Scientists pack terabytes onto DVDs

Posted by Rick C. Hodgin

Jerusalem (Israel) - A company called Mempile has developed a prototype storage device which uses a DVD-like disc capable of eventually storing up to 1 Terabyte worth of data in a semi-transparent disc.  It has 200 physical layers of about 5GB each on this disc.  That's enough to store more than 250,000 MP3s, 115 full-length DVD movies, or 40 full-length HD movies on each disc.  Researchers believe the technology will yield multi-TB discs that could fit in your coat pocket.

 

The process involved in storing data using this media is quite different from traditional CDs or DVDs which uses physically etched pits to represent 1s and 0s.  The multi-terabyte process relies on a photochemical reaction which takes place at the focal point of laser.  The unique material in use is what allows it to happen.

 

 

 

Each physical disc is made of a specially developed variant of the polymer polymethyl methacrylate (PMMA) known as ePMMA.  This material gives the disc itself a yellow hue but is transparent to the wavelength of light used in reading/writing.  The disc material is uniquely sensitive to particular wavelengths of laser light at varying intensities.  These properties are used for reading/writing 200 virtual layers contained within the media.

 

 

 

This disc itself is solid and potentially thicker than traditional CDs or DVDs.  Each layer exists only logically and is actually created by the read/write process itself at a spacing of 5 microns per layer.  Non-linear optics must be used due to the thickness of the material and the desire not to interact with other layers when reading or writing.  Mempile has overcome these limitations and perfected the technology.

 

To read and write data, a red laser at a specific wavelength and intensity is used.  At the focus point, a non-linear photochemical process occurs at the molecular level.  The company uses what they call a "two-photon absorption method" for writing.  This process involves the unique properties of the ePMMA media in that by slightly varying the input light intensity a notably different photochemical reaction takes place.

 

Basically, when the red laser is focused at some point at a given intensity it writes a 1 in that layer.  Decreasing the intensity only very slightly causes the non-linear photochemical process to react much differently and therefore write a 0.  By spinning the media and alternating intensities from the data stream, and by using the non-linear optics to access the many layers contained within, the large storage capacities are made possible.

 

 

Reading is handled similarly.  Data is made visible by exciting the ePMMA molecules at a given layer by a lower-intensity read laser.  Based on the excited state and its degree of illumination, either a 1 or 0 is read back.  The read process also uses a slightly out-of-focus laser so that no writing takes place in their two-photon absorption write method.

 

The technology is currently limited to WORM (Write Once, Read Many), though RW abilities are possible with only new recorders being needed.  The media itself would not change and the WORM limitations found in one recorder would be overcome by the RW abilities of another.  Current prototypes are limited to 600-800 GB with 1 TB coming soon through additional optimization of the process.  Researchers believe the technology will eventually max out about at 5 TB.  The media is expected to have a data life of at least 50 years.  Read more about the history of mass storage technology at The Future of Things.