Compact Disk Lasers

A pre-recorded compact disk is read by tracking a finely focused laser across the spiral pattern of lands and pits stamped into the disk by a master diskette. This tutorial explores how the laser beam is focused onto the surface of a spinning compact disk, and how variations between the height of pits and lands determine whether the light is scattered by the disk surface or reflected back into a detector.

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The tutorial initializes with the beam emitted from a red laser directed onto a single track on a compact disk by a prism/beamsplitter. The compact disk, although appearing to be stationary in the tutorial, should actually be envisioned as rotating. As the disk rotates, the beam encounters a series of pits and lands that determine whether the beam is reflected back through the lens system into the detector (from a land) or scattered (by a pit). As the sequence of pits and lands is read by the detector, a pattern of ones and zeros, representing the binary encoded data, emerges across the bottom of the window and travels from left to right. In order to reduce the tutorial speed, use the mouse cursor to translate the Applet Speed slider.

The compact disk is rotated by a drive system that regulates the rotation speed, so that data is read at the same speed, regardless of the track location being read by the laser (either on the inside or outer periphery of the disk). As a result, the rotation speed of the disk decreases as the laser detector slowly traverses from the center to the periphery of the disk.

Light from the laser beam must penetrate a thin protective polymer layer (not illustrated) on the disk before striking the reflective coating that contains pits and landings. As the disk rotates, light reflected from landings on the disk strikes the photo sensor producing a series of electrical pulses that are coordinated with a timing circuit to generate a stream of ones and zeros, which reproduce the binary code of information on the disc. At the bottom of the tutorial, a simulated bit reader continuously streams the binary code as it is retrieved from the disc.

Contributing Authors

Matthew Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.