ReWritable CD (CD-RW)

CD-RW disks look almost like a CD-ROM and are distinguishable from CD-R discs by their metallic gray color. They act like CD-ROM during reading, but also allow data recording for thousands of times.

Many materials can exist in more than one phase (water is a good example), and different phases may have different mechanical and physical, in particularly - optical properties. Often these materials could be switched from one phase to another by supplying the heat. This phenomena is used in CD-RW disks.

The typical phase-change alloy for optical recording has a stable, crystalline phase and a metastable amorphous phase with different optical properties. Recording on a CD-RW is accomplished by local melting the recording material, which is then being cooled quickly enough to quench in its amorphous phase. The cooling rate is apparently a strong function of thermal properties of the layer and surrounding layers, in particularly, their thermal diffusivity k = c/pa, where c is the specific heat, p is the material density, and a is material thermal conductivity. In general, the time t required for heat to diffuse a distance l in a material could be expressed as following:

    t ~ l2 / k.

To return amorphous material to its initial crystalline state, a process of annealing is used, when material is heated slightly below its melting temperature. Fast data erasure could be achieved if the annealing rate is very high and the temperature is slightly below its melting temperature.

There are the following most important basic requirements for an effective erasable phase-change material:

Different refractive index for crystalline and amorphous phases (for optical contrast)
Low melting point (low laser power)
Moderate thermal conductivity (fast cooling and quenching)
Rapid annealing below melting temperature (single-pass erasure)

The structure of the disk is somewhat similar to that of a CD-R, and have the similar polycarbonate substrate layer, protective layer, and reflective metal layer, plus two dielectric layers and a layer of phase-changing metal alloy. The dielectric layers prevent overheating of the phase-changing layer during data recording process. The data marks - the pits are formed inside the light-adsorbing phase-changing film and have different optical properties and different light reflectance. A typical structure of the CD-RW disk and the data reading process are shown in the next picture.

To simplify the head positioning mechanism, like in other CD storage systems, a special pre-groove is usually created on a blank CD-RW. The laser beam of the servo mechanism can follow this groove during both data reading and writing.

The CD-RW drive is different from the regular CD-ROM drive since its laser can operate on the different power levels. The highest level causes phase transitions in the recording material and is used for data recording. The medium level is used for annealing or erasure. And the lowest level of laser power is used for data reading - scanning the pits and lands without damaging the disk surface. Today's CD-RW use a so-called Direct Over-Write (DOW) method when the new data are just written on top of the old data. A standard CD-RW disk is more expensive that CD-R (still less than $10) and typically accommodates about 650 MB of data (or 74 minutes of audio recording).

The design of CD-RW itself makes them a perfect-writable storage, which is inexpensive and mobile. On the other hand, the distant future of CD-RW technology is unclear since new technology - DVD-RAM - is gaining momentum. With its mach higher storage capacity and data rates, DVD-RAM could become the "CD-RW/R killers" in the near future, when the price of these relatively new systems will go down...

For more details on data encoding and optical data reading process go to CR-ROM.

Read the White Papers .