32 Chapter 3 Laser Diode Driver Module Operation
left floating relative to earth ground to suppress AC power-on/power-off transients
that may occur through an earth-ground path. If the output circuit is earth-grounded
at some point (such as through the laser package and mount), the user must be careful
to avoid multiple earth grounds in the circuit. Multiple earth grounds may provide
circuit paths that shut-out the driver and may damage the laser.
3.4.2 Photodiode Feedback Connections
The photodiode signal is input at the 9-pin D-connector at pins 6 and 7, or the
photodiode BNC.
Many laser diode modules contain an internal photodiode that monitors the back-
facet emission of the laser. Usually, one side of the photodiode is internally
connected to either the laser anode or cathode. Figure 14 through Figure 17 show the
recommended connections and shielding for the various configurations of laser diode
modules and photodiode feedback schemes. The photodiode circuit is isolated from
ground and the laser circuit. Therefore, when using a 4-pin package with no common
connections, place a 1MΩ resistor between the laser diode cathode and the
photodiode anode to provide a bias return for the photodiode circuit.
7
6
8,9
4,5
3
+
-
Output
8500 module
Bias
Earth Ground
L.D.
P.D.
+
-
Figure 14 - Common Laser Cathode / Photodiode Cathode
7
6
8,9
4,5
3
+
-
Output
8500 module
Bias
Earth Ground
L.D.
P.D.
+
-
Figure 15 - Common Laser Cathode / Photodiode Anode
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