Intensity noise vs PRM position

The first plot compares intensity noise as measured on the ISS (in and out of loop) signals, on IM4_TRANS and on MC2_TRANS:

• BLUE = second loop off, PRM misaligned
• GREEN= second loop off, PRM aligned
• BROWN = second loop on, PRM aligned
• RED = second loop on, PRM misaligned

The conclusion is that the noise added by the PRM reflection is still there, even though it's not as bad as before. However, in the present configuration, closing the second loop does not give us good performance, since for example on MC2_TRANS we can measure intensity noise as large as 8e-6, worse than when the second loop is open at some frequencies (18 Hz).

Tapping on IOT2R

To better understand the origin of this noise, I aligned the PRM, misaligned the BS to avoid any IFO fringes, and closed the second loop. The attached figures 2 to 4 shows the out-of-loop ISS signal (SUM58) and another measurement of intensity noise using MC2_TRANS (both calibrated in units of RIN, but MC2 is sensor noise limited at 1e-7 /rHz level).

• In figure 2, I'm tapping with one finger on IOT2R enclosure, without opening it. The effect is very large, and in particular the 18 Jz bump and harmonics are rang up
• In figure 3, I openend the enclosure and I'm tapping gently with one finger on the bench, far from the beam dumps that are on the path. Still, there is quite an increase of noise
• In figure 4, I'm gently patting the top mirror of the periscope, without touching anything else. The noise is cleary increased, especially the 18 Hz peak and harmonics

In conclusion, there is still a lot of scattered / backreflected light coming from this bench, likely already at the level op the upper periscope mirror. We must dump these beam properly.

In the last figure, I misaligned the PRM mirror and I stroke the bench much strongly than before. There is no visible effect. So it's the REFL beam that causes the problem. In addition, we can conclude that the noise that we normally see in the out-of-loop signal is not coming from this bench, but it's something else, still unidentified. One interesting observation from the first plot, is that hen the PRM is aligned, some of the structures visible in the out-of-loop ISS signal are not there. So maybe they are again light from the REFL beam, which is scattered in-vacuum.