= ISC Meeting on 2020/03/09 14:00 - 15:00 =
Participants: Matteo Leonardi, Tomotada Akutsu, Masayuki Nakano, Koji Arai, Ryutaro Takahashi, Osamu Miyakawa, Shinji Miyoki, Koji Nagano, Hiro Yamamoto, Kiwamu Izumi, Yuta Michimura

Zoom meeting: https://zoom.us/j/6676627462

=== Next meeting ===
TBD

[[LCGT/subgroup/ifo/ISC/Meetings|back to Meetings page]]

== Minutes ==
 * DRFPMI trial was done when the alignment is not so good. With current alignment, DRFPMI might be possible [Masayuki]
 * We also need to consider if compound SRM is good or not. If not, we have to think about making a monolithic 0% SRM or 70% SRM (takes 0.5 years?). [Matteo]
  * Compound SRM was used for aLIGO O1 and O2, but KAGRA might have more issue with compound SRM since KAGRA have more HOMs at AS [Koji A.]
 * If ITM re-polishing is necessary to compensate TWE map, it takes 1 year with the most optimistic case to do them. [Matteo]
 * The effect of contrast defect and frequency/intensity noise coupling should be measured more directly. [Koji A.]
 * A/I for Michimura
  * Add these discussion to the document and send it to SEO
  * Draft: https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=11541

== Agenda ==
 * We want to know what to do for O4 with birefringence and ITM transmission asymmetry https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=11541
  * Cryogenic temperature?
  * SRM reflectivity: 0% or 70% or 85%
  * Polarizers in PRC and SRC?
  * ITM transmission asymmetry: re-coating necessary?
 * Summary of measurements so far
  * X-arm round-trip loss from reflectivity is 86(3) ppm (if we assume no birefringence) https://klog.icrr.u-tokyo.ac.jp/osl/?r=7307
  * ITMX and ITMY single bounce has 12% and 15% loss, respectively, including ITM transmission and losses to p-pol. Losses to p-pol measured at POP was 9.4% for ITMX and 4.6% for ITMY. Considering BS reflectivity is different for s-pol and p-pol., this corresponds to 6.1% for ITMX single bounce and 11% for ITMY single bounce. https://klog.icrr.u-tokyo.ac.jp/osl/?r=9314 https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=10369 https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=10388
  * Carrier power recycling gain for PRMI was about 3, which corresponds to PRC intra-cavity loss of 25% or so. Considering the sloshing effect, this is reasonable. http://klog.icrr.u-tokyo.ac.jp/osl/?r=9300
  * The mount of p-pol from ITMX is smaller by a factor of 3 with X-arm locked (mode healing effect). http://klog.icrr.u-tokyo.ac.jp/osl/?r=9393
  * Carrier power recycling gain when PRFPMI is locked is 12 for X-arm and 10 for Y-arm. http://klog.icrr.u-tokyo.ac.jp/osl/?r=12639
  * X-arm round-trip loss, finesse, and mode healing effect altogether gives X-arm loss on reflection give 86 ppm * ~1000 + 6.1 % / 3 = 11 % loss. So may be PRG of 10-12 for the carrier is reasonable. This is also consistent with X-arm reflectivity measurement done before we are aware of birefringence issue. https://klog.icrr.u-tokyo.ac.jp/osl/?r=7307
  * Power recycling gain for the sidebands are 2 or so (very rough estimate).
  * DRMI on 1f is fine. Lasts 30 min or so.
  * DRMI on 3f lasts 5 min or so. https://klog.icrr.u-tokyo.ac.jp/osl/?r=12535
  * CMRR for intensity noise. https://klog.icrr.u-tokyo.ac.jp/osl/?r=13028

 * Summary of calculations
  * Effect of ITM asymmtery to sensitivity: https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=10352
  * Effect of ITM asymmetry to ASC: https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=10359
  * Effect of inhomogeneity (without birefringence): https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.082005

 * List of measurements to be done before the vent for O4
  * Try DRFPMI locking for one more week.
  * Updated shot noise calculation https://klog.icrr.u-tokyo.ac.jp/osl/?r=12772
  * X-arm and Y-arm reflectivity/loss measurements with polarization considered
  * Re-measurement of CMRR for frequency noise https://klog.icrr.u-tokyo.ac.jp/osl/?r=13069
  * Measure power recycling gain for sidebands (POP 2f with FPMI locked and PRFPMI locked)
  * DRFPMI LSC and ASC sensing matrix
  * MICH/PRCL/SRCL to DARM coupling
  * Measure MICH contrast defect with MICH locked and FPMI locked
  * OMC cavity scan to scan mode content of AS beam
  * Measure if there are any structure to DARM sensitivity coming from compound SRM.
  * Scattered light measurement in PRC by sweeping PRM and PR2/3 deferentially.

 * List of calculations to be done
  * DRFPMI LSC and ASC sensing matrix with birefringence and ITM transmission asymmetry
  * Frequency noise and intensity noise coupling with birefringence and ITM transmission asymmetry
  * MICH/PRCL/SRCL to DARM coupling with birefringence and ITM transmission asymmetry

 * O4 Schedule https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=9209

 * Executive summary (tentative)
  * We are not sure yet whether birefringence is limiting our sensitivity and stability. DRFPMI locking probably possible with current interferometer.
  * Putting polarizers inside PRC and SRC might improve the sensitivity and stability, but it might introduce some extra issue (thermal lens, extra loss, scattered light etc).
  * May be better to focus on characterization of the interferometer and the preparation of better ITMs rather than the preparation of polarizers.
  * It is always good to prepare for the unexpected if we have manpower and money. Preparing polarizers now might save our lives in the future.
  * Measurement listed above must be performed before the vent for O4 to characterize the current situation and to make a plan for O4.