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AdV-COM (AdV commissioning (1st part) )
hoak, martynov, swinkels, ruggi, carbognani - 0:32 Saturday 27 May 2017 (37762) Print this report
noise investigations -- first shot at beam jitter coupling

Tonight we made a number of simple investigations of the noise:

  • Paolo fixed the SSFS glitches by switching back to the 2nd order boost.  This wasn't a problem for the rest of the night.  It looked like these glitches were happening on the integer second boundaries, so we suspected a cm command like the digital noise reset, but this 1) didn't seem to cause a glitch, and 2) doesn't necessarily happen on integer seconds.
  • We tried changing the OMC dither amplitudes, by up to a factor of 100.  This didn't change anything in the noise.  The OMC1 locking wasn't so happy with the dither line reduced by 100x, probably switching this loop to B1_DC will help (we did not try this).
  • OMC1 locking still required some help.  It looks like it is stopping the ramp just barely before the cavity reaches the full fringe, or it gives up due to flickers in the transmitted light power.
  • Bas improved the alpha tuning, which has now completely eliminated the MICH coherence with DARM.  This is not reflected in the calibrated strain channel.  We need to share MICH subtraction filters so that we aren't doing the same job twice.
  • We made a new PRCL control filter with a more aggressive rolloff, designed for a UGF of 25Hz. We gave the filter a try, but it didn't change the noise.
  • Turning off the MICH, PRCL, and DARM lines gave us another Mpc or so.  The sidebands of the DARM line are particularly bad.
  • Moving to LOW_NOISE_2 did not change the shape of the noise between 10 and 100Hz.  This should have reduced the DAC noise by 20x.  This transition was only 50% successful tonight, we might try some handoff tricks to make sure this switch is 100% successful.
  • Noise injections on the BPC gave some evidence that beam jitter noise might be within a factor of ten of the current noise floor, see below.

 

=== BPC Noise Injections ===

We made some quick injections to the BPC TX and TY dofs.  We were able to see a clear effect on DARM (and more in PRCL and the SSFS), see Figures 1 and 2.  The noise injections should be valid for +/- 25 seconds around the time of the plot.  It looks like the TX coupling is worse than TY, but they both could be too close for comfort.

A quick transfer function of BxC TX to DARM (Figure 3) shows the coupling in the 100-1000 Hz band is more than 1e-4.  A very simple projection (using 2e-4 coupling) shows the noise is not far below the DARM spectrum in this region, see Figure 4.  However, there is not usually any coherence between DARM and these channels, see Figure 5.  Anyways, we should make a careful set of BPC injections to all four dofs (TX TY X Y) and build a noise budget.

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swinkels, martynov, hoak, ruggi - 0:59 Saturday 27 May 2017 (37765) Print this report
Attached some more plots related to the alpha tuning: Fig 1, 2 and 3 show the longitudinal noise budget before tuning, after tuning and after increasing the roll-off of PRCL. Implementing a subtraction of PRCL noise should be done soon. Fig 4 shows that we reached a surpression close to a factor 100, but the coherence gets so low that the estimate might not be accurate. Fig 5 shows the difference between the online suppresion (as visible in the disappearing bump around 25 Hz) and the suppression in Hrec.
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