Today I reinstalled the reference samples and checked the proper positions.

With the surface sample I got the crossing point at z = 35 mm (maximizes AC) and z_IU = 68 mm (maximizes AC/DC).

I measured R_surface = 16.91/W and R_bulk = 0.6212 cm/W.

I installed back SHINKOSHA 7 and did a long z scan.

In the attached figure you can see the comparison between various signals for the previous long z scan (red) and current situation (black) taken at the mirror X and Y centers.

Note that the z axis has been shifted for the red using the surfaces signal and then interpolated to the new measurement (step size of 0.05 mm instead of 0.1 mm).

As expected, the absorption is larger now and it seems that we have something like at least a factor 1.4 increase.

However, I was expecting to see a somehow constant increase but this is not the case..

I started a XY absorption measurement at the mirror center (ie X = 399.08mm, Y = 122.175 mm and Z = 71.625 mm)

Katsuki, Marc

This is a summary of these past days activities.

As reported in entry 2863 we found out that the pump beam was larger than expected (48.5 um instead of 36 um).

In summary we had to act on the two lenses on the pump beam path to recover the good beam size and position following Jammt simulations.

These 2 lenses are now about 1 cm closer to the laser source.

During this realignment we also checked the the probe beam size to have a reference waist position.

We found out that it is not feasible to use the absorption DC photodiode together with the razor blade because there is scattering when we start to cut the beam that creates a spikes in the data and prevent a good fit of the data.

Furthermore, the probe beam is really large on this photodiode and is really astigmatic when setting up the imaging unit translation stage at z_IU = 0 mm (ie farthest from the translation stage).

In the end, we installed a power-meter in between the imaging unit lens and sphere and could get good data.

We also found out that a good step size for the translation stage is 20 um as it allows to get good enough resolution while not taking too long.

The attached figure reports the beam profiles of probe beam in vertical and pump beam in both vertical and horizontal directions.

The z axis is the same as Manuel's measurement (see elog 1089) ie the 0 mm is at 75mm from the breadboard.

We had to tweak a bit the tilt of the lenses to minimize the pump beam astigmatism.

We recovered the expected pump beam waist size and position so we will now switch to absorption measurement.

Note that probe beam power is 2.5 mW and pump beam power was about 150 mW for this measurement.

Nishino

I measured the beam profile between L2 and L3. I got better results than the previous one (see 2856).

*starting position is 25 mm away from L2.

Taking the first 30ns of measurement data, I did a FFT analysis of the data and got a power spectrum density (PSD). Then the time span is shifted by 1ns several times to get the PSD evolution. In total, the 200ns data is shifted by 170 times to get the signal PSD change as a function of time. This is shown in attached figure one.

The FFT has a bandwidth of 33MHz (since I used 30ns to make a FFT). Because the RF signal has a frequency of 110MHz, I took the frequency span of 99-132MHz to check the amplitude of RF switch output.

From this analysis, the fall time, which is the time that signal drops from 90% to 10%, is 10.6ns.

In addition, I also put a time-frequency-amplitude plot of this signal.

Pierre Prat (remote), Yuhang and Michael

We received the Minicircuits M3SWA-2-50DRB+ absorptive RF switch evaluation board. Nominally, it has a fall time of 4.6 ns, well within bounds of what we want (400 ns). In this case, the rise and fall times have been specified by the manufacturer as the time it takes to go from 10% to 90% of the peak voltage and vice versa. The circuit can accept high input power > 24 dBm at 100 MHz. It is powered by -5/+5 V supply. The switch is activated/deactiveated by a TTL (transistor-transistor logic) control signal. In short, voltages in a certain low threshold (0-0.8 V) are considered "OFF" and in a certain high threshold (2.1-5 V) are considered "ON". In this case, we can just use a square wave oscillating between 0 and 5 V, and then trigger the oscilloscope to follow the rise/fall of the RF signal.

Chip manual: https://www.minicircuits.com/pdfs/M3SWA-2-50DRB+.pdf
Evaluation board diagram: https://www.minicircuits.com/pcb/WTB-M3SWA250DRB+_P02.pdf
 

-- Test --

The RF switch was tested in the filter cavity clean room using the already present oscilloscope, function generator and RF amplifier(s). We brought a DC power supply to send -5V/+5V to power the RF switch, as well as a Tektronix AFG320 function generator to provide the control signal to the RF switch (0 to 5V square wave, checked at 1 Hz and 12 kHz). Both of these were tested first to make sure they give the required voltage and square wave signal.

A 500 MHz RF signal was sent from the filter cavity function generator to the switch -> RF amplifier -> oscilloscope. A 20 dB attenuator with 50 Ohm impedance was connected to the oscilloscope to prevent back reflection. Unfortunately, the first RF amplifier (Minicircuits ZHL2) we were using stopped outputting. We did take care to say the order in which you should make connections with the RF amplifier. I hope it is not permanently broken... 

-- Data --

The figure shows the fall time when a 1 Hz square wave is sent to the TTL port of the switch (rise time figure pending). The data is a bit low resolution. The lower half of the figure shows a zoom in of the timescale and indicates 10% of Vpk. This measurement doesn't seem very accurate, but regardless, the fall time is well below the target of 400 ns.

With these results, we moved the RF switch and the necessary electronics to the ATC cleanroom.

This work is on 20220303. 

I measured CCFC error signal with green 1/f, 1/f^4 filters, and CCFC. The green transmission beam spot is upper side of camera (new beam spot). The green FC injection power was 23mW. The parameters for 1/f and 1/f^4 filters are as follows. Fig 1 shows the green OLTF.

The measured CCFC error signal with green 1/f, 1/f^4 filters, and CCFC are shown in Fig. 2. The CCFC amplitude was 142mVpp. CCFC filter gain is 1000 with 30Hz LPF.

Measurement of CCFC OLTF

CCFC OLTF can be measured by injecting a signal to sum port of CCFC filter (SR560) and measuring CCFC filter input/output. The relation between this measurement and CCFC OLTF is as follows.

(CCFC filter input/output)*CCFC filter = - G_CCFC/(1+G_green)

G_CCFC = - (CCFC filter input/output)*CCFC filter*(1+G_green)

Since the CCFC filter and G_green are known, G_CCFC can be obtained. Fig. 3 shows green, CCFC OLTF. The crossover frequency between green/CCFC is 1.6kHz.

We tried to investigate possible explanations for this discrepancy.

First we performed along z scan to be sure that we are able to see the 2 surfaces of the samples.

We could find S1 at 34.8 mm and S2 at 122 mm along z.

We can see the ac/dc signal decreasing with an increase of z (same as Manuel's measurement) but the signal is roughly half of what he got.

We have the same chopper frequency, we're injecting pure s polarization, but differences are that he was injecting about 10 W vs our 8.5 W, he set the DC to about 2.5 V vs 4V now and in his computation he is using 1.16 /cm instead of the 1.04 /cm later measured and I'm not sure how the transmission was taken into account.

For reference Manuel's measurements and analysis are in the KAGRA#7 folder.

One strong possibility is that we have a too large pump beam size. Indeed Manuel found out that it could cause some factor discrepancy when he upgraded the setup.

We characterized the beam size with the razor blade as reported in figure 1. The beam waist is 48.5 um instead of the expected 35 um.

Following Jammt simulation that indicates that the beam waist of 35 um by moving the last lens by ~5mm we started to realign but without clear improvement so we'll continue on Monday.

Aso and Nishino did fit-check on Febrary 22nd.

There were 2 issues in the optical components

1.  the sizes of the screw head for OBS1 were too large and they touched both sides of the OBS1 mirror.
2.  the new mirror mount for OBS4 doesn't have a reference mark to set on the mirror

for the issue 1, Fukushima-san fixed it and Nishino did fit-check on March 4th.

Issue 2 is left on the date of this report.

I measured the beam profiles again on March 3rd. I took 10 samples for each point and used an average of them.

* start position is two holes (~50 mm) distant from the center of the BS.

I compared CCFC error signal with old/new green beam spot. The new green beam spot is upper side of camera as shown in elog2613. The old green beam spot is center of camera. 

The offset of BS pointing for old/new beam spot is as follows. 

The attached figure shows CCFC error signal. The CCFC amplitude was 118mVpp. The new beam spot is better than old beam spot below 10Hz.

As reported in elog2850, FC is sometimes very stable, but not very stable most of the time. I noticed that the BS coil output was too large (~20000). After BS offload with picomotor, FC got more stable. Maybe BS was touching somewhere and that could cause the unlock. However, FC still sometimes unlocks. It might be better to open PR chamber and check PR suspension.

I designed an optics aligenment using g-trace (see fig 1).

I measured the beam profile of the laser in the ATC clean booth. Yuhang and Michael are doing another experiment, so Aso-san and I will use the reflected light on the beam splitter(see figure 1).

Results:

* start position is two holes (~50 mm) distant from the center of the BS.

In order to get a more precise comparison between birefringence (integrated along z) and absorption (at a given z position) measurements, we performed absorption measurements at several z positions.

Note that in that case the shinkosha 7 orientation is still the same as the measurement done by Manuel ie arrow at the top and pointing towards the imaging unit.

All results are attached to this entry where I used same colorlimit as Manuel (0 to 200 ppm/cm) and similar colormap.

Similar patterns are visibles.

However, it seems that maximum absorption is quite lower than what was measured before...

One difference with the previous measurements is that we were using 0.5s waiting time and 70mm radius..

I'm now starting new measurements with differents lockin amplifier parameters to investigate this issue.

Note that the z values indicated here correspond directly to the translation stage values (therefore different than Manuel measurements where he corrected the z value to match the real position in the mirror)

Abe, Katsuki, Marc

We purchased a Soleil-babinet compensator that will be installed in PCI for future birefringence measurements.

We prepared a test setup on the optical table just in front of PCI clean room (the one where there is the reflectance measurement setup).

We will use the FC spare laser.

We installed 2 OD to have ~ 40 mW of power then 2 steering mirrors(Newport 5204)  to have proper alignment above a lign of holes.

We also brought the required componenents (1 PBS, 1 QWP and 4 HWPs).

As reported in elog2852, the CC detuning and CCFC demodulation phase should be adjusted. Since the CC detuning in elog2852 was 76Hz, first I changed CC PLL frequency by ~22Hz, but the shape of CCFC error signal was strange. Then I decided to change CC PLL frequency by 10Hz. The setting of CC PLL is as follows.

The setting of LEMO cables for demodulation is as follows.

Fig. 1 shows CCFC error signal. The CCFC calibration amplitude is 138mVpp. The mode matching is fixed to 0.9.

Today FC was quite stable and I could lock CCFC for the first time since last August! The CCFC filter gain is 1000 with 30Hz LPF. The Z correction, AA, BS pointing were engaged.

Fig. 2 shows the locking accuracy with CCFC.

First I aligned BAB to FC. The optimal p pol PLL frequency without green was 300MHz and BAB power before FC was 435uW. The maximum IR transmission of FC was 470.

Then I checked the CCFC error signal. I optimized the p pol PLL frequency to maximize the CCFC error signal. The optimal p pol PLL frequency with 20mW green was 240MHz and the CCFC error signal was 118mVpp. The setting of CC detuning is reported in elog2521. The setting of LEMO cables for demodulation is as follows.

The attached figure shows CCFC error signal. The mode matching is fixed to 0.9. We need to adjust the demodulation phase and CC detuning.

I found that CC PLL could not be locked. I changed the phase detector polarity of ADF4002 from negative to positive. Then the fast loop of CC PLL could be locked, but slow loop could not be locked.

Today I replaced the Qubig PD back to the one with DC output (simply called DC-Qubig later), whose change was done about one month ago (elog2801). Note that I just temporarily set up DC-Qubig, whose cables are still not deployed properly since it's hard to do by myself

After putting DC-Qubig back, to have a decent loop gain, I adjusted DDS2 channel CH1 amplitude from 1/4 to 1/2. After that, I took a measurement of open-loop transfer function. The unity gain frequency was around 13kHz. At the same time, the Rampeauto attenuation is 0, the Rampeauto gain is 8.

The setting for AA is shown in Fig.1. The setting for z_corr is shown in Fig.2. 

In the first minute of Fig.3, the filter cavity is controlled with PDH and z_corr. After using setting of Fig.1 for AA, the filter cavity transmission is stabilized. This figure shows about five minutes. But longer lock such as about 20min was observed tonight as well.