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David Notebook: Difference between revisions
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I tested this again by cutting the HeNe power with an ND filter to 2.57 μW as measured on the power meter. Using the formula for the lock-in, I calculated 2.63 μW of power.
I measured the power of the generated 633 beam using this technique and found a typical unchopped value to be 5 nW. About the same value as before, but I'm much more confident in the calculation technique. This gives an efficiency on the order of <math> 10^{-7} </math>, so the next steps should probably be better shaping the 780 beam since there don't seem to be many gains left to be made in alignment.
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 V peak-to-peak amplitude square wave should give a reading of 0.9 V on the lock-in <math>(4/(
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.
I walked the 780 beam while ramping (a small 633 signal is still detected when ramping since it flashes at a relatively low harmonic of the 500 Hz chopper speed). Presumably finding the max power when ramping will be the same as when locking. Increased the signal to 5.8 nW at the detector (accounting for chopper--I'm always going to do this since we could turn the chopper off and have a 5.8 nW beam, it would just be hard to detect), which means 5.8/.13 = 42 nW in the cavity.
Our current efficiency then is 21 nW/156 mW = <math> 1.35 * 10^{-7} </math> (use chopper-on value for 633 since we use the chopper-on value for 780).
Chopper frequency: 500 Hz (chopper is outputing the frequency to the lock in--one of several variations I've tried including using the lock-in to set the chopper frequency and using a function generator to set both frequencies. Having the chopper in charge seems to be the most stable).
Lock-In: signal input (using only built in amplifier)-- I (<math> 10^8 </math>), time constant-- 1 sec, slope/oct-- 12 dB, sensitivity-- 50 μV, reserve-- low noise, filters-- off
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