• KAGRA
• Subgroups
• MIF
• Simulation
• SimMeeting20210302

Simulation meeting 2021 Mar 2 17:00

Zoom2: https://zoom.us/j/6676627462

Participants

Kokeyama, Hirose, Wang, Yamamoto H, Aso, Miyakawa, June Gyu, Eisenmann, Somiya, Michimura

Agenda

Updates from Projects

• GUI
  • Lense sensing noise is implemented. Quantum noise is under way.
• Ifo model
  • the purpose of this project is to set up and provide FINESSE models for other specific project like the OMC scan.
  • The implementing of mirror maps has several procedures. Phase tunings of PRC and SRM are still under test after implementing maps.
  • Only mirror maps of test masses in the arms are implemented.
  • We don't have mirror maps for recycling mirrors. But we have maps for the BS. We couldn't find a way to implement the BS map, so in current model, the BS map is ignored.
  • The phase of PRM and SRM is usually set 0 or 90 degrees depending on resonance/anti-resonance and power/signal recycling for the carrier. However, the actual tuning is more complicated for recycling cavities as it also depends on the resonance and frequency of sidebands. Note that the PRC has something to do with the RF sidebands, while the SRC relates to the signal sidebands.
• OMC model
  • Some simulations have been done and June Gyu-san will show this on next meeting.
• Birefringence
  • We have developed the model which can calculate the birefringence angle based on the coupling matrix.
  • The birefringence angle of ITMX used for KAGRA is calculated. It is compared with the measurement of a discarded ITM (same growth method of installed ITMs). It seems that they have similar structures. But they are different mirrors and it is hard to see the effectiveness of the comparison. We don't have TWE maps for this discarded ITM to calculate its birefringence angle. But an interferometer is being set up which can take TWE map measurements in the future.
  • At the moment, we have TWE maps for a spare ETM and the birefringence measurement is done. We are going to compare simulations to measurements for this mirror.
• Further discussions related to birefringence:
  • Yamamoto H: Are these efforts for birefringence going to be used in O4? What's the best sensitivity with sapphire mirrors?
  • Aso: Yes. First of all, we need to know the limitation imposed by birefringence if we continue to use sapphire mirrors. Secondly, we need to make mirror for O5 (not O4) by summer which is an intense work. We are unlikely to make birefringence free material so soon. O5 looks far away but actually not so much. If you look at the schedule, we need to start the production of the O5 mirror. So we need to use birefringence mirrors. The best candidates right now are the two crystals produced by GTAT before, which still have birefringence but the birefringence pattern is much more smoother. What we can do is to produce new ITMs using this kind of crystal. What we want to do is 1) to make sure the imbalance of reflectivity between the two ITMs (the current two crystals have slightly different reflections which is bad), 2) the current polishing uses circular polarized beam done by Zygo for the measurement which is wrong. KAGRA uses s-polarized beam and has great inhomogeneity in the reflection due to this incorrect polishing. We need to make sure the new polishing uses the correct measurement. So These will solve two problems out of three. The third one the the birefringence which we cannot solve. The question is if it is worth doing. If we solve two problems out of three, do we gain something for O5? This question is very important, because if we want to make new ITMs starting very soon, we need a lot of money. So it depends on the decision whether to spend a million US dollar or not for the production of new mirrors but still has birefringence. Therefore, precisely assessing the problem of birefringence and its impact on the current sensitivity, and knowing the question how severely we will limited by the birefringence are very important and actually very urgent.
  • Michimura: I think the first thing we can try is to use the model to calculate frequency noise coupling and intensity noise coupling with and without birefringence. We can also take into account the reflectivity imbalance of ITMs.
  • Yamamoto H: For LIGO O2 run, the ITMX in Hanford site has one point absorber and that makes the transmission map of ITMs asymmetric, and make the alignment control difficult. So LIGO had to change the ITM with the one which doesn't have a point absorber to make the transmission symmetric. As Aso-san says the current ITMs in KAGRA has reflectivity non-uniformity which can impact on the control and introduce some noises, the reflectivity nonuniformity may be another thing that should be included in the calculation of noise. We can check the LIGO case in O2 as a real case example.
  • Yamamoto H: When a new crystal is made, what kind of non-uniformity is allowed, or what is the upper limit of the non-uniformity. These are questions that need to be consider. You don't need to worry about the point absorber. The non-uniformity of reflectivity in the ITM is the issue.
  • Somiya: I have a question for Aso-san. Have we estimated the impact of the two issues that can be solved in O5 by replacing the mirror?
  • Aso: I think Yamamoto-san estimates the reflectivity asymmetry impact. That's it, not the index of reflectivity and nonuniformity problem.
  • Somiya: That one has been reported in our paper, right? But there are some remaining issues and we need to combine them. I wonder if the birefringence impact is not so big while the other two impacts are big, I guess it maybe worse to pay a lot of money replacing the mirror.
  • Aso: Yeah, that's the question and it is really the relative thing.