Hardware Reference
In-Depth Information
light waves are now compressed to 500nm (780/1.55). One quarter of 500nm is 125nm,
which is 0.125 microns—the specified height of the pit.
Note
DVD drives use two different lasers: a 780nm laser for CD media and a 650nm laser for
DVD media. Consequently, a DVD drive could suffer a failure of one laser, causing it to no
longer read (or write) one type of media while continuing to read (or write) the other type of
media.
Drive Mechanical Operation
An optical drive operates by using a laser to reflect light off the bottom of the disc. A
photodetectorthenreadsthereflected light.Theoveralloperationofanopticaldriveisas
follows (see
Figure 11.3
)
:
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 disc by moving the reflecting mirror.
3.
When the beam hits the disc, its refracted light is gathered and focused through the
firstlensbeneaththeplatter,bouncedoffthemirror,andsenttowardthebeamsplitter.
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.
Figure 11.3
Typical components inside an optical drive.
