50
Three Laser Beams from One Laser
Although there is one laser, there are three spots of light on the disc’s
information layer. The three spots are made by diffraction grading inside
the laser assembly.
1ST
Order
Main
Laser
Beam
To
Objective
Lens
Defraction
Gradin
g
DVD
Laser
Beam
1ST
Order
2ND
Order
2ND
Order
A diffraction grading is made of a glass sheet with lines etched on it form-
ing channels. The channels are represented by the spaces between the
dotted lines in the diagram. As the main laser beam passes through a
channel, secondary light beams are produced. The first order beams are
the strongest and these are the two additional (side) spots that pass through
the objective lens onto the disc. These side spots are used for CD track-
ing.
1.1 cm
Single
Focus
Point
Info Layer
CD
DVD
Laser
Light
CD FOCUS CORRECTED
Objective
Lens