Terabyte on a CD

For years, Ortal Alpert tried to stay ahead of the game by 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 10 years later, Alpert’s dream led to the creation of a new optical technology.

Mempile, a company created by Alpert and a few of his colleagues in 2000, is now in advanced stages of developing its revolutionary optical technology, one with the potential to hold 20 times more data than the best existing optical technology does.

DVDs and high-definition (HD) optical media have one or two layers of data. Adding more layers using existing technology would be expensive — but more important, 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/Blu-ray discs reduce the amount of light that reaches the deep layers, making the amount of signal reflected from each successive layer smaller; and 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 changing the way optical media work — starting from the material of which the medium 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 or player. And the yellowish color is not a publicity stunt concocted by the public relations department but the result of the special properties of the material used by Mempile.

Using ePMMA, Mempile created a medium with about 200 virtual layers — created by the laser — five microns apart, each containing approximately 5 gigabytes of data. Although current prototypes are still in the 600GB to 800GB per disc range, Mempile is convinced that further optimization will enable it to reach its goal of 1 terabyte per 1.2-millimeter disc in the near future.

In With the New

But using specially designed polymers is just half the story. In order to make a medium that 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 wavelength.

In order to precisely illuminate a specific molecule inside the disc, Mempile uses what is known as nonlinear optics. In linear optics, the amount of light 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, the photons easily 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, a laser at a specific power excites the chromophore in a particular layer of the disc. In order to record data, a stronger light creates a different chemical reaction in the molecule. Mempile says that its technology also can be adapted to be rewritable in the future, but there’s not much of a market for it now.

According to Mempile, their product should be reliable, and different simulations and acceleration tests showed data lifetime of about 50 years. Although Mempile is planning to launch their first product using red laser light (which is a more mature technology), moving to blue laser light down the road will possibly allow the technology to achieve up to 5TB of data per disc.

Other Optical Storage Technologies

Several other companies are developing next-generation optical storage technologies. TDK recently announced a 200GB Blu-ray disc, which seems to be getting close to the limit of Blu-Ray media technology. A different path was taken by InPhase, which uses holographic technology to record data on a special medium currently capable of containing about 300GB. InPhase is working to increase the capacity of its medium and hopes to reach 1.6TB by early next decade. The main market for InPhase’s technology is among 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 also.

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 that is inherently different than that used by conventional CD/DVD/HD media, and likely will take longer to develop, the current market doesn’t seem ripe for such a revolution. In a time when 25GB or 50GB media are still just a small percentage of the consumer market, bringing in 1TB media doesn’t make sense to most manufacturers.

For that reason, Mempile’s technology likely will reach the market just after HD media becomes mainstream.

However, when this transformation occurs, we will reach a whole new stage in data storage. The invention of the CD-ROM made the challenge of storing documents (and to some extent images) irrelevant, because one disc could store more documents than most people write in their entire lifetimes. 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 simpler than it is today. A single TeraDisc will be able to store more than 250,000 high-resolution, high-quality pictures or MP3s, more than 115 DVD-quality movies and about 40 HD movies — not to mention an enormous 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, notebooks and digital video recorders.

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 appear. They could require hundreds of gigabytes per hour on entirely new display technologies, such as holographic displays, which could require even more storage space.