Main message: I've made another change to the squeezer laser current, which will hopefully give us a week or two without mode hopping, but it seems that we need to start planning on changing out the squeezer laser.
The first attachment shows a 60 day trend of the squeezer laser power, and the green power produced by the SHG degrading as the laser mode hopping becomes worse. Jeff B and I had made a small adjustment to the laser current last Friday, 49327 but this step was not large enough to fix the problem.
We have potentially three ways to quickly see that the laser is multi-mode, SHG scan, OPO scan, and by looking at the beatnote. Today it seemed like the OPO and SHG scans are both useful, but I could not tell the difference between the two current settings by looking at the beatnote. The second attached screenshots shows the SHG and OPO scans before and after lowering the laser current from 2A according to the laser controller to 1.857A on the laser controller. The readbacks for the laser current are consistently lower (shown in yellow in the screenshot) than the front panel readbacks. While watching both of these scans with the laser current set to the multi-mode value, I tried blocking the two main paths to see if that would change the behavoir. (While watching the SHG scan, I blocked the path which goes to the seed, clf and beatnote, saw not change in mode hopping in SHG scan. Then I blocked the SHG while watching seed scan of OPO, no change). So if the problem is due to laser feedback it is from a problem upstream of the split between the two paths, I looked through this path two weeks ago during Tuesday maintenance and again today.
This new current setting is low, which may mean that we cannot engage the noise eater with this setting. I changed the diode current nominal level to reflect the change.
A few more comments on the squeezer today:
Because the ref cav transmission was increased, there is now much more light coming to the squeezer fiber from the PSL, so I've increased H1:SQZ-FIBR_TRANS_DC_HIGH. I increased the limits on this PD, I do not think that we are in danger of saturating the beatnote diode which was why Daniel had enforced the limits in the first place (right now we have -3dBm of RF and half a mW of DC power).
The beatnote strength was initially decreased to -20dBm after I changed the laser current despite the increase in power from the PSL. I went back to the table and adjusted the alignment of the beam onto the beatnote diode, which brought the beatnote back up to -3dBm. Nutsinee had once before made a comment that changing the current on the squeezer laser seemed to change the downstream alignment, this could be consistent with that happening.
It seems that the SHG power servo has mostly been sitting at its rails for more than a week. For today I have reset the set point, so it is running. I also added a notification to the squeezer guardian that would at least tell the operators that it is railed. I think that this power stabilization would work more reliably if the laser were not running multi-mode.
The first attachment shows that the SHG power fluctuations which have been characteristic of the squeezer laser mode hopping have not returned (yet) after Tuesday's current change. The green power produced by the SHG has been dropping, along with the IR power transmitted by the SHG, but more slowly.
Keita suggested looking at the intensity noise out of the laser might be a usefull way to diagnose mode hopping while the cavities are blocked. The second attachment shows a spectrum from the laser power monitor diode, which is after the EOM. It does have a change in the spectrum for the low and high current times.
