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masahide.tamaki - 9:03 Thursday 18 August 2022 (21799) Print this report
GR beam Power Budget (TMSX)

[Ushiba, Tamaki]

Measurent

Fig1

We measured GR beam power budget (TMSX) on the points I showed in the Fig1 and took average of PD/QPD counts to know the relation between them.
These points are basically locate before/after the BS, but I forgot to measure at the point ⑥, which is after GBS1 (before GBS2).

As for count of PD/QPD, we took a 10-sec average (so 5 sec longer than the time in previous similar measurement) because the power was very sharky (after earthquake).
Moreover, the average was measured at each place because of the same reason.

* We used the PD-type power meter.
* We did the measurements with the GRD paused (the power meter blocked the beam which entered PD), so it was always run with care to ensure that GR lock was not lost. 

Results

The results are shown in Table1.

Place Power [mW] GR_PDA1_IN1 [cnt] GR_QPDA1_SUM [cnt] GR_QPDA_2_SUM [cnt]
① 2.02±0.42 1276.92 11086.4 9285.54
② 0.502±0.061 1227.53 10629.2 8834.94
③ 0.442±0.095 1208.10 10455.7 8685.53
④ 0.800±0.168 1252.26 10107.8 7551.74
⑤ 0.490±0.104 1297.99 11252.5 9341.58

＊ As for spec of BS or STMs, please see https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=8962.
① ≈ ② + ③ + ④ (There is a lot of fluctuation, which can be subtle.)
② + ③ ≈ ④ and ② ≈ ③  

Analysis (partway)

  1. We can see the responsibility of this GR PD (PDA100A2) is about 0.320 A/W @532 nm (Fig2 or https://www.thorlabs.co.jp/drawings/499f960e07673103-BB0C2227-DF16-A1AC-104D600C5463F9CA/PDA100A2-Manual.pdf).
    Then, (0.490±0.104) mW (laser power into PD) → (0.490±0.104) mW × 0.320 = (0.157±0.033) mA.
    Since 1 count = 610 μV in KAGRA ADC, 1298.0 - 10.8 = 1287.2 cnt → 0.78519 V (＊The value of PD count in the dark is 10.8 [cnt] according to the value in medm).
    So the gain is 0.78519 V / (0.157±0.033) mA = (5.00±1.05)×10^3 V/A.
    The value of gain step was set to 10 dB (Fig3), so this value shows the power measurement was not largely wrong (please see Fig4 or the link atattched above).
     
  2. I heared the GR power after Faraday Isolator is 7.1 mW (maybe klog17454?), so it seems 2.02 mW at ① is low. 
    As for this, I was told that there may be some reason considering the past measurement,.
    However, even if I read the past log, there were a few things that were not clear to me, so I will write about this tomorrow after asking a few questions.
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Comments to this report:
masahide.tamaki - 14:35 Thursday 18 August 2022 (21811) Print this report

Analysis (continuation)

memo

   

Detail

The GR beam power was 7.1 mW after FI (Ⓐ) according to klog17454.
I thought it decreases by the reflection or transmission (PR2 trans: ~0.9, PR3 refl: >0.9, BS trans: 0.994  ⇒ kagrawiki), so before IX (so Ⓑ) the power would be 7.1×(0.9×0.9×0.994)~7.1×0.81=5.8 mW.
However, in klog17444, one-way loss was reported to be 0.62 (calculated from the power of IX and EX refl like in the memo above).
In this log, it is said that the reason of loss is unknown yet, but anyway, the power at Ⓑ is 7.1×0.62=4.4 mW.

Next, the cavity transmission is calculated using the data in klog17444, and it is 0.9995~1.0 (see the memo above).
Then, the power at Ⓒ can be written as 4.4t_h mW, where t_h is high order mode transmissivity.

The alignment was not good yesterday, so we took PD count average again, and it was 1550 cnt.
Using the yesterday data, laser power at  Ⓒ can be estimated to be 2.0×1550/1277=2.4 mW.

Therefore, 4.4tht_h mW=2.4 mW  ∴ t_h=0.55.
I didn't consider the loss in TMS chamber, but I don't think it is large, so it seems that high oder mode trans is a little small.

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tomotada.akutsu - 20:24 Thursday 18 August 2022 (21816) Print this report

Just a note: in my measurement and calculation in 17444, the 0.62 loss is maybe before the "HR surface" of IX; this means the location where this happens is not necessarily "before" the IX physical body, as the IX HR is facing inside the arm cavity. Loss within the IX substrate body might be one possiblity (I'm looking for D. Chen or Abe's such analysis...).

masahide.tamaki - 21:57 Thursday 18 August 2022 (21820) Print this report
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