Operating A Laser In Constant Power (P) Mode; Entering Photodiode Responsivity Values - ILX Lightwave LDX-32420 User Manual

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C H A P T E R

Operating a Laser in Constant Power (P) Mode

Suppose you want to operate a laser diode at a constant 10 W level. The LDX-
32420 allows you to operate the laser current source driver in a Constant Optical
Power mode. In this mode, the LDX-32420 drives current to the laser to reach a
setpoint power value (in W). The control loop feedback parameter is photodiode
current which the LDX-32420 converts to optical power via a user-defined
photodiode responsivity number. If laser power changes due to internal or
environmental conditions, the LDX-32420 will increase/decrease the current to the
laser to maintain the power setpoint. The following paragraphs detail the
instrument configuration in Constant Power Mode.
To set the LDX-32420 in Constant Power mode, select P in the MODE section of
the front panel by toggling the SELECT push button [LASer:MODE:MDP].
Repeatedly pressing the (MODE) SELECT switch cycles through the current (I)
[LAS:MODE:ILBW], photodiode power (P) [LAS:MODE:MDP], or high bandwidth
current (I
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
photodiode parameter [LAS:CALMD XX], 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). Press the set button
then rotate the adjust knob to enter a new value for responsivity. When the correct
value is displayed, the number will be stored after 3 seconds.
A method to calculate the responsivity is suggested below.
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 LDX-32420
O P E R A T I O N
Operating a Laser in Constant Power (P) Mode
) [LAS:MODE:IHBW] control modes. The LED indicator shows the
HBW
Measure (with a calibrated detector) the output power of the device.
Measure the corresponding photodiode current.
Calculate the responsivity by dividing the photodiode current by the optical power noting
the units required are µA/mW.
Note: The LDX-32420 High Power 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 a current setpoint value (I
instrument is in Constant I
current and "-.-" for power measurement.
mode. The display will indicate A for photodiode measured
PD
) in micro amps (A). Therefore, the
PD