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44
Chapter 4
It is recommended that the connections to the module output be made using twisted
wire pairs with an earth-grounded shield available at pin 3, except MOPA modules.
For the MOPA module, use the ground post on the mainframe. The output terminals
of the module are 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 short out the driver and may damage the laser.
See section 4.1.3 for additional information on laser diode protection.
4.4.2
Photodiode Feedback Connections
The photodiode signal is input at the 9-pin D-connector at pins 6 and 7, or the
photodiode BNC. For MOPA modules, the photodiode signal is input at pins 12 and
13 of the 15-pin D-connector.
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 22 through Figure 25 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 1 MΩ resistor between the laser diode cathode and the
photodiode anode to provide a bias return for the photodiode circuit. The output
connector shown below is the 9-pin D-sub on standard laser modules.
Laser Diode Driver Module Operation
Although the intermittent contact circuitry works well in
helping to protect the laser diode, there is still a danger in
having poor connections, as no circuit can protect
It is possible with some modes of modulation, especially
square-wave, to trigger the intermittent contact circuit and
cause a shut-down. If this is the case for you, the
intermittent contact feature may be disabled in the laser
set up menu, with reduced laser protection in the case of
poor connections.
NOTE
completely.
NOTE
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