I have been recently wondering how is data stored on and retrieved from optical devices like CDs, DVDs, and Bluray. What makes these different storage types different from each other?
Optical Drives are used for the storage of data (Using Photochromism) in several disk formats with the help of laser diodes. Popular optical drive formats include CD-ROM, CD-R, CD-RW, DVD, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-R DL, DVD+R DL, BD-R & BD-RE. As we all know, computer understands binary data (either 0 or 1). Here, 0 (false value) says that there's no reflection whereas 1 (true value) says that there's reflection.
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 and it spins at about 200 to 4000 RPM (or more depending on drive-type) in the drive.
Store data: 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. The depth of the focus may then be changed and another entirely different layer of information written.
Retrieve data: In order to read the data, the laser causes fluorescence in the media. This is achieved by using a low-power laser 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.
In case of DVDs, Write-once disks (which come under WORM devices) have an organic dye recording layer between the substrate and the reflective layer whereas Write-over discs contain an alloy recording layer composed of a phase change material, most often Ag-In-Sb-Te (an alloy of Silver, Indium, Antimony, and Tellurium).
The size of individual chromophore molecules (photo-active color centers) is much smaller than the size of the laser focus (depends upon diffraction limit). The light therefore addresses a large number ($10^9$ max...) of molecules at any given time, so the medium acts as a homogeneous mass rather than a matrix structured by the positions of chromophores..!
Note: Thus, We could conclude that these disks have capacity variation based on:
- Type of optical media (CD - Polycarbonate, DVD - An organic dye, Bluray - Hard coating Tech)
- Nature of LASER diodes
- Wavelength of LASER used for read/write (CD uses 780nm IR, DVD uses 650nm Red & Blu-Ray uses 405nm Violet)..!
But nowadays, we're moving on towards portable media with high efficiency like USB, portable HDDs, etc. This paper explains something new in WORM devices. Also, Have a look over Compact-Disks, Digital Versatile Disks, Blu-Ray Disks in Wiki..!
On a physical level, they basically work on the same principle of encoding information in pits arranged in a spiral track on one or more reflective layers in the disc, and using them to modulate laser light reflected off these layers. The main differences are:
- The size of the pits and the pitch (spacing between adjacent iterations) of the spiral, with Blu-ray having the smallest features and CDs the largest.
- As a consequence of the above and a phenomenon of physics called diffraction (which causes light of longer wavelength to be impossible to be focused into a smaller spot than a minimum size), newer technologies must use light of shorter and shorter wavelengths (infrared for CDs, red for DVDs, then blue/violet for Blu-ray).
- DVDs and Blu-ray discs can also use multiple overlapping layers on one disc, separated by clear material. The laser can access different layers by changing its focus.
- To enable more precise tracking, the amount of transparent plastic between the layer(s) and laser has also decreased from 1.2 mm in CDs to 0.1 mm in Blu-ray discs. In fact, on CDs, the aluminium layer is on the opposite side of the polycarbonate disc from the laser, but the same side on Blu-ray discs, being protected only by a durable coating.
- Above points 1 and 3 mean that the electronics must work to tighter tolerances in terms of tracking and focus, and at a higher speed.
For more information, see http://www.myce.com/review/pioneer-bdr-205-blu-ray-drive-review-22946/DVD-Blu-Ray-Reading-Performance-9/