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David Notebook: Difference between revisions
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Jared and I have both thought that the visible power meter head has been giving somewhat high values (10% or so), so I'm skeptical of it's readings. However, further investigation of the lock-in reveals it is supposed to always multiplies the output by an additional 10V, regardless of the settings. Page 21 of the manual alludes to this, but I couldn't find a full description. I measured this value as closer to 9 though--I used a function generator to send in a square wave at 500 Hz with a max amplitude of 2V and a min of 0V, similar to the type of signal the photodiode should send in. With the gain setting on the lock-in as "1V" or "1μA", the output was 9V. A 100 mV average value square wave gave 0.9V.
Ok, the multiplicative factor of 9 is checking out more--it matters for the lock-in whether the input signal is a sine wave or square wave. For a sine wave, it singles out anything oscillating at the set frequency and gives the RMS voltage. For a square wave, it takes only the first
I've still been having trouble getting the lock-in readings to match the values I think it should have, whether I give it a signal from a function generator or from the photodiode. A 2 volt peak-to-peak amplitude square wave should give a reading of .9V on the lock-in (4/(π*2^.5) and an output of 9 V for the extra factor of 10. Page 31 in the manual gives this an an example. However, I am getting closer to .8V.
I also noticed that I am not getting perfect square waves from the laser/chopper/photodiode combo. While it works well at low frequencies around 40 Hz, at 500 Hz where I was running it the waves are distorted. This is likely because the response time of the photodiode is not fast enough. This should not affect optimization procedures, but it does mean that the Fourier component at 500 Hz does not have an amplitude factor of 4/π. I will either need to run the chopper slower if there is not too much noise at lower frequencies, or calculate a better value for the pre-factor before converting to an absolute power value for the 633 beam.
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