Recordable CD (CD-R)
A CD-R disk looks almost like a CD-ROM, but is gold on the label side and dark green on the data (recordable) side, unlike the silver coloring of the regular CD. The disks act like CD-ROMs during reading, but also allow data recording (once!) if used in special CD-R or CD-RW drives.
Initially, a CD-R is blank. The structure of the disk is somewhat different from that of a CD-ROM, but they both have a similar substrate layer, protective layer, and reflective metal layer. The difference is really in the organic dye layer. The CD-R drive is different from the regular CD-ROM drive since its laser can operate on different power levels. The highest level burns the pits on the disk surface, the lowest reads the pits and lands without damaging the disk surface. The recording spiral starts, like in CD-ROM, from the inner radius of the disk and continues towards the outer edge of the disk till all required data are recorded or the data limit is reached.
To simplify the head positioning mechanism, a special pre-groove is usually created on a blank CD-R. The laser beam can follow this groove during both data reading and writing. Standard CD-R disks are relatively inexpensive (less than $1-$2) and accommodates about 650 MB of data (or 74 minutes of audio recording). A typical CD-R structure and the data reading process are shown in the next picture.
Data marks - the pits - are formed inside the light-absorbing organic film and look like holes. The thermal conductivity of the organic material is very low (much lower than that of metals); very high temperatures can be achieved during less than a microsecond of heating by the drive's laser. The energy absorbed by that organic layer dissipates in the form of heat, causing local material ablation and the pit formation.
Even if the output of the laser diode of the drive is relatively small focusing of the laser beam at a very small area result in high power density, and the surface temperature can be increased by hundreds or even thousands degrees, depending on the thermal properties of the materials.
The design of CD-R itself makes them write-once media, since it is impossible to erase the pits from the disk surface using the same laser. This is why the CD-R is a relatively inexpensive option for permanent data archiving and backup.
Note: If you have a CD-R or CD-RW drive that records at 8X or higher speeds, pay attention to the disks you are buying: you need to buy specially rated discs with fast-acting dyes.
For more details on data encoding and the optical data reading process go to CD-ROM.
Read the White Papers.
