CHAPTER 13

692 Chapter 13 Optical Storage
As the disc rotates over the laser and receptor, the laser shines continuously while the receptor sees
what is essentially a pattern of flashing light as the laser passes over pits and lands. Each time the
laser passes over the edge of a pit, the light seen by the receptor changes in state from being reflected
to not reflected or vice versa. Each change in state of reflection caused by crossing the edge of a pit is
translated into a 1 bit digitally. Microprocessors in the drive translate the light/dark and dark/light
(pit edge) transitions into 1 bits, translate areas with no transitions into 0 bits, and then translate the
bit patterns into actual data or sound.
The individual pits on a CD are 0.125 microns deep and 0.6 microns wide (1 micron equals onemillionth
of a meter). Both the pits and lands vary in length from about 0.9 microns at their shortest
to about 3.3 microns at their longest (see Figure 13.2).
Figure 13.2 Pit and land geometry on a CD.
The pit height above the land is especially critical as it relates to the wavelength of the laser light used
when reading the disc. The pit (bump) height is exactly 1/4 of the wavelength of the laser light used
to read the disc. Therefore, the light striking a land travels 1/4 + 1/4 = 1/2 of a wavelength further
than light striking the top of a pit. This means the light reflected from a pit is 1/2 wavelength out of
phase with the rest of the light being reflected from the disc. The out-of-phase waves cancel each
other out, dramatically reducing the light that is reflected back and making the pit appear dark even
though it is coated with the same reflective aluminum as the lands.
The read laser in a CD drive is a 780nm (nanometer) wavelength laser of about 1 milliwatt in power.
The polycarbonate plastic used in the disc has a refractive index of 1.55, so light travels through the
plastic 1.55 times more slowly than through the air around it. Because the frequency of the light passing
through the plastic remains the same, this has the effect of shortening the wavelength inside the
plastic by the same factor. Therefore, the 780nm light waves are now compressed to 780/1.55 =
500nm. One quarter of 500nm is 125nm, which is 0.125 microns—the specified height of the pit.
Drive Mechanical Operation
CD-ROM drives operate in the following manner (see Figure 13.3):
1. The laser diode emits a low-energy infrared beam toward a reflecting mirror.
2. The servo motor, on command from the microprocessor, positions the beam onto the correct
track on the CD-ROM by moving the reflecting mirror.
3. When the beam hits the disc, its refracted light is gathered and focused through the first lens
beneath the platter, bounced off the mirror, and sent toward the beam splitter.
4. The beam splitter directs the returning laser light toward another focusing lens.
5. The last lens directs the light beam to a photo detector that converts the light into electric
impulses.
6. These incoming impulses are decoded by the microprocessor and sent along to the host computer
as data.