27.8

*Compact Discs, Digital Video Discs, and the Use of interference

The compact disc (CD) and the digital video disc (DVD) have revolutionized how text, graphics, music, and movies are stored for use in computers, stereo sound systems, and televisions. The operation of these discs uses interference effects in some interesting ways. Each contains a spiral track that holds the information, which is detected using a laser beam that reflects from the bottom of the disc, as Figure 27-37 illustrates. The information is encoded in the form of raised areas on the bottom of the disc. Under a microscope these raised areas appear as “pits” when viewed from the top or labeled side of the disc. They are separated by flat areas called “land.” The pits and land are covered with a transparent plastic coating, which has been omitted from the drawing for simplicity.

Figure 27-37 The bottom surface of a compact disc (CD) or digital video disc (DVD) carries information in the form of raised areas (“pits”) and flat areas (“land”) along a spiral track. A CD or DVD is played by using a laser beam that strikes the bottom surface and reflects from it.

The physics of retrieving information from compact discs and digital video discs. As the disc rotates, the laser beam reflects off the bottom and into a detector. The reflected light intensity fluctuates as the pits and land areas pass by, and the fluctuations convey the information as a series of binary numbers (zeros and ones). To make the fluctuations easier to detect, the pit thickness t (see Figure 27-37) is chosen with destructive interference in mind. As the laser beam overlaps the edges of a pit, part of the beam is reflected from the raised pit surface and part from the land. The part that reflects from the land travels an additional distance of 2t. The thickness of the pits is chosen so that 2t is one-half of a wavelength of the laser beam in the plastic coating. With this choice, destructive interference occurs when the two parts of the reflected beam combine. As a result, there is markedly less reflected intensity when the laser beam passes over a pit edge than when it passes over other places on the surface. Thus, the fluctuations in reflected light that occur while the disc rotates are large enough to detect because of the effects of destructive interference. Example 10 determines the theoretical thickness of the pits on a compact disc. In reality, a value slightly less than that obtained in the example is used for technical reasons that are not pertinent here.

| Example 10 | | | Pit Thickness on a Compact Disc

The laser in a CD player has a wavelength of 780 nm in a vacuum. The plastic coating over the pits has an index of refraction of . Find the thickness of the pits on a CD. Reasoning As we have discussed, the thickness t is chosen so that in order to achieve destructive interference. Equation 27.3 gives the wavelength in the plastic coating as .
Solution The thickness of the pits is

The pit thickness calculated in Example 10 applies only to a CD. The pit thickness (and other dimensions as well) are smaller on a DVD because the lasers used for DVDs have smaller wavelengths (635 nm, for example). The fact that the pit dimensions are smaller is one of the reasons that a DVD has more information storage capacity than a CD, from 7 to 26 times more, depending on the type of DVD.
The physics of the three-beam tracking method for compact discs. As a CD or DVD rotates, the laser beam must accurately follow the pits and land along the spiral track. One type of CD player utilizes a three-beam tracking method to ensure that the laser beam follows the spiral properly. Figure 27-38 shows that a diffraction grating is the key element in this method. Before the laser beam strikes the CD, the beam passes through a grating that produces a central maximum and two first-order maxima, one on either side. As the picture indicates, the central maximum beam falls directly on the spiral track. This beam reflects into a detector, and the reflected light intensity fluctuates as the pits and land areas pass by, the fluctuations conveying the information. The two first-order maxima beams are called tracking beams. They hit the CD between the arms of the spiral and also reflect into detectors of their own. Under perfect conditions, the intensities of the two reflected tracking beams do not fluctuate, since they originate from the smooth surface between the arms of the spiral where there are no pits. As a result, each tracking-beam detector puts out the same constant electrical signal. However, if the tracking drifts to either side, the reflected intensity of each tracking beam changes because of the pits. In response, the tracking-beam detectors produce different electrical signals. The difference between the signals is used in a “feedback” circuit to correct for the drift and put the three beams back into their proper positions.

Figure 27-38 A three-beam tracking method is sometimes used in CD players to ensure that the laser follows the spiral track correctly. The three beams are derived from a single laser beam by using a diffraction grating.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.