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AdV-DET (Local controls for all benches)

mwas - 10:03 Friday 20 December 2019 (47992)

bench motion scattered light coupling for O4

The transfer function between bench motion and DARM due to scattered light have been computed for O3. Here the same is done for O4 by adding the SR mirror and doubling the laser power. The transfer functions can be found on the svn.

Figure 1 shows the result for SDB1/SDB2 (light scattered at the anti-symmetric port), the coupling is expected to be slightly lower below 50Hz, slightly higher between 50Hz and 100Hz, and a factor 3 lower above 500Hz. The high frequency difference is presumably due to the change in ITF optical gain due to the presence of SR.

Figure 2 shows the result fo SIB1 (light at the symmetric port). The coupling between 20Hz and 200Hz increases by a factor 2.

Figure 3 and 4 show the result for SNEB and SWEB. The phase coupling remains the same, but the radiation pressure coupling path is reduced by a factor 3.

Images attached to this report

47992_1576832164_Screenshot from 2019-12-20 09-46-42.png

47992_1576832170_Screenshot from 2019-12-20 09-46-56.png

47992_1576832175_Screenshot from 2019-12-20 09-47-07.png

47992_1576832180_Screenshot from 2019-12-20 09-47-15.png

Comments to this report:

mwas - 10:33 Wednesday 04 March 2020 (48611)

The choice of dark fringe offset for O4 has an impact on the coupling of scattered light to the sensitivity.

Figure 1 shows the detection port, with a nominal dark fringe offset in solid line, and a 4 times bigger offset (in terms of B1 power) as dashed line. The amount of scattered light coupling is multiplied by 2, which is the expected behaviour from having more light at the dark port that can be backscattered

Figure 2 shows the injection port. More surprisingly here also the coupling increases, between 50Hz and 300Hz, by up to a factor 2. It is not clear to me why this is happening, but it seems to be due to some opitcal response zero being shifted to higher frequency for a larger offset.

Images attached to this comment