CD-RW - Mechanism of Action

Mechanism of Action

Rewritable media can with suitable optical drive according to some manufacturers be re-written up to 100 000 times. The CD-RW technology is based on the phase change technology, so the degree of reflection reached is only 15–25%, compared to the 40–70% reflection from CD-R discs. The properties of the medium and the write and erase procedure is defined in the Orange Book Part III.

To keep rotational speed precise any track have a slight superimposed sinusoidal excursion of 0.3 µm at a frequency of 22.05 kHz. In addition an 1 kHz frequency modulation is applied to provide the recorder with an absolute time reference. The grooves have a width of 0.6 µm and pitch of 1.6 µm.

The media for CD-RW has basically the same layers as CD-R media. The reflective layer is, however, a silver-indium-antimony-tellurium (AgInSbTe) alloy, which has in its original state, a polycrystalline structure and reflective properties. When writing the laser beam uses its maximum power (8 - 14 mW) to heat the material to 500–700 °C. This causes liquefaction of the material. In this state, the alloy loses its polycrystalline structure, assumes an amorphous state and lose its reflectivity. The lost reflectivity serves the same function as bumps on a manufactured CDs and the opaque spots on a CD-R which will be read as a "0". The polycrystalline state of the disk forms the trenches, which is read as "1". The scanning signal when reading is created by strong or weak reflection of the laser beam, just like recordable CDs. Unlike amplification and extinction of the laser light by superimposing the reflected light with the emitted as with pressed CDs. To erase the disk, the write beam heats the amorphous regions with low power to about 200 °C. The alloy is not melted, but returns to the polycrystalline state and is thus again reflective.