The optical disc revolution started with CDs and then moved on to DVDs, and we're in the midst of the next-gen battle between HD DVD and Blu-ray. Since the birth of the CD 25 years ago, we've gone from 600MB to a whopping 50GB of storage capacity on these little, convenient and versatile discs. But for those who desire more space on a highly portable medium, new technology from a company called Mempile in Jerusalem promises to blow these limits away. The company claims that they can store up to 1TB (1,000GB) on an optical disc with the same dimensions—only slightly thicker—than a regular DVD and will be able to store 5TB once the jump to blue lasers is made. The 1TB disc is divided into 200 different layers, each comprising 5GB of storage space. Unlike standard multilayer DVDs, the layers aren't physically stacked and stuck together. The Mempile discs are solid and use a specially developed variant of the polymer polymethyl methacrylate (PMMA)—a mixture of Perspex, Lucite, and Plexiglass—known as ePMMA. It's this polymer that gives the discs a distinctive yellow color. When recording data to the disc, the laser focuses on one of the virtual layers and, using a photochemical reaction, modifies only a part of the plastic to represent a "1" or leaves it alone to represent a "0". This approach uses three dimensions in the polymer to store data rather than the two dimensions used by DVD. The technology is currently limited to WORM (write once, read many) although the company hopes to have read/write drives available in the future.

                                                  OPTICAL MEDIA
Optical media - such as the compact disk (CD) - are storage media that hold content in digital form and that are written and read by a laser; these media include all the various CD and DVD variations, as well as optical jukeboxes and autochangers. Optical media have a number of advantages over magnetic media such as the floppy disk. Optical disk capacity ranges up to 6 gigabytes; that's 6 billion bytes compared to the 1.44 megabytes (MB) - 1,440,000 bytes - of the floppy. One optical disk holds about the equivalent of 500 floppies worth of data. Durability is another feature of optical media; they last up to seven times as long as traditional storage media.
Optical media are used for storing digital sounds, images and data. There are three main families:
  • The commercially issued, mass produced, CD family including the digital audio CD- both 12cm and the "single" 8cm disc - CD-ROM, CD-I and CD-V and the analogue Video Disc.  
  • Optical disks and tapes that can be recorded on once.
  • Re-recordable disks.
Mass Produced Discs:-
The mass-produced discs of the CD family have the digital information in the form of microscopic pits pressed into a polycarbonate base which is coated with a light reflective layer. This reflective layer is usually of aluminium, but gold and silver are also used. A transparent lacquer is then placed over the reflective surface to protect it. This surface also carries any label information. As the data on members of the are impressed, they cannot be altered or rewritten.
Because of the high costs to setup the production of a pressed disc, the discs are only used when large numbers of copies are required (over about 100), for example, encyclopaedia or sound recordings. The higher the number of discs issued, the lower is the unit price. The storage capacity of a 12cm CD is about 650 MB or one hour of audio. The average access time is about 300 ms with a double speed player, 250 ms with quadruple speed and 130 ms with sextuple speed.
The first disc in the family to be developed was the 30cm analogue LV (Laser Vision) Disc for video. This usually consisted of two discs stuck back-to-back to form a double sided disc with one hour of video per side. A sub-format was developed which could store up to 54000 still video images per side. The LV disc was the most successful of several attempts to generate market acceptance but is expected to be superseded by the DVD (Digital Versatile Disc or Digital Video Disc) that is being launched in 1997.
The DVD is the same diameter as the CD (12cm) but, by using a laser with a shorter wave length, the storage capacity of one layer is increased by a factor of seven to 47 GB. Additionally, a dual layer structure will be possible, read by two different laser wave lengths, thus doubling the capacity to 9 GB. In principle, by glueing two such double layer disks together like the LV video disks, a total capacity of 18 GB can be achieved. The disk is intended for the storage of data-reduced video-films or, like CD-ROMs, texts and multimedia data with, however, considerably higher storage capacities.

Optical Tape:-
Optical tape is made by ICI and packaged in a cassette for use as a WORM format data storage tape. The tape drives are made by EMASS in the USA and supplied in Europe by GRAU Storage Systems. Kodak are about to launch a competing system.
The tape contains a dye layer which changes its state when a high power laser beam is applied and can be read by a lower power laser - the same basic method as for CD-Rs. Because the tape is a sequential carrier, the access time can be quite long. In compensation, the storage capacity of one tape is considerably greater than a disc (up to 100GB).

Rewritable Optical Media:-
In contrast to the preceding optical media, data on rewritable optical disks ("Erasable"), Magneto­Optical (M/O) and Phase­change, can be altered or deleted many times. There are rewritable optical disks in the 525 inch format and, more recently, in the 35 inch format. The most common still are the magneto­optical discs, where a laser beam in the write mode heats the inner layer of the optical disk and thus changes the polarity of a magnetic coating. The resulting microscopic magnetic marks of different polarity can be read as a bit stream by a low­energy laser beam in the read mode. A more recent recording technology is the Phase­change where the carrier layer is coated with a thin semi­metal film, which can be both in an amorphous and in a crystalline state. A laser beam in the write mode can change single spots to either an amorphous or a crystalline state so that, again, a digital bit stream is created. The Phase­change may replace M/O in the future.
Rewritable optical disks have a short access­time (600 milliseconds). The storage capacity has steadily increased up to the current 26 GB.
The Stability of Optical Carriers:-
The main factors that affect the stability of carriers and the retrieval of information can be summarised as:
  • Humidity and temperature.
  • Mechanical deformation.
  • Dust and dirt of all kinds.
For some carriers there are additional factors:
  • Light
  • Stray magnetic fields.
Humidity is, as with other data carriers, a most dangerous factor. In the case of optical media it has a hydrolytic action on components such as the protection layer of CDs and a corrosive influence on all metal components including metallic reflective layers. As a secondary effect, high humidity levels (above 65% RH) encourages the growth of moulds and fungi which can obstruct the reading of optical information.
Temperature, as with all other data carriers, determines the speed of (deteriorating) chemical reactions. More importantly, it is responsible for dimensional changes which may be of concern, especially in the case of multi-layer media.
Mechanical integrity is of utmost, and underrated, importance. Even microscopic scratches can hinder the reading laser beam, as do fingerprints and other foreign matter. Mechanical bending of discs cause microscopic cracks which again divert the laser. While the WORM and MO-disks developed as computer storage media are housed in cartridges which only open when inserted into the respective players, the representatives of the CD-family must be handled with utmost care, keeping mechanical integrity in mind.
Dust and dirt affects the proper reading of the recorded information. Cigarette smoke will accumulate on the disk surfaces and may hide information. The CD-family is again more exposed to this danger than those disks that are protected by cartridges.
Light may affect the dye layers used in recordable and erasable disks.
Stray magnetic fields must be kept away from magneto-optical disks.
3D optical data storage is the term given to any form of optical data storage in which information can be recorded and/or read with three dimensional resolution (as opposed to the two dimensional resolution afforded, for example, by CD).
This innovation has the potential to provide terabyte-level mass storage on DVD-sized disks. Data recording and readback are achieved by focusing lasers within the medium. However, because of the volumetric nature of the data structure, the laser light must travel through other data points before it reaches the point where reading or recording is desired. Therefore, some kind of nonlinearity is required to ensure that these other data points do not interfere with the addressing of the desired point.
No commercial product based on 3D optical data storage has yet arrived on the mass market, although several companies are actively developing the technology and predict that it will become available by 2010.
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 approximately 10 layers. 3D optical data storage methods circumvent this issue by using addressing methods where only the specifically addressed voxel (volumetric pixel) interacts substantially with the addressing light. This necessarily involves nonlinear data reading and writing methods, in particular nonlinear optics.
3D optical data storage is related to (and competes with) holographic data storage. Traditional examples of holographic storage do not address in the third dimension, and are therefore not strictly "3D", but more recently 3D holographic storage has been realized by the use of microholograms.Layer-selection multilayer technology (where a multilayer disc has layers that can be individually activated e.g. electrically) is also closely related.
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.
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 (in this example), a similar procedure is used except this time instead of causing a photochemical change in the media the laser causes fluorescence. This is achieved e.g. by using a lower laser power or a different laser wavelength. The intensity or wavelength of the fluorescence is different depending on whether the media has been written at that point, and so by measuring the emitted light the data is read.
It should be noted that the size of individual chromophore molecules or photoactive color centers is much smaller than the size of the laser focus (which is determined by the diffraction limit). The light therefore addresses a large number (possibly even 109) of molecules at any one time, so the medium acts as a homogeneous mass rather than a matrix structured by the positions of chromophores.

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 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 or Cube. 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.

Drive design:-

A drive designed to read and write to 3D optical data storage media may have a lot in common with CD/DVD drives, particularly if the form factor and data structure of the media is similar to that of CD or DVD. However, there are a number of notable differences that must be taken into account when designing such a drive, including:
Laser. Particularly when 2-photon absorption is utilized, high-powered lasers may be required that can be bulky, difficult to cool, and pose safety concerns. Existing optical drives utilize continuous 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 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 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. reading laser, writing laser, signal; sometimes even two lasers are required just for writing). Therefore, as well as coping with the high laser power and variable spherical aberration, the optical system must combine and separate these different colors of light as required.
Detection. In DVD drives, the signal produced from the disc is a reflection of the addressing laser beam, and is therefore very intense. For 3D optical storage however, the signal must be generated within the tiny volume that is addressed, and therefore it is much weaker than the laser light. In addition, fluorescence is radiated in all directions from the addressed point, so special light collection optics must be used to maximize the signal.
Data tracking. Once they are identified along the z-axis, individual layers of DVD-like data may be accessed and tracked in similar ways to 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:
Destructive reading. Since both the reading and the writing of data are carried out with laser beams, there is a potential for the reading process to cause a small amount of writing. In this case, the repeated reading of data may eventually serve to erase it (this also happens in phase change materials used in some DVDs). This issue has been addressed by many approaches, such as the use of different absorption bands for each process (reading and writing), or the use of a reading method that does not involve the absorption of energy.
Thermodynamic stability. Many chemical reactions that appear not to take place in fact happen very slowly. In addition, many reactions that appear to have happened can slowly reverse themselves. Since most 3D media is based on chemical reactions, there is therefore a risk that either the unwritten points will slowly become written or that the written points will slowly revert to being unwritten. This issue is particularly serious for the spiropyrans, but extensive research was conducted to find more stable chromophores for 3D memories.
Media sensitivity. 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.

                                   HOW OPTICAL DISC WORKS?
CDs, DVDs, and the current HD media have a number of physical similarities. Each uses a 1.2mm (4/100 inch) piece of clear polycarbonate plastic with microscopic bumps arranged as a single, continuous, spiral track of data. Optical media requires a player composed of a fast-spinning drive motor for spinning the media, a laser (infrared, red, or blue, depending on the player and media), and a tracking mechanism that moves the laser beam to follow the spiral track (with a resolution in the scale of submicrons). 
The function of the player is to focus the laser on the track of bumps. In a CD, the laser beam passes through the polycarbonate layer of the media and is reflected off the aluminum layer and hits an opto-electronic device that detects changes in light. Since the bumps in the media reflect light differently than the rest of the layer, the opto-electronic sensor detects that change in reflectivity and translates this difference into digital information (zeros and ones).  
The main difference between the current types of optical media is the size of the data track, bumps and the wavelength of the laser used for reading the data. In a CD, each track is about 1.6 microns wide and each pit has a depth of about 0.11 micron and a minimal length of about 0.834 micron. A DVD shrinks this almost by half with a 0.74 micron track and a pit length of 0.4 (interestingly, the pit depth of a DVD is a bit deeper at 0.12 micron). As we already mentioned, the 650nm wavelength (down from 780nm of a CD) allows the DVD to read this extra information. HD media does even better with a pit length of about 0.2 for HD-DVD and 0.15 for Blu-ray. 
Another important technical change is the size of the laser spot which had to be reduced to read the ever smaller pits in the media. The original CD had a spot size of about 1.6 microns, which shrunk to 1.1 microns on a DVD—and even further in HD (0.62 micron in HD-DVD and 0.48 micron in Blu-ray). Besides the size of the data and reading apparatus, the data’s location inside the media also changed over time. While the original CD had only one layer located in the innermost part of the 1.2mm thick polycarbonate plastic (fairly close to the label), in a DVD (and HD-DVD) the data surface is located in the middle of the media. Blu-ray however is very different in this respect, locating its data surface on the opposite side of the label. Although each media type has a different location for its data surface, the overall volume taken up by the data inside the media is very small and the majority of space could be considered wasted.  

So far we discussed read-only memory (ROM) media. However CD-R/RW, DVD-R/RW and similar HD media are in widespread use. Unlike ROM media, which is made in one go by commercial pressing machines, R/RW media use laser light to record the data onto the disc. In “write once” media (R) the burner turns the laser writer on and off according to the way the ones and zeros should appear on the disc. Some describe the operation of the laser as darkening the material to encode a zero and leaving it translucent to encode a one, although a more accurate description might be to say that the laser changes the volume of the disc in a specific location (“filling” the pit). A rewritable (RW) media is more complicated as it is based on phase-change technology. The phase-change element is a chemical compound made out of silver, antimony, tellurium, and indium (other compounds exist as well, including organic dyes). When the compound is heated above its melting temperature (about 600 degrees Celsius, or 1,112 degrees Fahrenheit), it becomes a liquid; at its crystallization temperature (about 200 degrees Celsius, or 392 degrees Fahrenheit) it turns into a solid. The crystalline form has less volume, so it leaves the pits “empty” while the noncrystalline form has a larger volume, so the pits are full. When the pits are full, there is constructive interference between reflection from the pits and their surrounding which means more light is reflected. When the pit is empty, there is a destructive interference, which means the light is reflected in a lower quantity.

In blank media, all of the material in the writable area is in the crystalline form, so light will shine through this layer to the reflective metal above and bounce back to the light sensor. In order to write information on the disc, the burner uses its write laser, which is more powerful and can heat the compound to its melting temperature. The melted spots have the same function as the bumps on conventional optical media. Nonreflective areas on the RW media indicate a zero, while areas which remain reflective indicate a one (here as well, things are a bit more complex in practice, and data on a disc is encoded as a series of lines having different lengths similar to Morse code called Run Length Limited or RLL for short). 
Since the days of the first CDs, optical media increased its capacity by 75 times (from 650 MB in a CD to 50 GB in dual-layer Blu-ray media). However the demand for more storage space continues, and with ever larger hard drives now reaching capacities of 1TB and beyond, an appropriate next-generation optical media is in the making.  

                                   MEMPILE TECHNOLOGY
For years Ortal Alpert tried to stay ahead of the game buying the latest hard drives and optical drives to store his ever growing library of data. In the mid 1990s, Alpert came up with a novel idea for storing data, and he decided to start his own company. Almost ten years later, Alpert’s dream lead to the creation of a new optical technology, one with the potential to hold 20 times more data than the best existing optical technology. 
In late April 2007 the TFOT team visited the offices of Mempile, 20km Northwest of Jerusalem, Israel. Mempile, the company created by Alpert and a few of his colleagues in 2000, is now in advanced stages of developing its revolutionary optical technology, which we had a chance to see. When we first set our eyes on the see-through yellowish disc we were a bit surprised. Was the choice of color the idea of the PR department looking to draw attention to the new media, we inquired? The answer we received took us straight into the heart of Mempile's technology and made us realize that looks could very well be deceiving. 
In a DVD or HD optical media, there are either one or two layers of data. Adding more layers using existing technology would be expensive — but more importantly, it would have to get around a very basic problem: it’s difficult to read information embedded deep inside this kind of media. The current semireflective layers used to store data on CD/DVD/HD-DVD/BD reduce the amount of light that reaches the deep layers, making the amount of signal reflected from each layer smaller, after a few layers the amount of light reflected becomes so small and so noisy that reading the data becomes nearly impossible. 
Overcoming this basic limitation of existing optical media is the goal Mempile set for itself, and the way to achieve it is by completely changing that way optical media works — starting from the material of which it is made. Mempile developed a special variant of the polymer polymethyl methacrylate (PMMA) known as ePMMA. After several years of trial and error, Mempile was able to develop this unique polymer, which it claims is almost entirely transparent to the specific wavelength of the laser used by its recorder/player. The yellowish color of the media is thus not a publicity stunt but the result of the special properties of the material used by Mempile. 
Using ePMMA, Mempile was able to create a media with about 200 virtual (i.e., created by the laser) layers, five microns apart, each containing approximately 5 GB of data. Although current prototypes are still in the 600–800GB per media range, Mempile is convinced that further optimization will enable it to reach its goal of 1 TB per 1.2mm disc in the very near future. 
But using specially designed polymers is just half the story. In order to make a media which could actually store all this data and effectively retrieve it, the old method of reading and writing on optical media had to be abandoned. Instead of the pits and flat surfaces representing zeros and ones, Mempile chose to implement a photochemical process, which happens when an ePMMA molecule is precisely illuminated by a red laser of a specific a wavelength.  

In order to be able to precisely illuminate a specific molecule inside the disc, Mempile uses what is known as nonlinear optics. In linear optics the amount of light which is absorbed by an object is directly proportional to the amount of light used, in nonlinear optics the amount of light absorbed does not stand in direct proportion to the amount used — instead, a small decrease in the amount of light used will result in a dramatic decrease in the amount of light absorbed. The process that Mempile uses to write and read data is called two-photon absorption and is nonlinear in nature. When the laser beam is focused to a small radius on the disc, it is very easy for the photons to excite the ePMMA molecules (chromophores), but when the radius of the beam increases even slightly, it becomes very improbable for two photons to be absorbed by a chromophore, so no writing or reading can occur. Nonlinear optics is required in this case because in a 200-layer disc, linear optics would cause some of the light to be absorbed by the layers above the intended one resulting in errors and loss of signal. 
In order to read data Mempile uses laser at a specific power which excites the chromophore in a particular layer of the disc. In order to record data, a stronger light is used which creates a different chemical reaction in the molecule. Mempile told TFOT that its technology can also be adapted to perform RW in the future, but market demand for such a product does not seem to be huge. 
According to Mempile their product should be very reliable, and different simulations and acceleration tests showed data lifetime of about 50 years. Although Mempile is currently planning to launch their first product using red laser (which is a more mature technology), moving to blue laser further down the road will possibly allow the technology to achieve up to 5 TB of data per disc. 

There are currently several other companies developing next-generation optical storage technologies. TDK recently announced a 200GB Blu-ray disc, which seems to be getting closer to the limit of Blu-ray media technology. A different path was taken by InPhase, which TFOT covered in 2006. InPhase uses holographic technology to record data on a special media currently containing about 300 GB. InPhase is working on increasing the capacity of its media and hopes to reach 1.6 TB by early next decade. The current main market for InPhase’s technology is professional users who are willing to pay extra for a fast and large backup storage system. Mempile is looking toward both the professional market and the consumer market and hopes to launch its first product early in the next decade. 
Although this might seem like a long time to wait, there are some good reasons behind this decision. Besides the fact that Mempile developed an entirely new technology which is inherently different than that used by conventional CD/DVD/HD media, and hence bound to take longer to develop, the current market doesn’t seem ripe for such a revolution. In a time when 25/50GB media are still just a small percentage of the consumer market, bringing in 1 TB media doesn’t make sense from the point of view of most manufacturers. For that reason we shall probably see Mempile’s technology on the market just after HD media becomes mainstream.  
However, when this transformation occurs, we shall reach a whole new stage in data storage. The invention of the CD-ROM made the question of storing documents (and to some extent images) irrelevant, as one disc could store more documents than most people write in their entire lifetime. The DVD allowed for the first time saving full movies (without the need for excessive compression). Only with the recent introduction of HD media did it become possible for higher-resolution movies to be saved on one disc. When Mempile’s technology reaches the market, it will make storing all major data types irrelevant. A single TeraDisc will be able to store over 250,000 high resolution, high quality pictures or MP3s, over 115 DVD-quality movies, and about 40 HD movies — not to mention an unimaginable number of documents. Mempile also sees its technology being used as a network-based backup technology, allowing users to save data from a variety of devices, including desktops, laptops, and digital video recorders (DVRs). 
Although many people find it hard to imagine the need for such space on a single disc, it is not inconceivable that by the time Mempile’s technology reaches the market, even higher-resolution video formats will start to appear, requiring hundreds of Gigabytes per hour, on entirely new display technologies, such as holographic displays, which could require even more storage space.   
                  TERABYTE CONCEPT
When you compare them to this new disc from Mempile, they don’t have much of a chance. The product is only a concept at this point, so it’s still quite unsure if and when the disc will be available in mass production We have just started to get to know blu-ray and HD DVD with pretty good storage capabilities.
Mempile’s TeraDisc™ optical media solution will enable low-cost, high-capacity (>1 TeraByte) permanent storage on a DVD-size disc.
The TeraDisc is made of a material which is highly responsive to two-photon writing and reading. This allows us to write anywhere in that we can focus a red laser onto the disc, e.g. multiple layers. However, many other properties of the material have to be optimized to allow this to work properly. Especially the written points, and written layers have to remain transparent after writing, without which it would be very difficult for the reading process to see the 200th layer through 199 written, nontransparent layers.
When a red laser is focused to a small spot inside the TeraDisc, we can choose if we probe the state of this material (reading , low power) or alter it (writing at higher power). This is very similar to the way a regular CDR works, except for the fact that this is now done in 3D. 
Archiving — in consumer and enterprise markets where rich media content is growing exponentially. Home storage needs are growing exponentially and we are beginning to see 1TB hard-disk drives entering the home networking market. There are no solutions for archiving personal content other than low-capacity optical media. The TeraDisc fills this void. In enterprise markets, compliancy requirements are increasing, compounded by high-resolution content being produced. Healthcare, government, video surveillance, etc., are all searching for low-cost solutions that will provide high data reliability over increasingly longer periods of time for rich media content. Mempile believes that libraries of TeraDiscs will meet these archival needs. 
                TWO PHOTON CHEMISTRY
Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.
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.
Positioned to become the 2-photon optical storage standard, Mempile's TeraDisc solution: 
  • Fills a void in the fest-growing consumer market where no high-capacity archiving solutions exist.
  • Leads to significant growth in removable archiving activity in enterprise, healthcare and public sector markets.
  • Provides significant advantages over existing optical storage offerings.
  • Has the potential to dislodge alternative storage options from their traditionally entrenched positions.  
  • Has great synergy with rapidly increasing digital-content trends in the home, health, enterprise and  government markets.   
The demand for storage capacity is doubling on an annual basis. Driven by high capacity applications, data proliferation, broadband web access, networked homes, multi-channel access to digital information and - more than all – ubiquity, the need for a robust solution providing high capacity, reliable and user-friendly removable archival storage at a reasonable price is becoming acute.
Existing optical storage technology is reaching its physical limitations with blue-laser technologies expected to hit the 200GB barrier around 2010. Mempile is able to record 1TB while providing truly random data access, creating a significant increase in capacity at greatly reduced marginal costs.

                                            BLU RAY DISC
Blu-ray Disc (also known as Blu-ray or BD) is an optical disc storage media format. Its main uses are high-definition video and data storage. The disc has the same dimensions as a standard DVD or CD.
The name Blu-ray Disc is derived from the blue-violet laser used to read and write this type of disc. Because of its shorter wavelength (405 nm), substantially more data can be stored on a Blu-ray Disc than on the DVD format, which uses a red (650 nm) laser. A dual layer Blu-ray Disc can store 50 GB, almost six times the capacity of a dual layer DVD.
Blu-ray Disc was developed by the Blu-ray Disc Association, a group of companies representing consumer electronics, computer hardware, and motion picture production. The standard is covered by several patents belonging to different companies. As of March 2007, a joint licensing agreement for all the relevant patents had not yet been finalized.

Competition from HD DVD:-

The DVD Forum (which was chaired by Toshiba) was deeply split over whether to go with the more expensive blue lasers or not. In March 2002, the forum voted to approve a proposal endorsed by Warner Bros. and other motion picture studios that involved compressing HD content onto dual-layer DVD-9 discs. In spite of this decision, however, the DVD Forum's Steering Committee announced in April that it was pursuing its own blue-laser high-definition solution. In August, Toshiba and NEC announced their competing standard Advanced Optical Disc. It was finally adopted by the DVD Forum and renamed HD DVD the next year, after being voted down twice by Blu-ray Disc Association members, prompting the U.S. Department of Justice to make preliminary investigations into the situation.
HD DVD had a head start in the high definition video market and Blu-ray Disc sales were slow at first. The first Blu-ray Disc player was perceived as expensive and buggy, and there were few titles available.This changed when PlayStation 3 launched, since every PS3 unit also functioned as a Blu-ray Disc player. By January 2007, Blu-ray discs had outsold HD DVDs, and during the first three quarters of 2007, BD outsold HD DVDs by about two to one.
Some analysts believe that Sony's PlayStation 3 video game console played an important role in the format war, believing it acted as a catalyst for Blu-ray Disc, as the PlayStation 3 used a Blu-ray Disc drive as its primary information storage medium. They also credited Sony's more thorough and influential marketing campaign. More recently several studios have cited Blu-ray Disc's adoption of the BD+ anti-copying system as the reason they supported Blu-ray Disc over HD DVD, an opinion supported by Paul Kocher, Cryptography Research's president and chief scientist.
Blu-ray Disc was locked in a format war with HD DVD until Blu-ray Disc's victory on February 19, 2008. On that day, Toshiba — the main driving force behind HD DVD — announced it would no longer develop, manufacture and market HD DVD players and recorders, leading almost all other HD DVD supporters to follow suit.
                     HD DVD
DVD (also known as "Digital Versatile Disc" - see Etymology) is a popular optical disc storage media format. Its main uses are video and data storage. Most DVDs are of the same dimensions as compact discs (CDs) but store more than six times as much data.
Variations of the term DVD often describe the way data is stored on the discs: DVD-ROM has data which can only be read and not written, DVD-R and DVD+R can be written once and then functions as a DVD-ROM, and DVD-RAM, DVD-RW, or DVD+RW holds data that can be erased and thus re-written multiple times. The wavelength used by standard DVD lasers is 650 nm.
DVD-Video and DVD-Audio discs respectively refer to properly formatted and structured video and audio content. Other types of DVDs, including those with video content, may be referred to as DVD-Data discs. The term "DVD" is commonly misused to refer to high definition optical disc formats in general, such as Blu-ray Disc and HD DVD. As a result, the original DVD is sometimes called SD DVD (for standard definition).

 DVD capacity:-

Single layer capacity
Dual/Double layer capacity
Physical size
12 cm, single sided
12 cm, double sided
8 cm, single sided
8 cm, double sided
The 12 cm type is a standard DVD, and the 8 cm variety is known as a mini-DVD. These are the same sizes as a standard CD and a mini-CD, respectively.

1.A single TeraDisc will be able to store over 250,000 high resolution, high quality pictures or MP3s, over 115 DVD-quality movies, and about 40 HD movies and unimaginable number of documents.
2.Archiving in consumer and enterprise markets where rich media content is growing exponentially.
3.Used as a network-based backup technology, allowing users to save data from a variety of devices, including desktops, laptops, and digital video recorders (DVRs).

While Blu-ray and HD DVD battle over the sub-100GB space and   holographic storage companies try to get things going around 300GB, a company called Mempile is working to ship optical discs, the same size as standard DVDs and that will ultimately contain a full terabyte of data.
The TeraDisk is a removable disk the size of a single DVD. It uses a new optical technology that allows it to store 300GB more data than the blue-laser technologies will be able to in 2010. Mempile uses a two-photon technology that allows it to record in three-dimensions and write data to transparent virtual layers over the entire surface of the disk. As many as 100 layers can be recorded and read.

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