The laser beam is directed to the disc using severaldifferent methods, depending on the system preferredby the manufacturer. One type of system deflects thelaser beam off a semitransparent mirror, through thelenses, and onto the disc. When the laser beam strikesa land, the reflected light passes through thesemitransparent mirror into the photodetector.LensesThe lenses in a CD-ROM drive are used to focusthe laser beam onto the compact disc. When the laseris turned on, the beam tends to diverge as it travels awayfrom the source. The beam first passes through acollimating lens that reduces the divergence. Thebeam then passes through the objective lens, where itis focused onto the surface of the disc.The final component used to focus the beam on acompact disc is the disc itself. The diameter of the laserbeam as it exits the objective lens is approximately1 mm. The refractive properties of the clear plasticmaterial of the disc further focus and reduce thediameter of the laser beam so that it is 1.0 µm when itreaches the information surface of the disc. This finefocus of the laser is one of the factors of the highdurability and reliability of the compact disc.Tracking and FocusingOnce the optical head is positioned over the area tobe read, a system is needed to properly hold the opticalhead on the track and maintain proper focus. Errors intracking and focus can occur because the compact discis not perfectly flat. Several methods are used todetermine tracking and focus.In the optical head system described earlier in thischapter, the reflected laser beam passes through thesemitransparent mirror. The reflected laser beam isnext split into two beams by a prism. These two beamsare directed to the photodetector. The photodetectorconsists of four photodiodes. Figure 11-4 shows howthe reflected light strikes the photodiodes if the trackingFigure 11-4.—Photodiodes detecting tracking and focus of the laser beam.11-5
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