How CD-Recordable and CD-ReWritable Dyes Work
by Kaz Chigita and Katherine Cochrane
The cyanine and phthalocyanine dyes used in CD-Recordable discs are photosensitive organic compounds, similar to those used in making photographs (which explains why companies like Kodak and Fuji are in this business). When a CD-R disc is recorded, the dye is heated by the writing laser and becomes opaque (or absorbtive) through a chemical reaction to the heat (and possibly to some mingling with nearby semi-melted polycarbonate according to some sources), so it's not likely that any process will be developed to reverse the process and make previously burned CD-Rs reusable.
CD-ReWritable uses a different kind of data-bearing layer from that in ordinary CD-Recordable, one which uses a phase change process to alter its state from a reflective state to a light absorbing state rather than an irreversable chemical change as in cyanine-based CD-R. This phase change CAN be reversed to make the area erasable and reusable.
The phase change technology used in CD-ReWritable does not incorporate any magnetic field like those in Magneto Optical disks. Rather, it alters the state of the recording layer from crystaline to non-crystaline, (and vise versa) so that the cystaline portion can allow the metalized layer to reflect the laser better while the non-crystaline portion absorbs the laser beam, so it is not reflected. As in pressed CDs and CD-Recordables, these alternating binary states create the data-bearing signal.
In order to achieve these effects in the recording layer, the CD-ReWritable recorder use three different laser powers:
- The highest laser power, which is called "Write Power," creates a non-crystaline (absorbtive) state on the recording layer.
- The middle power, also known as "Erase Power," melts the recording layer and converts it to a reflective crystaline state.
- The lowest power, which is "Read Power", does not alter the state of the recording layer, so it can be used for reading the data.
PD (phase change) uses similar technology to record data on PD discs, but the data layer compound mixture is different from that used in CD-Recordable. The dye compounds in CD-Recordable and CD-ReWritable provide better modulation of the laser wavelength than the PD data layer, vital for the CD family. More detail is available in Orange book part III.