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17874-D03
T-Cube Laser Diode Driver
Setting the Watts/Amps Calibration Factor
The displayed power is d erived from the photodiode current, and the relationship
between these parameters will differ between laser diodes. Therefore, the power
display must be tuned to the diode being driven before the output power of the laser
diode can be shown or adjusted accurately.
17)On the GUI panel, click the CONST P button.
18)Turn the OUTPUT knob to decrease the drive current to a value less than the Max
Current limit. (At maximu m current limit the optical power may not be constant
because if the feedback loop senses a drop in the optical power, it will be unable
to compensate for it by increasing the laser current.)
19)Note the power reading on the power meter and the Photodiode current displayed
on the GUI.
20)Calculate the Watts/Amps calibration factor as:
Measured Power (mW)
PD Current (mA)
21)Click the Settings button and select the ‘General’ tab.
22)Enter the calibration factor into the ‘W/A Calib Factor’ field.
23)Click the ‘Persist Settings to Hardware’ checkbox, then click OK.
5.5
Modulation of the Laser Diode Output
It is possible that an application may demand a time dependent laser current (I
LD
) or
photodiode current (I
PD
) instead of the normal constant output. This is achieved by
connecting a modulating signal to the rear panel MOD IN connector. The voltage
range is -10V to 10V, input resistance is >10k
Ω
.
The external input is always scaled so that 0 to 10V is equivalent to 0 to ILIM (in open
loop) or 0 to max optical power (in closed loop), regardless of the value of I
LIM
or P
Max
.
For example, if the maximum laser current is set to 75mA, then the 0 to 10V will result
in an output current of 0 to 75mA (i.e. the transconductance of the system is not
constant, but depends on the initial set up). Note that this is in addition to the current
already set by the software or the OUTPUT control knob. The contribution of the
external input can also be negative, i.e. a negative voltage on this input can reduce
the output current or power.
The procedure for modulating the laser diode output is as follows:
1) Switch on the unit and select either constant current or constant power mode - see
2) Turn the OUTPUT pot to set the desired value.
3) Connect the modulation source to the rear panel MOD IN connector, ensuring that
ground loops are avoided.
