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AOS (Optical Levers)
rana.adhikari - 14:40 Monday 16 September 2019 (10537) Print this report
PRM Optical Lever: aluminum foil air shield on laser side

Rana, Jenne

Saturday, during the AS WFS investigation and REFL beam clipping investigation, we constructed a wind shield enclosure for PRM. It has reducethe 1 Hz noise for the PRM optical lever by a factor of ~8.cool

In the attached PDF, you can see the spectra of the pit/yaw spectra. Nakano-san tells me that they are all calibrated into micro-radians, so I have labeled them as such.

For PR2 and PR3, the loops are closed and so the noise below 10 Hz is suppressed. However, it seems like to me that the loops are mostly sensor noise limited (increasing the gain will not reduce the RMS motion of the mirror).

For PRM, the BLACK trace shows the open loop spectra before shield and the Blue-Gray trace shows after wind shield. For this, we're using the QPD labeled as TILT. Later, we will mix in the QPD of the other Gouy phase to cancel the large longitudinal-to-yaw coupling (otherwise, these cannot be used for feedback stabilization).

By the fact that the noise reduction was so much means that most of the wind-susceptibility comes from the laser side (not surprising). The QPD side is difficult to work on.

also, I forgot to reconnect the PRM OL beam position sensor cablesfrown, so that will have to be done next time we enter the mine.


QPD Comments:

  1. The QPDs should be mounted in a box, not on translation stages. We want to have EMI shielding for sensitive low frequency circuits. Some opamps have a large RFI downconversion problem.
  2. I recommend not to use ANY translation stages on these QPDs. They are not long term stable. Instead we should use the stable mounts (e.g. Thorlabs Polaris) for steering onto the QPDs.
  3. The optics should be moved away from the edge of the breadboard so that a proper box can be attached the breadboard.
  4. The box used for the MMT trans is too thick. For the OL, we can just use aluminum framing (as is used on the PRM OL laser side) and some steel sheet metal. Soft rubber gaskets should be used to attach the sheet metal to the frames. The sheet metal should have a few small holes for the cables to come out and the holes should include rubber grommets so that the cable holes don't leak so much air.
  5. All cables from the Optical lever laser and QPD should be strain relieved by attaching them strongly to the optical lever pier so that the vibration from the cable is minimized.
  6. The current 'round steel pier' approach is probably not very mechanically stable, since the pier's foot is so small relative to the moment of inertia of the pier. Ideally, the whole system should have a 'flagpole' mode resonance above 30 Hz.
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Comments to this report:
tomotada.akutsu - 15:01 Monday 16 September 2019 (10538) Print this report

Thank you so much, Rana and Jenne; actually as you mentioned, it would be difficult to work for covering the QPD side. For example, some pillars for supporting the cover would be required later...

rana.adhikari - 15:56 Monday 16 September 2019 (10540) Print this report
rana.adhikari - 19:59 Monday 16 September 2019 (10544) Print this report

this plot shows the comparison of PR2 & PR3 w/ oplevs on/off. There are problems...

  1. The PR2 & PR3 loops are suppressing the error signals below the level of the sensor noise (which seems to have a 1/f noise of ~5 nrad/rHz @ 1 Hz). i.e these loops are injecting noise in the 1-10 Hz band.
  2. The PR2 Yaw loop is making 20 dB of gain peaking at 5 Hz.
  3. There is a lot of acoustic noise. We need more serious low pass filters for the 30-100 Hz band. There is not much BBH SNR below 30 Hz, so lets not worry about 10-30 Hz for now.
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tomotada.akutsu - 20:21 Wednesday 18 September 2019 (10589) Print this report

The cables to the PSD at PRM were re-connected today, Sum->Sum, Pitch->Y, Yaw->X.

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