Sheila, Jenne, Keita
Summary: We have some evidence that our sensitivity can be better with a higher DARM offset.
Details:
Last Thursday I took some data with the squeezer off at different DARM offsets, 55086 to compare with the data that Jenne took 54969.
- The first attachment is just a GDS calib strain spectrum, and show data from the two DARM offsets that we used on Thursday and some data from a no squeezing reference time at our nominal 10pm DARM offset (from 54822). The calibration lines at 17Hz, 410Hz, and 1.1kHz all line up to within 1% of each other in these three times in GDS.
- The second plot shows the coherence between strain and the LSC control signals. The SRCL feed-forward is only well tuned for our nominal DARM offset, as expected. The PRCL coherence is slightly increased by changing the DARM offset and MICH FF seems not to depend much on the DARM offset. Jenne remeasured the feed-forward on Friday with the DARM offset at 14pm 55108.
- The third attachment shows the same GDS spectra as the first attachment, but with an estimate (best possible) or the sensitivity we could get with perfect SRCL subtraction for all three cases (strain*sqrt(1-coh)). There are several frequencies where the 14pm sensitivity is just a bit better than 10pm.
The high frequency noise is consistent with the measured change in optical gain and the dark noise:
- The 4th attachment shows kappa c and the DCPD current plotted against the x0 offset (the darm offset in picometers used for the DCPD normalization.) Kappa C (the normalization for the sensing function used to compensate for time dependence in optical gain) is calculated by demodulating DARM err, which is measured after the normalization by x0. This means that ideally, if the DCPD normalization by x0 was perfect, kappa C would not change with the DARM offset. Kappa C does decrease for the 5pm DARM offset, which could be because of junk light that isn't sensitive to DARM. In the lower panel of the 4th attachment the curve which is drawn there is a parabola defined by the 14pm measurement, just as a visual aid.
- The 5th attachment is shows how the sensitivity is improved at high frequency. Only the 5pm (gray colors) and 14pm data (reddish colors) are shown here. The sum of dark noise and shot noise explains the degradation of the high frequency sensitivity for the low DARM offset, mostly because of the loss of optical gain. This is from DCPD sum data calibrated into strain using pyDARM and correcting for changes in the fringe offset, kappa C, and the cavity pole frequency. With an FFT length of 10 seconds, the calibration lines agree within 3%, the plots attached are with a 3 second FFT so that we can get more averages for the cross correlation.
The last two plots show the low frequency sensitivity and the impact of SRCL subtraction, comparing our nominal DARM offset of 10pm to the candidate new offset of 14pm.
- The 6th attachment shows the DCPD sum data while the 7th attachment shows the same thing for the DCPD cross correlation. You can flip back and forth between them to see the difference if you like.
- The SRCL noise is estimated based on coherence with the DCPD sum and is the same in both plots.
- For the gray (10pm) case, the dashed line showing the measured sensitivity is directly on top of the sensitivity with the estimated SRCL noise subtraction, for the 14pm case there is a significant improvement from the estimated subtraction. Where the solid maroon traces are below the gray traces, there is some improvement in the sensitivity with the reduced DARM offset.
I went back and got a few more data points of DCPD current vs optical gain from the time when Jenne moved the DARM offset (54969). Attached is a new version of the 4th attachment above, where the optical gain is plotted against the DCPD power along with a fit to the data (from both times). There are only 2 parameters in the fit, the quadratic coefficient and an offset from junk light which doesn't change with the DARM offset or contain any DARM signal.
This fit suggests that we have 1.7mA of photocurrent from junk light, which would mean 1.95mW of junk light (without the data from Jenne's ealier test we get 1.5mA). This can be compared to the 1.7mW that Craig found in 51273, with 20W of input power.
Summary: I've made an estimate of low frequency noise that could be explained by intensity noise on the junk light that we have on the DCPDs. It is below the DARM residual but within a factor of a few.
If we assume that the low frequency increase in noise at 5pm compared to 14pm is due to intensity noise on the junk light, and assume that intensity noise stays the same when the DARM offset is changed, we can make an estimate of where this noise is when we are operatoing at 10pm.
The first attachment shows the GDS strain data from above with the power based SRCL subtraction, you can more clearly see that there is a low frequency sensitvity difference. If this is due to intensity noise on the junk light, we can use the difference here to estimate the intensity noise. The DARM PSD in displacement is
DARM PSD(5 pm) = Intensity PSD /(optical gain (5pm)^2) + PSD of noises which are independent of DARM offset
and similar for 14pm DARM offset. We can find the Intensity PSD by comparing the two DARM PSDs and knowing the optical gain.
Intensity PSD = [DARM PSD(5pm) - DARM PSD(14pm)]/[1/optical gain(5pm)^2 - 1/optical gain(14pm)^2 ]
The RIN estimated this way is around 5e-8 at 40Hz, shown in the second attachment. The third attachment shows the estimated intensity noise scaled by the optical gain at our nominal DARM offset of 10pm. We can compare this to the attached noise budget residual This is a noise budget for Jan 20th, although we haven't updated the coupling measuremetns used here in several months. The reason that the noise budget misestiamted the quantum noise around 200 Hz is that we don't have the frequency dependence of the squeezer modeled correctly here). The peak at around 48Hz in this noise budget total is from a vibration noise estimate from the PEM website before the 48Hz peak was fixed, this peak should go away when we update that. The message is that the noise I'm estimating to be caused by junk light intensity noise is about half of our noise budget residual at 50Hz, and about a third of the residual at 40 Hz. We would need 4 noise sources of this size to explain our residual at 50Hz, and 8 noise sources this size to explain our residual at 50Hz).
