Aritomi and Yuhang

In the past few weeks, we measured homodyne shot noise few times, we found the shot noise level changes from -133.2dB to -133.7dB.

We monitor the BAB intensity for almost one hour on Monday this week. We found BAB intensity changed by about 5% during this time (attached figure 1). By doing 20*log10(sqrt(1.05))-20*log10(sqrt(1)), we found this can introduce 0.2dB shot noise change.

Although BAB and LO both come directly from main laser, this is not a test done for a real LO. We decide to check the power of LO before and after IRMC soon.

Marc, Yuhang

In entry 2545 the filter was not exported properly and was actually just a gain...

This time we used the proper filter and got the correct 4 open loop transfer functions of automatic-alignment as shown in the 4 attached figures.

It seems that for input & end pitch, the 11 Hz resonance is really too close to unity gain.

And the frequencies around 1 Hz have similar behavior.

We'll tune the filter in order to reduce this effect.

Marc, Michael, and Yuhang

We have done a calibration for pointing loop with a new method. We move the locking point of pointing loop and check how much BS needs to compensate.

Pointing pitch:

we move pointing pitch, as attached figure 1 [1,0] subplot, from  -24 to -34 [counts]. Then we found BS pitch, as attached figure 1 [0,0] subplot, needs to move from 275.8 to 326.0 [counts] to compensate. Taking into account the calbration of oplev, we got  calibration factor for pointing loop on the base of BS.

The pointing precision was estimated by a RMS integral of pointing loop error signal spectra. We also found that high frequency spectrum can be better if we center beam on PSD, which is shown in attached figure 1 [0,1] and [1,1] subplots. However, the RMS of these two seneriaos are the same, which is reasonable since the dominating noise is at low frequency. In conclusion, we got pitch pointing precision as 0.43urad.

pitch pointing loop calibration factor 1.3 urad/count.

Pointing yaw:

We have done the pointing yaw calibration in the same manner with pointing pitch. The details can be found in attached figure 2. In conclusion, we got yaw pointing precision of 0.65urad.

yaw pointing loop calibration factor 1.5 urad/count.

Note: to take a quicker measurement, we have increased pointing loop gain by factors of 3 and 5 for pitch and yaw. So you can see the BS compensation finishes within 1 minute. We have also tested the spectrum of poiting pitch/yaw don't show difference when gains are increased.

I consulted with Takahiro Yamamoto-san about if it is OK to delete data from \full directory and he said there is not important setting but just saved data inside here.

So I deleted the data from 2020/07/30 to 2020/11/21.

Now we have 63% in use, which means 37% space is released.

A similar measurement was done in elog2350, I and Michael got quite similar result even with a time scale of 1 hour. However, if I remember and understand well, Aso-san had a comment that different homodyne angle may affect our measurement, which is what we have seen. It seems important to understand why detuning becomes not so stable if we measure with different homodyne angle.

However, the same measurement may be interesting to be made for without CCFC but with AA/pointing.

To investigate the detuning stability, we fixed the homodyne angle to squeezing quadrature and measured CCFC error signal and FDS three times with half an hour interval (attached figures). As you can see, the CCFC error signal and FDS are stable and the detuning fluctuation is ~2Hz.

The strange thing is that although the CC detuning is almost optimal, the CCFC error signal with demodulation phase of 23deg seems good detuning for FDS.

This 15Hz peak can be also amplitude noise.

To better calibrate, probably we should move slowly the pointing loop offset and see how much BS oplev signal response.

In this measurement, PR pointing loop was used instead of BS pointing. The detuning fluctuation is still ~10Hz.

sqz_dB = 10.7;                    % produced SQZ (dB)

L_rt = 120e-6;                    % FC losses

L_inj = 0.31;                     % Injection losses 

L_ro = 0.24;                      % Readout losses

A0 = 0.05;                        % Squeezer/filter cavity mode mismatch

C0 = 0.05;                        % Squeezer/local oscillator mode mismatch

ERR_L =   1e-12;                  % Lock accuracy (m)

ERR_csi = 30e-3;                  % Phase noise (rad)

Note: In this calculation, shot noise level is assumed to be -133.2 dBVrms/rtHz, but actually it was -133.7 dBVrms/rtHz. So the result in this log is wrong...

Marc, Michael and Yuhang

The pointing loop control precision is an important parameter, which tells us the light beam spot moving range on filter cavity end mirror. The beam pointing loop is a very slow loop. To measure this beam pointing precision, we need to measure the error signal of this loop.

1. Calibration

We did calibration by sending a line for BS pitch/yaw. By overlapping BS oplev signal line and FC transmission PSD line, we can calibrate PSD pointing to the base of BS. The BS oplev calibration is from Eleonora elog1874.  Attached figure 1 and 2 show this overlap and the calibration factor of PSD.

2. Pointing precision

We took time series and spectrum of pointing loop error signal. By using calibration factor, we found our pointing precision for pitch is 4.04urad, for yaw is 10.10urad (from time series measurement). From spectrum measurement, pitch precision is 3.36urad, yaw precision is 11.44urad.

Remarks:

1. This measurement is done with PR and BS oplev open. Now the PR/BS oplevs are noisy above 10Hz, the closing of oplev loop may introduce noise but the resonance peak should be anyway reduced. It will be interesting to compare the pointing noise when PR/BS oplev is on/off.

2. This measurement is doen when BS pointing is closed. It has been found that pointing introduces noise for BS oplev when pointing loop acts on BS. So it will also be interesting to do the same measurement when pointing loop acts on PR.

3. Attached figure 5 and 6 show the comparison of BS oplev pitch/yaw and FC transmission PSD pitch/yaw. We can see PSD signal is higher than oplev after calibration, which means there are other noise sources are dominating PSD signal instead of BS pointing.

I compared the BS and PR pitch and yaw oplev while either closing the pointing loop on BS or on PR.

It seems that closing the loop on BS is introducing noise on BS pitch and yaw above ~4Hz.

 On the other hand, the pointing loop closed on PR only slightly increases PR yaw noise between 1 and 6 Hz.

Therefore it seems better to close the pointing loop on PR from now on.

Today I sent 15 Hz line on input and end mirrors pitch and yaw to calibrate the AA signals.

The 4 attached figures show the comparison of Oplev signal with calibrated AA signals.

This line can also be used to estimate the SNR of both Oplev and AA signals.

Marc, Michael, Yuhang

Today we wanted to do 2 activities in parallel :

1 compare the oplev and AA snr

2 measure the OPO NL gain.

We locked the FC with all loops on green for activity 1 and locked OPO while sending 10 Hz line with 1 Vpp on GR phase shifter for activity 2.

We found out that this caused an increase of Oplev (especially end mirror) and AA noises level as shown in figure 1. The green and brown reference curves are without beam dump, the blue and red are with deam dump and have the expected noise level.

By moving a beam dump from in-between the GR phase shifter and the GRMC to just before the OPO we could remove this new noise.

Therefore it seems that the green back-scattered light is the culprit.

We'll do further check to understand the coupling path and mechanism of this noise.

Marc, Michael, Yuhang

Today we added the PR pointing loop to medm.

It is basically a copy & paste of BS pointing loop except that we had to change the pitch gain sign.

We could close the loop and things seem stable.

On medm there are now 2 buttons to close pointing with feedback on either BS or PR.

However, as pointed in entry 2493 PR is drifting more than BS so closing the pointing loop on PR instead of BS should be useful for long term stability.

I calculated the IR misalignment effect on CCFC error signal in I phase.

As you can see, even if the IR mode matching changes between 90% and 100%, the zero crossing point of CCFC error signal (detuning) changes by only ~ 0.06*54 = 3 Hz. So this effect cannot explain ~10Hz FDS detuning fluctuation.

The Bayesian method to evaluate parameter in a function is a well-known prevalenty used method for parameter estimation in many research of fields. (such as gravitational wave detector signal analysis) You can check from this link, they did a simple demonstration that Bayesian method gives a more accurate parameter estimation. In our experiment, we are trying to fit the FDS measurement result to FDS degradation model to get information of cavity detuning and homodyne angle. To achieve this goal, we were using a least square curve fit method but the error of many parameters were not considered. Therefore, to better evaluate the filter cavity detuning information, I would propose to use Bayesian method.

To do Bayesian estimation, there is a package called 'emcee' developed in python environment. The method to do this Bayesian estimation is called 'MCMC'. To use this package, we need to move the FDS degradation function from matlab to python. I did this based on the FDS code provided from Eleonora. To make sure the python function I wrote is correct, I did the comparison of python and matlab results as the  attached figure 1. I found the python code I wrote gives quite the same result with Eleonora code.

Then I made a preliminary run of the mcmc code I wrote. (It was taking ~30 hours, but can be reduced if I use parallel calculation) Based on the result of MCMC, I made a violin plot of the cavity detuning fit result estimation, as attached figure 2.

I would like to do this analysis for the future measurement of FDS.

Using Inventor 2015 I tried to design the holder for KAGRA QPDs that will be used for all our OpLev sensors.

Figures 1 and 2 show the 2 parts I'm forseeing to use. Basically (as in figure 1)  a vertical spacer to mount the QPD board using 4 screws, a long hole to use a L to fix to a translation stage and 2 holes on top to fix the 15pin D sub connector.

Figure 2 shows the 15 pins D sub holder.

The main point is that there will not be vertical or horizontal translation stage (similar to current situation). I'm also planning to put a laser line filter on the translation stage along the beam axis.

If this preliminary design is fine, I'll translate it into documents usable by manufacturer.

Aritomi, Marc, Michael, Yuhang

Yesterday during the work on CCFC, we found out that the FC can be unlocked if we touch some cable of the rack or some ground of mixers also connected to the rack.

The rack power supply ground is connected to tama ground. Using a powermeter we checked the voltage between the power supply ground and the ground cable and found that it was zero...

We might have to investigate directly on the rack.

Also we found out that even if the signal generator is turned off, if it is connected to some voltage driver it might introduce noise and cause some unlocks.

Although the locking accuracy is 1.8Hz with CCFC, the FDS detuning fluctuation is ~10Hz even with CCFC/Z correction/pointing/AA. The possible reasons are parameter estimation error and IR misalignment change and FC macroscopic length change.

sqz_dB = 10.5;                    % produced SQZ (dB)

L_rt = 120e-6;                    % FC losses

L_inj = 0.31;                     % Injection losses 

L_ro = 0.24;                      % Readout losses

A0 = 0.05;                        % Squeezed field/filter cavity mode mismatch

C0 = 0.05;                        % Squeezed field/local oscillator mode mismatch

ERR_L =   1e-12;                  % Lock accuracy (m)

ERR_csi = 30e-3;                  % Phase noise (rad)

Note: In this calculation, shot noise level is assumed to be -133.2 dBVrms/rtHz, but actually it may not be true. So the result in this log could be wrong.

All AA filters are 'DC_damp2' with an additional gain of 0.002 for yaw and 0.005 for pitch.

I plotted as an example in the figure 1 both OLTF and filter of AA input yaw. But similar results for all other dofs.

The right  column shows the product of the OMTF with the associated filter.

It can be seen that the gain never reach 1.... This might be due to poor coherence during the measurement (even if it is not possible to drastically increase the excitation while keeping the FC locked).

Another possibility to do this measurement could be to estimate Type-C suspension TF from simulation and use a single line excitation to estimate the WFS gain.