Sheila, Koji, Robert, Evan, Alexa, Dan
We have made several measurements of backscattering from the OMC. It seems like the reflectivity of the OMC is smaller by a factor of about 20 than what was seen at LLO, and it seems that backscatter from the OMC is probably not limiting our DARM spectrum.
Two nights ago, we measured fringe wrapping by exciting the OMC suspension in the longitudnal direction, as well as by exciting OM1. (related alogs 17910 17904 17882) The attached plots show the DCPD RIM, with the DARM loop supression removed, with the excitations on.
- First, we noticed that the osem calibration must be wrong, for both OM1 and the OMC. For the OMC our knee frequency around 45 Hz indicates that indicates that our excitation amplitude was about 35.25 um. The osem readback indicates that our exitation amplitude was 11.8um, if we assume that this path length is double passed the osem is underestimating the motion by about 50%. The story for OM1 is similar, the osem indicates that OM1 was moving with an amplitude of 6.25 um, but the path length modulation must have been about 20 um based on the knee frequency. (Tobin's thesis, p 45)
- When we excite the OMC, we see two scattring shelves, the second has a finge velocity twice that of the first path. For the first shelf we get an amplitude reflectivity of 1e-5, for the second shelf it is 1e-6. These are eyeball fits, but they seem to be good to within about 15%. At LLO (alog 13607) similar measurements showed an r = 2e-4 for their first shelf.
- One speculation is that the first shelf could come from scattering within the OMC itself, and the second from the OMC refl path. Yesterday (after these measurements were made) Koji realinged the OMC refl QODs, and the amplitude of the second shelf is reduced today.
- FOR OM1 we only see one shelf, because OM1 is not the scattering source. With this we are only modulating the path length between the IFO and the scattering source. However, we measure the same reflectivity as the first shelf when exciting the OMC, so the two measurements are consisitent.
Tonight Jeff made a test of turning off the HAM6 sensor correction, as was done at LLO (third attachment) (LLO alogs 16814). The spectrum is attached, but we do not see the dramatic fringe wrapping seen at LLO. We would expect the impact to be smaller here than in LLO because our scattering amplitude is smaller and it is also likely that the microseism could have been smaller here.
Today we Robert Koji and I made injections into all 6 DOFs on the OMC to see fringe wrapping. We saw nothing by exciting roll or vertical, we were able to produce shelves by exciting L, T, P and Y. The last screenshot attached shows the sectra with the various excitations on. For the record, here are times, all excitations were at 0.2 Hz, into the test filter banks. While I've attached spectra of these, several off these shelves were moving around durring the measurement because of some lower frequency motion.
| DOF | ampltide counts | time April 16-17th UTC |
| L | 20000 | 23:47-23:51 |
| T | 20000 | 23:59-0:05 |
| V | 20000 | 0:14-0:18 |
| P | 2000 | 0:24-0:29 |
| Y | 200 | 0:31-0:35 |
| R | 2000 | 0:39-0:42 |
Thanks to Sheila for logging my scattering measurements, apologies for not putting it up myself. A few more relevant comments on it: - The experiment involved turning off both the HAM6 *and* HAM5 sensor correction (independently). - HAM6 shows no affect but HAM5 caused lots of non-stationary noise, from which the captured HAM5 curve is only a representative bump/glitch/effect. - Just after I got that spectra, the IFO broke lock. This is why I didn't get an ASD of the GS13s/CPSs on the ISIs exposing the full region where sensor correction ON/OFF should have an impact (down to ~0.1 [Hz], since we're using the Hua, FIR sensor correction on all DOFs on all the HAMs). My locking skills are still minimal, so I wasn't able to bring the IFO up past 1f DRMI (the ISC_LOCK message complain of to little light on AS90, I tried nudging the BS out of ignorance, and that re-broke the DRMI lock, and the next automation attempt failed during ALS acquisition and I gave up).
There was some question about the shotnoise RIN level in the ISC meeting. We have ~20mA total current on the OMC DCPDs.
This corresponds to the shotnoise of 4e-9/rtHz. It is consistent with these attached plots.
DCPD1 and DCPD2 are perfectly coherent around the shelves.
At Stefan's suggestion, here's the coherence between DCPD1 and DCPD2 around the injection shelves. The coherence is almost 1 for the first shelf. As for the second shelf the coherence is not as perfect but it is almost 1 at the highest peak of the scattering shelf, and the flat part of this shelf is already pretty close to the noise floor.
We're either looking at something that comes through the OMC (as opposed to large angle scattering reflected by some random thing and unfortunately falling on the DCPDs), which is more likely, or something that come from the opposite side of the BS for the DCPDs, which sounds unlikely.
