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David Notebook: Difference between revisions
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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]
I found the response curve to be Voltage <math> = 1.77ln(</math>Power<math>) - 6.00 </math> with Power in nW for the output of the lock-in amplifier set at 50 mV sensitivity using the UDT photodiode and 780 nm light.
Accounting for the sensitivity, the equation becomes Voltage*Sensitivity <math> = .0921ln(</math>Power<math>) - 0.3224 </math>The photodiode is about 86% as sensitive at 633 as 780, so 633 powers should be 16% higher. Then for 633 light, we should have Voltage <math> = 1.16*Voltage*Sensitivity <math> = .0921ln(</math>Power<math>) - 0.3224
So rearranging and simplifying, we have: Power<math>=e^{(</math>V*sensitivity<math>+0.3224)/0.0921}</math> with power in nW. This is the chopped power.
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