Virgo Logbook

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

tacca - 12:08 Thursday 15 November 2018 (43586)

B1 beam

Figure 1 shows the B1 beam during O2, while figure 2 show the B1 beam in these days. The shape seems to be no more Gaussian. Figure 3 and figure 4 show the same images plotted in log scale. It is visible that, in these days, B1 has some HOM components, clearly visible increasing the integration time and the gain of the camera (figure 5).

Checking B1s2 (figure 6 and figure 7), it can be noticed that the beam reflected by the second OMC is changed wrt the past, as expected considering the substitution of the first OMC. In B1s2 the same HOM components present in B1 are clearly visible.

Images attached to this report

Comments to this report:

mwas, tacca - 18:55 Thursday 15 November 2018 (43596)

In the attached figures, the images of B1 and B1s2 in different ITF conditions.

2018 June 25th, Input power = 13 W, old OMC1, BNS range = 35 Mpc:
- Figure 1 = B1 linear scale -> not perfectly Gaussian
- Figure 2 = B1 log scale -> HOM structures not very clear
- Figure 3 = B1s2 log scale -> almost circular
2018 August 28th, Input power = 25 W, old OMC1:
- Figure 4 = B1 linear scale -> saturated, almost circular with some HOM ring around
- Figure 5 = B1 log scale -> some HOM structures
- Figure 6 = B1s2 log scale -> many HOM structures
2018 September 24th, Input power = 25 W, new OMC1, first lock after replacement:
- Figure 7 = B1 linear scale -> saturated
- Figure 8 = B1 log scale -> some HOM structures, similar distribution to the current one
- Figure 9 = B1s2 log scale -> many HOM structures, similar distribution to the current one

Images attached to this comment

Allocca, Chiummo - 13:11 Friday 16 November 2018 (43602)

This HOM was visible also before O2 (see fig 4 in 38907).

The HOM visible in transmission from the first and also the second OMCs could be the mode of order ~12 (tbc) which is pretty close to the carrier resonance in the OMC cavity. The process could be that the HOMs present on B1p are injected in the OMC and they get projected on the OMC basis, therefore scattering light in the 12th HOM which is close to the carrier TEM00 resonance.

This was already noticed in the past, when a scan of the OMC was performed (#41846), although the data from the cameras are no longer available for that period. Maybe now the situation could be worse with the new OMC. This can be seen by doing a full scan of the first OMC (the usual scan performed to lock the OMC could not go on the right side to catch this mode) and looking at the cameras close to the carrier resonances: in this way one can see if the same HOM (12) is closer to the resonance than before or maybe more excited.