Optical Disc Drives

The word optical should make you think about the eye and how we see the world in terms of reflected light. Optical media work in a similar way. Lasers write data to the disc and read data from it. Optical media include CD's and DVD's.

How it works

CD burners operate on the same essential technology that modern DVD burners do. A modern DVD burner employs two lasers at different times: one for DVDs, one for CDs. A laser sled (1), on sliding rails (2), is driven by a motor (3) that moves it radially relative to the disc hub (4). (Another, variable-speed spindle motor, located under the hub, spins the disc itself.) A diode inside the laser assembly emits a laser beam, which is focused through a lens (5). Scaling the laser's power to different levels allows the drive to read discs (using a low power setting) or write them (at high power).

As for the discs themselves, a commercial pressed CD-ROM disc starts as a molded platter of polycarbonate (6), a tough plastic. Its surface is laced with microscopic pits (7) that represent data, arranged in a tight spiral like an LP record's groove. (DVDs' tighter spiral partly explains their greater capacity.) Atop the polycarbonate is spun a microthin layer of reflective material (8), often an alloy containing aluminum, silver, or gold, topped by a lacquer or other protective coating (9) and a label surface (10).

A reading laser beam partly scatters when it strikes pits in the spiral. When it hits land (11) on the disc, as the flat spots between the pits are called, it bounces cleanly back into an optical pickup in the laser assembly. The drive electronics translate this pit-versus-land data stream into binary code and, in turn, into actionable bytes of data.

Ready to write a disc? Insert a writable or rewritable disc into the drive, and the drive's firmware detects the disc type, determines the media's parameters in a lookup table, and deploys an appropriately powered writing laser. It writes data from the hub outward in a spiral, but writable discs can't be physically "pitted" like pressed commercial ones. Instead, "R"-type write-once CDs and DVDs have an organic dye layer, backed by a reflective layer and protective/label surfaces. When the writing laser hits the dye, it "burns" nonreflective spots, which are later readable as light-scattering pits.

Rewritable discs (CD-RWs, DVD±RWs) work similarly, but they substitute a mutable "phase change" chemical layer for the dye. The chemical is clear in one state, opaque in another; a properly calibrated laser melts a pattern of pit-like nonreflective spots into the layer. A laser set to a different strength, however, can eradicate the pattern, allowing re-use of the disc.

This technology is also used for Blu-ray discs.