To compare least square fit and mcmc fit in a fair way, it is necessary to make both of them have both four parameters free with the four parameters defined in elog2618.

The information of mcmc fit has been already summarized in elog2618. The fit of least square information is summarized in the attached four figures.

Figure 1 and 2 are FDS with detuning ~200Hz. Figure 3 and 4 are FDS with detuning ~70Hz.

The least square fit gives similar result with mcmc if detuning is around 200Hz. However, the least square fit gives not-expected and seems-unresonable result as figure 3 and 4. By just changing the fitting method from least square to mcmc, we extract information more precisely and more reasonably.

For detuning around 200Hz data, the fit result of generated squeezing level and optical losses are

For detuning around 70Hz data, the fit result of generated squeezing level and optical losses are

Michael and Yuhang

In this elog, we compare the published FDS fit result and the new mcmc method we are using.

Interesting result! By the way, how is the fitting result of generated squeezing and optical loss for each curve? Are they consistent with each other?

Michael and Yuhang

We took FDS with filter cavity GR control about two weeks ago. The measurement contains 12 effective data with 6 for detuning around 200Hz and 6 for detuning around 70Hz. The data below around 70Hz is contaminated by back scattered noise. To have some margin from back scattered noise, we start fit from 100Hz.

The mcmc code needs a good enough initial value and corresponding range. We start with a least square fit with detuning, homodyne angles free and other parameters fixed. The fit result was used as initial value for mcmc code. The least square fit results are attached as figure 1 and 2.

We used the result of least square for mcmc and set four parameters to be free, including homodyne angle, detuning, optical losses, generated squeezing level. The result is attached as figure 3 and 4. The FDS with 200Hz detuning has more information about the squeezing quadrature rotation. Therefore, the error of fitting result is more precise. But the FDS with 70Hz detuning has less information, which makes the fit result has larger error on detuning.

The mcmc result gives more stabilized detuning, which means data favors a more stable detuning. The least square mothod gives larger detuning change may just comes from the fact that we are fixing other parameters but leave only two free.

First I checked IR injection alignment. There was yaw misalignment and the mode matching was 89%. 

After the alignment of yaw, the mode matching became 92% as follows. The injected BAB was 447uW. The misalignment is more or less fine, but LG is a bit larger than before.

By the way, during the alignment work, I noticed that the injection BAB power drifted a lot between 435uW and 465uW within a few minutes.

Then I locked CCFC and measured FDS (attached figure). CCFC calibration amplitude was 124mVpp, which is somehow lower than before. CCFC gain was 1000 and CC2 mass feedback gain was 3. The CCFC was stable and it kept locking during FDS measurement other than the squeezing quadrature. The 50, 100Hz bumps and detuning drift still exist.

Finally, I checked the nonlinear gain as follows. The nonlinear gain was 4.5 which corresponds to the generated squeezing of 10.2dB.

I will replace the electronics for CCFC to investigate the 50 and 100 Hz bumps.

By using TAMA demodulator, I monitor its output with two identical RF frequency signals as inputs. The signal drifts from 83.1 to 81.9.

Marc, Michael, and Yuhang

When we lock filter cavity with GR, IR detuning has change related to alignment. When GR automatic alignment (AA) and pointing loop is closed, IR detuning change can be stabilized.The filter cavity pointing loop is working mainly to fix the injection beam on end mirror. AA works to align filter cavity to the incident beam.

To check how detuning change for different alignment condition, we can change the pointing. By pointing the incident beam to different positions on end mirror and keeping AA loop closed, we can get AOM frequency for each point on end mirror when BAB is on resonance. The change of position on end mirror gives us a dependence of detuning as end mirror beam hitting point. In this way, we call it a detuning map for end mirror.

The changed parameters are not only beam hitting position on end mirror, the other changed parameters are input/end mirror angles and cavity length. A typical change for input angle is 40urad, end angle is 10urad and cavity length is 0.2um. Since the optical axis of filter cavity is almost the same for GR and IR, the GR AA should work also for IR. In addition, GR length control should also work for IR. Therefore, the map we get should just depend on beam hitting position of end mirror. The corresponding map is attached.

Today, I went to filter cavity end room and centered the FC GR transmission camera in yaw. But I didn't move pitch.

The pointing offset is still a good reference, since I didn't move PSD.

The filter cavity IR detuning is monitored from 2021-07-08 11:40 to 2021-07-09 11:40. (The time used in DGS is JST minus 9 hours) The minute trend data is saved in standalone desktop/detuning/20210709.

The screenshot of this monitor is attached.

Although there are many peaks in the detuning data, only 4 of them come from the unlock of filter cavity. Others are due to the suspended mirror sudden position changes but pointing loop has limited bandwidth.

We see change of detuning even when fc length is controlled. 

The FC length control change may also come from the main laser frequency change, which is due to we use laser frequency as a reference at low frequency. Especially, we don't have a reference cavity as used in gravitational wave detectors, such as KAGRA.

Abe-san, Aso-san, Marc, Michael

For reference, ETMY cleaning is summarized in KLOG entries : 17219, 17271, 17292, 17311, 17397, 17409

We brought the ETMY inside PCI room using the crane.

We added HEPA filters and put an ion gun at the end of the pressured air.

We remove optics from the small optical table on the side of the PCI setup and installed ETMY box on it.

Using a strong green light and a strong white flashlight we inspected and slightly cleaned the AR surface using ultra pure water and the ion gun.

On this table, we installed ETMY inside its holder using 4 jacks.

In order to avoid incident, we removed the entire imaging unit optical table to ease the ETMY installation on the translation stage.

Before doing so, we had installed pairs of forks on 3 of the 4 pillars to be sure to recover the same imaging unit position.

We installed ETMY on the translation stage (the additional weight due to the jig is negligeable because the translation stage can hold few hundreds kg).

We removed the HR surface first contact while using the ion gun.

To avoid scratching ETMY surface or magnets, we decided to let a metal ring at the edge of the mirror surface.

We reinstalled the IU and turning on the probe beam showed that this beam was still hitting well on the IU optics.

I don't have so much pictures but we'll add beautiful ones to this entry.

This morning  we set up translation stage limit along the Z axis. We are now not letting ETMY get closer than 1 or 2 cm to both side.

There are now really strange troubles with Zaber that does not recognize the translation stage (or any com port) even if it is working fine in labview..

We will solve this issue before any translation stage motion.

Here is the calibration performed before the installation of ETMY on PCI setup :

AC_surfref = 0.45825;
DC_surfref = 3.987;
P_in = 30.3e-3;
abs_surfref = 0.22;

R_surf = 17.24 /W

The AC peak is located at Z=39.6 mm.

It was noticed that filter cavity z-correction was feeding back some high frequency components recently.

I modified the filters and now less high frequency components are feed to end mirror.

The new filter is called dc_damp2 (gain is adjusted so that we can use gain 1 in medm). Let's use this filter in the future.

Poles: 1e-4, 0.1(1), 20, 20, 300

Zeros: 0.04, 0.05(1), 3

The comparison of signal sent to end mirror is attached.

Marc and Yuhang

Recently, we found a new spot of filter cavity elog2573, which makes the IR locking accuracy much better (the spectrum below~3Hz reduce by up to a factor of 10). At the beginning, we thought we found a more stable optical axis. However, we did a test of GR length correction signal when using old and new spot, which shows pretty similar spectrum at frequency region below ~3Hz (attached figure 1). Since the GR length correction signal below ~3Hz tells us mirror motion information, this means the mirror motion is similar for the old and new spot.

Meanwhile, the correction measured in this time is different from elog2312. Especially, it seems more high frequency signal is sent to end mirror.

When we leave z-correction loop open, if we send 1000 excitation at 0.1Hz to channel "END-len-ex2", we get 32.4 counts from PZT mon(1620counts from figure REF4, which is amplified by 50), which means 3240 counts are sent to main laser (PZT mon is 100 times smaller than the signal sent to main laser PZT). 3240/1000*0.31=1V. It corresponds to 2MHz of main laser frequency change. It corresponds to 2um change in cavity length.

When we leave z-correction loop closed, if we send 1000 excitation at 0.1Hz to channel "END-len-ex2", we get 1934 counts from z-correction loop correction signal. (there are still 10 counts sent to PZT, but since it is so small, we neglect it). Therefore, 1934 counts corresponds to 2 um length correction.

For correction signal sent to end mirror, the calibration factor is thus 1.034 um/kcount.

(the data of this calibration is saved in standalone desktop/detuning/20210628)

Because Pin ~3W seems quite convenient to measure viewport absorption (as it makes the 2 surfaces visible on the AC signal), we performed again the absorption measurement of the spare viewport.

Figure 1 shows a long Z scan of the translation stage with the 2 surfaces visibles and at the expected position.

Figure 2 and 3 show the absorption of respectively the spare and cleaned viewport.

The spare viewport shows some dusty spots (some visible by eye+ strong green light).

The cleaned viewport shows more dirty spots  (also some visible by eye+ strong green light).

However, it seems that there is no more large stain pattern visible.

In the last week, resonant condition between GR and IR changed by around 20Hz.

To check if there is any correlation with suspended mirrors, I checked the oplev signal of all mirrors. Basically all mirrors are staying in the same orientiation, but end mirrors have quite obvious drift during the last week. In fact, this drift seems to be not really because we are not really correcting it by the coils. So we need to investigate why end mirror oplev is behaving like this.

The high frequency noise is same for old and new beam spots, but is increased for 50Hz detuning compared with the one on resonance. This noise difference could be explained by the cavity pole effect. The cavity pole effect for 50Hz detuning (half detune) is smaller than the one on resonance by a factor of ~sqrt(2). Please check P.50 of LIGO-T1800447 for the cavity pole effect of detuned cavity.

I measured FDS with CCFC with old beam spot. However, the result is similar to the one with new beam spot...

So the bump around 50Hz and larger detuning fluctuation will not be related to the beam spot position.

We had better FDS spectra before with old beam spot. I don't know why it is worse now...

I was using AOM scanning speed as 4000Hz/1.7s in the calibration. However, since the scanning speed for IR is 1/2 of the value for GR, the figure in the old elog was wrong.

Calibration for the measured spectrum should be: calibration = 2000/1.66666*11.5/11.2 #Hz/V (PDH: 11.2mV/11.5ms) (AOM: 4000Hz/1.66666s)

There was also problem for the calibration for off-center on-resonance, I modified the plot by using a more reasonable calibration. It comes from the center on-resonance. The new plot is shown in the attached figure.

We can see the new stable optical axis makes especially the low frequency length noise reduced. However, the high frequency noise is increased a bit.