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CD
– LASER RW Systems
CD stands for Compact Disk and denotes an optical storage medium
used to store video, audio and data. CD-ROM disks can be written once and only
read thereafter. CD ROM drive uses LASER technology to read data from it. A semiconductor LASER generates a high
intensity light wave of stable wavelength = 780 nm. A lens system is used to direct
the LASER towards the disk over approximately 1 micron diameter spot as shown
in the figure.
The intensity of the reflected light from metallic reflection
layer, received by photo sensors gives the information of binary data stored in
CD. There are two different surfaces called land and bit from which reflection
occurs. The pit is approximately 0.12 micron deep compared to land and
reflected intensities are about 25 % and more than 70 % respectively (Figure
b).
Every time laser beam travels from land to pit or pit to land
there is a change in intensity of reflected light. This change is read as binary digit 1 and a
constant intensity reflected light is interpreted as 0. The pit width is such
that there is at least 2 and at most 10 zeroes between every 1. This is achieved by converting every 8-bit
byte into a 14-bit value, a process called Eight to Fourteen Modulation (EFM). Such arrangements make it easy for the read
laser to detect bits and synchronization. Data corresponding to a small portion
of the track is shown above label in Figure a.
CD-RW
CD-RW or CD-Read Write gives user facility to write and erase
data many times. CD-RW uses an active layer of Ag-In-Sb-Te alloy that has a
poly crystalline structure making it reflective (reflectivity 25 %). Writing data
on disk uses highest power of laser that heats up selected spots to 500°C –
700°C. At this temperature the chemical
structure liquefies losing its polycrystalline structure and on cooling solidifies
to an amorphous state that has reduced reflectivity of 15 %. The reading
process is like CD-ROM and CD-R that notes the differences in reflectivity of
the storage surface.
To reverse the phase or erase data, the laser operates at a
lower power setting and heats the active material to nearly 200°C. This reverses
the material from its amorphous to its polycrystalline state and then becomes
reflective again. This is illustrated in
the following figure.
