44 Chapter 5 Tips and Techniques
point, the driver will attempt to drive the necessary current to reach the
set point, and hit the current limit so hard that a hard limit condition will
occur, shutting down the laser.
Im Limit and Po Limit
Photodiode current and power limits, on the other hand, are implemented in
software. Because of this software implementation, it is possible to exceed
the photodiode current or power limits for a brief period of time before the
condition is detected by the microprocessor. These conditions are checked
once per second, so the latency time from the time the condition occurs to
when the output would be shut off is at most two seconds. For more detailed
information see LASer:TOLerance section of the manual (Computer
Interfacing chapter).
5.3 Modulating
Laser modulation is supported by many of the Newport laser diode
controllers, and allows external control of the laser operating current. By
supplying an input voltage, the operator can set the operating current
anywhere within the full range of the laser controller, although the output
will still be limited by the current limit.
The modulation 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.
Using the Modulation Input as the Set Point
It is possible to fully control the laser set point through the modulation input
by setting the laser current at zero from the front panel and turning on the
laser output. The modulation input is then summed with the set point (which
is zero) to give the actual set point. The 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. The set point on the front panel could also be set at 50% full
scale and the current could be modulated between zero and full scale by
inputting a
±
5V modulation signal.
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