Dan Gould (ANU), Georgia, Sheila, Daniel, Vicky
Following up observations of scattering transients during PSAMS actuations with intentional scatter tests, using longitudinal excitations on various ZMs in the SQZ path.
Sheila's screenshot attached below: When we open the beam diverter, we see 14 uW on the OFI monitor PD, which is 1% of what reaches SFI2. LLO:57943 AS_C reports that there is 140 mW from the IFO to HAM6, so we are getting 1% of the light headed towards HAM6 scattered to HAM7 at the current OFI temp. The current OFI set temp is 25 C.
From the frequency cut-off and amplitude of the scatter shelf, we can calculate the amount of scattered light from this 5000 cnt, 0.3 Hz ZM5-L excitation and go from there. Attached screenshot shows DARM with excitations on various ZMs. Seems like, shaking btwn OPO-FC1 doesn't scatter, but shaking btwn OPO-IFO path shows scatter.
(Louis, with help from Shiela & Keita)
I modeled the scatter and estimate the reflectivity at ~900 ppm (asd) [which translates to ~0.81 ppm in power]. The data from OMC DCPD_SUM_OUT has been calibrated to RIN and closed-loop DARM corrected using pydarm_modelparams_PostO3_H1_20220614.ini. The ZM4 was shaken with OSEM RMS L ~ 3um, but I estimate something closer to 4.9 um rms. I'm not sure where the discrepancy is coming from.
The red and green curves in the attached plots are the DARM-corrected OMC DCPD SUM_OUT channel during nominal operation and during the ZM4 injection, respectively, in RIN/rtHz. The blue curve is the modeled scatter at ZM4. The scattering model and nominal DCPD SUM_OUT channel are summed and shown in orange.
Code and data used for this is in /ligo/louis.dartez/Documents/archive/SQZ_scattering_20220824
Wouldn't a change of 3um in the ZM4 position make a ~6um length change in the light path? And 12um if it is doubled passed?
At this time ZMs were in positions ZM4(-218, 309), ZM5(-396, 426), ZM6(880, 816)
@Daniel
Yes, a 3um change at ZM4 corresponds to a total length change of ~6um in the light path (per reflection). This is accounted for in my script (now moved to gitlab).
While chasing down possible sources for the 4.9<--->3 um rms discrepancy, it was suggested that maybe we were actually looking at the second shelf (i.e. the double bounce / double reflection shelf).
The attached plot shows the scattering model adjusted so that the "double bounce" shelf matches DCPD_OUT during the ZM4 injection, placing the single-bounce shelf cutoff at ~25Hz.
For the ZM4 injection to have resulted in two shelves, we would expect to see /something/ near 25Hz in the green trace to convince us of the presence of a scattering shelf. This does not seem to be the case and I think we can safely assume that the 50Hz shelf present in the initial injection corresponds to the single-bounce scenario.