Optical Storage

In computing and optical recording, an optical disk is a flat, usually circular, disk which can contain audio, video or data encoded in microscopic pits (or bumps) on a special material (often aluminium) on one of its flat surfaces. The encoding material sits atop a thicker substrate (usually polycarbonate) which makes up the bulk of the disk and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disk surface and extending from the innermost track to the outermost track. The data is stored on the disk with a laser and can be accessed when the data path is illuminated with a laser diode in an optical disk drive which spins the disk at speeds of about 200 RPM up to 4000 RPM or more depending on the drive type, disk format, and the distance of the read head from the center of the disk (inner tracks are read at a faster disc speed). The pits or bumps distort the reflected laser light; hence most optical disks characteristically have an iridescent appearance created by the grooves of the reflective layer. This side of the disk contains the actual data and is typically coated with a transparent material, usually lacquer.

The Technologies

Today’s optical storage alternatives can be divided into two camps, UDO (Ultra Dense Optical) and Blu-Ray.


An Ultra Density Optical disc or UDO is a 133.35 mm (5.25″) ISO cartridge optical disc which can store up to 60 GB of data utilising a design based on a Magneto-optical disc, but using Phase Change technology combined with a blue violet laser, a UDO disc can store substantially more data than a magneto-optical disc or MO, because of the shorter wavelength (405 nm) of the blue-violet laser employed. MOs use a 650 nm-wavelength red laser. Because its beam width is shorter when burning to a disc than a red-laser for MO, a blue-violet laser allows more information to be stored digitally in the same amount of space. Current generations of UDO store up to 60 GB, and a 120 GB version of UDO is in development.

Originally an optical disc storage medium developed as a replacement for the Magneto-optical disk, Ultra Density Optical was developed and announced by Sony on November 1 2000. It was later adopted with heavy investment by Plasmon, a UK technology company with extensive experience with computer archival backup systems and solutions and is now owned by a US company called Alliance Storage Technologies Inc (ASTI) based in Colorado, USA.

Currently UDO is being championed by its development partners ASTI/Plasmon, and Mitsubishi Chemical, parent company of the Verbatim media storage brand.

There are three versions of UDO: a True WORM (Write Once Read Many), an R/W (Re-Writable), and Compliant WORM (shreddable WORM).


Blu-ray Disc is an optical disc storage medium designed to supersede the standard DVD format. Its main uses are for storing high-definition video and data, with up to 25 GB per single layered, and 50 GB per dual layered disc. The disc has the same physical dimensions as standard DVDs and CDs. The name Blu-ray Disc derives from the blue-violet laser used to read the disc. While a standard DVD uses a 650 nanometer red laser, Blu-ray uses a shorter wavelength, a 405 nm blue-violet laser, and allows for almost six times more data storage than a DVD.


The blue-violet laser’s shorter wavelength makes it possible to store more information on a 12 cm CD/DVD-size disc. The minimum “spot size” on which a laser can be focused is limited by diffraction, and depends on the wavelength of the light and the numerical aperture of the lens used to focus it. By decreasing the wavelength, increasing the numerical aperture from 0.60 to 0.85, and making the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller spot. This allows more information to be stored in the same area. For Blu-ray Disc, the spot size is 580 nm. In addition to the optical improvements, Blu-ray Discs feature improvements in data encoding that further increase the capacity. All Blu-Ray Disc media is required to use hard-coating.

Although the Blu-ray Disc specification has been finalized, engineers continue to work on advancing the technology. Quad-layer (100 GB) discs have been demonstrated on a drive with standard, unaltered optics. In August 2006, TDK announced that they had created a working experimental Blu-ray Disc capable of holding 200 GB of data on a single side, using six 33 GB data layers. Its planned market launch is in the 2010–11 time frame

In December 2008, Pioneer Corporation unveiled a 400 GB Blu-ray Disc (containing 16 data layers, 25 GB each) that will be compatible with current players after a firmware update. Its planned market launch is in the 2012–13 time frame. Ongoing development is under way to create a 1 TB Blu-ray Disc as soon as 2015.