Further physics - The working principle of CD-ROM
Xia Yun-jie (Translation by Kwok Yee-shan)    

Fig.1  CD-ROMs
Along with the development of information technology, many families now have their own computers at home. New personal computers are usually equipped with CD-ROM drives for multimedia uses. You must have used CD-ROMs. But do you know the working principle of CD-ROM? What are its advantages over other storage devices?

The full name of CD-ROM is Compact Disc - Read Only Memory. It is a read only information storage device. Roughly speaking, a CD-ROM consists of three layers. It has a transparent substrate as its base. On the base there is a reflective metal layer which carries information. On top of it, there is a protective coating. We know that information like sound, graphics and computer software can be digitized, that is, the data can be represented by binary numbers, which are made up of a series of '1' and '0' digits. This information is recorded on the metal layer. If we magnify the surface of the metal layer, we can see 'pits' and 'lands' in circular tracks.

We use laser beam to read the data. When the beam travels from the direction of the base and reaches the pits, the pits on the metal layer reflects the beam like a mirror. The depth of the pits is one-forth of the wavelength of the laser. The laser rays reflected by the pits and the lands have a path difference of half the wavelength (Fig. 2). It leads to destructive interference, that is, the two rays cancel each other and no reflected ray is resulted, and this produces a '1' digit. The path difference is zero for two rays both reflected by the pits, or both by the lands. No destructive interference occurs and we say it is a '0' digit. Therefore we have digit '1' at the edge of pits and digit '0' elsewhere. This is the basic mechanism of information storage and retrieval using CD-ROMs.

Destructive interference at the edge of a pit
Fig. 2  Destructive interference occurs when laser is reflected by the edge of a pit, giving a '1' signal.
As laser beam can be focused very sharply, we can make very narrow tracks, typically in the order of wavelength of the laser, so the storage size of a CD-ROM is much larger than that of a floppy disk. We can store a lot of data like a whole movie or many softwares in a single CD-ROM, thus it speeds up the development of multimedia computers. No mechanical contact is required to read the data from a CD-ROM and it has a protective layer on it. Therefore a CD-ROM is very durable, which is one of its greatest advantages.

At present, we use semiconductor lasers, whose wavelengths are in infrared region, for reading CD-ROMs. If we use violet or even ultraviolet lasers, the tracks on CD-ROMs can be ten-folds narrower. Correspondingly, the capacity can be increased a lot. However, these products are not fully developed yet.