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AdV-COM (AdV commissioning (1st part) )

mwas - 11:40 Friday 01 November 2019 (47501)

Dark fringe offset tuning

Last Sunday, Oct 27, we could gain 2Mpc in sensitivtiy by reducing the dark fringe offset to have 1mW instead of 2mW per B1 PD.

Figure 1. Today this was not the case, reducing the dark fringe offset has lowered the BNS range. Myabe because of the additional bumps there is in the spectrum, that could be more impacting with a lower dark fringe offset.

This data can be also analyzed later to measure the contrast defect.

However, figure 2-6 show that the HOM on the B1 camera is also decreasing with the power on B1. So this will measure a lower limit on the contrast defect. It will be important to understand where this HOM comes from, is this something produced by the OMC? So slight clipping on some of the folding mirros, an artifact of the camera, a real HOM at the input of the OMC? If it is a real HOM at the OMC input it is strange to see its power scale with the dark fringe offset.

Images attached to this report

47501_1572604539_Screenshot from 2019-11-01 11-35-18.png

47501_1572604628_Screenshot from 2019-11-01 08-50-39.png

47501_1572604638_Screenshot from 2019-11-01 08-58-28.png

47501_1572604647_Screenshot from 2019-11-01 09-30-07.png

47501_1572604659_Screenshot from 2019-11-01 09-40-59.png

47501_1572604668_Screenshot from 2019-11-01 10-10-12.png

Comments to this report:

mwas - 14:51 Wednesday 13 November 2019 (47628)

I have analyzed the data from the dark fringe offset reduction to measure the contrast defect.

Figure 1 shows the signal of interest (B1 power, ITF optical gain, and RF DARM error signal)

Figure 2 shows looking at the relation between optical gain and B1 power and offset of 130uW seem to be present on B1 PD1.

Figure 3 shows the comparion between the RF DARM error signal and the B1 power, the relation here is also quadratic as the RF DARM error signal should be proportional to the DARM offset in meter (and the B1 power is quadratic with the DARM offset in meters). This yields an offset of 120uW.

After the unlock I could check that the PD itself has an offset of -7uW, so the numbers above correspond to actually 137uW and 127uW. Given that B1 PD1 sees only half of the power, the total contrast defect light is about 260uW. This is much higher than the ~120uW we had before increase of input power during the October commissioning break

Images attached to this comment