Another Heimann Sensor / Boston Electronics delivered to Paco.
This unit (purchased May 2020/ / Delivered Aug 5th, 2020) has a FZ-Si window on it.
We don't know how it is.
% Eqn 41 of
% "Doppler-induced dynamics of fields in Fabry–Perot
% cavities with suspended mirrors", Malik Rakhmanov (2000).
% read in ringdown timeseries:
at = importdata('tek00000.csv');
I wanted to push the limits and see when NN subtraction performance starts to break by changing the number of seismometers and the size of the array. For aLIGO, 10 seismometers in a doubly-wound spiral around the test mass with outer radius 8m is definitely ok. Only if I simulate a seismic field that is stronger by a factor 20 than the 90 percentile curve observed at LHO does it start to get problematic. The subtraction residuals in this case look like
The 20 seismometer spiral is still good, but the 10 seismometer spiral does not work anymore. It gets even worse when you consider arrays with circular shape (and one seismometer at the center near the test mass):
This result is in agreement with previous results that circular arrays have trouble in general to subtract NN from locally generated seismic waves or seismic transients (wavelets).
I should emphasize that the basic assumption is that I know what the minimum seismic wavelength is. Currently I associate the minimum wavelength with a Rayleigh overtone, but scattering could make a difference. It is possible that there are scattered waves with significantly smaller wavelength.
The attached PDF shows the thermal noise of a short, low mass silicon cavity.
The thermal noise uses the gwincdev BQuad suspension noise term, I've disabled all the stages except the last (so it is a simple pendulum). The frequency noise is the noise budget of Dmass' silicon refcav.
The mass is 10g, the cavity length is 0.1m. The ribbons are 2mm by 0.05mm, 0.5m long.
I made them so thin because for some reason the gwinc model starts going nuts with the violin modes at high frequencies. (you can see that above ~150Hz). The frequency where it goes nuts comes down lower if I make them thicker. I don't know what's happening in the model but I guess it's not physical.
So this experiment looks a little like Thomas Corbitt's cantilever, it's not so similar to what would go in LIGO but it makes the thermal noise big enough to be seen above frequency noise.
In order to accentuate the thermal noise in a silicon test cavity, it would be nice to make the ribbons a little bit thicker. Sadly, the BQuad thermal noise model seems to explode when the fibers get thicker.
The three plots I will show have the following parameters in common:
4 fibers, single pendulum silicon suspension @120K. 10g mirror mass, 5cm fiber length, 2cm cavity length. The fiber width is 2mm and the fiber thickness varies in the three plots.
The first shows a fiber thickness of 0.05mm. The second has 0.1mm, and the third is 0.2mm.
As one can see, the model sort of goes more and more nuts as the thickness is increased. I don't really understand the model enough to know why this is the case, but it seems that to have a believable noise budget we might need to make a thermal noise model from scratch, rather than using gwincdev.
The source for what I've been using to calculate thermal noise.
I made an estimate for frequency noise requirement for a laser that can be used in crackle experiment. With some assumptions, I came up with df = 3x102 [Hz/rtHz ] for the requirement.
The two beams from both arms are recombined at the output port of a Michelson interferometer. If it is operated at dark port, the output signal will be linear with the differential length between the two arms.
some assumptions in the calculation:
This will be a requirement for the planned ecdl.
Is a HeNe laser good enough? I'm not sure about HeNe frequency noise level, and I haven't found it in literature that much. I checked here,see fig 5, HeNe f noise is not so bad compared to NPRO noise (10^4 /f Hz/rtHz).This feels a bit counter intuitive. But if it is real, it should be ok for the measurement around 100 Hz and above.
you have to overlay the estimated displacemnt noise with the existing L1 noise bud or else we cant tell what the importance of the result is
There we go. Based on the noisemon data at L1 and H1, I calculated the DAC noises at those sites, using roughly the same approach as described in 1847.
I used the coherence between the master channel and the noisemon channel to calculate the total noise going into the coils.
Then I converted the ADC noise and noisemon noise to DAC volts and subtracted them from the total noise. I compared the result of the subtraction, which should be DAC noise, at least in the passband (20-100Hz), with the G1401399 model and made a noise budget, shown in attachment 1. We can see that, as designed, the DAC noise is sufficiently amplified so that it dominates over the noisemon noise or the ADC noise in the passband.
Next, I projected the DAC noise to strain noise and summed them up for all the four channels in all the four stations.
Finally, I compared this with the interferometer noise spectrum based on data in L1:OAF-CAL_DARM_DQ and H1:CAL-DELTAL_EXTERNAL_DQ. I calibrated these data with calibration files here. The results are shown in attachment 3. All the data and scripts are included in attachment 4, where analysis.py is the script that does the job. Based on the plots, it seems DAC noise could be potentially a limiting factor for the interferomter sensitivity.
The coil driver states for L1 is LP off, ACQ off (state 1). For H1 is LP on, ACQ off. The LISO files calculating the current transfer functions and the voltage transfer functions are attached in attachment 4.
I used a resolution of 1mHz in the diaggui measurement. The data files are too large so I can not upload them here. I am figuring out what to do.
Note: I fell into a few traps during the calculation. Many of them was about data and transfer functions. I have been more careful about what data is used in these calculations. For example, the noisemon data downloaded from the sites when MASTER was off still has DAC noise in it. I thought it was ADC noise + noisemon noise before and used it for subtraction. Another example, the transfer function measured at the sites has all the noise in it. We do not see the noises in the passband but ADC noise dominates at high frequencies. If you use this transfer function to figure out how much noisemon noise contributes, you result will be tampered by the noises, like ADC noises at high frequency. Last example, if you use the noisemon noise data measured in the digital system in our lab, you should be aware that, although it does not have DAC noise (I disconnected DAC when measuring the noises), it also has ADC noise. Therefore, it would be better to use data from SR785 or LISO simulations (which has been shown to agree with each other). I drew a diagram in attachment 2 to help thinking about what data or transfer functions should be used.
for some reason the DAC noise estimate is too high, it can't really be so large compared to the real DARM curve (see the noise budget curves from LLO - there are other noise sources besides DAC noise)
I hve modified the code to plot nicer and also to remove some divide by zero problems. There is also still some warnings about other divide by zero - those should probably be fixed by examining how better to handle it when the coherence goes to zero.