Operating A Laser In Constant Power Mode, P; Operating A Laser In Constant Power Mode, P; Entering Photodiode Responsivity Values - ILX Lightwave LDX-3200 Series User Manual

OPERATING A LASER IN CONSTANT POWER MODE, P

Suppose you want to operate a 3 mW laser at constant 1 mW level. The LDX-3200 Series instrument allows you to
operate the laser current source driver in a Constant Power mode. In this mode, the LDX-3200 Series instrument
drives current to the laser to reach a set point power value (in mW). The control loop feedback parameter is
photodiode current that the LDX-3200 converts to optical power via a user-defined photodiode responsivity number.
If laser power changes due to internal or environmental conditions, the LDX-3200 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 LDX-3200 in Constant Power mode, select P in the MODE section of the front panel
by toggling the SELECT push button. Repeatedly pressing the (MODE) SELECT switch
cycles through the current ( I), light power ( P), or high bandwidth current ( I
modes. The LED indicator shows the selected mode.

Entering Photodiode Responsivity values

A method to calculate the responsivity is suggested below.
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 LDX-3200 Series Precision Current Source 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
instrument is in Constant I
PD
Next, configure the display so that it reads in mW by pressing and releasing the (DISPLAY) SELECT switch until the
mW annunciator is lit.
Adjust the Constant Power set point of the LDX-3200 to 1.00 mW. To do this, turn the adjust knob to vary the set
point. When the adjust knob is turned, the LDX-3200 automatically changes to set point mode as indicated by the
flashing annunciator. Adjust the set point to 1.00 mW. After an adjustment has been made, and the adjust knob is
released, the set point mode will time out in three seconds. The display will then return to the measurement display
mode (I, I , V, or P ).
PD PD
Before the laser output is enabled, the power limit for the laser under test should be set. In Constant Power mode, this
is accomplished by setting a Power limit, LIM P value. The LDX-3200 uses the responsivity value entered to calculate
a power limit as a function of monitor current (Ipd). This is a software limit only.
HOW TO OPERATE YOUR LDX-3200
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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 (PARAMETER) 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 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
(PARAMETER) SET pushbutton.
mode. The display will indicate "-.-" for the power measurement.
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set point value. Therefore, the
PD
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HBW
CHPT 2