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==Daily Log==
'''3/16/15'''
Major findings of the last month:
We got the interferometer set up again and, surprise, there's no linewidth narrowing from the ebay mirrors. So I switched back to the layertec mirrors (the two curved ones--again it's unclear why it doesn't work well with one curved and one flat). The linewidth looks around 20 KHz, which is at the limit of what the interferometer can measure, so the narrowing is potential less. Some sources suggest you need a substantially longer fiber delay than the coherence time of your last (like 6 times as long) but I've seen it quoted that being equal is fine too. It's clearly narrowing substantially though.
The layertec mirrors never locked very well. I attributed this to bad slow feedback for a while (all the piezo would ever do is rail). It seems like the actual problem though was etaloning between the fiber launch and some other element. Essentially the pre-locking cavity peaks were inside a larger envelope of some much lower finesse cavity. This explains the strangely high sensitivity the walking mirrors seemed to show. I put an isolator in which mostly fixed the problem. It seemed to substantially reduce the error signal and transmitted signal though--way more than the 20% or so lost through the isolator. I'm unsure why since it the shape of the beam shouldn't be affected. But the cavity locks okayish for now--upwards of 10 minutes when working well. Improvement can certainly be made, but for now I'm working on the Raman cavity since the basic features I want from the pre-locking cavity are there (much longer lock times than the Raman cavity and substantial linewidth narrowing).
Little luck with locking the Raman cavity. I've been working on making a slow-feedback locking circuit (see Zach's description under equipment list) but am still having a few issues. I borrowed on of his completed circuits though and haven't had luck, but still have a few things to try. I had thought our old slow box was broken since the power supply always went a little crazy when I plugged it in, but I realized today that the manometer is actually drawing way more current than it should. Plugging the slow box in just always pushed the power supply over the edge. The -15 V line was actually running closer to -11 V. I'm not sure how long this has been going on. I've just unplugged the manometer for now since I don't really need it when I'm just operating under vacuum.
The low pass filter after the mixer on the PDH setups seems to be greatly lowering the error signal. Like most things, I'm not sure why. I'm taking them out for now on the grounds the added signal is pretty much too high frequency to do anything in the servos, and any detrimental effect it has is probably greatly outweighed by the decrease in SNR.
I'm getting worried this isn't going to work. I guess that was a risk when I started this, but I really thought if we got the laser linewidth on the order of the cavity linewidth we could lock with just slow feedback. I don't really see how that's different from what Nick does with the SHG cavity. At least I've learned a ton, and I've really only been trying something that might potential work for a day or two. I can always table this whole things for a bit and come back to it in a few months. I think I might be starting to become frustrated with it to the point where I'm not working effectively. Probably in another week or so I'll start trying to set things up to modulate the pulsed orange laser, which is something I think I can do.
'''2/11/15'''
Got very poor locking by using the old 1555 slow feedback box on the Raman cavity while the laser was locked to the low finesse cavity. I was having trouble sending two error signals to the Vescent box since they seemed to interfere with each other, so I'm trying this method for now. Having the laser locked to the low finesse cavity seemed to help stability with the Raman cavity, but not for transmitted power. I think we're not getting any or enough linewidth narrowing, so I'm going to swap the cavity mirrors again. I think the problem with the Layertec mirrors wasn't that they were too high fiensse, but that we weren't impedance matched (I still don't think we understand that very much though). So I'm going to use the two curved mirrors which should be pretty identical and presumably more reflective than the ebay mirrors.
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