I'm trying to understand the measured RIN in the setup. The evidence suggests that the measured RIN in 100Hz- 6kHz, is real intensity noise and not associated with alignment + jitter.

==Problem==

As seen in PSL:1329 that we might be limited by RIN at high frequency, I tried to figure out what cause the shape of the RIN around kHz to be mechanical -like peaks. So the problem can be minimized, and does not have to rely on ISS that much.

==Assumption==

My assumption was that they were from mirror mounts, because

1. the frequencies were close to mechanical resonances of the mounts, see PSL:818, PSL:824 for examples. The power coupled into the cavity would reduce, and thus causing the peaks in the RIN measured behind the cavity. And
2. the shape changes during the time of measurement. During the day, the shape is like a big bumb, while during the night, around 2-3am, the level is smaller and the individual peaks shows up instead (may be because of the lower seismic)

==Measurement==

So to test this, I measured RIN before and after ACAV (NOTE:ACAV path has PMC in it),  when

1. The beam was well aligned to the cavity (DC from REFL PD =94mV, total level ~ 1.6V)
2. The beam was misaligned a bit (DC from REFL PD = 175mV)

above, beam path in front of ACAV, before the beam enters ACAV. The PD for RIN measurement is circled in blue.

above, beam path behind ACAV.

If the measured RIN was from the jitter, RIN after the cavity should change with the alignment, and RIN before the cavity should not change much. I made sure that the spotsize on both PDs are significantly smaller than the PD to make sure that any jitter in front of the cavity should not change the power level that much.

==comments about the result==

• The result agrees with the assumption at low frequency (DC - 30Hz),
  
• However, from 100Hz and above, the measured RIN from four cases are very similar.
• The measured coherence between the two PDs are similar in both cases (aligned and misaligned), I plotted the one from the misaligned beam. It shows that at low frequency, RIN behind and after the cavity are not caused by the same mechanism, the one behind PD might suffer more from jitter. However, at 100Hz and above, anything observed before the cavity is seen behind the cavity as well. This rules out the assumption that the alignment change due to the motions(resonant peaks) from optics.
  
• As a comparison, I plotted RIN measured around 3am in brown trace, see PSL:1329, the level is smaller than those measured in the evening. It still makes me think that it is related to seismic but not alignment. I have to think about what other seismic driven mechanism might cause this intensity noise.
• The level of RIN seems to change as well, I looked at Evan's measurement, earlier today[/PSL:1330]. The bump around 1kHz is ~ 2e-6 1/rtHz, while for my measurement, it is close to 1e-5.  I'll try to investigate more to find out what change the level of RIN.

==To do next==

• Pick up the beam some where before PMC to check the RIN level and see where the peaks occur
• Install PD behind RCAV, see RIN from RCAV
• update noise budget: add RIN induced noise from ACAV and RCAV.