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.

According to Pierre, the GAIN PIEZO in ACTUATOR module controls also the saturation of PIEZO ELEC signal and should not be small value. Pierre suggested to set this gain as 7 and adjust the servo gain with INPUT ATTENUATOR in SIGNAL module. I set the GAIN PIEZO as 7 and the INPUT ATTENUATOR as 0.2 for 1/f^4 filter. UGF is 13 kHz and phase margin is 60 deg (Fig. 1).

Fig. 2 shows the correction signal (CH1) and error signal (CH2). The correction signal does not saturate even at 2.36V (4.72V for PIEZO ELEC) and FC gets more stable. However, FC still often unlocks even without saturation.

As described in elog2846, FC easily unlocks due to saturation of FC GR correction signal sent to laser PZT.

Fig. 1 shows the FC GR correction signal (CH1) and error signal (CH2) with FC servo gain of 0.8. The FC GR correction signal is half of the signal sent to laser PZT. When the FC GR correction signal reaches +/- 0.44V (signal sent to laser PZT reaches +/- 0.88V), the FC unlocks. This saturation can be relaxed by Z correction as shown in Fig. 2 and we could lock FC for a few minutes with Z correction today.

Fig. 3 shows the same measurement with FC servo gain of 2. The error signal becomes noisy because of too high gain, but the saturation does not occur at +/- 0.44V.

Fig. 4 shows the FC GR OLTF with servo gain of 0.8 and 2. UGF is 14kHz with gain of 0.8 and 23kHz with gain of 2. 

I confirmed that BS pointing is working. The setting for center in camera is as follows.

After the recent recovery of FC alignment, FC GR lock is very unstable. I turned off the END picomotor driver, but it didn't change the situation.

I checked the FC GR correction signal and laser PZT signal. The FC GR correction signal is PZT mon signal amplified by 50 with SR560. For laser PZT signal, I divided the signal sent to laser into two. One is sent to laser and one is for monitor. The relation between FC GR correction signal and PZT signal is as follows.

FC GR correction signal = PZT mon*50 = (1/100*PZT)*50 = 0.5*PZT

Fig. 1 and 2 show the FC GR correction signal (CH1) and PZT signal (CH2). As you can see, CH1 with 500mV range and CH2 with 1V range are completely overlapping, which means the FC GR correction signal is exactly half of the PZT signal as expected.

Fig. 1 shows that when the FC GR correction signal reaches +/- 1.1V (PZT signal reaches +/- 2.2V), FC unlocks (the signal becomes 0). According to a manual of laser, the PZT can accept +/- 65V (PZT tuning coefficient is 1MHz/V and PZT tuning range is +/- 65MHz). So the PZT signal is well within the PZT range.

Fig. 2 shows that FC also unlocks even without saturation.

We will investigate the reason of this unlock.

The attached figure shows the dark noise and shot noise of CCFC error signal. The CCFC error signals were measured on 20210622 and the dark noise was measured on 20220218. Note that the DC port of CCFC PD should be terminated with 50Ohm for lower CCFC dark noise. This dark noise was measured by blocking the light before CCFC PD. This dark noise includes the noises of PD, RF amplifier, and mixer.

Estimation of shot noise

Measured CCSB power at CCFC PD when CCSB are on resonance: 0.8 uW

Typical efficiency of InGaAs photodiode at 1064 nm: 0.45 A/W

According to Yuhang's simulation, transimpedance of PD at 14MHz is 5900 V/A

Shot noise at 14MHz before amplification: sqrt(2*A*q)*R = sqrt(2*0.45*0.8e-6*1.6e-19)*5900 = 2e-9 V/rtHz

Shot noise at 14MHz after amplification: 2e-9*10^(34/20) = 1e-7 V/rtHz

Shot noise of CCFC error signal = amplitude of CCFC error signal*(shot nosie at 14MHz after amplification/amplitude of 14 MHz signal) = (154mVpp/2)*(1e-7 V/rtHz / 0.045 V) = 1.7e-7 V/rtHz 

where the amplitude of CCFC error signal and 14MHz signal (-14dBm = 0.045 V) are measured values.

I tried to measure the shot noise directly by injecting BAB to CCFC PD. The injected BAB power was 73uW. However, the shot noise could not be measured since the dark noise was still limiting.