
313
8
Digital Coherent Optical Receivers
Chapter 7 has presented the detection of optical signals by direct conversion of optical
s
ignal
po
wer
i
nto
e
lectronic
c
urrent
i
n
t
he
p
hotodiode
(
PD)
a
nd
t
hen
e
lectronic
p
ream-
plication.
T
his
c
hapter
d
escribes
t
he
f
undamentals
o
f
c
oherent
r
eceivers
i
ncluding
t
he
d
etection
o
f
o
ptical
s
ignals
b
y
m
ixing
o
ptical
elds
w
ith
t
hat
o
f
a
l
ocal
o
scillator
(
LO),
a
h
igher
a
mplitude
l
aser
o
f
t
he
s
ame
o
r
d
ifferent
f
requency
s
o
t
hat
t
heir
b
eating
p
roduct
would result in demodulated signals preserving the signals in both phase and amplitude
c
haracteristics
i
n
t
he
e
lectronic
d
omain.
W
hen
t
he
f
requencies
o
f
t
he
s
ignal
a