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Since the refle~ted light coming into the PBS is the horizontally
polarized beam, the direction of the light is bent 90°. The projected
beam and reflected beam are separated each other by the PBS. The
PBS serves to prevent a back-talk that is resulted _from the feedback
of the reflected beam back into the laser tube.
OBJ. LENS DISC
P.B.S.
.
.
·--r
~
CYLINDRICAL
-·•
--\
LENS
S P ~ T ··c;:~N~RI~~"";
I
~";-/
LENS
,
~
PHOTO DIODE
FACEPLATE
LENS
IN FOCUS
PHOTO DIODE
I
I
~ ~ ~ = = {
\J:
,
--:;s::t
~✓--
O=
Fig.
2-12.
Focus detection using cylindrical lens
The reflected beam that is separated by the PBS is passed
through a cylindrical lens and landed on the photo diodes. The
characteristics of the c.ylindrical lens are such that the shape of the
beam landing on the diodes is circular only when the beam is perfect-
ly focused onto the videodisc by the objective lens. If the beam
becomes unfocused because of the up and down movements of the
disc, the shape of the beam ·becomes elliptical when landed on the
diodes.
0
0
0
0
0
0
0
0
0
0
0
TRACKING BEAM
MAIN BEAM
0-PIT
0
0
Fig. 2-13. 3-spot astigma method
15
The reflected light landed on the diodes actually contains the
following three information.
1.
Luminance variation of the refle\;ted light resulted, from pits
-
Video and audio multiplex FM signal
2.
Deformation of the reflected spot shape focused on the photo
diodes
-
Focus error signal
3.
Luminance difference between the two tracking beams among
three beams that are landed on the photo diodes
-
Tracking error signal
The photo diod.es has a multiple elements structure as shown in
Fig. 2-14 to detect the FM signal and the servo control signal con-
tained in the reflected light.
;.
PBRF
SIG.
(A+BHC+OI
FOCUS
ERROR
>-' - ·~• __
TRACKING
L - - - - - - - - - - < l > - - ~ ~ : + ~
ERROR
Fig. 2-14. Photo diode
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