
343Digital Coherent Optical Receivers
ber
l
oss α
=
0
.2
d
B/km,
t
he
a
mplier
g
ain
i
s
G
=
2
0
d
B,
t
he
n
oise
gure
(
NF)
i
s
6
dB,
t
he
o
ptical
w
avelength
λ
=
1
553nm,
a
nd
t
he
b
andwidth
o
f
o
ptical
lter
a
nd
e
lectrical
lter
a
re
s
et
t
o
Δ
λ
=
0
.1nm
a
nd
7
G
Hz,
r
espectively.
T
he
O
SNR
i
s
d
ened
a
s
t
he
r
atio
b
etween
t
he
s
ignal
po
wer
a
nd
t
he
n
oise
po
wer
i
n
t
wo
po
larization
s
tates
w
ithin
a
0
.1
n
m
b
andwidth.
A
s
s
hown
i
n
F
igure
8
.23a,
t
he
r
eceiver
s
ensitivity
o
f
t
he
M
L
p
rocessor
h
as
i
mproved
b
y
a
bout
0
.5
a
nd
3
d
B
o
ver
P
E
a
nd
d
ifferential
d
emodulation,
r
espectively,
a
t
t
he
B
ER
l
evel
o
f
1
0
−4
. Besides, it is noteworthy that differential demodulation has