Dan, Jeff, Sheila
Tonight we had some more fun noise hunting. Turning the power up to 8 Watts was easy, but didn't improve the noise so much since we are mostly not shot noise limited.
- First, when we got to DC readout we noticed that someone did a nice job damping the ETMY violin mode, reducing the peak in the asd by about a factor of 2. This allowed Dan to turn on one stage of whitening in the OMC DCPDs, which helped improve the noise floor.
- We further reduced the gain of the ETMX oplev damping (by a factor of 2 for a gain of -5). This reduced coherence around 10 Hz and helped the DARM noise floor.
- We have been running all day with the ESD linearization on, this seems to be fine.
- We had the ISS second loop on for the first long lock of the night, it has been off ever since.
This gave us a lower noise floor, we sat here from 5:28:10 UTC Feb 13th to 5:41:29 UTC, with the intent bit undisturbed.
We then made a single destructive attempt to reduce the ESD bias voltage, we'll have to come back to this. :)
On the next lock we decided to try increasing the input power. This was suprisingly easy, we have now been sitting at 8 Watts input power since 7:03:43 UTC. We have been adjusting the BS alignment by hand every 10-15 minutes or at 10 Watts. We have watched the AS36Q WFS signals while we do this, and can see that they are not always correct (zero does not always give us the best AS90). BS yaw dramatically improves the intensity noise coupling. It seems as though the IFO would stay locked like this all night if we sat here and adjusted the BS once in a while. We are leaving it locked and undisturbed.
In the attached plot, the green trace is earlier today, The blue trace is with 1 stage of DCPD whitening on and the ISS second loop on, which is injecting noise at around 700 Hz. The red trace is now, with 8 Watts input power, BS YAW oplev damping gain reduced, and the ISS second loop off. You can see that we are not really shot noise limited, as increasin the power by a factor of 2.8 only gives us a small improvement at high frequencies. Something is clearly wrong with the calibration.
The attached plot illustrates eight hours of progress today. (Times are approximate.)
At 3:30pm we were using the in-vacuum POP photodiode for MICH, PRCL, and SRCL.
At 9:30pm the violin damping ninjas had allowed us to enable one stage of whitening on the DCPDs. The ISS second loop was on for this lock stretch. During this lock the intent bit was set to OBSERVATION for about ten minutes (see times above), and ISC_LOCK Guardian was in the 'DC READOUT' state.
At 11:30pm we had increased the power from 2.8W --> 7.8W. But, we forgot to turn on the ISS second loop. Tweaking the BS alignment reduced the beam jitter coupling at 100-300Hz and around 700Hz. There is a lot of coherence above 1kHz with the LSC DOFs, we believe that we are not dark noise or shot noise limited in this region.
As we increased the power we changed the DARM offset, gain, and OMC scale factor & gain to maintain good phase margin. At 7.8W we could not incerase the DARM offset to provide ~16mA in the carrier mode at the OMC, the DCPDs would saturate from the violin mode. The last lock stretch had a DARM offset of 2e-5 and a DCPD sum of ~12mA.
We noticed the 1/f^3 noise between 30 and 70Hz was breathing on a several-second timescale. The noise here is somewhat coherent with MICH, we wonder if it is also due to ESD DAC noise or DAC zero crossing glitches. We have left the IFO locked in 'DC READOUT' with the intent bit set to observe. DetChar, please examine the data for glitches associated with major-carry transitions.
Glitch investigation: Keep in mind that there are about a dozen IPC erros per second on the controls signal sent to the ETMs.
Shot noise dominates the noise floor in kHz range, there was something wrong about the power scaling and/or scaling with DARM offset.
The first attachment shows that, in calibrated spectrum, the Pcal line in 7.8W (red) was a factor of 1.6 larger than in 2.8W (blue).
There should have been something wrong about power scaling somewhere, and if you want to make a fair comparison between 7.8W and 2.8W, 7.8W should be brought down by a factor of 1.6.
If you take this into account, 7.8W noise floor would be about a factor of 1.7 smaller than 2.8W for, say, f>7000Hz. (The top panel in the second attachment shows that 7.8W data is 5% or so smaller than 2.8W, and 1.6/0.95 = 1.7.)
sqrt(7.8W/2.8W)=1.7.
Further, in the same plot, the bottom and the middle panel shows that both in 7.8W (red, blue) and 2.8W (green, brown) the noise floor of DCPD SUM is very close to the NULL stream except for many structures, and PDB/PDA coherence is very low except these structures. (Also, in the middle pane of the second attachment, the black trace is the dark noise with high Z and 1 stage whitening, which is a factor of 5 smaller than 7.8W noise.)
Therefore we're confident that the noise floor for kHz range is already dominated by the shot noise.
Overall scaling is another problem which should be fixed separately.