The measurement finished and when I plotted the data I realised, there was no data from LC1 at 0-5V and LC2 from 5-25V and LC1 at 5-25V and LC2 at 0-5V. 

started the measurement for both the above setting with 50mV resolution. 

Owing to the issue in saving data, polarization states are generated again. We have put a BS infront of the two LC. Now, the polarization states are being generated after the "LC1, LC2, BS". This configuration will be treated as one optical element from now on.

1. Both LC 0-5 V with 15mV resolution

2. Both LC 5-25V with 50mV resolution

All, done with LC @ 30degC. Data is being saved with average order of 10.

foldername: 'C:\Users\ShalikaSingh\Atamapc\OneDrive\LC-Experiment\Measurement Data\Polarization states\20240305'

filename: Tue, Mar 5, 2024 8-25-32 PM.txt

There was issue with how data was saved. Even though there was averaging implented, there was no resetting of averging buffer after voltage change. Now, the data are being saved after the corrections.

New folder: C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Beam Splitter Calibration\20240305

Only LC, No BS:

Tue, Mar 5, 2024 7-45-18 PM.txt

Back face

transmission:

Tue, Mar 5, 2024 8-19-02 PM.txt

reflection:

Tue, Mar 5, 2024 8-03-47 PM.txt

Tue, Mar 5, 2024 8-13-04 PM.txt

Tue, Mar 5, 2024 8-16-36 PM.txt

According to the calculation it seems that for the last scan from 5-25V it will take time

1. with 0.1V resolution--> 5hrs

2. with 50mV resolution--> 24hrs

The scan finished and hence. I will begin another scan with 50mV resolution from 5.02V-25V.

10mV step with averaging of 10th order will take around 30hrs. So, I changed. Now the configuration is as below in format: LC1,LC2,Resolution

1. 0-5V, 0-0.52V, 10mV

2. 0-5V, 0.52-5V, 20mV

3. 5-25,5-25V, 0.1V

foldername:

C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Polarization states\20240301

[Marc, Katsuki]

We tuned input polarization to 0deg azimuth and ellipticity with QWP and HWP.

Katrsuki-san made a 15% hydrogel and we started acquiring birefringence data.

However, the translation stage acting on the pusher was not too well fixed so it was moving a bit after every motion and also applying force on diagonal.

We used sand paper to increase the translation stage holder hole diameter and used zip tie to stabilitze it.

Then, we measured 10% hydrogel mixture at the bottom and the top of the cell container.

After small motions were nothing happen, we can see a clear linear relationship between birefringence and pusher position.

When the laser is hitting the bottom of the cell container, there is an intermediate phase where the effect is linear but smaller.

The pusher motion to force was calibrated using a kitchen scale.

data are saved in the cell birefringence folder :

#first 2 measurements below were done with pusher not too well fixed..
foldername = r'C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\cell birefringence'
#15%
filename = 'Mon, Mar 4, 2024 2-14-41 PM.txt'
#10%
filename = 'Mon, Mar 4, 2024 3-30-28 PM.txt'
#10% and better way to apply force
filename = 'Mon, Mar 4, 2024 3-56-22 PM.txt'
#same as before but laser beam close to top of box
# filename = 'Mon, Mar 4, 2024 4-30-33 PM.txt'

All measurements were made with the plastic cell container.

For the glass cell container we should use a .8 or .9 mm diameter pusher (vs 1 mm for the plastic one).

We have put a BS infront of the two LC. Now, the polarization states are being generated after the "LC1, LC2, BS". This configuration will be treated as one optical element from now on.

1. Both LC 0-5 V with 10mV resolution

2. Both LC 5.001-25V with 20mV resolution

All, done with LC @ 30degC. Data is being saved with average order of 10.

foldername: 'C:\Users\ShalikaSingh\Atamapc\OneDrive\LC-Experiment\Measurement Data\Polarization states\20240301'

filename: 'Mon, Mar 4, 2024 2-13-11 PM.txt'

The VI was modfied to access two Polarization camera from 2 different VI and save data in two VIs at the same time. 

But, there is issue now that the graphs are not displayed in 2 different VI at the same time. 

I have set the laser to emit atleast 100mW. As such the power reaching the LC was 8-9mW. The mirror infront of the LC path was replaced with a 50:50 BS 10Q20HBS.33S. Now, the power is around 4mW. The input polarization before LC was tuned to be around 0+/-0.004deg in azimuth, and 0+/-0.003deg ellipticity.

An input polarizer was kept infront of the setup. In the past, sometimes the polarization changes overtime. So, for historical reasons, I kept the input polarizer back. After pol, azi 0+/-0.002 and ell 0+/-0.004 deg.

To estimate BS birefringence, LC is bein used as in polarimetry setup.

The BS Jones matrix has 4 parameters. Hence, 8 unknowns, taking real and imaginary parts into account for each element.

The BS was kept on removable magnetic mount after the pair of LC. The LC1 was scanned in voltage range 0 to 25V and LC2 from 0-5V, each with 1V step. This leaves us with atleast 8 to 10 measurements to create 8 equations. The measurements were taken with averaging of 10 order.

Calibration of BS back face

Measurement foldername : C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Beam Splitter Calibration\20242902

1. Modulation from LC, without BS.

C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\Beam Splitter Calibration\20242902\

Fri, Mar 1, 2024 3-53-37 PM.txt

2. The camera is placed in the transmission of the BS.

filename: Fri, Mar 1, 2024 5-55-12 PM.txt

3. Camera in reflection of BS. The measurements were taken by removing BS 3 times. 

filename:

Fri, Mar 1, 2024 6-23-25 PM.txt

Fri, Mar 1, 2024 6-26-41 PM.txt

Fri, Mar 1, 2024 6-29-09 PM.txt

I checked the polarization of the CC beam by looking at its transmission and reflection through the first polarized beam splitter on the injection path. It is almost completely reflected with very low transmission, so basically is s-pol. I guess the polarization wouldn't suddenly change for no reason but might as well check anyway.

I replaced the battery for the fiber PD and it turns out that fixed the issue with the disappearing CC PLL. I thought this fiber PD was outputting an electronic signal based on what I saw connecting it to the oscilloscope, but it seems that in fact the battery had run out. I used a Panasonic 12V A23 size battery from Yodobashi Camera, though Thorlabs only recommends Energizer ones from their website. Also the optimal laser temperature is about 37.96 C, somewhat less than what is listed on the wiki. If this remains consistent I will update the wiki. 

A scanning amplitude is sent to the CC1 error signal via the knob on the control servo to make it a sinusoidal shape, and from there we see the peak to peak level that is representative of the total squeezing (once the scan amplitude knob on the servo is turned past a certain number, the Vpk of the error signal is independent of the scan amplitude). I measured 560 mVpk at 25 mW green injection, which is about twice as much as what it was last time, the difference being that I optimized green alignment to the OPO using nonlinear gain of BAB last week.

I attempted to measure squeezing at 195 (39*5) MHz ppol but the glitch noise is extremely bad. The shot noise level while blocking squeezing is stable (-132 dBm, maybe slightly unbalanced homodyne but not important right now), so the noise has nothing to do with the homodyne path (we thought it might be an issue with homodyne power supply). I could barely make out that I was in a squeezed quadrature before the noise level glitched upwards by about 15 dBm.

After some time, the CC loops were behaving strangely, so I unlocked and relocked and noticed the green phase shifter HVD was sending 0.0 V and that the "LOCK" and "RAZ" lights on the GRMC/MZ servo boards were flashing. I saw that the green mode cleaner was *again* misaligned, which has become a repeating issue whenever large changes in PZT voltage get sent to the green phase shifter. I quickly recovered MZ/GRMC alignment to 92% mode matching but now the CC1 error signal was making some weird non sinusoidal shape. It was getting late so I didn't confirm, but I suspect that once again the green has misaligned to the OPO.

This CC glitch noise and green path hysterisis is very annoying and is severely impeding the ability to do priority tasks (ML data taking and OPO replacement). There are 4 points of suspect performance in my opinion - the CC PLL, green phase shifter, CC loop stability and CC loop signal to noise ratio. I personally do not believe the CC PLL is the issue since we measured the phase noise recently and it didn't have any of the glitch behaviour. We also tried to swap the boards ADF4001/ADF4002 used for CC and ppol, although now that I think about it I didn't do it correctly, but now I know the correct procedure - when the PLL software is loaded and the board is selected, I thought the initial screen was the correct settings, but in fact it is wrong and you have to load the correct settings for CC/ppol. Also, the signal frequency is set in the DDS software, but we have to input f*3 for CC frequency and f/5 for ppol frequency (i.e. above I input 39 MHz for a 195 MHz ppol lock). Last time we tried, I applied "division by 5" when switching the CC to the PLL board, but in fact the correct loaded settings also do the multiplication/division of the number coming from DDS, so I should have put 21 MHz (multiplication by 3). But even so, like I said, I want to check the CC control loop properties since I am more inclined to believe that these are the problem.

I designed a holder for 30mm diameter optics that can be installed inside the 2 inch holder of PCI.

The CAD file is in the workstation folder 'PCI holder'. There are 2 versions of this component :

'30mm_holder' was thought to be 3D printed but it might be too dangerous to use during absorption measurement

'30mm_holder_v2' is compatible with aluminium machining. 

The main difference between the 2 is that a slit to install a o-ring to stabilize small length component was replaced by a through hole in the v2.

The part will be manufactured at ATC.

A long cylinder will be 3D printed to be inserted into the through hole.

This holder should be useable for all the 30mm diameter sapphire we currently have (30mm to 120mm length).

Thanks to the help of hirata-san and Kanzawa-san for the 3d printing, I could assemble the required parts for this experiment.

The files are saved in the workstation in the cell birefringence folder. Current versions of the parts are the '...._v3'.

Note that it was nice to have the pillar screws of the part a bit smaller than M3 to make a tight fit of the screw.

I had to use sand paper for the on the pusher as the width of the cell container is not uniform along its height.

I confirmed the good assembly on the LC table.

This is the latest version of the optical setup.

I measured the power of the CC laser at various points.

According to the wiki, it should be:

325 mW at reflection of 98% beam splitter
3.3 mW at transmission of 98% beamsplitter (to fiber and PLL)
15 mW to OPO 

I set current and temperature as per the wiki (1.185 A, 38.15 C) and measured:
CC output after QWP, HWP - 320 mW
98% reflection -  59 mW
To fiber - 0.50 mW
To OPO - 3.2 mW

So it seems power levels are ~ factor 5 below nominal

I set current to 1.175 A and 1.195 A and measured again to see if it was mode hop, but got roughly the same results, slightly lower/higher respectively. 

Last time I could not see a signal from the OPO REFL PD which takes the CC error signal. This time I used a power meter to the oscilloscope, which resulted in the same thing. There is only a DC level and no reflection peaks. By contrast, the ppol transmission spectrum is fine (2V peaks as per wiki).

Perhaps it seems like the polarization is bad (before the 98% BS there is QWP -> HWP -> lens -> FI -> HWP) but I find it hard to believe that it spontaneously went to some bad polarization, without touching anything  after it was fine earlier on the day of the problem.

Yesterday we found that we could only get about 1 dB squeezing and ~4dB antisqueezing at 25 mW injection. Inspecting the homodyne showed that some bright leakage from OPO was interfering with the measurement. Initially I thought this was CC related, and it goes away when I block CC.

While checking alignments to OPO, I decided when sending BAB to also check green alignment - when sending green to locked OPO, the transmitted BAB beam will be amplified and deamplified according to the nonlinear gain. I found that green was heavily misaligned to OPO in yaw. The nonlinear gain before adjustment was less than half of the optimal amount (actually quite less, maybe 1/3 to 1/4). I reoptimized the green alignment to OPO by maximizing the nonlinear gain of transmitted BAB. I also checked PLL frequency and temperature. The optimal temperature did not change (7.117 kOhm). The optimal ppol frequency was found to be 39*5 = 195 MHz, although it has been seen on several occasions that this causes a large amount of noise in the CC2 error signal, so we usually slightly offset to 205 MHz. The optimal temperature/ppol without green was found to be 7.117 kOhm/240 MHz, the same as written on the tape label next to the ppol monitor (2024-01-09). A rough reading of the power meter in BAB transmission gives NLG 980 uW/310 uW ~ 3.2, which is consistent with previous measurements although a bit lower.

Anyway, this isn't the first time the green path randomly received large yaw misalignment. We may need to watch out for this in the future. A suggestion from Yuhang was that the green phase shifter gets kicked a lot and has some hysterisis.

I decided to check squeezing with 195 MHz ppol. However, I completely lost CC PLL output. Both CC fiber PDs are receiving about 1.1 mW power. Both are outputting some signal, although I am not sure what is supposed to be correct in this case. On the battery operated fiber PD I see about 115 mV and on the power supplied PD I see about 12 V. Like I said, I don't know if it's correct, but it's something. Somehow I cannot see anything on CC PLL MON while adjusting CC temperature. Also, the CC laser power reaching the fiber coupler is only 500 uW, but should be 3.3 mW from the wiki. We should probably buy a spare power supply for the battery operated fiber PD (after the Thorlabs ban ends...)

We have several labviews for different purpose, eg. single LC characterization, Dual LC control etc. 

But, some functionalties present in some (eg, the averaging present in rotating HWP VI) was not present in all. It was more like adding functionalities whenever and wherever. Hence, we never unified the design and the various available options. 

So, now we have 

1. Averaging (till 10th order) working in all. 

2. Button to scale power from W to mW in running VI.

3. Also, we can see in plots, both the averaged data and non averaged data. 

4. All, the VIs are changed to lock mode, so that nothing can be changed. Its for safety so that nothing is changed while using the VI. The VI are in run mode. Unlock and change to edit mode to have access for changing. 

Let's call this sample T60#1.

tuned z_iu = 44.13mm

aligned DC

Y edges at 74.2 and 171.45 (DC = 0.402V)

X edges at 340.5 and 313.91 (DC = 0.215V)

Y_center = 122.825 mm ; X_center = 327.05 mm

z edges at 41 and 76 mm ie Z_center = 58.5 mm

long z scan with Pt = 4.78 +/- 0.17 W : Wed, Feb 21, 2024 1-28-46 PM.txt

map at z_center Wed, Feb 21, 2024 2-29-03 PM.txt lockin sensitivit 50mV ; Pt = 4.82 +/- 0.17W

measurements will be repeated at z = 68.5 and 48.5 mm during the next days.

z = 68.5 mm : Thu, Feb 22, 2024 10-03-11 AM.txt, Pt = 4.73 +/-0.17W

z = 48.5 mm : Thu, Feb 22, 2024 10-46-08 PM.txt, Pt = 4.84+/-018W

Marc, Michael

GRMC/MZ lock had problems but was somehow recovered by adjusting the gain and threshold of the control servo (I don't know how that worked).

We looked at squeezing in an attempt to see and check the glitch noise. Indeed, it was present again in the CC2 error signal, as usual. We recovered squeezing but could only measure about 1 dB squeezing and 3 dB antisqueezing, which is very low for 25 mW green injection (should be ~ 5 dB squeezing).

We looked at the spectrum of CC2 error signal. It has quite a broad bump that rolls off about 10-15 kHz, which is about the width of the FC control loop, among other things.

Homodyne shot noise spectrum seems ok but it was found that there is some bright spot leaking from the OPO to the homodyne. The homodyne was rebalanced to the local oscillator and the erroneous homodyne signal remained. The homodyne should just be noise around 0 with a rough amplitude of about 1 mV, but instead there is a strong oscillation with an amplitude of about 50 mV that goes away when I block CC to OPO. Measuring this bright spot gives only 5-10 uW (really sensitive sensor card it seems). Still it seems there is some error with homodyne. Perhaps we should double check and optimize CC and BAB to OPO, as well as green alignment (ppol seems fine).

I reinstalled the 2 inch sapphire (let'scall it C19.7#1) previously measured.

moved z_iu to 61.46 mm

z edges 24.8 and 36.8 mm ie z_center = 29.8mm

realigned DC at z_center

long z scan with lockin sensitivity 5mV and Pt = 4.78 +/-0.17 W

Wed, Feb 21, 2024 9-49-47 AM.txt

map with 0.5mm step, 15mm radius, lockin sensitivity = 20mV, Pt = 4.79 +/- 0.17 W

Wed, Feb 21, 2024 9-59-54 AM.txt

Marc, Michael

We looked at a measurement of the CC PLL phase noise to see if there were any visible glitches. This measurement works as outlined previously and in Yuhang's thesis 4.25-4.31. To reiterate, we can inject a local oscillator and the relevant PLL beat signal into a mixer. There is a 2*f component rejected by a low pass filter 1.9 MHz. If the LO and the signal are the same frquency it can be shown that the output is an average DC offset (which can be set to zero via LO/signal relative phase) and the PLL phase noise assuming a sufficiently clean LO. The spectrum is monitored on the spectrum analyzer and the amplitude is calibrated by offsetting the LO by some small frequency inside the LPF passband to bring back the 2*f oscillation.

We took a measurement of the CC_PLL_MON channel (7 MHz) combined with DAC3 DDS3 (what is normally ppol LO) also at 7 MHz. The calibration factor K_mix * A_1 * A_2 / 2 comes from the peak to peak amplitude when sending mixer out to the oscilloscope with LO = 7.000 1 MHz -> 2.80 mV. We also took low and high frequency PSD (have not postprocessed yet). During this process we could not see any glitches. Actually, this measurement really isn't necessary in and of itself, it was mostly just some curiosity I did for the KAGRA filter cavity project, but it is where the CC glitch noise problem first became apparent.

The previous measurement of this phase noise used ppol LO as the local oscillator, but now that we have removed the 78 MHz EOM we have a spare DDS output channel, so we can try this phase noise measurement again while applying squeezing. We confirmed and locked the CC2 loop, which measn that we are generating squeezing, but somehow we did not see any glitches anymore in the CC2 error signal.  While it would be fortunate that this noise source has disappeared, it would be better to know where it came from. 

However, after some time GRMC and MZ refused to lock. MZ became very misaligned and there was no error signal from GRMC reflection PD. Perhaps in our switching of local oscillator cables the MZ PZT got kicked - the alignment and PDH error signal of GRMC was rmostly recovered just by a yaw adjustment of the MZ non-PZT mirror, and then I optimized the PDH phase (1.11 V peak to peak PDH signal at 110 degrees in DAC2 DDS2). The GRMC mode matching was recovered to 592 + 22.4 + 20.8 + 8.8 -> 91.9%. However, GRMC and MZ still do not lock. I don't know what is wrong but I guess it has something to do with DDS signal connection and or on/off status.