Page 462 - Introduction to Information Optics

8.4. Bit-Pattern Optical Storage 447

systems such as computers and other communication equipment. However,
this 1-D information is formatted in 2-D or 3-D in an optical storage system
in order to achieve the maximum storage density allowed. A straightforward
method for storing the optical information represented by a bit pattern is to
record the bit pattern directly.

8.4.1. OPTICAL TAPE

A siring of bits can be recorded on a tape. The drawback of a tape system
is that the data cannot be quickly accessed at random. The tape must be
wound from the beginning before the desired data can be read. Optical tape is
traditionally applied to movie film to store the movie's sound track.

8.4.2. OPTICAL DISK

The most successful optical storage medium is the optical disk. Figure 8.1
shows the basic structure of an optical disk. The recorded signal is encoded in
the length of the pit and the spacing of pits along the track. The distance
between two adjacent tracks (track pitch) is 1.6 /mi. The width of a pit is equal
to a recording spot size of 0.5 to 0.7 /mi. The light source used in an optical
disk system is usually a GaAlAs semiconductor laser diode emitting at a
wavelength of 0.78 to 0.83 /mi. The spot size of the readout beam is determined
by the numerical aperture (NA) of the objective lens. Typically, //NA = 1.55
is chosen, so the effective diameter of the readout spot is approximately 1 /mi,
The spot size is larger than the width of a pit, but a single readout spot does
not cover two tracks.

8.4.2.1. Read-Only Optical Disk

For a read-only optical disk, such as a compact disk for music recording
(CD) or a read-only memory compact disk for computers (CD-ROM), the

light spot

Fig. 8.1. Basic structure of an optical disk.