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David Notebook: Difference between revisions
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The photodiode is definitely too slow for the chopper. The rise time is plenty fast, but it seems there is a fairly long decay time--the waves were asymmetric, with the first half looking good and the second half looking like an exponential decay. They got worse with increasing chopper frequency. I switched to a [[Thorlabs PDA10A Si Amplified Fixed Detector|Thorlabs PDA10A]], which has no problem even at 1000 Hz. At 633 nm, it has a listed responsivity of about .38 A/W.
Now I can compare the HeNe power measured by the power meter and the lock-in. The power meter read 199 μW just before the PVC pipe. I then took out the 780 notch filter before the photodiode and had the photodiode output to the lock-in and the lock-in output to an oscilloscope. The oscilloscope read 6.7 V. I had the sensitivity on the lock-in to 1 V, so using the formula from yesterday for offset square waves, the peak-to-peak voltage from the photodiode should be 1.49 V. Checking the photodiode on the oscilloscope confirmed this was accurate to about 5%. Then
<math> P=\frac{V}{(.38 Amps/W) * (10^4 V/Amps)} </math>
With the .38 Amps/W as the responsivity of the photodiode and the 100000 V/Amps is the transimpedance gain of the photodiode.
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