How does the technology behind DVD/ CD-RW (rewritable CDs) work, and what makes them different from regular CDs and CD-Rs?

-Karthik Gurumurthy

I’ve always been fascinated by how rewritable CDs actually work. The technology behind them is quite clever when you think about it.

All CDs and DVDs fundamentally work by using marks on the disc that appear darker than the background, which are detected by shining a laser on them and measuring the reflected light. In traditional store-bought CDs or DVDs, these marks are physical “pits” permanently imprinted into the surface of the disc.

For CD-Recordable (CD-R) discs, the process is different. When you burn a CD-R, the computer’s writing laser creates permanent marks in a layer of dye polymer in the disc.

What makes CD-Rewritable (CD-RW) discs special is that the changes to the recording surface are reversible. The key to this technology is a special phase-change material – an alloy made of silver, indium, antimony, and tellurium. What I find remarkable about this alloy is that unlike most solids, it can exist in either of two distinct solid states: crystalline (with atoms neatly arranged in an organized array) or amorphous (with atoms in random positions).

The crystalline state reflects more light, while the amorphous state reflects less. When the disc is manufactured, this alloy layer starts in the crystalline state. During writing, a laser heats specific spots to about 700°C, causing those areas to melt and rapidly cool into the amorphous state. These areas then appear as dark spots when the disc is read.

The truly ingenious part is how erasure works. By heating the same spots to a lower temperature (around 200°C), the material reorganizes back into its crystalline state, effectively erasing the marks. According to manufacturers, a typical CD-RW disc can be rewritten up to 1,000 times and should last about 30 years.