I measured a 10 kHz modulated white light using a photodiode (Thorlabs PDA10A):
Unexpectedly, the photodiode output sensitively depends on the orientation and position of the photodiode. For two positions with subtle difference, I got two measurements, as:
This issue is not due to the driver or the signal generator. Everytime I moved the photodiode away from the stable position to a new position, likely there will be random phase superpositions as above left.
Also, I have tried changing light source to a 632.8 nm laser diodes, yet same thing happens.
If leaving the stable status running for long time (>10 min), the random phase superposition happens again. However I am not sure if this is because of the driver or the photodiode.
The photodiode I used, there is a piece of cover glass in front of the sensing area. I am not sure if multi-reflection could cause this effect.
Why this random phase issue happens?
How can I suppress it?
I have used a lens to focus the light onto the photodiode.
I cannot reproduce the stable status anymore, even with my previous setup. So something must be wrong with the driver.
With an FFT on 4kHz data, we see: (note the waveform itself is not as good as it looks)
- For the driver part, I was using analog modulation. Is analog modulation possible to produce such a random phase phenomenon?