Mempile - Terabyte on a CD
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 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.
Mempile disc in the lab
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 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 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