© 2018 Thorlabs
3 Operating the ITC4000 Series
33
3.7 LD Modulation Settings
The ITC4000 provides a modulated operation of laser diodes in constant current and constant
power mode. The modulation signal can be generated internally or by an external source.
An internal function generator offers an AC coupled sine, triangle or square signal. The modula-
tion frequency range depends on the model - it is 20 Hz to 150 kHz for ITC4001, 20 Hz to 130
kHz for ITC4002QCL, 20 Hz to 100 kHz for ITC4005, ITC4005QCL and 20 Hz to 50 kHz for
ITC4020. The modulation depth can be set from 0.1% to 100%. In constant current mode, this
percentage refers to the peak to peak value of the output current in relation to the nominal max-
imum current of the laser diode controller.
Example
: A DC current of 10 A (ITC4020) is modulated from 8A to 12 A with 20% modulation
depth.
In constant power mode, the bandwidth may be limited by the constant power loop characterist-
ics (for optimizing the speed of the constant power loop, please refer to section
). In this mode, the percentage refers to the peak to peak value of the op-
tical power in relation to the selected power range.
The external modulation input allows a precision power level control or a wavelength tuning of
the laser.
When using an external modulation source, please connect the external source to the
"MODULATION IN" BNC jack (R5) on rear of your ITC4000 and switch the modulation source
to External. The modulation signal is DC coupled and added to the current set point (in con-
stant current mode) or to the power set point (in constant power mode).
The resulting values for the laser current "I
LD
", the photodiode current "I
PD
" and the thermopile
voltage „U
TH
"
are calculated as:
I
LD
= I
LDset
+ I
LDmax
* U
MOD
/ 10V
(in constant current mode)
I
PD
= I
PDset
+ I
PDmax
* U
MOD
/ 10V
(in constant power mode, photodiode sensor)
U
TH
= U
THset
+ U
THmax
* U
MOD
/ 10V
(in constant power mode, thermopile sensor)
with:
I
LDmax
maximum laser current (e.g. 5A for a ITC4005
)
I
PDmax
maximum photodiode current (depending on selected range)
U
THmax
maximum thermopile voltage (depending on selected range)
I
LDset
set value in constant current mode
I
PDset
set value in constant power mode, photodiode as sensor
U
THset
set value in constant power mode, thermopile as sensor
27
