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Optical measurement (8753ES)

Question asked by degemermat on Mar 19, 2015
Latest reply on Mar 20, 2015 by degemermat
Hi all, I am trying to measure an optical signal and have a few questions.

- my DUT (an LED) is connected to port 1 of the VNA (8753ES), and receives a DC bias + the VNA signal
- the DUT emits an optical signal collected with a photodetector (photodiode + transimpedance amplifier, with 50ohm termination) which is connected to port 2
- I measure the amplitude of S21 as the received optical signal

* First, I am wondering if there is any sense in calibrating port 2 for S21 (at least for amplitude), and if so how I should proceed.

* Second, I am not sure how to interpret the S21 signal. In principle S21 is a ratioed power measurement, but what I am really measuring here is an absolute optical power. What unit am I then looking at?

* Finally, the photodetector converts optical power to voltage, which I want to emasure. But the VNA measures power. Is it then correct that the photodetector's voltage is proportional to the sqrt of the VNA's S21 signal?

Thanks, and sorry if there are dumb questions here!