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TOPIC 2CD-ROM DRIVES
TOPIC 3CD-ROM APPLICATIONS

Fire Controlman Volume 03-Digital Data Systems
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The laser beam is directed to the disc using several different methods, depending on the system preferred by the manufacturer. One type of system deflects the laser beam off a semitransparent mirror, through the lenses, and onto the disc. When the laser beam strikes a   land,   the   reflected   light   passes   through   the semitransparent mirror into the photodetector. Lenses The lenses in a CD-ROM drive are used to focus the laser beam onto the compact disc. When the laser is turned on, the beam tends to diverge as it travels away from the source. The beam first passes through a collimating lens  that reduces the divergence. The beam then passes through the  objective lens,  where it is focused onto the surface of the disc. The final component used to focus the beam on a compact disc is the disc itself. The diameter of the laser beam  as  it  exits  the  objective  lens  is  approximately 1 mm. The refractive properties of the clear plastic material  of  the  disc  further  focus  and  reduce  the diameter of the laser beam so that it is 1.0 µm when it reaches the information surface of the disc. This fine focus of the laser is one of the factors of the high durability and reliability of the compact disc. Tracking and Focusing Once the optical head is positioned over the area to be read, a system is needed to properly hold the optical head on the track and maintain proper focus. Errors in tracking and focus can occur because the compact disc is  not  perfectly  flat.  Several  methods  are  used  to determine tracking and focus. In the optical head system described earlier in this chapter, the reflected laser beam passes through the semitransparent mirror. The reflected laser beam is next split into two beams by a prism. These two beams are directed to the photodetector. The photodetector consists of four photodiodes. Figure 11-4 shows how the reflected light strikes the photodiodes if the tracking Figure  11-4.—Photodiodes  detecting  tracking  and  focus  of  the  laser  beam. 11-5






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