Chapter 5 Tips and Techniques 45
5.3.1 Grounding with Modulation
Most modulators have a BNC output that is connected to earth ground. This
is not a problem as long as the laser anode or cathode is not also connected
to earth ground. However, in many systems, it is. When it is, the following
must be considered:
When the anode is tied to earth ground, this effectively connects earth
ground to the laser power supply, which can be up to +10V dc. When the
BNC on the modulator is connected to the modulation input, it biases the
input to the laser power supply voltage. Since the common mode input range
is only about +12V dc, the modulation signal range is reduced. 12V-10V
leaves only 2V for the modulation. Thus, it may appear that the modulation
input is not working properly.
5.3.2 Current Limit when Modulating
A special limiting circuit prevents overdriving the Laser Diode current when
using external modulation.
The diagram below shows a sine wave of approximately 200 kHz that is
being clamped at I
lim
.
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