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136
Tips and Techniques
7.4.2
Voltage Limit
A special limiting circuit prevents overdriving the Laser Diode voltage.
When the voltage limit is reached, the output current is shut off to prevent
damage to the laser diode.
7.4.3
Operating at or Near Io and Vcomp Limits
Because of the sensitivity of the limit circuits, operating at or near the limit
unless necessary is not recommended. AC line transients, RF interference, or
static can be enough to trigger these limits. Triggers for hard limits and
voltage compliance limits vary, but can include the following:
7.5
External LD Analog Control
External analog control allows external control of the laser diode operating
current. By supplying an input voltage, the operator can set the laser diode
operating current anywhere within the full range of the laser controller,
although the output will still be limited by the current limit.
The analog control input takes a voltage range of 10V. Each 100 mV
represents 1% of the driver's maximum operating current, which gives a
range of 0% at 0V to 100% at 10V.
7.5.1
Using the LD Analog Control Input as the Set Point
It is possible to fully control the laser diode set point through the analog
control input by setting Laser Diode Driver subsystem unit into this analog
control mode via USB command (i.e., LAS:MODE command, see Section
5.5) The analog input voltage can then be ranged from 0V to 10V to give the
full range of current output. Note, however, that the current limit is still
active, and will not allow the current to exceed the current limit. Also note
that the laser diode driver reverts back to normal CW mode (i.e., it's no
longer in Analog Control Mode) after a system reset or power cycle.
7.5.2
Grounding Considerations when using the LD Analog
Control Input
Most signal generators and oscilloscopes have a BNC output that is
connected to earth ground. This is not a problem as long as the laser anode or
cathode are not also connected to earth ground. However, in many systems,
The second circuit monitors the actual current drive and, if that exceeds
the limit set point, shuts down the output. This form of limit is called the
hard limit. The Laser Diode Driver subsystem will always be shut down
on a hard limit.
Static discharges, which may cause enough noise to trigger the circuit.
Turning on the laser when its set point is at or near the limit. Turning on
the output with the set point at the limit can cause a small overshoot in
the drive current, which the limit circuitry may pick up as a hard limit
and shut down the laser.
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