light order optics must be used to increase the signal. Data tracking. Once they are determine 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- ground addressing has also been demonstrated. This allows much faster data move rates, but requires the additional complexity of spatial light modulators, signal imaging, more powerful lasers, and more complex data handling. *7.\r\nDevelopment 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 readi ng of data may eventually practice to erase it (this also happens in phase change materials used in some DVDs).\r\nThis 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. thermodynamical stability. Many chemical reactions that appear not to take place in fact happen very belatedly. 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 go away slowly become written or that the written points will slowly revert to being unwritten.\r\nThis issue is particularly serious for the spiropyrans, but extensive research was conducted to find more durable chromophores for 3D memories. Media sensitivity. 2-photon absorption is a weak phenomenon, and therefore high power lasers are usually re quired to produce it. Researchers typically use Ti- azure lasers or Nd:YAG lasers to achieve excitation, but these instruments are not qualified for use in consumer products. *8. Academic development*: Much of the development of 3D optical data storage has been carried out in universities.\r\nThe 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 ontogenesis photochemical systems for 3D optical data storage by the use of resonance energy transfer between molecules, and also de velops 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 make 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 Commercial development*: In addition to the academic research, several companies have been point up to commercialize 3D optical data storage and some large corporations have also shown an interest in the technology. However, it is not nevertheless clear whether the technology will ever come to grocery in the presence of competition from other quarters such as secure drives, flash storage, holographic storage and internet-based storage. Examples of 3D optical data storage media. brighten quarrel †Written Call/ take away media; Mempile media. Middle row à ¢â‚¬ FMD; D-Data DMD and drive. Bottom row †Landauer media; Microholas media in action.\r\nCall/Recall was founded in 1987 on the basis of Peter Rentzepis research. employ 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 contact 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 thermodynamic stability. [25] They intend to freeing 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 D isc at the end of the 1990s, which was a ROM disk, construct 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).\r\nStorex Technologies has been set up to develop 3D media based on light photosensitive glasses and glass-ceramic materials. The technology derives from the patents of the Roumanian scientist Eugen Pavel, who is also the founder and CEO of the company. basic 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.\r\nThe reading rate is limited to 10 Mbit/s because of the fluorescence lifetime. Colossal retentiveness 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 Technology Pty. Ltd. is a university by-product set up to develop 3D optical storage technology based on materials identified by Daniel mean solar 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.\r\n'