O P E R A T I O N S
Functions and Features
7/01
LDC-3700 Series
31
C H A P T E R
2
Using the Laser Compliance Voltage Adjustment
For applications where you need to have critical protection of the laser we recommend the following.
Using a 1
Ω
resistor or other non−critical load, set the LASER voltage limit as described in Chapter 2.
Then with the LASER output on, slowly increase the LASER current and watch the LASER voltage
measurement. (Press the
LASER DISPLAY V
push button to see the voltage). At about 0.25 volts
below the LASER voltage limit value, the VOLTAGE LIMIT warning indicator will become lit on the
LASER display
. Continue to adjust the current up until the voltage limit is reached and the output is
shut off. Record the voltage at which the LASER output shuts off. It should be within the specification
of the LASER voltage limit accuracy. However, there is typically some offset error. For example, if
the LASER voltage limit is set to 5.0 volts, the actual voltage where it shuts off may be 4.9 volts. This
error term of 0.1 volts should be considered when setting the LASER compliance voltage limit.
The closer the LASER voltage limit shut off point is to the operating voltage of your laser, the faster
the circuit will work in the event of an open circuit. Some experimentation may be necessary for
optimum results.
Using the LDC-3700B Controller’s Trigger Function
For applications where you need to synchronously initiate a measurement task from a remote
instrument with the LDC−3700B Controller, the controller offers a trigger output signal. The TTL
pulse is initiated with any change in set point of the laser current source and after the analog output
has settled. A typical application for utilization of this feature is laser characterization tasks where
an L−I curve is generated. For a programmed step in laser current, a light measurement can be
triggered for each step in laser current. The trigger output is enabled only in remote mode via the
IEEE 488.2 GPIB Interface.
Note:
The minimum step change time required for a corresponding trigger pulse is 20 mS.
Care should be used with respect to the timing of any set point commands in relation to the
actual hardware function. The INC and DEC commands have a fixed minimum step time of
20 mS which allows for the hardware initiation of a set point change and the settling of the
output. The LDI command does not. With this command, some trigger pulses may be
missed if the program step time is less than the 20 mS minimum.
A one shot trigger pulse will occur on power up of the instrument due to the states of the processor
I/O.
LASER DISPLAY
I
I
PD
P
PD
V
SET
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