
291Direct Detection Optical Receivers
Knowing
t
he
i
nput
o
ptical
s
hape,
t
he
e
qualizer
t
ransfer
f
unction
f
or
a
g
iven
p
ulse
s
hape
c
an
b
e
d
etermined
b
y
m
ultiplying
t
he
t
ransfer
f
unction
o
f
t
he
r
eceiver
a
nd
t
hat
o
f
t
he
e
qualizer.
I
t
i
s
n
oted
t
hat
t
he
3
d
B
b
andwidth
o
f
a
f
eedback
r
eceiver
i
s
g
iven
b
y
A/R
f
C
w
hich
i
s
a
bout
A
t
imes
t
he
b
andwidth
o
f
t
he
r
eceiver
w
ithout
f
eedback.
T
hus,
t
he
d
emand
o
n the equalizer is much less for this case.
7.5.3.3 Photodiode Shot Noise
The
o
utput
n
oise
po
wer
d
ue
t
o
t
he
r
andom
uctuation
o
f
t
he
m
ultiplied
P
oisson
n
ature
o
f
t
he current produced by the PD can be written [4] as:
i qG a h t kT h t t d
sN
x
k
k
p B d I
2 2 2
= ℜ
′