Digital Signal Processing Reference
In-Depth Information
E(f)
E(
t)
i(t)
2
f
c
Photo
Detector
LPF
10GHz
i(f)
2f
c
2f
c
-
0
400 THz
Fig. 6
A block diagram model for a photodetector receiver is shown as an ideal square-law
detector followed by a low pass filter
2.3.1
O/E Detectors
Typical optical detectors include the avalanche photodiode (APD) and the PIN
diode, each of which produce an electrical current proportional to the incident
optical power. Intensity modulation can be modeled as
E
,
and dispersion, can be viewed as a bandlimited square-law device, approximately
producing an output current
(
t
)=
m
(
t
)
cos
(
2
π
f
c
t
)
t
t
2
|
m
(
t
)
|
2
dt
i
(
t
)=
R
−
τ
|
E
(
t
)
|
=
R
dt
2
t
t
−
τ
where
85
A/W for APD and PIN receivers), owing to the finite integration bandwidth of the
detector, where 1
R
is the responsivity of the detector (typically between 0
.
7A/Wand0
.
f
c
is typically on the order of 10 GHz, for a 10 Gb/s link, for
example. The double-frequency term induced by the quadratic detector is therefore
filtered out, leaving only the baseband demodulated intensity signal
m
2
/
τ
(
t
)
,
in the
absence of dispersion. Since
m
(
t
)
>
0 for intensity modulation, the data can be
completely recovered from
m
2
(
t
)
by a simple CDR, in the absence of noise and
dispersion.
2.3.2
Analog Front-End (AFE)
The AFE refers to the linear signal processing blocks in the receive path. The back-
plane AFE consists of a variable gain amplifier (VGA) for signal amplification.
Usually, the receive amplifier also provides some high-frequency peaking in order
to compensate of the channel roll-off, and limits out-of-band noise. In EDC-based
optical links, the AFE consists of a cascade of the TIA and the VGA.
