Kiwamu, Sheila
Today we saw that the power on the als refl PD drops when we misalign the ITM. (ITM misalinged, 13000 counts, ITM alinged, fringing up to 18000 counts). This would suggest that the ETM has a low reflectivity for green.
We went out to the end station and measured 34mW going into the chamber, and by misaligning the ETM measured 13mW returning. According the Keita the TMC efficiency is 90% each way, if this is correct for the polarization we are injecting the ETM reflectivity is 47%. (or if we assume the etm transmitts 24% of the green, it could mean that the TMS efficiency one way is 70%) We also measured 10mW rejected by the Faraday and 1.4mW in the hartman path.
The fringes that we saw in the reflected PD DC output and the signal out of the demod seem consistent with a verry low cavity finesse.
It may be worth checking the polarization of the light leaving the table. Keita noted that there was more green light in the IR QPD path this time than in end Y: alog 8705 However, the polarization should only change the efficiency of the TMS, it would not explain the signal on the refl PD.
According to the coating report that Betsy sent the ETM transmission at 532 is 24% https://dcc.ligo.org/DocDB/0059/C1103233/002/Coating%20Characterization%20Report_ETM08.pdf
Alexa and stefan measured the polarization at the bottom of the periscope to be spol: 8558
Looking at the data from last night more carefully, with the ITM misaligned we get 12675 counts on refl B LF, the top of the fringe is around 17500 counts and the bottom of the fringe is 15300 counts. The attached plot shows the ratio of the top of the fringe to the prompt reflection, and the bottom of the fringe to the prompt reflection, with predictions assuming the ITM R=99% and no other losses. Both the top and the bottom of the fringe are consistent with the ETM R=73%, not so different from the 76% on the nebula page. The other ETMs on the nebula page (including livingston) are similar. It seems like at least several of our ETMs are out of spec (spec was 3%-15%) , the cavity fringing is not terribly far off from what we would expect given the measurements from LMA. Also, estimating this using the fringes means that our measurement is sensitive to mode mismatch and misalingments, and we don't know how our mode matching or alingment are right now.
If the ITM has a reflectivity of 99% (we don't have a measurement of this, but that was intended) and the ETM 76%, we would get a cavity finesse of 22, and the reflected power on resonance would be about 93% of the reflected power on resonance. The cavity pole would be at 840 Hz. This will probably mean that the performance of our PDH lock is worse, but this may not be a serious problem since the noise of the PDH lock wasn't limiting us in HIFO Y.
The efficency of the TMS table still seems to be worse than expected. Assuming the ETM has R=76% , our measurement of the reflection off the ETM indicates that we loose 50% of our power in the TMS, or 70% of the power on each pass through the TMS. At end Y (alog 3077) Keita measured 61% of the injected green light returning to the table, so this was at worse 78% efficiency each pass through the TMS.
Summary:
the ETM is out of spec, but this was reported by LMA in the coating documentation, and pretty much explains the low finesse cavity we have.
TMS efficiency is worse at end X than at end Y for reasons we do not know yet.
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=3077
Look at the above alog entry from OAT and its followup below that.
I was wrong about silver mirror reflectivity (I looked at Thorlabs, Newport and CVI catalog and they're more like 97% or less rather than 98%), and this 1% difference makes a huge impact because we have four mirrors double path so it's 8 reflections.
Anyway, all included, TMS itself is supposed to have 67% double path efficiency.
During OAT the double path efficiency was supposed to be 66% including the 99% ETM but was measured to be 61%, but note a large uncertainty regarding the protected silver mirror coating. If we put this discrepancy in the silver coating reflectivity, it is 96% per silver coating.
One thing to note is that we changed the coating vendor of some (but not all) of the TMS optics, we now use Newport protected silver mirrors for F1 instead of Edmund for all but H1 TMSY.