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MIF (General)
Haoyu Wang - 0:46 Wednesday 31 August 2022 (21953) Print this report
polarization measurements at POP and POS

[Takafumi Ushiba, Yuta Michimura, Haoyu Wang]

 

After installing the whitening filter for POS PDs (IR s-pol and IR p-pol), see klog#21952, we did power measurements for the s-pol and p-pol with the current configurations for POP and POS. We didn't change any optics yet. The POP power is measured with CCD cameras following the same method in klog#21759. The power at POS is measured with the two DCPDs for s-pol (K1:LSC-POS_SPOL_DC_OUT16) and p-pol (K1:LSC-POS_PPOL_DC_OUT16). We found the POS s-pol and p-pol signals fluctuate quite a lot. We checked their spectrum and found this fluctuation comes from the peak at around 120Hz. So we take the average of the 10 second data for each signal for four times, and then do the average of the 4 data slices.

  POP SUM POP EXP POP SUM/EXP POP_P SUM POP_P EXP POP_P SUM/EXP POS s-pol POS p-pol
ITMX single 4.9e6 750 6533.3 8.2e6 10000 820 1919±10 50.7±0.4
Xarm locked 4.8e6 750 6400 7.5e5 10000 75 1933±21 23.8±0.5
ITMY single 4.7e6 750 6266.7 4.6e6 10000 460 1785±5 342±1
Yarm locked 4.8e6 750 6400 6.5e5 10000 65 1830±10 139±5

 

After the measurements, we worried the gate valve in the tube may introduce clipping of the beam and we wanted to measure the mode matching ratio. We do this with the green laser with CARM and DARM locked. Then we add offset to the green CARM gradually and watch the TMS PDs for both arms. The mode mismatch is quite small (<1%). The misalignment (TEM01) is around 10% for both arms.

Comments to this report:
Haoyu Wang - 2:06 Thursday 01 September 2022 (21965) Print this report

We calibrated the measured beam power at POP and POS for reflection/transmission imbalance for s-pol and p-pol. The following table shows the calibrated power of s-pol and p-pol. For power at POP, we calibrated the effect of central BS (R:T=50:50 for s-pol and R:T=20:80 for p-pol [1]) and the effecct of RBS1 (T=48% for s-pol and T=78% for p-pol). For power measured at POS, we only calibrated the central BS effect. The round-trip loss is calculated using this method. If there is birefringence, in principal, we should use the total power (s-pol + p-pol) to calculate the round-trip loss. If we only use s-pol, the round-trip loss will be under estimated.

Measrement at POP        
  POP SUM/EXP POP_P SUM/EXP Calibrated s-pol Calibrated s-pol Total power P-pol content Round trip loss (ppm)
X single 6533.3 820 27222.08 1314.103 28536.19 4.605039% 72.55784 (Use s+p power)
X lock 6400 75 26666.67 120.1923 26786.86 0.448699% 27.11463 (only use s-pol)
Y single 6266.7 460 26035.31 2948.718 28984.03 10.17359% 80.28122 (Use s+p power)
Y lock 6400 65 26589.12 416.6667 27005.78 1.542879% -19.1902 (only use s-pol)
               
Measrement at POS        
  PD s-pol PD p-pol Calibrated s-pol Calibrated p-pol Total power P-pol content Round trip loss (ppm)
X single 1919 50.7 3838 253.5 4091.5 6.195772% 33.36619 (Use s+p power)
X lock 1933 23.8 3866 119 3985 2.986198% -3.66163 (only use s-pol)
Y single 1785 342 3570 427.5 3997.5 10.69418% 49.959 (Use s+p power)
Y lock 1830 139 3660 173.75 3833.75 4.532116% -23.5668 (only use s-pol)

 

For comparison, we calibrated the measured beam power at POP by Matteo in klog#21759 and klog#21777.

  POP SUM/EXP POP_P SUM/EXP Calibrated s-pol Calibrated p-pol Total power P-pol content Round trip loss (ppm)
X single 6750 859.6 28043.21 1377.564 29420.77 4.682284% 300.6861 (Use s+p power)
X locked 5125 177.6 21292.06 284.6154 21576.68 1.319088% 271.9342 (only use s-pol)
X single 6800 886.5 28250.93 1420.673 29671.61 4.787988% 330.4912 (Use s+p power)
X locked 4933.3 292.7 20495.64 469.0705 20964.71 2.237429% 309.4608 (only use s-pol)
               
Y single 7250 484.9 30120.48 3108.333 33228.82 9.354331% 77.74787 (Use s+p power)
Y locked 7276.1 126 30228.92 807.6923 31036.61 2.602386% 0.444444 (only use s-pol)
Y single 7163.7 468.1 29761.94 3000.641 32762.59 9.158743% 56.81537 (Use s+p power)
Y locked 7337.9 114.3 30485.67 732.6923 31218.36 2.346992% -22.5745 (only use s-pol)
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