
149
5
Optical Direct and External Modulation
This chapter deals with the modulation of light wave sources by either direct or external
m
odulation.
F
or
d
irect
m
odulation,
t
he
g
eneration
o
f
t
he
l
ight
w
ave
w
ithin
t
he
c
avity
o
f
t
he
l
aser
i
s
m
anipulated
b
y
i
njection
o
r
w
ithdrawing
o
f
e
lectrons
a
nd
h
ence
s
timulating
t
he
e
mission
p
rocess.
O
n
t
he
o
ther
h
and,
i
n
t
he
c
ase
o
f
ex
ternal
m
odulation
t
he
l
aser
i
s
t
urned
o
n
a
t
a
ll
t
imes
a
nd
t
he
g
enerated
o
ptical
c
ontinuous
w
aves
a
re
t
hen
m
odulated,
b
y
frequency, phase, or amplitude through an optical modulator. The properties of modulated
s
ignals
a
re
d
escribed.
T
he
u
ses
o
f
t
hese
t
ransmitters
i
n
o
ptical
c
ommun