Philippe Nguyen, Robert Schofield

Input beam jitter coupling has been an important noise source for LHO during aLIGO, prompting many efforts to reduce motion of optics on the PSL table and periscope, as well as off-line subtraction of jitter noise from the GW channel. We had hoped that replacement of ITMY, which had a central defect, would reduce input beam jitter coupling by increasing arm symmetry and common mode rejection of input jitter. Figure 1 shows that this seemed to have happened, with  nearly a factor of 10 reduction in the coupling function (meters of DARM per beam diameter of input beam jitter).

Figure 2 shows the expected level of jitter noise in DARM from ambient vibration levels in the PSL. Jitter noise was much reduced in each successive aLIGO run, but in O3, jitter coupling still produced a small peak in DARM just below 500 Hz. The plot shows that the noise at the O3 peak frequency should be a factor of nearly ten smaller and is now nearly at 1/10th of the DARM level. The second page shows that the LHO ambient jitter noise is now similar to that at LLO. The magnitude of the coupling reduction may help set the scale for the importance of test mass defects in jitter coupling.

In contrast with the coupling reduction at the PSL, I could see no significant difference in the vibration coupling at HAM5-6. We think this coupling is due to scattering noise from the septum.