Design Note HFDN-18.0 (Rev.1; 04/08)
Maxim Integrated Products
Page 4 of 9
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Recall that the overall response of a feedback
system has a zero at each pole of its feedback
network. Therefore, the overall gain of the
feedback part of Figure 2 rises above 100MHz,
and its output becomes extremely noisy. Another
way of looking at it is that the feedback becomes
ineffectual above 100MHz, so the system output
includes all the noise of the high-gain amplifier
running without feedback.
2.1 Automatic Power Control
Something useful can be achieved by abandoning
the quest for large loop bandwidth, and replacing the
high-gain amplifier by an integrator:
τ
s
A
1
amplifier
.
(6)
When
montior
laser
1
1
output
optical
desired
output
optical
actual
A
A
s
τ
+
=
,
(7)
the system becomes a low-pass filter. If the
integrator time constant
τ
is chosen long enough, the
average optical power output P
av
becomes equal to
the desired average output, independent of the detail
of the data pattern in the modulation waveform.
However, all the high-frequency information in the
modulation waveform is filtered out, so P
max
, P
min
,
and the difference between them are not controlled.
Automatic power control (APC) is achieved, but not
automatic modulation control (AMC).
Laser driver systems incorporating APC are quite
common. The user can program the average optical
power output, and this will be maintained
automatically despite variations in the laser.
However, peak-peak optical output and extinction
ratio are not controlled, and must be set up and
tweaked for each individual laser.
3 Automatic Modulation Control
via a Pilot Tone
Maxim’s MAX3865, achieves automatic modulation
control by adding a small pilot tone to the laser
current. This pilot tone is a square wave with a
frequency of about 1MHz (low enough that it can
pass through the monitor diode without attenuation).
Figure 3 shows a simplified block diagram. The
control loops for laser bias (average) current and
modulation (peak-peak) current are separated.
The automatic power control (APC) feedback loop
holds the bias or average current in the monitor
diode constant, and is basically the same as Figure 2.
The desired (pre-scaled) bias current enters at the
center-left in Figure 3. (The pre-scalar is omitted
Figure 3. Conceptual block diagram of a laser driver with APC and AMC
