We've had several locklosses throughout the day due to the 18040 Hz ETMY PI mode.
This last lock at 25W, I finally found a combination of settings that seemed to be actively damping it according to the OMC DCPDs. (Using the QPD as the error signal, ETMY_PI_DAMP_MODE4, -60deg +30deg -3000gain). In the OMC DCPD spectrum, the peak was going down. According to the RMS monitor of this mode in the DCPDs, I was winning. However, according to the RMS monitor of this mode in the ETMY QPD, the mode was still going up. We lost lock because of this. I don't know of any DQed high freq channels for the QPDs, so we can't look back in time at the spectrum to see if that helps illuminate the situation.
Anyhow, if someone can help me understand how a mode can seem like it's going down according to the OMC DCPDs, but still actually be increasing according to the transmon QPDs and causing a lockloss, that would be great.
> Anyhow, if someone can help me understand how a mode can seem like it's going down according to the OMC DCPDs,
> but still actually be increasing according to the transmon QPDs and causing a lockloss, that would be great.
I guess there could be many explanations, but one easy one is that the dirt which produces the OMC signal might be alignment dependent. That is, for odd (n+m) optical modes the OMC transmission will depend on the OMC alignment. Since the OMC DC signal should (in a perfect world) be zero for these higher order modes, it might not be surprising that it sometimes actually approaches zero.