KAGRA Logbook

CAL (General)

takahiro.yamamoto - 1:41 Thursday 03 April 2025 (33198)

1.5dB change in the optical gain on the latest OLTF

Abstract

I evaluated a current optical gain from the latest OLTF.
A change in optical gain is -1.5dB, so current sensitivity may be overestimated as ~1.5dB.
But I have no good idea now to explain this change.
So I gave up to update the front-end calibration today.

Details

DARM OLTF was measured by Kenta in this evening. So I estimated a latest optical gain and it changes as -1.5dB from last calibration update in Oct. (see also Fig.1-3).

When Ushiba-kun measured OLTF in March, he found a 2dB increase in the optical gain. At that time, the OLTF measurement was not completed due to the lockloss and he added -2.08dB gain in LSC_OMC_DC in order to keep calibration without the update of the calibration parameters. On the other hand, I heard from Kenta that he added +2dB gain in LSC_OMC_DC to keep a same UGF as the previous measurement today. +2dB gain added today and -2.08dB added in March are almost cancelled out each other. And also, commissioners kept UGF among Feb., Mar., and today. So there is no reasonable explanation of a -1.5dB change (filter changes without klog post, a change in the actuator efficiency, or else?).

Due to some tiresome troubles on fotonpick, I could not finish a check of all DARM loop components by 10pm and completing it remotely is too tough work. Not that it's practical to do the update remotely in the first place without a help from someone in the control room even if I can complete a check. So I gave up the update of the front-end calibration today.

Images attached to this report

33198_1743611869_Screenshot from 2025-04-02 21-32-43.png

Comments to this report:

takahiro.yamamoto - 14:41 Thursday 03 April 2025 (33213)

According to Fig.3 in the original post (klog#33198), optical gain is increased as 1.5dB (246.601dB -> 248.094dB). So current sensitivity seems to be underestimated as 1.5dB not overestimated.
The original post is a talk in my sleep.

Changes from Feb are as follows.
IM_LOCK_L (Fig.1)
- notch filter was switched from FM2 to FM9
- notch design in FM10 was modified

MN_LOCK_L (Fig.2)
- notch filter was switched from FM8 to FM9

LSC_OMC_DC (Fig.3)
- FM2 (+2dB) and FM3 (-2.08dB) were added.

So there is no change in the overall gain due to the digital filters and 1.5dB difference should come from an increase of optical gain or actuator efficiency. Temperature of IM was ~10K different between Feb (T1 cursor) and yesterday (T2 cursor) as shown in Fig.4. cross hair shows a time around previous best(?) sensitivity discussed in klog#33015 and klog#33099. Though I have no idea to the temperature dependency of the actuator efficiency around 80K, we may be able to predict that a 1.5dB change comes from the optical gain or actuator efficiency by the comment from CRY folks. (According to Komori-kun's comment in the morning briefing, mismatch in the shot noise level is consistent with an assumption that 1.5dB change comes from one in the optical gain...?) If it's difficult to conclude by a discussion only with current results, it had better to do the full calibration after IM temperature becomes stable.

BTW, the full calibration process requires non-short time (~2-3 hrs. per once). So frequent execution of the full calib. process steals a lot of time from commissioning activity. From the view point of the time sharing, improving the stability of mirror temperature and cry-cooler becomes important if we need to take care about temperature dependency of the actuator efficiency around current temperature.

Images attached to this comment

33213_1743658123_Screenshot from 2025-04-03 13-40-20.png

takahiro.yamamoto - 20:51 Friday 04 April 2025 (33260)

I updated the calibration parameters including FMs on LSC_OMC_DC and LOCK_Ls.

There is no change for the fact that it's better to check the actuator efficiency in the current temperature once.
On the other hand, change in the filter design should be updated from the view point of storing the history of filter changes.
Current displacement (though IFO is being in down today) is being served with most likely parameters based on the measurement on Apr. 2.