Page 463 - Introduction to Information Optics

448 8. Information Storage with Optics

recorded data cannot be changed after the disk is manufactured. The process
of optical pickup is as follows: When there is no pit, the light will be fully
reflected to the detector. When there is a pit, the focused beam will cover both
pit and the surrounding land. Both pit and land are coated with high-
reflectivity material such as aluminum (Al), so light is reflected from both the
pit and the land. The depth of the pit is made such that the phase difference
between the reflected light from a pit and the land is n. Consequently, there is
destructive interference at the detector, and less light is detected. The pit: depth
is typically 0.13 /mi.

8.4.2.2. WORM Optical Disk

A WORM (write once, read many times) disk consists of either a
polycarbonate or hardened glass substrate and a recording layer made of a
highly reflective substance (dye polymer or tellurium alloy). As with other
optical disks, the recording layer is covered by clear plastic to protect the
recording medium. WORM disk systems use a laser beam to record data
sequentially. A write beam burns a hole (pit) in the recording medium to
produce a change in its reflectivity. In contrast to the previously discussed
read-only optical disk or CD, the WORM optical disk modulates directly the
reflected intensity of the readout beam.

8.4.2.3. Magneto-optic Disk

An erasable and rewritable optical disk is best represented by a magneto-
optic (MO) disk. The MO disk makes use of MO recording material that at
room temperature is resistant to changes in magnetization. The magnetization
of the material can be changed by placing it in a magnetic field and heating it
to its Curie point, about 180°. To erase and to write new data onto a MO disk,
the heat of the write laser beam brings the recording material to the Curie
point. A bias magnet then reverses the magnetization of the heated area that
represents a bit. A low-power linearly polarized laser beam can be used to read
the data on the MO disk. According to the Kerr magneto-optic effect, the
polarization of the reflected readout beam will be rotated to the left or right,
depending on whether the magnetization of the recording material is upward
or downward. At the present time the typical rotation is less than 1 .

8.4.3. MULTILAYER OPTICAL DISK

While an optical disk is a 2-D optical storage device, a multilayer optical
disk is in the class of 3-D optical storage. A new structure for an optical disk