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--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 19:36, 3 December 2014 (CST)
Made a little bit of headway today with the PDH.
* Tuned up the FP cavity modes; ended up moving the cavity and mode-matching lens a fair amount, probably about an inch, putting the distance from the front mirror very close to where ABCD predicts it should be. Lesson learned: turns out it is much easier to move the lens than the cavity. Was able to get it pretty much single-mode at times.
* Strung together the PDH electronics and it worked! well we could see an error signal anyway. Error signal looks pretty much like the Black paper. Did not have to struggle with the phase-shift in order to get a proper-looking error signal.
* Stuck the lock-box in the set-up. We can turn on the fast feedback and see the central resonance broaden, as we expect, as we zoom in on the peak (i.e. adjust the piezo DC and shrink the ramp), the fast feedback intermittently locks on its own. This is pretty encouraging, so at least the FFB seems to be working. However, when we turn from ramp to lock, it doesn't lock. There is a problem with the slow (piezo) feedback branch. Probably the first thing to look at is the switch.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 22:13, 2 December 2014 (CST)
Spent much of the day messing with the Fabry-Perot cavity. I removed the cavity and checked to see what the beam looks like in the position of the cavity; it looks very similar to what my ABCD matrix script predicts if i remove the input coupler, so i am confident that my mode-matching is decent. I mentioned before that i did not see a Finesse above about 20, it turns out these were not the cavity resonances we were looking for. Nick suggested looking at more extreme orientations. I turned the cavity 90deg in its mount, i.e. rotated the whole can, and then searched for more peaks. Then i saw much sharper peaks, finesse>500. My leading theory is that when i glued the piezo back on, i must have glued it off-center. This led to the cavity resonance mode being at a weird angle and thus hard to find. Perhaps the crappy peaks we were seeing were etaloning between a cavity mirror and the windows? or some other cavity-like effect? Those peaks were very clean, we didn't see neighboring modes, they also benefited slightly from turning the polarization, two differences from what we see now.
Hopefully we can get the electronics all set up now that this cavity is working better.
--[[User:Zjsimmons|Zjsimmons]] ([[User talk:Zjsimmons|talk]]) 17:46, 1 December 2014 (CST)
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