Since we've had more mode hopping of the squeezer laser in the last few days, I went out this morning to change the current and temperature settings.
Initial settings: 1.936 A, 33.72C Final settings: 2.107A, 31.68C
Before starting, I could clearly see that the laser was mulitmode by scanning the SHG. Increasing the laser current slightly (2.015A) was enough that I could no longer see the second mode, but the beat note dropped in strength to -25dBm at the PFD monitor, and it became difficult/imposible to move the laser temperature to bring back the beatnote. Watching the SHG scan, I moved the current up without following with the temperature, and found that it just started to become multimode when the current reached 2.234A, near the max.
I set the current to about half way between the two places where I saw mode hopping, 2.107A, which is fairly close to what Nutinsee had in January 46587 (around 33.67C). When we found the beatnote there it was very small and seemed to move a lot, so we tried a large temperature search, and found a good beatnote at 31.68C.
The attached screenshot shows a history of the mode hopping behavoir of this laser over the last several months, each time the SHG trans and green power monitors become noisy, we adjust the laser current, and things are better for a while before it starts hopping again. It seems that the change last week (48361) didn't help much.
This is the laser diode current plotted over the past 400 days. The jumps are associated with our tweaking. We might expect the current to be constant in between, but it isn't.
This plot shows a couple of SHG locks during a 20 minute period with a bad diode laser current. The SHG initially locks with high conversion efficiency which then drifts down over several minutes. Once relocked, it starts again at high efficiency. While the laser diode current is stable, we see a correlation with the laser output power. I am wondering, if we are back feeding some of the SHG reflected light which in turn aggravates our mode hopping problems. We should consider a second Faraday isolator in the IR path.
