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Re: Laser linewidth issue:
We finally made a little progress on this problem. So it appears there is a problem with the fast feedback, some sort of ground issue. Deniz showed pretty dramatically that just touching the BNC ground to the laser feedback terminal on the driver totally messed up the error signal. As much as i would like the electronics to not be responsible for problems, it appears that they are. I
*If we tune up the gains, piezo gain basically low as workable, fast feedback not too low but not too high(where it looks best when ramping), we get a reasonably stable lock. If we turn the fast feedback gain too low we see wiggles at the piezo resonance freq in the transmission. If the fast feedback gain is too high we see slow oscillations at around 300Hz, evidently the noise on the laser that was responsible for the more or less pulsed behavior of our green light.
*The Fabry-Perot apparently led us astray. Either we were looking at the peaks while the fast feedback was connected (i could swear it wasn't) or the cavity linewidth is not as good as we thought. I thought that the linewidth changed slightly when we tuned up the fast feedback by doing the threshold test, indicating not being limited by the cavity linewidth, but maybe that was wishful thinking and we were actually limited by the cavity linewidth.
*Then again, that's why we wanted to look with the interferometer, which worked very well. With the fast feedback disconnected, we observed a lindewidth of about .5MHz, very reasonable. However, with the fast feedback connected, even if it's off, the linewidth is more like 5-10MHz. Even just touching the ground of the feedback connection BNC connector ground totally disturbs it.
*One piece of information that doesn't jive is the frequency of the noise oscillations, ~300Hz? it's not a line freq or harmonic. One idea: perhaps there is some sort of feedback or cross-talk between the two feedback branches. What if the piezo feedback is modulating the cavity transmission a bit, i.e. gain at the SHG piezo resonance freq is not sufficiently low, there's some wiggle. If this modulation is being fed back some to the laser, maybe that's responsible to the increase in lindewidth. I don't know if something along those lines could be the mechanism, but if it were, swapping the piezo could really help. That's our next idea priority.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 16:45, 7 July 2014 (CDT)
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