Aritomi and Yuhang

In the beginning of yesterday's recovery of squeezing measurement, we measured LO spectrum with homodyne and found lots of peaks at the low frequency region. We checked the clipping and the centering of beam on homodyne PD. It was fine.

However, since we recognized that the frequency of 24.5Hz comes from scroll pump. And 20Hz comes from clean room fan. The 13.5Hz peak comes from bench horizontal mode. We decide to put back the board wihch is taken away because the demodstration requirement we had on Monday. After that, the measurement of LO spectrum becomes very clean.

Maybe the balance of weight on top of bench helps to remove the bench horizontal mode peak. And the board isolate the sound wave propogates to the mirrors and PDs. These help to remove peaks. But the source of peaks at 16Hz, 18Hz and 33.8Hz are still unknown.

Simon

(This is report from Yesterday)

I inspected the OSTM mirror substrate by my eyes in the TAMA clean-room (absorption bench) to make sure that there are no major damages (pictures attached).
So far, it looks good. I packed the substrate again and left it in the shelf in the anteroom.

[Yuhang, Eleonora]

Today we worked on the recovery of the FC after summer break. We recovered easily the alignment towards the end mirror but once we aligned the input we couldn't find any flash.

Since from the signal we suspected a saturation of the stanford research used to amplify oplev signals in the end, we went to check and we found a major water leakage in the end room.

The floor below and around the vacuum chamber (which is a bit lower with respect to the rest of the room) was completely covered with water (~1cm deep).

While switching on and off the air conditioner we observed that a discrete amount of water started to drop from it along and wall, reaching the floor. We suspect that the drain system of the air condition is not working properly.

We tried to dry the water with some paper but a part of the water is still there. 

Anyway we decided to go on with the recovery: we zeroed the oplev signal, so that the stanford where not saturating anymore and we could close the loop properly. Than we realigned the end mirror, found the flashes and lock the cavity.

To realign the end mirror, we did the usual trick that is to let the beam pass through the second target hole and look for the reflection from the end mirror on the back of the second target. 

Tomorrow we will remove the remaining water and ask Takahashi-san if he can check with us the air conditioner.

As suggested by Matteo, our squeezing measurement issue can be related to homodyne. The idea is to check the visibility of homodyne by using two PD of homodyne and compare them. To check the visibility, I used BAB and IRMCtra to make the beat note. For the BAB maximum transmission, the new PLL locking frequency I got today is 315MHz (without green).

For homodyne PD close to IRMC:

BAB is 500mV

LO is 1.835V

In this case, I found PD is saturated. So I put an OD 0.5 filter in front of IRMC. After this, LO becomes 644mV.

The beat note is shown in the attached figure 1. In this case, visibility is 90.37%

For homodyne PD far from IRMC(everything is the same apart from this PD):

BAB is 498mV

LO is 647mV

The beat note is shown in the attached figure 2. We could see that it is saturated. This is strange because this PD should have the same response with the other. Or we should not use this homodyne in this way because it is designed to use both PD at the same time. Anyway, we should investigate if this is a problem or not.

Today I checked the alignment of SHG, GRMC, OPO, IRMC and homodyne.

Among them, only OPO and homodyne is misaligned. So every misalignment is related to OPO.

Homodyne visibility is measured as 90.37%. (The situation of homodyne will be reported in the following entry)

Fig.1 SHG spectrum

Fig.2 GRMC spectrum

Fig.3 OPO p-pol spectrum

Fig.4 OPO p-pol spectrum(after alignment improvement)

Fig.5 IRMC spectrum

Fig.6 OPO CC spectrum

Fig.7 OPO BAB spectrum

Fig.8 OPO BAB spectrum(after pitch alignment improvement)

Kubo-san, Yoshiyuki, Simon

I inspected today the big furnace at the ATC (located besides the exhaust hood) to see how we can use it and whether it is available or not. Please see also the pictures attached.

The model is: ISUZU DSTR-314K
max. temp.: 1220 degrees Celsius

The inner area seems not very clean. So, in order to use it for KAGRA substrates, we need to create some kind of enclosure to save the substrates from pollutions (especially penetrating atoms/ions).

I will try to find a manual but it seems that the actual usage is relatively simple. Also, since almost nobody uses it now, we can take our time.
Before using it, however, we have to write an Email to Kubo-san and Yoshiyuki.

Here I attach the auto-alignment telescope design circulated by Yuefan. Just copy Yuefan design. 

The injection telescope for squeezing injection is changed(entry 1546) because of the existing CVI lens we have is limited. It is good to know the robust of this new telescope. Thanks to the code EleonoraPolini sent me, I easily performed this test.

Since only the injection telescope is changed, I only did the test for this injection telescope. Compared with the previous design(entry 1366), the robust is similar. So I think we can be happy.

Aim: We made some measurements with the quadrants to check if both RF and DC are working fine

For testing the quadrant, we took a beam from the main laser with a beam splitter (BS1). Scheme in figure 4.

At the first tried, we just put another beam splitter (BS2) after the first one, then EOM in one of the path with a modulation frequency of 23MHz. One lens was put between BS1 and BS2 in order to have the beam waist inside the EOM. After combining two pathes with BS3, we put a filter with OD=1 to avoid injecting too much power on the quadrant. Then we place the quadrant with the beam more or less centered.

But we could not see any beats from the RF output of the quadrant. We thought one possible reason could be the phase modulator makes 2 sidebands (one above and one below the light frequency), both of them interfere with the non-modulated beam. Probably two beat signals cancel each other almost perfectly because the beam path is the same (there is no such thing as a cavity that can treat the two sidebands differently).

So we decided to add the AOM on the non-modulated path with modulation frequency of 80MHz. With two beams of similar size and well aligned on the quadrant, we could see the 80MHz beat in the spectrum. 

Then we demodulated the RF signal with the box. By checking the in-phase(I) and quadrature(Q) of one of quadrant output , we could see that not only the power between I and Q is changing, but also the total power (both signals are moving up and down together).

Since our oscilloscope was not able to calculate the total power of two signals, we decided to put the second quadrant at the other output of BS3 and accquired one group of data while we knocked on the bench. The raw data we got show in figure 1. From this figure, we confirmed the quadrants are working fine. The I and Q are perfectly out of phase for two quadrants. The fact that the I and Q signals of the same QPD do not have the same peak-to-peak amplitude could be explained by a path length change of less than one wavelength.

Then Martin did some analysis of the data.  In figure 2, he calculated the I^2+Q^2 for both quadrants, and also the sum of them. The amplitude of two quadrants have more or less each others inverted signal. The fluctuation in the sum is smaller than that of the individual amplitudes. Hence the total power is almost conserved. The remaining fluctuation in the sum could be due to unequal beat signal amplitude (for instance due to a difference in ND filter, or a difference in overlap of the beams on the different QPDs).

In figure 3, the phase of both quadrants are moving in the same way, it could be caused by the path length difference before BS3, so it has same effect for both the quadrants. But there is this factor of 2 difference in the phase changing, which we didn't really understand. Part of it could be caused by the small DC offset of the raw signal.

After testing the RF signal of the quadrants, we also tested the DC with the galvo. We simply put the galvo in front of one of the quadrant (see the bench picture in figure 5), and did some rough alignment to make sure that the beam is inside the diode. Then by sending four quadrant DC outputs to the galvo controlling board, two outputs of this board will be used to control the x and y direction of the galvo. Then we checked the x and y error signal through the monitor port of this board. As long as the galvo loop is closed, the galvo is able to bring the error signal back to 0, which means the beam is centered on the quadrant.

Conclusion: With two quadrants, we got figure 1 that confirm both of them are working as we expected. For the DC, the galvo is able to center the beam on the quadrant. Now everything has been removed from the bench and packed. We will send them to NAOJ today or latest next Monday.

I checked the CVI lens we should buy if we want to replace the thorlabs lens now we are using between OPO and homodyne.

The result is attached in the figure. Since we could find a lens with the focal length similar to thorlabs focal length, the simulation result is similar to the previous simulation. In this case, the lens we should buy from CVI is PLCX-25.4-46.4-UV and PLCX-25.4-70-UV.

Also if we have again almost 100% match from OPO to AMC, the telescope design for filter cavity injection should be not be influenced.

If we just consider this is a factor of 11 between green and IR filter cavity locking accuracy as pointed in entry 760 figure 6, we could estimate the IR locking accuracy of the filter cavity. (in green locking case)

As reported in entry 1486 and entry 1513, we have green filter cavity locking accuracy of ~4Hz. This is corresponding to 4e-12m.

Then by considering the factor 11(because of cavity pole of green and IR is different), the locking accuracy of IR should be 0.36e-12m. In this case, it meets the requirement of filter cavity length fluctuation.

But we still need to confirm the measurement of entry 1486and 1513.

If we consider an error of reflectivity of HR coating of PPKTP(0.02%). We could have a very different estimation of OPO round trip loss and OPO escape efficiency.

Note that here we use transmissivity of OPO of 0.2% as a prior (as entry 1538).

Escape efficiency is T/(T+L) where T is transmission of output coupler and L is intra cavity loss. So escape efficiency should be 0.08/(0.08+0.00425) = 95%. Calculation in Marco' s thesis seems wrong.

From Marco thesis, the escape efficiency is 0.92/(0.92+0.00425). It is 99.5%, it seems fine in that case.

I was using a database considering all the focal lenghts commercially available, not only the ones on Thorlabs. Did you check also the robustness of the injection telescope with the two lenses that close?

The last version of injection telescope was version 3 of entry #1366.  

Simon

For some reasons (probably Windows related), the absorption-bench PC had a reboot Yesterday evening (around 19:08). Therefore, the running measurement of ETMY abs. map was interrupted. Fortunately, nothing bad happened to the system itself and I could easily recover the setup and start another measurement today.
Fingers crossed...

Meanwhile, I have started to take spectra of the Sapphire samples that we will use as new calibration samples to have a measurement on their absorption index. Actually, I did such a measurement already on Tuesday and wanted to finish it today with the other samples but for some reason, the settings in the spectrometer were totally different from Tuesday and I couldn't get any stable signal from the spectrometer. I asked Mitsui-san from ATC for help but he is now in vacation...
Anyway, attached to this report is the preliminary result of Blue-Sapphire No1.
According to the measurement, we can expect an absorbance of 0.106 at 1064nm.

Simon

From the measurements last week I obtained the calibration of the system:

R_surf = AC_surfref/(DC_surfref*P_in*abs_surfref) = 24.9 [1/W]
where AC_surfref = 0.75V, DC_surfref = 4.15V, P_in = 0.033W and abs_surfref = 0.22

R_bulk = AC_bulkref/(DC_bulkref*sqrt(T_bulkref)*P_in*abs_bulkref) = 0.535 [cm/W]
where AC_bulkref = 0.083V, DC_bulkref = 4.7V, T_bulkref = 0.55, P_in = 0.033W and abs_bulkref = 1.04/cm

As can be seen, the calibration factor of the surface reference seems to be a lot higher than for the measurements done in July (-> 16). Probably, because I did not pay attention to the orientation of the surface sample
The bulk-sample, however, is not so far away from the values of past calibration measurements (-> 0.604 in July). That gives me at least some confidence about the calibration so far.

Furthermore, today I restarted the laser and put the ETMY test-mass on the translation stage again.
I started a Z-scan with ~2W input power to check the positions of the surfaces. Also from this measurements, the crossing point seems to be shifted by 2mm (center is now at 77!), which is consistent with the calibration.

I increased the laser power to ~10W and ran a rough map to check that the test-mass can hold that power in the whole map area. Fortunately, there seems to be no problem so far. Apparently, we were successful in removing all the soot and FC remains.

I started the mapping at the test-mass' center.

Simon

I have successfully reset the absorption bench by removing all the additional optics which we put for the polarization measurements.
After that I adjusted the optical-unit (putting it back to calibration position @70mm) and exchanged the ETMY test-mass with the calibration-sample holder. I used partly the empty space on the optical-unit table to lift the heavy test-mass.

During calibration, I recognized that the bulk-sample is still inside the holder. However, by that time, I already finished to take some calibration measurements. So, I changed it with the surface-sample and took the respective measurement. Anyway, in both cases the calibration looks very good.
The crosspoint, however, seems to be shifted by 2mm compared to our calibration for ETMX, but is in very good agreement to measurements taken in February this year. I did not do a further analysis of the calibration measurements yet (maybe I do this during weekend, let's see). But just by the eye, the AC/DC values look good.

During the weekend, I shut-down the laser-power as there is nothing to do. For the actual test-mass, I need to be there for carefully increasing the laser-power, which will take some time, I guess.