Kenwood DP-1100 B Service Manual page 23

D P - 1 1 0 0 B II
4 SEMICONDUCTOR LASER (LASER DIODE)
4-1 P R I N C I P L E O F L E D L U M I N E S C E N C E
A LED is f o r m e d w i t h a P-N j u n c t i o n c o m p o s e d of an n-type
s e m i c o n d u c t o r w h i c h a l l o w s electric c o n d u c t i o n w i t h elec­
t r o n s a n d p-type s e m i c o n d u c t o r in w h i c h holes serves electric
c o n d u c t i o n . A p p l y i n g a v o l t a g e in t h e f o r w a r d d i r e c t i o n , elec­
t r o n s in t h e n-type s e m i c o n d u c t o r are i n j e c t e d into t h e p-type
s e m i c o n d u c t o r , a n d holes in t h e p- t y p e s e m i c o n d u c t o r are
i n j e c t e d into t h e n-type s e m i c o n d u c t o r . Red l u m i n e s c e n c e is
e m i t t e d w h e n e l e c t r o n s i n j e c t e d into t h e p-type s e m i c o n d u c ­
t o r c o m b i n e w i t h holes.
Green l u m i n e s c e n c e is e m i t t e d w h e n holes injected into t h e
n-type s e m i c o n d u c t o r c o m b i n e w i t h e l e c t r o n s .
Electrons
N-Type Region
PN Junciton
2 6
II. FUNDAMENTALS
Conduction band
Light
Valence Band
Holes
P-Type Region
Fig. 4.1
4 - 2 L A S E R D I O D E
A laser d i o d e , as m e n t i o n e d 4 - 1 , is t h e s a m e as an LED in
t e r m s of r e c o m b i n a t i o n l u m i n e s c e n c e of carriers, b u t different
in t h a t t h e light e m i t t e d is a c o h e r e n t laser light, t h e phase of
w h i c h is u n i f o r m (single w a v e l e n g t h ) .
4 - 3 P R O P E R T I E S R E Q U I R E D F O R A L A S E R D I O D E
(1) Oscillation W a v e l e n g t h
A c c o r d i n g t o a C D ' s p r o p o s a l , t h e r e s h o u l d be t h e f o l l o w ­
ing relaiton b e t w e e n a w a v e l e n g t h of a laser d i o d e a n d
t h e n u m b e r of a p e r t u r e of lens N A :
X / N A = 1 . 7 5 j i m
A s l o n g as t o d a y ' s G a A I A s material is u s e d , it is difficult
t o m a k e a laser d i o d e h a v i n g a w a v e l e n g t h shorter t h a n
a p p o r x . 7 6 0 n m , b u t a laser d i o d e w i t h higher
7 8 0 n m c a n be m a d e in mass p r o d u c t i o n .
T h e r e f o r e , N A = X / 1 . 7 5 /*m = 0 . 4 4 6
A s t h e result, t h e o b j e c t i v e lens in t h e p i c k u p used in DP-
1 1 0 0 B / I I has been d e s i g n e d for a p p r o x .
N A = 0 . 4 7 ± 0 . 0 1
(2) O p e r a t i n g C u r r e n t
A n laser d i o d e has a t h r e s h o l d c u r r e n t I, w i t h
oscillation starts, a n d w i t h a c u r r e n t larger t h a n this
t h r e s h o l d level, a light p o w e r P increases linearly w i t h i n ­
crease of a c u r r e n t I. F u r t h e r m o r e , if k e e p i n g drive at a
fixed c u r r e n t , t h e light o u t p o u t is greatly varied d u e t o
t e m p e r a t u r e i n c r e a s e . T h e r e f o r e , c o n t r o l is a l w a y s d o n e
so t h a t t h e light o u t p u t is kept c o n s t a n t .
I, (30mA)
Current
Fig. 4 . 2 Oscillating C h a r a c t e r i s t i c of a L a s e r Diode
t h a n
w h i c h
T , < T
2