Bots, Bureaucrats, Interface administrators, Suppressors, Administrators, Upload Wizard campaign editors
573
edits
David Notebook: Difference between revisions
Jump to navigation
Jump to search
→Daily Log
|
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
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 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 as an example. However, I am getting closer to 0.8V. Values for simple sine waves are off by about the same amount. This lock-in should be
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.
Lasers are dangerous--we should set up a curtain around the computer and Wednesday afternoon lab laser tag should probably be cancelled.
'''7/15/14'''
Further 780 alignment optimizations have raised the 633 power at the photodiode to 1.9 nW (3.8 accounting for beam chopper). I put in a 780 notch filter right before the photodiode to make adjusting the 780 alignment easier--the scattered 780 at the photodiode changes when adjusting its alignment, which washes out a change in the 633 signal. Hopefully this will make changes in 633 easier to detect.
| |||