
231Advanced Modulation Format Optical Transmitters
in
t
he
s
ignal
c
onstellation
d
iagram.
F
or
c
ontinuous
P
SK
s
uch
a
s
t
he
M
SK,
t
he
p
hase
e
volves
c
ontinuously
o
ver
a q
uarter
o
f
t
he
s
ection
a
nd
r
esults
i
n
a p
hase
c
hange
o
f
π/
2
b
etween
o
ne
p
hase
s
tate
t
o
t
he
o
ther.
T
his
i
s
i
ndicated
by t
he
i
nner
b
old
c
ircle
a
s
s
hown
i
n
Fi
gure
6
.11b.
6.4.2
O
ptical DPSK Transmitter
Figure
6
.12
s
hows
t
he
s
tructure
o
f
a
4
0
G
b/s
D
PSK
t
ransmitter
i
n
w
hich
t
wo
ex
ternal
L
iNbO
3
MZIM are used. Operational principles of an MZIM were presented earlier. The
M
ZIMs
s
hown
i
n
F
igure
6
.12
c
an
b
e
o
f
e
ither
s
ingle-
o
r
d
ual-drive
t
ype.
T
he
o
ptical
D
PSK
t
ransmitter
a
lso
c
onsists
o
f
a
n