Relating Optical Power To Analog Voltage - ILX Lightwave FPM-8200 User Manual

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C H A P T E R

Relating Optical Power to Analog Voltage

The most direct way to relate optical power to analog voltage is to read the power
from the front panel or by GPIB while noting the voltage. If you set up a GPIB
automated system with a datalogger or voltmeter, you can directly correlate power
and analog output voltage in your data.
You can also get this information from calibrated detector response data. You can
get the detector response data in two ways:
• The GPIB query RESP? tells you the calibrated detector response in mA/mW for the
• The factory calibration certificate includes a table of detector response at every 10 nm.
To convert analog voltage to optical power, follow these steps:
1
2
3
4
5
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 FPM-8200
FR O N T PA N E L O P E R A T I O N
The FPM-8200 Back Panel
currently set wavelength. You can get the response for any wavelength by first using the
WAVE command:
For example: WAVE 1480; RESP?
This sequence returns the calibrated detector response for 1480 nm.
Make sure optical power input is ON.
Connect a voltmeter to the analog output.
Press and hold LOCAL/ADR and AUTO/MAN to display the gain range setting.
Using Table 2.1 on page 14, convert the voltage to detector current. For example, if the
display shows "--3--", the instrument is set to range 3. pressed UP three times, the
FPM-8200 is on range 3. A voltage reading 5.0V = 50% of scale = 5 µA.
Divide current by detector response to get optical power. For example, using 5 µA current,
and 0.01193 mA/mW detector response:
0.005mA
0.01193mA/mW
This example is for 5.0V, range 3, detector responsivity 0.01193 mA/mW. Refer to
Appendix A, Conversion Tables for converting mW to dBm.
Accuracy of the analog output is not a factory specification. However you will find it to be
stable and reliable.
=0.419mW =−3.77dBm