OPERATIONS
Operating from the Front Panel
20 LDC-3700 Series
CHAPTER 2
Operating a Laser in Constant Power Mode, P
Suppose you want to operate a 3 mW laser in constant power at 1 mW. The 3700B Series Controller
allows you to operate the instrument current source driver in a Constant Power mode. In this mode,
the controller drives current to the laser to reach a set power value in mW. The control loop feedback
parameter is photodiode current which the controller converts to optical power via a user defined
photodiode responsivity number. If laser power changes due to internal or environmental conditions,
the controller will increase/decrease the current to the laser to maintain the power set point. The
following paragraphs detail the instrument configuration in Constant Power Mode.
To set the LDC−3700B in Constant Power mode, select P in the LASER MODE section of the front
panel by toggling the SELECT push button. Repeatedly pressing the (LASER MODE) SELECT
switch cycles through the current (I), light power (P), or high bandwidth current (I
HBW
) control
modes. The LED indicator shows the selected mode.
Entering Photodiode Responsivity values
The photodiode responsivity, CAL PD, is used to convert between photodiode current and optical
power of the laser diode. To enter the CAL PD parameter, select CAL PD with the SELECT push
button by toggling down past LIM P. The indicator becomes lit when CAL PD is selected and the
display indicates the responsivity parameter in µA/mW (not enunciated). Rotate the adjust knob in
the ADJUST section while holding the (PARAMETER) SET push button to enter a new value for
responsivity. When the correct value is displayed, store the number by releasing the SET push
button.
To calculate the responsivity:
1 Measure (with a calibrated detector) the output power of the device.
2 Measure the corresponding photodiode current.
3 Calculate the responsivity by dividing the photodiode current by the optical power noting
the units required are µA/mW.
The LDC−3700B Controllers can be put into a special mode with the CAL PD parameter set to zero.
When the CAL PD value is zero, the LASER output will be controlled to the I
PD
set point value,
therefore, the instrument is in Constant I
PD
mode. The LASER display will indicate "−.−" in P
PD
mode.
Next, configure the display so that it reads in mW by selecting P
PD
in the LASER DISPLAY section
of the front panel (directly underneath the LASER display). The LASER DISPLAY switch section is
used to select the I (laser current), I
PD
(photodiode current), P
PD
(Laser power), or V (forward
ON
OUTPUT
I
HBW
I
SELECT
LASER M ODE
SAV E
CONST
TEC
CAL PD
RECA LL
SET
SELECT
PARAM ETER
LASER
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