Marc and Yuhang

We have checked the important parameters and green power as following:

SHG temperature: 3.09kOhm

AOM setting: 109.036MHz, 5.5dBm

green power before AOM: 50.1mW

green power before FC injection: 24.9mW

Yuhang and Michael

Unfortunately in 2398 the measurement was performced incorrectly. Random noise from the spectrum analyser was injected into the "Perturb IN" port of the IRMC servo controller rather than the IN port of the IRMC HVD. This mistake was corrected, and we measured the following

• Power spectrum of dark noise, LO jitter and phase shifter excitation noise from the X, Y and T (Total) channels of the 500 Ohm position sensitive detector (PSD) - figures 1, 2 and 3 respectively
• Transfer functions and coherence of source noise to the X, Y and T channels of the PSD - figure 4
• Transfer functions of source noise to the IRMC error signal (at EPS1), correction signal (at Servo Out) and IRMC reflection - figure 5

The high frequency numbers for pitch are now much more in line with 1904. Notably, the mid frequency discrepancy between 2398 and 1904 still remains, where the mid frequency noise magnitude in this rearranged configuration is much higher than for 1904 despite having similar levels at the extremes of the measurement window. The PSD of yaw noise from the excited phase shifter is now signifcantly reduced compared to 1904, although the LO jitter is slightly above the PSD dark noise this time. We also measured the T-channel of the PSD to obtain the amplitude noise of the LO beam.

There is quite a lot of coherence from the source noise to both T-channel noise and pitch noise, especially amplitude noise above 30 Hz. The T-channel noise transfer function and coherence is fairly similar to the IR reflection, which is to be expected since they essentially just amplitude noise transmitted and reflected from the IRMC.

Source noise coupling to the error signal increases above 100 Hz, while the correction signal transfer function does not follow said increase. Howver, they have similar coherence across all frequencies.

Just as a note, when I checked and realigned the IRMC at the start of the measurements for the day, it seemed to have drifted out of alignment with a pitch HOM prominent in the reflection spectrum.

Marc and Yuhang

Marc provided us a code written by Julia Casanueva Diaz, which was used by Marc to do FIS auto alignment for Virgo.

After reading this code together, I found out what can be improved for the construction of driving matrix. Especially, I finally understand what is the meaning of matrix of inversion. The idea is the change of basis after inversion. With this in mind, we checked again the sensing matrix and inverted it. We got following driving matrix

With this driving matrix, we did preliminary check of Input and End mirror pitch/yaw reconstructed spectrum. Compared with the old measurement (elog2263, elog2245), it seems the reconstruction is more reasonable (attached figure).

More investigation and optimization will be done soon.

Marc and Yuhang

After moving picomotors, filter cavity alignment was recovered.

Spectrum of all mirrors were checked as the attached figure. No touching induced peaks were found.

For the Labview control for INPUT mirror picomotor, I have some comments.

***notice: check point of reflected beam is inside PR chamber

1. Pitch positive is to move the reflected beam to the left.

2. Yaw positive is to move the relfected beam to the up.

(currently, the input mirror pitch and yaw picomotors are swapped)

As reported in entry 2399, there was an earthquake in March 16th around 5 am.

I went to check the setup and it seems that at least the imaging unit is misaligned (moved by few 10 um in distance to the translation stage and the red beam was misaligned by ~ 1 cm to the left on the photodiode). I'll try to characterize the beam again after maximizing the red and IR beams on the photodiode/powermeter.

Also, I suspect that there was some troubles with the computer as :

- several errors messages on the LabView program referring to troubles with the translation stage

- waveplate moved around the earthquake time but no errors

- the chopper frequency changed to 1kHz.

Michael and Yuhang

We completed the setup as per 2393. The spectrum of aligment noise from excitation of the IRPS was made and compared to 1904.

The measurement was taken using a 500 Ohm PSD placed after the IRMC and homodyne flipping mirror. We measure the dark noise, followed by the jitter noise when transmitting the local oscillator, and then the jitter noise when the phase shifter PZT is excited by random noise from the spectrum analyser, applied via the high voltage driver. In the original measurement of 1904, the PZT was excited using 200 mVpk random noise, where the beam had a horizontal angle of incidence of ~ 45 degrees. Here, the beam has perpendicular incidence, so we only use 200/sqrt(2) ~ 140 mVpk excitement for the yaw measurement, but keep 200 mVpk for the pitch measurement.

For yaw, we seen that the noise floor seems to be the same or perhaps slightly better than 1904, it is hard to tell just from the picture. However, the new measurement reaches the noise floor at approximately 30 Hz, compared to ~ 10 Hz in the old measurement. Also, both measurements have similar values at either end of the measurement frequency window, but the new measurement looks to be a lot higher in the mid frequencies than that of the old measurement. The PD dark noise is consistent in both cases.

For pitch, the measurement is very strange. Unlike yaw in this case and both degrees in 1904, the LO jitter noise does not converge to the PSD dark noise at high frequency. Both the LO jitter and excitation jitter seem to be about 15-20 dB higher than their yaw counterparts.

Afterwards, we measured the transfer function of input excitation (from the spectrum analyser) to measured PSD noise. We see that the pitch transfer function has significantly more coherence from the excitation to the PSD. The yaw coupling at 140 mVpk excitation is also much lower than for pitch excitations.

A few other notes:

- The IRMC alignment seems to drift quite a lot. Last week it had to be relocked about 4 or 5 times during the afternoon. It also had dropped to about 1.6 mW transmitted power before starting the measurement described in this elog. I realigned it for the optimal transmission each time (1.8 mW).

- The phase shifter is not quite flat, it has some pitch tilt. As a result, the incoming alignment is not perfectly horizontal, but rather is adjusted so that the outgoing beam maintains a constant height of 76 mm.

- PSD output voltage was zeroed to within 0.5 mV for both X and Y

Aso-san, Marc

The air leakage of the gate valve was indeed due to a loose screw.

Aso-san tighten it more and no more audible sound nor air coming out.

On Japanese standard time 04:56 JST 16 Mar. 2021, an earthquake happened close to Tokyo area. (attached figure one) 

I checked the oplev signal during this time in the saved data of DGS. PR, input and end mirrors reached the maximum of oplev sensing range. Besides, this oscillation lasted for almost 20 seconds and the exponential decay lasted for almost 40 seconds. (attached figure two)

After the earthquake, all mirrors drifted away from their original position. (attached figure three)

According to the above table, we are going to move picomotor to recover mirror position.

Before moving picomotor, I also checked the spectrum of all mirrors pitch/yaw. No touching induced peaks were found. (attached figure four)

Marc, Yuhang

Yesterday we wanted to check again QPD demodulations pahses before implementing the new driving matrix.

However, the BS pitch seemed to be totally out (180 offset required to get the beam on the end camera).

We had a look on one week data trend of every mirror oplev signal (attached pdf).

This large BS offset has 3 origins as indicated by the red arrows :

1. An earthquake not too far from Tokyo in March 3d caused a -500 pitch offset of BS (also visible on IN and slightly on END but not so evident effects on PR )

2. Switching the air conditioning to 'cold mode' cause a 1500 pitch offset of PR. Indeed, PR is located directly under the air conditioning.

3. Strong rain/storm on March 13th cause a further 500 pitch offset of PR

Therefore we decided to use PR pitch picomotors to move PR back to its previous good position (~300 steps).

But this made the required BS offset -180... This is quite close to coils saturation (it corresponds roughly to 20 000 counts) so we might have to move again BS picomotors...

After the movement of PR picomotor we took the oplev signals (fig 1) where green/brown are references and blue/red yesterday data.

We can see that PR seems fine. However, comparing the low frequency spectrum of BS and PR between reference and yesterday datas, PR low frequency spectrum changed quite more than BS one. It seems that PR is far more sensitive to seismic activities than BS?

We also found out that there is an air leakage on the BS gate valve. It might be due to a loose washer pointed by Yuhang in Fig 2. It might be another after effect of the large earthquake as the air leakage sound was not so easy to hear.

Marc, Yuhang

Yesterday we spent some time to tweak a program made by Julia Casanueva for Virgo automatic alignment and adapt it to our case.

Then, we tried to measure the sensing matrix.

However, the green beam was not anymore centered on the QPDs..

We found out that the last steering mirror before the QPDs board was not well fixed. We fixed it back but we could not recover the alignment by acting only on this mirror.

It could mean that another mirror on the green reflection of the FC has been misaligned but we couldn't find which one so we acted on the both galvos to recenter the beam on the QPDs.

We measured the sensing matrix by injecting a 2Hz line on Input and End mirror pitch (300 amplitude) and yaw (200 amplitude) and extracted a driving matrix and phases to get all signal on I quadrature.

Today, we saw that the phases are tuned at the level of each segment so we will have to tweak a bit more the program as it gives the optimal phases at the level of pitch and yaw (not individual segment).

Anyway, today we tuned each segment phase 'by hand' to get all signal on I quadrature by either looking at the 11 Hz resonance of pitch or injecting a 2 Hz line. During this measurement the misalignment of the FC affected the accuracy of the measurement so we had to realign it, but also there were moment with large changes of the optimal phases...

The new phases are :

We completed the arrangement of the optics described in 2393.

A bit of misalignment from an ND filter in front of the 250mm lens was corrected, and some slight account had to be made for the phase shifter not being perfectly vertical (i.e. introducing pitch misalignment from horizontal incidence). The IRMC mode matching was optimised through alignment and fine tuning of the 75mm and 250mm lens positions, and we recovered the transmission of 1.79 mW of TEM00 from the IRMC (power meter measurement).

We will compare the new result to that of 1904

Michael and Yuhang

We worked on replacing the phase shifter for the beam going into the IRMC, highlighted in figure 1. Afterwards we will also look at the one going into GRMC. The angle of incidence causes beam jitter noise when the phase shifter acts on the beam. We decided to replace this with a perpendicular incidence setup as per the sketch in figure 2, using a PBS and waveplates.

We have reference for the values of the IRMC (390 µm beam waist, 1.8 mW transmitted power). Using the reference waist and the distances of the holes, the beam should be collimated before the 250 mm lens (fig 3). So there is not much need for complicated rearrangement of the lens positioning. We just have to move the 250 mm forward by however much the path length is from the PBS to the phase shifter and back. Fine tuning of the lenses can be done via the mount. A rough indication of the distances is shown in fig 4.

We took the following items:

PBS: CVI PBS-1064-100

HWP: CVI QWPO-1064-08-2-R10

QWP: CVI QWPO-1064-09-4-AIR-R10

During the aligment we made sure to recheck the beam propagating along the west end of the table to the 250 mm lens, to make sure it aligned with the holes and was consistent with the more recently aligned beam height from the SHG (76 mm).

Using the power meter, the power before the PBS was 4.14 mW. After double passing through QWP and PBS, it was ~ 3.8 mW.

Akutsu-san, Marc, Matteo

In agreement with Akutsu-san we plan to use the ATC clean room to assemble the new OPO.

Fortunately, there is an IR laser installed on one corner that we can use. After the laser source, there are some optics (Faraday Isolator, lenses) so that the beam should be collimated.

The procedure to turn on the laser is the following :

- Connect power supplies

- Turn on the +5V supply (largest black button)

- Press 'stand-by' button on the laser source to get out of the standby mode

There are few components around this laser setup that we can displace for the assembly (scale, forks, pillar) so that with 2 steering mirrors we have enough space to use this laser.

PS :

- We have to bring new clean suits and we can also bring the old ones of this clean room for the next time we clean the clean suits

- we'll need a periscope, beam profiler, mounts, forks, few lenses, safety glasses, etc ...

Marc, Matteo

Last Friday I flipped the surface reference sample and tried to further tweak the alignment without much improvement.

Yesterday, we decided to measure the beams profile.

Unfortunately the automated LabView program had some error and we had to do measurement one step at a time...

I installed first the razorblade vertically (2 cm in front of the last hole of the translation stage toward the imaging unit) and scanned both the red and IR beams on the Y/Z axis.

[Note that connecting the DC output of the powermeter went smoothly despite troubles to read the powermeter through the other cable]

Then I turned the razorblade horizontally (54mm above the translation stage) and scanned both the red and IR beams on the X/Z axis.

Actually the razorblade cut the beam not on the razor edge... meaning that we have to take into account the 28mm width of the blade.

I finished to write some MATLAB code to automatically extract all measurement data from yesterday, fit them using a program wrote by Manuel.

But It is still needed to check the screenshots 1 by 1..

Marc, Matteo

Yesterday we removed the first contact applied on KAGRA spare, flipped the mirror and applied first contact on the other surface (Fig 1)

Michael and Yuhang

We replaced the big mirror mount in front of AMC with a compact one. (attached figure 1: the replaced mirror. attached figure 2: the gap between replaced mirror and board)

We have made homodyne DC cable, homodyne power cable, AMC DC PD power cable go through the appropriate hole. (figure 3: extended cable on homodyne power side. figure 4: current cables situation around homodyne)

To close bench, we still need to drill holes on board to let GR inj/ref and IR inj/ref go through (three holes are expected). 

Marc, Michael and Yuhang

We have characterized some signal and noise of CCFC PD in elog2384. However, the pump power was 30mW in that case. According to the simulation and experiment, for the current system, 18mW of pump power would be better. Therefore, we did more test with 18mW pump.

In the attached figure, some signal and noise levels are shown. From this plot, 18mW pump power seems to be not large enough. Indeed, we also tried to close anyway the loop with such small error signal. But we couldn't find a good filter configuration to make the loop closed proporly.

I confirmed the coating of the surface reference sample is on the side with the written part. Therefore the correct orientation is with that side first (i.e. facing the periscope).

The discrepancy between the current and old measurement as well as the slight asymmetry can be caused by not optimal alignment of pump beam.

Aritomi, Marc, Matteo

Today we cleaned a bit the computer room and brought the KAGRA spare to the clean room. We putted first contact on the surface facing outside the protection box.

Then, we followed the realignment procedure described on the wiki and installed the surface reference sample to realign the setup with the goal of reproducing the calibration done in elog 1619 : R_surf = AC_surfref/(DC_surfref*P_in*abs_surfref) = 16.9 [1/W]

First, we putted the surface with the sample name facing the laser (PS : mirror center in z is 38 mm). Results are presented in figure 1 and gives (with abs_surfref = 0.22) : AC_surfref = 0.17V, DC_surfref = 3.91V, P_in= 16mW -> R_surf = 12.4 [1/W]

Then, we flipped the reference sample (PS : new center at 33.25 mm) to check if it could explain the discrepancy wrt to elog 1619.

During a scan, there were some troubles with some connections and we got several error messages from LabView... We had to restart the computer to use LabView again but there is still some troubles with the power-meter ( detected but not able to connect to the computer)

The result of this configuration is presented in figure 2 and gives : AC_surfref = 0.31V,  DC_surfref = 4.01V, P_in= 15mW -> R_surf = 23.4 [1/W]

This time the AC measurement is not as symmetrical as expected so we'll further tune the alignment tomorrow.