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David Notebook: Difference between revisions
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I'm trying to calibrate the chopper/lock-in system to get better measurements of the 633 power and make sure our conversion efficiency numbers are accurate. I cut the power of the HeNe beam with irises to 20 nW as measured on the power meter (filters might have been better so that the intensity is reduced too, but I couldn't find a good combination to get the power I wanted. It probably won't matter since there is a lens in front of the photodiode anyway). However using the lock-in and my previous procedure to convert photodiode signals to power, I calculated only 1.25 nW.
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 its readings. However, further investigation of the lock-in reveals the display signal is always multiplied by an additional factor of 10, regardless of the gain 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 "
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 Fourier component (4/π sin(ωt)) and then the RMS value (multiplied by <math> 1/ \sqrt{2} </math>). Page 31 in the manual is helpful. So for a square wave pulsed laser at 500 Hz like I'm sending in, the output voltage should be
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
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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