Reading Process Of Optical Disc - LG GCE-8240B Service Manual

3) CD-RW Disc
• It is composed of polycarbonate layer, alloy(silver, arsenic) layer, aluminum reflectivity layer, protective layer.
• An crystalized alloy layer is transformed into noncrystalized by the laser heat. Therefore, writing and reading
is enabled by the difference of reflectivity.
• It is possible to overwrite about 1000 times.
• Laser Wavelength : 780 nm, Laser Power (Read) : 1.0mW
• Recording Power : Erase (4~18mW), Write (15~40mW)
• When disc rewriting, new data is overwritten previously recorded data.
• Polycarbonate layer has a Pre-Groove which make a track.

4. Reading process of Optical Disc

Focusing
Lens
Laser Spot
at Constant
Read Intensity
Reflected
Light
Signal
Laser Beam
Previously Recorded Marks
I
I
3
top
Groove
Lens
θ
Numerical aperture: NA=nsin θ ,
n: Refractive index
Focus depth : H = λ /NA
laser spot diameter : D = λ /NA
Groove Land
I I
11 G
Laser Spot
Position
(Time)
Label Printing
Protective Layer
Dielectric Layer(TL)
Reflective Layer
Dielectric Layer(UL)
Substrate
(Polycarbonate)
H
D
Beam
Spot
2
Mirror
I I
L 0
17