
271
7
Direct Detection Optical Receivers
This chapter describes the fundamental understanding of the detection of optical signals
u
sing
d
irect
t
echniques.
I
n
t
his
d
etection
p
rocess,
t
he
l
ightwave
p
ower
i
s
a
bsorbed
b
y
t
he
s
emiconductor
a
ctive
a
rea
i
n
w
hich
t
he
a
bsorption
w
ould
r
esult
i
n
t
he
g
enerationof
p
ositive
a
nd
n
egative
c
arriers
w
hich
a
re
t
hen
c
ollected
b
y
e
ither
t
he
c
athode
o
r
a
node
o
f
t
he
p
hotodetector
(
PD)
j
unction
u
nder
r
everse
b
ias.
O
n
t
he
o
ther
h
and,
t
he
d
etection
of the optical input eld can be mixed with a local oscillator so that the total eld can
b
e
d
etected
i
n
t
he
P
D
u
nder
t
he
s
quare-law
r
elation.
T
his
s
quare
l
aw
r
esults
i
n