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AdV-DET (Phase camera (2 to be developed))
schaaf, beuzekom - 11:09 Thursday 16 August 2018 (42451) Print this report
RF stability of PC signals

The amplitudes and powers of the PC signals are not stable on time scales of 10 min, and most definitely not on time scales of hours. As the effects are the same for all sidebands and the carrier, the DC of the corresponding main diode has been used in the past to correct for the drifts in the _PWR channels, but this situation is not satisfactory. To debug the problem we added new channels to dataDisplay:

1. <EPRB/EDB>_<B4/B1p>_PC_<112MHz/56MHz>_PDH_PWR, these channels demodulate the beat of the 56 MHz upper and lower sideband / sideband with carrier in the interferometer beam as is the case for the main diodes.
2. <EPRB/EDB>_<B4/B1p>_PC_<demodfreq>_REF_PWR, these channels contain the electrical reference needed for the phase images.

In the attached Figure a comparison of the PC and the main diode signals is shown, observations so far:

1. RF channels containing the reference beam do not follow the main diodes
2. RF channels not containing the reference beam do follow the main diodes
3. DC channels do follow the main diodes
4. The electrical reference is stable

Point 1 is the problem to be solved.
Point 2 seems to indicate that it is not an electronic problem, or interferometer beam problem.
Point 3 shows that it is not the reference beam amplitude.
Point 4 shows that it is not an unstable reference beam frequency (reference beam is at 80 MHz).

Remaining suspects are:

- Reference beam polarization
- something else we did not think of?

We will contact Eric to help with further investigation of the reference beam.

Images attached to this report
Comments to this report:
genin, derossi - 10:29 Thursday 30 August 2018 (42581) Print this report

In order to understand the origin of RF reference fluctuations, we have moved the AOM+splitter box located in the Atrium outside of the rack in order to see if surrounding electronics could affect the behaviour of the reference beam.

THis action has been made on August 28 around 12.25LT.

schaaf, tacca, beuzekom - 17:17 Thursday 30 August 2018 (42587) Print this report

Moving the AOM box out of the rack seems to have no effect on the observed drifts, attached is the situation during last night.

Images attached to this comment
derossi, genin - 11:16 Tuesday 04 September 2018 (42613) Print this report

Another trial to understand the origin of these fluctuations will be to check wheter they are caused by the splitter box. We then opened the AOM-splitter box (at around 9h45 LT), we unplugged the DET fiber coming out from it and we plugged it inside (look at the picture, on the right). We also unplugged the AOM output fiber and we plugged it in front of the DET fiber, so that we can noe monitor the beam bypassing the splitter box.

Images attached to this comment
genin - 17:49 Tuesday 04 September 2018 (42619) Print this report

a side effect is that the flag "PC" on the DMS is red just because no light reaches the EIB_PD anymore.

This flag should not be considered until we connect back the 3 ways fiber splitter.

schaaf, tacca, beuzekom - 10:27 Wednesday 05 September 2018 (42623) Print this report

The reference beam is only reaching EDB currently (B1p PC, see first figure). As a consequence no images or PWR channels are available on B4.

The change in setup seems not to have solved the instabilities (see second figure).

Images attached to this comment
genin - 15:41 Wednesday 05 September 2018 (42627) Print this report

Around 13.15 UTC I put back the 3 ways fiber splitter in the distribution of the pick-off beam.

Now the beam should arrive on both B1p and B4 PCs.

 

schaaf, beuzekom, tacca - 10:07 Tuesday 06 November 2018 (43417) Print this report

The instability of the RF channels has vanished. It is not yet fully understood why. Changes to the setup since the last check are:

- polarization matching of interferometer and reference beam on the bench has been optimized. During this procedure it was found that the diode on B4 seems almost only sensitive to s polarization, this is unexpected and will be investigated further.

- the generator for the reference beam was replaced for the measurement of the squeezing fiber, then the initial generator was placed back (so we are now back to initial state). Maybe some connection was bad (fiber or cable?).

Attached are the plots that show that the trend with the PC and the B4 diode agree.

Non-image files attached to this comment
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