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David Notebook: Difference between revisions
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I'm having a hard time getting the Thorlabs photodiode to work. It's noisier and has a smaller sensor. I was previously able to detect a signal with it, but have had trouble the last few days. The UDT sensors one is much easier to work with, but the problem is that it's slower and so doesn't produce square waves when chopping above about 20 Hz. I need square waves so I can properly calculate the power, but there is too much noise at such low frequencies. Presumably the lock-in reading is linear with laser power even with non-square waves, so I'm just going to try to send in a beam with known power to the UDT photodiode and infer the relationship.
Well, the photodiode has a very non-linear response, both when looking at the output from the lock-in amplifier and with the chopper off and just looking on an oscilloscope. I checked this for powers in the mW range down to μWs, so I don't think it's just a saturation issue. The Thorlabs photodiode seems fairly linear.
It seems the UDT photodiode has a logarithmic response at low powers. I can't find anything in the datasheets about this, but plotting about 10 points from 6 to 800 nW of input power vs voltage makes this fairly clear. I'm going to use a regression equation to infer powers from the voltage then. I'm likely in the 1-10 nW range, but the data is noisier here since it's hard to accurately measure down to nW with the power meter. I'm only going to use the data from 60-800 nW, which gives a good fit. Hopefully the response is similar at lower powers. Data is [https://wiki.physics.wisc.edu/yavuz/images/5/56/UDT_5DP_response.xlsx here]
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