Optical Storage Technology

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                             INTRODUCTION
The origins of the field date back to the 1950s, when Hirshberg developed the photo chromic spiropyrans and suggested their use in data storage. In the 1970s, Barachevskii demonstrated that this photochromism could be produced 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. 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.
One of the reasons that computers have become increasingly important in daily life is because they offer unprecedented access to massive amounts of information. The decreasing cost of storing data and the increasing storage capacities of ever smaller devices have been key enablers of this revolution. Current storage needs are being met because improvements in conventional technologies such as magnetic hard disk drives, optical disks, and semiconductor memories have been able to keep pace with the demand for greater and faster storage.
However, there is strong evidence that these surface-storage technologies are approaching fundamental limits that may be difficult to overcome, as ever-smaller bits become less thermally stable and harder to access. Exactly when this limit will be reached remains an open question: some experts predict these barriers will be encountered in a few years, while others believe that conventional technologies can continue to improve for at least five more years. In either case, one or more successors to current data storage technologies will be needed in the near future.
An intriguing approach for next generation data-storage is to use light to store information throughout the three-dimensional volume of a material. By distributing data within the volume of the recording medium, it should be possible to achieve far greater storage densities than current technologies can offer.
For instance, the surface storage density accessible with focused beams of light is roughly 1/ (2 Wave length). With green light of roughly 0.5 micron wavelength, this should lead to 4 bits/sq. micron or more than 4 Gigabytes (GB) on each side of a 120mm diameter, 1mm thick disk. But by storing data throughout the volume at a density of 1/ (3Wave length), the capacity of the same disk could be increased 2000 fold, to 8 Terabytes (TB).

Optical Storage Technology Seminar Reports
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 high intensity.

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 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 perhaps ~10 layers. 3D optical data storage methods circumvent this issue by using addressing methods where only the specifically addressed voxel 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, but operates on different principles.
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 micrometers, allowing >100 layers of information to be stored on a single disc.
In order to read the data back, 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.

             PROCESSES FOR WRITING 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 photo bleaching, 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 in principle. Also, multilevel recording, where data is written in ‘grayscale’ rather than as ‘on’ and ‘off’ signals, is technically feasible.
 Writing by 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 absorption 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.

  Two-photon absorption
   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 dependant 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.
Another approach to improving media sensitivity has been to employ resonant two-photon absorption. 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 one-photon absorbance is a linear process, and therefore risks compromising the 3D resolution of the system.
Two photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from its ground state to an excited state. The first TPA process was observed in doped europium salts
Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process, and therefore is most efficient at very high intensities.

In non resonant TPA two photons combine to bridge an energy gap larger than the energies of each photon individually. If there were an intermediate state in the gap, this could happen via two separate one-photon transitions in a process described as "resonant TPA", "sequential TPA", or "1+1 absorption". In non resonant TPA the transition occurs without the presence of the intermediate state.
  The "nonlinear" in the description of this process means that the strength of the interaction increases faster than linearly with the electric field of the light. In fact, under ideal conditions the rate of TPA is proportional to the square of the field intensity. This dependence can be derived quantum mechanically, but is intuitively obvious when one considers that it requires two photons to coincide in time and space. This requirement for high light intensity means that lasers are required to study TPA phenomena. Further, in order to understand the TPA spectrum, monochromatic light is also desired in order to measure the TPA cross section at different wavelengths. Hence, tunable pulsed lasers (such as frequency-doubled Nd: YAG-pumped OPOs and OPAs) are the choice of excitation.

 

Optical Storage Technology Seminar Reports

Description: A two-photon 3D optical data storage system consisting of a bichromophoric mixture of diarylethene and fluorene derivative as the storage medium is demonstrated here. Binary information bits were recorded throughout all three dimensions of the storage medium by two-photon localized excitation on the diarylethene molecules, transforming the closed form of diarylethene into the open form. The readout method is based on the modulation of the two-photon fluorescence emission of fluorene by the closed form of diarylethene
TPA with light intensity as a function of path length or cross section x as a function of concentration c and the initial light intensity I0. The absorption coefficient α now becomes the TPA cross section β with unit GM (after discoverer) equal to 10-50cm4.s.photon-1molecules-1
Optical Storage Technology Seminar Reports

Micro fabrication
One of the most distinguishing features of TPA is that the rate of absorption of light by a molecule depends on the square of the light's intensity. This is different than OPA, where the rate of absorption is linear with respect to input intensity. As a result of this dependence, if material is cut with a high power laser beam, the rate of material removal decreases very sharply from the center of the beam to its periphery. Because of this, the "pit" created is sharper and better resolved than if the same size pit were created using normal absorption. In the case of two-photon polymerization, the material is polymerized only near the focal spot of the laser, where the intensity of the absorbed light is highest. This makes TPA attractive for 3D micro fabrication

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 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 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 Burning (PSHB):
 Persistent spectral hole-burning has been utilized as a means for possibly achieving high-density optical storage, which also allows the possibility of spectral multiplexing to increase data density. Persistent spectral holes are formed in inhomogeneously broadened absorption lines when a photo induced change occurs in the subset of absorbers that are in resonance with a narrowband laser beam. If the photo reacted centers do not absorb at the original wavelength, a dip in absorption or spectral 'hole' is formed that may be detected by subsequent measurement of the absorption line.
Divalent samarium (Sm2+) and trivalent europium (Eu3+) ions in glasses are of special importance for their properties of persistent spectral hole-burning (PSHB), which is promising as an extremely high-density optical memory using a wavelength region in addition of spatial two-dimensions. PSHB of the rare-earth ions with 4f 6 configuration is conceptually based on a single site excitation and photochemical reaction of the ions in their inhomogeneous distribution of 5D0-7F0 energies in glasses. The homogeneous line width is ~ 0.1 cm-1 at 77 K, high density data storage at a light spot (~1μm), of ~ 30 bit/spot at room temperature and ~ 1000 bit/spot at 77 K, may be achieved. It is believed that the PSHB of Sm2+ ions is a photo-ionization of Sm2+ + hν giving Sm3+ + e-; the electron generated is captured in a defect site in glasses neighbouring to the photo-reacted Sm2+ and a persistency of the spectral hole with very narrow homogeneous width is eventually obtained.
Optical Storage Technology Seminar Reports

 

However, PSHB media currently requires extremely low temperatures ranging from 1.5K to50K to be maintained in order to avoid data loss.

Microholography: where tiny holograms are used to store data. In micro holography, focused beams of light are used to record submicron-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.
Void Formation: where microscopic bubbles are introduced into a media by high intensity laser irradiation. Standard set-up consists of a laser providing amplified femtosecond pulses  and an optical microscope is used for recording voids inside glasses under tight focusing conditions using an objective lens with a numerical aperture of NA = 1.35. The diameter of the focal spot was estimated as D = 1.22λ/NA at the 1/e2-level by intensity.
The void inside glass represents a kind of ultimate density modulation created by a laser pulse: the empty volume surrounded by a shell of densified material It is technologically important to establish the conditions of formation of such photo-modification for micro-structuring, creation of new phases inside the densified region with altered chemical properties, as well as to assess damage resistance of glasses at extremely high irradiance (> 10 TW cm–2). Voids of sub-micrometer cross-sections can be recorded inside glass by tightly focused single ultra-short pulses without crack formation. The mechanism of void creation is a micro-explosion which triggers shock and rarefaction waves from the focus. Recently, the scaling between the void diameter and the pulse energy has been established in the case of crystalline sapphire and high-purity viosil silica. It has been demonstrated that the radius of a shock-affected region, rsh, is given by:
Optical Storage Technology Seminar Reports
Where Eabs is the absorbed energy and Y is the Young modulus
Chromophore Poling: where the laser-induced reorientation of chromophores in the media structure leads to readable changes. Optical engineering of the photonic properties of polymer films provides efficient and convenient ways to store information within photo-sensitive materials. Azo-dye doped or side-chain polymers have attracted tremendous attention over the years because of their photo physical properties which can be exploited in many non linear optical applications. In this context, azo-doped poly (methyl methacrylate) (PMMA) polymer film is a model system which has been widely exploited for optical switching, holographic storage or optical memories where the required order of the chromophores can be achieved not only by applying an electric field but also via optical poling. In the field of optical data storage, the crucial parameter is spatial resolution. This need has been specifically addressed by the use of femtosecond laser sources, which, because of their high peak power, are able to give rise to multiphoton processes localized within a sub-wavelength volume in the vicinity of the focal point. This has been achieved by performing orientational hole burning through two-photon absorption in films of poly (methyl methacrylate) doped with Disperse Red 1 (DR1) that can subsequently be detected through confocal differential reflexion microscopy. A more sophisticated approach has recently been proposed by the Zyss group which encodes information by an all-optical poling technique in which the angles of polarization of the two irradiating fields are varied. The resulting spatial changes in the symmetry of the quadratic susceptibility tensor lead to a modulation of the detected SHG intensity when scanning the sample with the IR beam alone thus using a nonlinear optical phenomenon for the read-out stage as well. One possible way to combine the advantages of these two distinct techniques would consist in using two-photon isomerisation in a photo-assisted poling scheme followed by a simple SHG read-out stage. However, the critical step would remain the orientation of chromophores in a small volume and over a limited time span. Therefore, we propose to start out with the even simpler method of writing optical data into a previously corona poled film by locally disorienting the polar order, now using two photon isomerisation to randomize the initial orientation of the chromophores. Again, data retrieval will be performed by monitoring SHG intensity while scanning the sample with an IR beam. In our approach, the information is being encoded into the succession of localized areas which have been disordered or not. This takes advantage of the fact that it is by far easier to induce disorder than to create order, and that the former is more irreversible. In addition, we will show that the intensity thresholds will be low enough to allow the erasing of the data by heating the sample and the rewriting of new data after repoling it.
Role of wave length
Amount of data that can be stored is dependent on the wave length of light used. Optical refraction limits the size of focused laser beam, so a spot of the order of the wave length is used represent the presence of information; therefore wave length limits the density of data storage.
Optical Storage Technology Seminar Reports
Optical Storage Technology Seminar Reports
                    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:
1. TWO photon EXCITATION FLUORESCENCE
This method is essentially two-photon microscopy. Two-photon excitation may in some cases be a viable alternative to confocal microscopy due to its deeper penetration and reduced photo toxicity.
Two-photon excitation employs a concept first described by Maria Göppert-Mayer (b. 1906) in her 1931 doctoral dissertation. The concept of two-photon excitation is based on the idea that two photons of low energy can excite a fluorophore in a quantum event, resulting in the emission of a fluorescence photon, typically at a higher energy than either of the two excitatory photons. The probability of the near-simultaneous absorption of two photons is extremely low. Therefore a high flux of excitation photons is typically required, usually a femtosecond laser.
Two-photon microscopy was pioneered by Winfried Denk in the lab of Watt W. Webb at Cornell University. He combined the idea of two-photon absorption with the use of a laser scanner. In two-photon excitation microscopy an infrared laser beam is focused through an objective lens. The Ti-sapphire laser normally used has a pulse width of approximately 100 femtosecond and a repetition rate of about 80 MHz, allowing the high photon density and flux required for two photons absorption and is tunable across a wide range of wavelengths. Two-photon technology is patented by Winfried Denk, James Strickler and Watt Webb at Cornell University. he most commonly used fluorophores (A fluorophore, is a component of a molecule which causes a molecule to be fluorescent. It is a functional group in a molecule which will absorb energy of a specific wavelength and re-emit energy at a different (but equally specific) wavelength) have excitation spectra in the 400–500 nm range, whereas the laser used to excite the fluorophores lies in the ~700–1000 nm (infrared) range. If the fluorophore absorbs two infrared photons simultaneously, it will absorb enough energy to be raised into the excited state. The fluorophore will then emit a single photon with a wavelength that depends on the type of fluorophore used (typically in the visible spectrum). Because two photons need to be absorbed to excite a fluorophore, the probability for fluorescent emission from the fluorophores increases quadratically with the excitation intensity. Therefore, much more two-photon fluorescence is generated where the laser beam is tightly focused than where it is more diffuse. Effectively, fluorescence is observed in any appreciable amount in the focal volume, resulting in a high degree of rejection of out-of-focus objects. The fluorescence from the sample is then collected by a high-sensitivity detector, such as a photomultiplier tube. This observed light intensity becomes one pixel in the eventual image; the focal point is scanned throughout a desired region of the sample to form all the pixels of the image.
2. confocal detection
This method is essentially confocal laser scanning microscopy. It offers excitation with much lower laser powers than that of two-photon absorbance, but it has some potential problems because the addressing light interacts with many other data points in addition to the one being addressed.

Introduction to Confocal Microscopy - Confocal microscopy offers several advantages over conventional wide field optical microscopy, including the ability to control depth of field, elimination or reduction of background information away from the focal plane (that leads to image degradation), and the capability to collect serial optical sections from thick specimens. The basic key to the confocal approach is the use of spatial filtering techniques to eliminate out-of-focus light or glare in specimens whose thickness exceeds the immediate plane of focus. There has been a tremendous explosion in the popularity of confocal microscopy in recent years, due in part to the relative ease with which extremely high-quality images can be

obtained. In fact, confocal technology is proving to be one of the most important advances ever achieved in optical microscopy.
Optical Storage Technology Seminar ReportsConfocal Microscope Scanning Systems - Confocal imaging relies upon the sequential collection of light from spatially filtered individual specimen points, followed by electronic signal processing and ultimately, the visual display as corresponding image points. The point-by-point signal collection process requires a mechanism for scanning the focused illuminating beam through the specimen volume under observation. Three principal scanning variations are commonly employed to produce confocal microscope images. Fundamentally equivalent confocal operation can be achieved by employing a laterally translating specimen stage coupled to a stationary illuminating light beam (stage scanning), a scanned light beam with a stationary stage (beam scanning), or by maintaining both the stage and light source stationary while scanning the specimen with an array of light points transmitted through apertures in a spinning Nipkow disk.
Electronic Light Detectors: Photomultipliers - In modern wide field fluorescence and laser scanning confocal optical microscopy, the collection and measurement of secondary emission gathered by the objective can be accomplished by several classes of photosensitive detectors, including photomultipliers, photodiodes, and solid-state charge-coupled devices (CCDs). In confocal microscopy, fluorescence emission is directed through a pinhole aperture positioned near the image plane to exclude light from fluorescent structures located away from the objective focal plane, thus reducing the amount of light available for image formation. As a result, the exceedingly low light levels most often encountered in confocal microscopy necessitate the use of highly sensitive photon detectors that do not require spatial discrimination, but instead respond very quickly with a high level of sensitivity to a continuous flux of varying light intensity.
3. phase CONTRAST TECHNIQUE
This method usually employs a phase contrast microscope. No absorption of light is necessary, so there is no risk of damaging data while reading, but the required refractive index mismatch in the disc may limit the thickness (i.e. number of data layers) that the media can reach due to the accumulated random wave front errors that destroy the focused spot quality. As light travels through a medium other than vacuum, interaction with this medium causes its amplitude and phase to change in a way which depends on properties of the medium. Changes in amplitude give rise to familiar absorption of light which gives rise to colors when it is wavelength dependent. The human eye measures only the energy of light arriving on the retina, so changes in phase are not easily observed, yet often these changes in phase carry a large amount of information. The same holds in a typical microscope, i.e., although the phase variations introduced by the sample are preserved by the instrument (at least in the limit of the perfect imaging instrument) this information is lost in the process which measures the light. In order to make phase variations observable, it is necessary to combine the light passing through the sample with a reference so that the resulting interference reveals the phase structure of the sample. This was first realized by Frits Zernike during his study of diffraction gratings. During these studies he appreciated both that it is necessary to interfere with a reference beam, and that  that to maximize the contrast achieved with the technique, it is necessary to introduce a phase shift to this reference so that the no-phase-change condition gives rise to completely destructive interference. He later realized that the same technique can be applied to optical microscopy. The necessary phase shift is introduced by rings etched accurately onto glass plates so that they introduce the required phase shift when inserted into the optical path of the microscope. When in use, this technique allows phase of the light passing through the object under study to be inferred from the intensity of the image produced by the microscope. This is the phase-contrast technique.
                         MEDIA DESIGN
The active part of 3D optical storage media is usually an organic polymer either doped or grafted with the photo chemically active species. Alternatively, crystalline and sol-gel materials have been used.
1. Media form factor
Media for 3D optical data storage have been suggested in several form factors:
a. 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.
b. 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.
c. CRYSTAL or Cube. Several scientists have suggested of small solids that store massive amounts of information, and at least in principle this could be achieved with 3D optical data storage.
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. Different methods such as hot stamping and photo polymerization are used in the creation of 3D data storing discs such as fluorescent multilayer disc

                          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:
1. 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 head of the optical drive.
2. VARIABLE Spherical Aberration Correction. Because the system must address different depths in the medium, and at different depths the spherical aberration induced in the wave front 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, and adaptive optics.  
3. DETECTION. The detection system is very different from that in a CD or DVD, and requires operation with much lower signals. When fluorescence is used for reading, special light collection optics may be used to maximize the signal.
4. 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.
                       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 is the result of the limited financial backing that 3D optical storage ventures have received, as well as technical issues including:
1. 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.
2. 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.
3. LASER: As we have noted, 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 Nd: YAG lasers to achieve excitation, but these instruments are not suitable for use in consumer products.
Nd: YAG      Neodymium doped Yttrium Aluminium Garnet
Ti sapphire    Titanium doped sapphire

Commercial development
             Optical Storage Technology Seminar Reports
Examples of 3D optical data storage media.
Top row - Written Call/Recall media, Mempile media.
 Middle row – FMD, D-Data DMD and drive.
 Bottom row - Landauer media, Microholas media in action

1. FLOURESCENT MULTILAYER DISC
It is an optical disc format developed by Constellation 3D that uses fluorescent, rather than reflective materials to store data. Reflective disc formats (such as CD and DVD) have a practical limitation of about two layers, primarily due to interference, scatter, and inter-layer cross talk. However, the use of fluorescence allows FMDs to have up to 100 layers. These extra layers allow FMDs to have capacities up to a terabyte, while maintaining the same physical size of traditional optical discs.
       An example of an FMD
                             An example of an FMD
Operating principles
The pits in an FMD are filled with fluorescent material. When coherent light from the laser strikes a pit the material glows, giving off incoherent light of a different wavelength. Since FMDs are clear, this light is able to travel through many layers unimpeded. The clear discs, combined with the ability to filter out laser light (based on wavelength and coherence) yield a much greater signal-to-noise ratio than reflective media. This is what allows FMDs to have many layers. The main limitation on the number of layers in a FMD is the overall thickness of the disc. A 50 GB prototype disc was demonstrated at the COMDEX industry show in November 2000. First generation FMDs were to use 650 nm red lasers, yielding roughly 140 GB per disc. Second and third generation FMDs were to use 405 nm blue lasers, giving capacities of up to a terabyte.
   
 2. TAPESTRY MEDIA
Tapestry Media is a digital optical disc about the size of a DVD with a capacity of 300GB. It will go on sale in 2009, according to its American developer, InPhase Technologies.
Traditional DVDs record data by measuring microscopic ridges on the surface of a spinning disc. Two competing successors to the DVD format — Blu-ray Disc and HD DVD — use the same technique, but exploit shorter wavelengths of light to fit more information onto the surface.
The Tapestry system uses micro holography that is light from a single laser split into two beams: the signal beam and the reference beam. The hologram is formed where these two beams intersect in the recording medium.
The process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator, which translates the electronic data of 0s and 1s into an optical "checkerboard" pattern of light and dark pixels. The data is arranged in an array or "page" of around a million bits.
At the point of intersection of the reference beam and the signal beam, the hologram is recorded in the light sensitive storage medium. A chemical reaction occurs in the medium when the bright elements of the signal beam intersect the reference beam, causing the hologram.
By varying the reference beam angle, wavelength or media position many different holograms can be recorded in the same volume of material.Tapestry media is capable of storing up to 1.6TB with a data transfer rate of 120 MB/s (960 Mbit).

3. TERADISC
Mempile, a leader in next generation optical storage technology, announced today that it has proven its TeraDisc technology to be capable of storing up to one Terabyte (TB) of data. The company recently demonstrated this concept to several Japanese CE manufacturers by recording and reading over 100 virtual layers on a single DVD-size optical disc.
The demonstration attendees were amazed to see this breakthrough which showed Mempile’s capability of recording at least 500GB of data on what appears to be a simple plastic transparent disc – 300GB more than the announced roadmap of competing blue-laser technologies in the year 2010.
Existing optical media store the data through the use of light-reflective semi-transparent technologies. While increasing in capacity, even the newer blue-laser technologies are nonetheless limited to a very small number of layers.  The partial reflection from the multiple layers leads to signal reduction simultaneously raising background noise and coherent interferences. 
Mempile’s patented non-linear two-photon technology allows for 3D recording of transparent virtual layers on the entire volume of the disc. Mempile’s recent demonstration proved that more than 100 layers could be recorded and read – showing storage capabilities of slightly less than 300GB over a thickness of 0.6 mm of active material.   By increasing this active material to the thickness of a DVD, 1.2 mm, Mempile will be able to demonstrate the recording and reading of at least 500GB of data.  Future optimization will allow the recording of 200 layers and of up to 5GB of data per layer.
Due to the increase in data retention and compliance requirements, there is also a growing need for very reliable, removable and cost-effective storage solutions such as Mempile’s in the healthcare, financial, government and enterprise vertical markets. Each of these sectors now require archival storage technologies that can hold a high-capacity of information, are secure, user-friendly and are permanent yet removable and affordable.  Mempile’s technology is easily integrated into existing hardware manufacturing and software design processes making it a natural fit for these markets. 
.  A Mempile disc contains light sensitive molecules (chromophores) capable of switching between two distinct states upon the application of light. Due to the nonlinear nature of the light-matter interaction, when focusing the applied light inside the material using a lens, only those molecules present near the focal point will interact and switch state. This provides for true three-dimensional accessing of small volumes within the material, allowing the writing of data bits selectively within the bulk of the material. Reading is performed in a similar way, where light that does not result in writing excites the chromophores making them emit light. The amount of light emitted is highly sensitive to there being "written" or "unwritten" molecules near the focal point, allowing this process to be used as a reading mechanism.

   4. VERSATILE MULTILAYER DISC
High Definition Versatile Multilayer Disc's or Versatile Multilayer Disc (VMD or HD VMD) is a high-capacity red laser optical disc technology designed by New Medium Enterprises, Inc. VMD is intended to compete with the blue laser HD DVD and Blu-ray Disc formats and has an initial capacity of 20 GB to 40 GB per disc
Although initial details are sketchy, it appears that the format uses 5 GB per layer, similar to standard DVDs. The larger formats come from adding more layers. Whereas DVDs hold up to 2 layers per side, standard VMD’s can use 4 layers, for 20 GB of storage. There are also reports of 8- and 10-layered versions which can hold 40 and 50 GB, respectively.

  5. STACKED VOLUMETRIC OPTICAL DISC
The Stacked Volumetric Optical Disk (or SVOD) is an optical disk format developed by Hitachi/Maxell, which uses an array of wafer-thin optical disks to allow data storage of around 1TB. Each "wafer" (a thin polycarbonate disk) holds around 9.4GB of information, and the wafers are stacked in layers of 100 or so, giving overall data storage increase of 100x or more. SVOD will likely be a candidate, along with HVDs, to be the next-generation optical disk standard.
                                           

                                           ADVANTAGES

  • A high definition movie requires about 13 GB of storage with                           compression so it can fit in a single disc, and there is enough space to add some extra contents such as out-takes, additional scenes, etc.
  • Enables dramatic improvements in piracy protection, by taking                  advantage of the multiple layers of information.
  • Highest  optical capacity
  • Lowest cost per gigabyte
  • Highest data bit density of any storage device
  • Lowest power requirements per gigabyte
  • Long storage life
  • Have highest data transfer potential

 

 

                                 Conclusion

Computers have become very important in our life because they provide us access to storage of large amount of information. Conventional technologies have very limited amount of storage and their storage capacity cannot be increased any further. Since storage needs are increasing at a faster rate and conventional technologies are not able to keep the pace with demand for greater and faster storage requirements. So a new type of data storage technique with increased capabilities is required. These needs can be fulfilled by 3D Data Storage Devices. Further they are cost effective in the sense that they have the lowest cost per byte. They will also have faster data transfer rates compared to current technologies. 3D Data storage devices will have wide range of applications in fields such as satellite data storage, space researches, digital libraries, defense where large amount of data storage capacity is required. Hence we can undoubtedly say that 3D Data storage provides an effective solution for tomorrow’s storage needs.

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