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David Notebook: Difference between revisions
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Also it basically refuses to lock by adjusting the cavity piezo and you have to use the DC offset on the lock-box instead. Who knows?
The necessary locking settings are very different even at just 0.1 atm. I have the first integrator all the way down at 2 kHz. The proportional gain and optical feedback I've had to adjust all over the place. I had it locking well briefly and was getting about 150 mW, but lost it pretty quickly when trying to optimize it. The window of "good" settings seems much smaller with gas. The laser piezo is especially difficult--with the auxiliary servo gain too high (or just the overall gain too high), it maxes out the piezo, but when just a bit lower it doesn't seem to do anything. In fact I would get locking signals that were reminiscent of ramping with just the fast feedback on back in the old days (suggesting that the slow feedback wasn't doing much).
I did finally figure out where I would get weird signals from the transmitted 1555 photodiode (sometimes negative, sometimes positive when locking; the signal would quickly jump up and then fall down to center around zero even with the output power fairly constant). They seemed like symptoms of AC coupling even though everything as set to DC, but it turned out the photodiode is premade to AC couple. That's slightly annoying because it makes it hard to tell how good a lock I have without measuring it, but a new photodiode it expensive (we don't have any extras at this wavelength), so I'll make do for now.
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