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OPERATION
Operating a Laser in Constant Power (P) Mode
18 LDX-3232
CHAPTER 2
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-3232 High Compliance 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
PD
setpoint value. Therefore, the
instrument is in Constant I
PD
mode. The display will indicate
"-.-" for the power measurement.
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 setpoint of the LDX-3232 to 1.00 mW. To do this, turn
the adjust knob to vary the setpoint. When the adjust knob is turned, the
LDX-3232 automatically changes to setpoint mode as indicated by the flashing
annunciator. Adjust the setpoint to 1.00 mW. After an adjustment has been made,
and the adjust knob is released, the setpoint mode will time out in three seconds.
The display will then return to the measurement display mode (I, I
PD
, V, or P
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-3232 uses the responsivity value entered to calculate a
power limit as a function of monitor current (I
PD
). This is a software limit only.
To set the Constant Power limit, use the SELECT push button in the
PARAMETER section to select LIM P. When the LIM P is indicated by the
accompanying LED, the display will indicate the limit setpoint in mW. Press and
hold the (PARAMETER) SET push button and rotate the adjust knob to set the
limit to 1.50 mW. When the desired value is reached, release the (PARAMETER)
SET push button to store the new limit value.
The LIM P is the absolute limit of optical power, as detected by the unit via the
monitor photodiode feedback loop. In order for this limit to be meaningful, the
monitor photodiode responsivity (CAL PD) must be adjusted correctly. This limit is
a software limit only. The LASER output is normally turned off if this limit is
reached (See LASer:ENABle:OUTOFF command in Chapter 4 for instructions on
disabling this function). The maximum LIM P value is 5000.0 mW.
Also, in constant power mode the LASER current limit and voltage limit values are
in effect. Setting of these limits is described later in this chapter.