
296 Optical Fiber Communication Systems with MATLAB
®
and Simulink
®
Models
7.5.4.2.1
Noise Generators
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 and (2) photocurrents generated by the photodiode.
A
b
iasing
c
urrent
I g
enerates
a s
pectral
c
urrent
d
ensity
S
I
given by
S
d i
df
qI
I
I
= =
( )
2
2 in A /Hz
2
(7.65)
where
q
is
the
electronic
charge.
The
quantum
shot
noise
i
s
2
generated by the PD by an
optical
s
ignal
w
ith
a
n
a
verage
o
ptical
po
wer
P
in
is given by
S
d i
df
q i
Q
s
s
= =
2
2
2 in A /Hz
2
(7.66)
When
A
PD
i
s
u
sed
t ...