
447
11
Digital Optical Modulation Transmission Systems
Since the invention of optical bers, the modulation of light waves for transmission in
o
ptical
c
ommunication
s
ystems
h
as
b
een
b
ased
m
ainly
o
n
t
he
o
n–off
s
witching
o
f
t
he
i
ntensity
o
f
t
he
w
aves,
r
ather
t
han
m
anipulating
t
he
p
hase
o
r
f
requency
o
f
t
he
eld
o
f
t
he
w
aves.
I
n
t
he
1
980s,
i
nterest
i
n
ex
tending
t
he
r
epeater-less
d
istance
f
rom
4
0km
t
o
l
onger
t
han
6
0km
u
sing
c
oherent
r
eception
a
nd
t
ransmitting
t
echniques
a
ttracted
t
he
use of modulation in the carrier wave. However, sufciently narrow line width laser and
i
ntegrated
o
ptical
c
omponents
t
hat
w
ould
f
acilitate
t
he
d
evelopment
o
f
s
uch