NAOJ GW Elog Logbook 3.2
Marc and Yuhang
Marc and I were suspecting that the beam may not hit on the center of mirror. To check that, we decide to scan BS pitch/yaw and check IR transmission.
Today I did this check. I found two resonant beams while scanning BS, which can be seen from the attached video (two videos show the situation of pitch and yaw separately).
Yaw: https://drive.google.com/file/d/1ahDx6MTfYqxdoFXbyH0PbrLRPybCtIhp/view?usp=sharing
Pitch: https://drive.google.com/file/d/1N6N3BdUr3WIG_GeNLJeDQDmCbeHzdTXm/view?usp=sharing
By pointing the incident beam towards the center of mirror, these two resonant beams get closer. In the end, they still don't overlap and separate by roughly a HWHM. In the end, the pointing to the center has a precision of 2urad, considering the distance of 300m, this is 0.6mm precision.
After centering the beam, I measured the IR length noise by using BAB. Attached figure 1 shows their difference. We can see that the low frequency length change is greatly reduced. Note: the calibration of IR length is done by taking the slope of PDH signal as attached figure 2, 3, 4, 5 for on resonance and detuned case, which may introduce some error. Especially, the calibration of the green line (beam off-center and on resonance) was using a PDH signal with opposite slope compared with others.
| old center for pointing | new center for pointing | |
| pitch | 24 | 4.5 |
| yaw | -8 | 16 |
The attached figure 6 shows the new location of beam now. This moves the beam by 6mm in both yaw and pitch on end mirror. We should go to end room and center beam on both camera and PSD.
In addition, the AOM frequency changed by 2415Hz, which indicates the GR and IR length difference is 2415/2e-12m = 1.2075nm. This tells us, with the residual pointing error of IR, we have 1nm GR/IR length difference after beam moves 6mm on mirror. This phenomon explains the IR detuning change issue for different alignment of GR.
I was using AOM scanning speed as 4000Hz/1.7s in the calibration. However, since the scanning speed for IR is 1/2 of the value for GR, the figure in the old elog was wrong.
Calibration for the measured spectrum should be: calibration = 2000/1.66666*11.5/11.2 #Hz/V (PDH: 11.2mV/11.5ms) (AOM: 4000Hz/1.66666s)
There was also problem for the calibration for off-center on-resonance, I modified the plot by using a more reasonable calibration. It comes from the center on-resonance. The new plot is shown in the attached figure.
We can see the new stable optical axis makes especially the low frequency length noise reduced. However, the high frequency noise is increased a bit.
The high frequency noise is same for old and new beam spots, but is increased for 50Hz detuning compared with the one on resonance. This noise difference could be explained by the cavity pole effect. The cavity pole effect for 50Hz detuning (half detune) is smaller than the one on resonance by a factor of ~sqrt(2). Please check P.50 of LIGO-T1800447 for the cavity pole effect of detuned cavity.