THORLABS ITC4000 Series Operation Manual page 30

ITC4000 Series
2.02 A (ITC4002QCL), from 5 mA to 5.05 A (ITC4005 and ITC4005QCL) and from 20 mA to
20.2 A (ITC4020).
Setting the photodiode current limit:
The feedback photo current will be limited to this preselected value, so the optical output power
will be clipped by a hardware circuit. This feature protects the optical setup against excessive
optical power. If the photodiode range is switched from the high to the low range, the photodi-
ode current limit will be constraint to the maximum value of the low range (if higher then the
maximum before). When switching back to the high range, the constraint value will remain and
must be increased if necessary.
Setting the thermopile voltage limit:
The feedback voltage will be limited to this preselected value, so the optical output power will
be clipped by a hardware circuit. This feature protects the optical setup against excessive op-
tical power. If the thermopile range is changed from a higher to a lower range, the thermopile
voltage limit will be constraint to the maximum value of the lower range (if higher than the max-
imum before). When switching back to the higher range, the constrained value has to be in-
creased if necessary.
Setting the optical power limit:
The optical power limit can be set in Watts if the responsivity of the connected sensor is known
and entered. The resulting photodiode current or thermopile voltage will be calculated by the
ITC4000 Firmware.
Note
The photodiode current, the thermopile voltage, and the power limit values shouldn't be set to
zero, otherwise the laser current may be limited by these power limits in constant current mode.
If a power limit gets active in constant current mode, the operation mode changes over to con-
stant power mode.
The response time of these limits is dependent on the control rate of the power feedback loop.
If the feedback loop speed is adjusted to a very low level, the limit reaction will be accordingly
slow. If a quick reaction is desired, the feedback loop speed should be optimized as described
below. In any case, the laser current limit should be set to an appropriate value as an additional
safeguard for the laser.
The maximum limit values are dependent on the connected sensor and the selected range for
this sensor:
Optical sensor type Measurement range
current
current
voltage
voltage
voltage
voltage
Setting the constant power feedback loop:
In this menu topic, the feedback source for the constant power loop (photodiode or thermopile)
can be set. In addition, the feedback loop speed can be limited to minimize oscillation and over-
shoots. For DC operating laser diodes, the speed should be set to the minimum value (0.1%).
For modulated laser applications with a photodiode as sensor, the optimum feedback loop
speed can be identified by using an internal square wave modulation (100Hz 20% modulation
28
2mA
20mA
10mV
100mV
1V
10V
Limit range
0 to 2.02 mA
0 to 20.2 mA
0 to 10.1 mV
0 to 101 mV
0 to 1.01 V
0 to 10.1 V
© 2018 Thorlabs