
281Direct Detection Optical Receivers
7.4.4.1 Shot
N
oises
Electrical
shot
noises
are
generated
by
the
random
generation
of
streams
of
electrons
(cur-
rent).
In
optical
detection,
shot
noises
are
generated
by
(1)
biasing
currents
in
electronic
devices
a
nd
(
2)
p
hoto
c
urrents
g
enerated
b
y
t
he
p
hotodiode
For
a b
ias
c
urrent
I, t
he
s
pectral
c
urrent
n
oise
d
ensity S
I
is given by
S
d i
df
qI
I
N I
=
=
,
2
2
2 A /Hz (7.8)
where
q
is
the
electronic
charge.
The
current
i
N,I
represents the noise current due to the
biasing
c
urrent
I.
7
.4.4.2 Quantum
S
hot
N
oise
The average current
i
s
2
generated by the PD by an optical signal with an average optical
power P
in
is given ...