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D P - 1 1 0 0 B II
(1)
C o l l i m a t o r Lens
Diffused light b e a m s are c h a n g e d t o parallel light b e a m s .
Light b e a m s d i s t r i b u t e d in oval pattern is c h a n g e d t o a p ­
p r o x . c i r c u l a r d i s t r i b u t i o n s .
(2)
Polarized Prism
Light polarized in parallel t o a surface is r e f l e c t e d , a n d
light polarized in vertical is passed t h r o u g h t h e p r i s m .
(3)
114 W a v e l e n g t h Plate
(4)
Objective Lens
NA
WD
(5)
4 - d i v i d e d p h o t o d i o d e s
C o n v e r t s light into an electrical signal
2 8
II. FUNDAMENTALS
0 . 4 7
NA = Sin0
1.5/xm
Disc
Pit surface
5 - 2
F O C U S E R R O R A N D T R A C K I N G E R R O R
T o read tiny pits ( w i d t h : 0 . 5 / i m , length 0 . 9 # 3 . 2 / * m )
disc by m e a n s of a laser s p o t , t h e l o c a t i o n m u s t be precisely
c o n t r o l l e d t o f o l l o w surface a n d axial d e v i a t i o n s of t h e disc
c a u s e d by r o t a t i n g t h e disc f o r p l a y b a c k . For this p u r p o s e ,
(1)
Focus error
a n d
(2)
T r a c k i n g error
m u s t be d e t e c t e d . T h e d e t e c t i o n m u t h o d s for b o t h errors w i l l
be given b e l o w .
5-2-1
F o c u s Error Detection
W h e n a light b e a m is passed f r o m a high refraction material t o
a l o w r e f r a c t i o n m a t e r i a l , a r e l a i o n , as s h o w n in Fig. 5 . 2 , is
existed b e t w e e n t h e i n c i d e n t angle a n d refleciton ratio at t h e
b o u n d a r y of t h e materials. A s c a n be seen f r o m t h e g r a p h s ,
t h e reflection ratio will c h a n g e rapidly as t h e i n c i d e n t light
angle c h a n g e s in t h e area w h e r e t h e i n c i d e n t angle is slightly
less t h a n t h e critical a n g l e .
Strength
of
Reflection
(Rp-Rs)
Incident light angle 0
Fig. 5 . 2
Refleciton c h a n g e s rapidly at angles d o s e
to critical angle.
o n a
Reflection Surface
S P o l a
" " 1 L ,
g h t
Critical Angle
( 4 1 . 8 ° )

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