Also, in the cross polarizer setting, I saved some data of power. 

foldername= r'C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\CrossPol'
filename= 'Thu, Jan 25, 2024 3-37-57 PM.txt'

After beam characterization, the following components were setup.

The faraday isolator we have is FI-1060-5SC-HP and has aperture of 5mm. QWP and HWP was placed before the FI to tune the input polarization to be linear. The azimuth and ellipticity were both (0.0±0.01)º . The faraday isolator alignment was done at power of around 15mW. The isolation was ~83 dB after tuning of the polarizers of the FI. Also, the transmission from FI was ~94%. The FI has aperture of 5mm hence it was placed within 200 mm from the laser shutter. The beam characteristics were used for optimal positioning.

After the FI, two lenses of f = -50 mm, and 75 mm were installed to have a collimated beam and were placed within 210 mm to 230 mm. The choice of lens was done using the Q parameter previously obtained in 3413 and 3410 in Jammt.

After the FI the beam is steered towards BSN11(R=10%).  The transmission line is intended to used for be used for pockel cells.

The reflected beam from above is incident on BST11(R=70%). The transmission of this will be used for LC setup. The reflection is incident on power meter to monitor laser beam fluctuation. See the new setup for better info. I had to remove BST11 and is not present in picture, as I accidentally touched it and had my fingerprint.

For the LC path, we use QWP and HWP to gain perfect linear polarization light. The azimuth and ellipticity were both 0.0° ±0.01°.

Finally, a polarizer was installed before the camera. The setup is now in cross polarization polarizer configuration and ready to characterize LC.

The beam before lens was characterized using the ABCD matrix and q parameter of beam from 3410. The beam waist of laser is around 39 to 59 mm and is of size 137 um.

See plot for further visualization.

Date: Jan 17, 2024

Place: ATC North

by Yohei Nishino & Munetake Otsuka

System (1)Laser from Cavity ->Mirror ->BS->Mirror->Lens(f100)->PD with (3)Gain Dial->(2)Electric Output

The beam waist of laser was measured at 11.3 mW and 101 mW of power.

A plano convex lens(f=50mm) was placed at around 100mm after the laser shutter. The evolution of beam size (after the lens) depending on the position was measured. The corresponding plots are here. 

1. P =11.3 mW plot

2. P = 101 mW plot

The beam waist is found to be same for 11mW and 101mW power and is at [56.3,57.5 ± 0.003,0.004] mm for the major and minor axis respectively.

The major and minor axis respectively are [76.5,88.5 ± 3E-6,6E-6] µm at waist

This was my mistake. The cavity length was 7.5 cm, which makes the cavity pole half. We are building the cavity again with the proper length, 15 cm.

 I used a lens of F=50 mm infront of the power meter as the beam size was increasing with increase in power and it would have damaged the cascading of the power meter.

The power meter used here has max allowed incident laser of 2W.

Our previous laser was removed in the past month. The laser LIGHTWAVE 126-1064-100 has been setup for our experiment. It is being powered by the LIGHWAVE 126-OPN-PS. See laserPS and laserhead to view connections. Turning the key, turns on both the laser power supply and laser head. Its recommended to use STANDBY button to turn the laser off, instead of using the key. Also, turning off the Power supply will reset everything to default configurations set by the company.

As this laser is too old, the manual is difficult to find online.

Acoording to the manual we have maximum emission level of 2W and can atleast provide a specified power of 100mW. This doesn't mean we can't have power below 100mW. We can lower the current to have lower power. The laser starts emitting at around 0.58A.

The plot for current vs. Power shows a linear increase in power with increase in current.

Yuhang, Michael

2024-01-10

We realigned green to OPO using a rather rough method of checking the bright spots on the OPO reflection FI that separates BAB and ppol IR.

We replaced the cable for the correction signal of the CC PLL slow loop to the CC laser temperature. We took a 5m cable and made sure it wasn't broken. This fixes one issue of the CC PLL but there are still others as mentioned previously.

We changed the ppol PLL frequency for green injection to OPO and once again found it had shifted a large amount , to 205 MHz at 37.5 mW green injection (it was previously 240 MHz). We went back to our usual 25 mW green injection, but the ppol frequency this time didn't change much. The CC1 error signal which characterizes squeezing level was about 302 mV (should be about 320-340). We tried to optimize the ppol frequency but then ended up with large oscillations in the CC1 error signal (seems about 760 ns). It seems like it "should" be 185 MHz, but it is more stable at 205 MHz so we left it there. It should be noted that previously the CC PLL performance was improved by switching off rampeauto for the FC green lock. Now we need to have that on again so we go back to having CC issues. The noise seems to be common to CC1 and CC2 error signals. But there was not so much time to investigate this issue and this entry marks the end of Yuhang's visit... for now.

To do list of items noted during Yuhang's visit:

• Coherent control investigation - why is there glitch noise in the CC error signals? It seems to be an electronics problem regarding either the CC PLL board or the FC green lock servo.
• Take a more accurate optical loss/phase noise measurement (sqz vs antisqz at different green pump)
• Recover FDS and CCFC
• Cable management, especially at the bottom of the rack among the Nikhef RF amplifiers and DDS boards
• Figure out how to properly set the SR785 spectrum analyzer for open loop transfer function measurements - we seemed to not get the correct reading versus the old spectrum analyzer. The SR785 has a bit uncomfortable interface in my opinion but is also much faster.
• Figure out proper settings regarding transmission/reflection PD -> INV/NON INV -> +/- -> Error signal sign -> Lock threshold sign
• Fix SHG 3rd order mode mismatch
• Replace and retest new OPO PZT
• Update optical layout and wiki items
• Get rid of the superfluous green EOM (78 MHz, previously used for green lock of GRMC but now redundant) - requires complete overhaul of green beam shaping though. Would be quite a pain, especially because mode matching optimization is very time consuming

Yuhang, Aritomi, Michael

From 2024-01-09

We optimized IR transmission from the OPO. The BAB transmission through the OPO when locked for IR is 320 uW. However, the ppol frequency went to 245 MHz - previously, the "IR only" ppol frequency for OPO was 220 MHz, so it has changed quite a lot in a short time, so this OPO/ppol frequency issue might be independent of green.

To align the filter cavity for IR, we do 1) align to reference targets on PR window, 2) align injection beam with vacuum tube targets, 3) check reflection on squeezer bench, 4) lock FC for green and adjust detuning of IR (green AOM) to find IR resonance, 5) align IR to maximize IR FC_TRA. This constitutes locking of the filter cavity ready to measure FDS. The infrared reflection signal is obtained through a 0.5% transmission polarizer, so quite difficult to see. Anyway after just a small adjustment of input steering yaw I could see IR reflection on a sensor card on the squeezer bench already, so we really didn't need to do much intermediate alignment. It was moving quite a lot (there was an M5 earthquake in Niigata at the time). The reflected IR power at first glance was about 240 um, 75% reflection. 

Eventually we could see flashing of the IR at about 109.037 135 615 000 MHz on the AOM detuning. On second inspection of the targets in the vacuum tube the GR/IR overlap seems quite bad though, so it will have to be fixed.

Yuhang, Michael

From 2024-01-08

OPO kept locking to higher order mode until I flipped INV switch on the servo. I should really figure out how the INV, +/- and threshold work together because it seems it isn't written down properly anywhere, but it's important for setting the proper behavior of the lock threshold to not catch HOMs, and depends on the sign of the error signal and use of transmission or reflection specturm.

To we keep BAB injected into OPO. The transmission is not so good, about 52 uW (should be about 400).

The green beam was aligned to the filter cavity using the targets inside the vacuum tube and the coils were offloaded to keep the best position around 0V actuation. The filter cavity was then internally aligned via reflection through the second target and to the squeezer bench. The END SR560s were turned to 1000 Hz LPF, Gain 100, low noise mode, 6dB/oct, DC coupling.

Filter cavity locks for green but is not super stable. If we put 1/f^4 filter it unlocks. We changed sideband frequency to 88 MHz so this might affect PDH.

We took the transfer function of the green FC lock (Source -> Signal:Perturb, Ch1 -> EPS2, Ch2 -> EPS1). The unity gain frequency is about 12.5 kHz, should be a bit higher - the gain was adjusted to give 13.256 kHz.

There is some weird behavior on the MEDM screen where the displayed FC_GR_TRA goes up when the filter cavity is unlocked. It seems to be an electronics issue regarding the displayed value.

Yuhang, Michael

From 2024-01-03 to 2024-01-05

We took the replacement OPO from the ATC cleanroom to TAMA and (finally) intended to replace it.

We decided to do a quick check for squeezing and afterwards just swap out the OPO. Since BAB, CC and ppol all end up being commonly aligned to the OPO via 2 steering mirrors after the recombination beam splitter, we decided we could just take it out, replace it and then recover the alignment with either of those beams. ppol is probably the most straightforward since it has its PD connected to one of the monitor channels on the OPO servo board. We should probably get a camera to check the shape of the SHG mode mismatch but for now we will just use transmission spectrum to align.

We drew lines around the old OPO to refer to exactly where to put it. After realigning the ppol reflection to itself we looked to scan the transmission spectrum, but the HVD would not turn on. After extracting the HVD -> OPO connection it turns out the HVD is fine, so the problem is somewhere in the electrical connection to the PZT. 

It seems the new OPO PZT is broken. So we had to put the old one back in. I accidentally burned a mark in the POM casing (the ppol laser is < 50 micron radius at the entry point of the OPO housing). After readjusting the alignment and mode matching of ppol to the old OPO, it was seen that the mode matching was improved by a small amount compared to the reference level shown on the wiki, so there is no issue with the crystal. There are two lenses on a ruler rail before the OPO - the positions before taking out the OPO were 170/264, and after replacing were 168/262, with 91.3% mode matching.

The old OPO PZT gives about 800 nF capacitance (about the same as the datasheet) while the new OPO doesn't give a reading.

I have taken two power meters from ATC clean room. They are now in Tama. 

They will be used to characterize our laser. 

Schematics can be found here. All filter shapes are written. Actuation gain of PZT and temperature actuator can be found in previous works, Niwa's master thesis for example.

I measured the open loop transfer function of the main-cavity system.

UGF is ~2 kHz and phase margin is 80 degree.

See the whole plot and local plot around UGF.

Let me leave a brief note about "frequency response analyser" of Moku:Lab, especially the definition of the vertical axis of the upper panel (see this screen shot).

Since it uses a unit dB, sometime I'm (and someone should have been) confused if this means in power or amplitude; as a conclusion, it means power.

To confirm this I used 1st-order low pass filter in SR560 with cutoff of 100 kHz as a reference. The screen shot is the measured transfer function of that. 

At 100 kHz, you see the decrease of gain by -3 dB. This means:-3 dB loss in power (=in Moku:Lab) corresponds to *1/sqrt(2) in amplitude.

More generally if the output gain of Moku is G dB, the amplitude gain g in log-scale should be calculated as:

g = 10**(G/20)

Ex.) -20 dB in Moku:Lab corresponds to 1/10 in amplitude.

I measured the finesse of cavity by scanning the laser frequency through the temperature actuator. The result shows that the finesse is twice lower than the designed value.

Designed power transmissivity and loss of mirrors from Layertec are:

T_in = 4000 ppm, T_end = 35 ppm, L_in=L_end=30 ppm,

therefore the cavity finesse should be ~1500. With cavity length l=15 cm, cavity pole should be:

T_in*c/4/l/2/pi = 320 kHz.

However, the measured cavity pole was 635 kHz i.e., finesse is =750. You can see the Lorentzian plot here. Calibration factor from scanning time to frequency is derived from the RF sideband frequency of 15.24 MHz.

This result means that cavity loss is ~4000 ppm, which makes this cavity totally useless for speed measurement.

Some items that were taken from TAMA to ATC clean room were brought back.

1. Beam Profiler and PC
2. Power Meter Display Monitor

See image here.

PS: Since the problem of not being able to upload images/files to elog persists, I decided to upload pics in my google drive, and put a link in the elogs. I would recommend others to do so too, because in near future everyone will face difficulty when they will wish to refer back to certain things.

Yuhang, Michael

From 2023-12-30

Don't accidentally send BAB and LO to homodyne at the same time otherwise you will get a large DC signal :O

Replace the power supply of the CC fiber PD

We realigned the LO and BAB to alignment mode cleaner and rebalanced the homodyne in order to reduce the optical loss for the mass data taking. We did some other checks. The CC PLL fiber PD for whatever reason is using a hard-to-find battery (A23 12V). We really should replace it with a power supply. While checking the green path we found that the SHG has some ridiculous 3rd order mode mismatch. It's about 30%. It cannot be reduced by beam steering, laser line filter or adjustment of mode matching lens position, and also doesn't appear to be induced by clipping anywhere. Still, it generates enough green power so it's workable for now. We saw that GRMC could not lock, and it turns out it wasn't even aligned. This is seemingly due to the reason mentioned previously, that the GRPS PZT had some hysterisis from sending the too large wrong signal. In fact, it could be realigned just by adjusting yaw from the steering mirror just past Mach-Zender. This mirror normally shouldn't be used for alignment to GRMC since it is common alignment for arms in the Mach-Zender, but in this case it worked because the PZT introduced a common misalignment. While the SHG has some weird issue, the GRMC still sends 25 mW to OPO at the appropriate MZ offset setting.

We checked CC alignment to OPO (send CC laser to OPO, block LO, check homodyne). It seems fine. So we decided to check again the ppol PLL frequency for 25 mW green and found that the CC1 error signal becomes quite large. The optimal ppol frequency, representing the OPO co-resonance condition between p-pol and s-pol, changed from 160 MHz to 250 MHz to 75 MHz in the space of about 10 minutes. This is some very strange behaviour, later we asked Matteo but he didn't have a suggestion other than multimode laser mode hop, which we are quite sure we do not have at the relevant laser current/temperature settings. Anyway we just decided to keep going on the basis that we had to take data before Hsien-Yi left. So the 25 mW green gives ppol frequency 75 MHz, for now. CC1 error (indicating squeezing level) was 384 mV, a bit larger than last time. 

Anyway, the appropriate signals were sent to DGS:

CC2 eps1 (representing local oscillator phase -> homodyne angle): K1:FDS-FC_GR_CORR
Ramp signal from function generator: K1:FDS-FC_GR_ERR
Homodyne (from SR560 with gain 1000): K1:FDS-ADCspare_1

We ended up taking data at MZ offset 4.1, 4.2, 4.3, 4.4, 4.45. For MZ offset 4.3 and 4.4 I had to reduce the SR560 gain from 1000 to 500 since it was overloading, and then again to 200 for the highest green injection power. This will also increase the proportion of dark and ADC noise. Also, when changing from 4.2 -> 4.3, the ppol frequency went from 75 MHz to 240 MHz - normally it is 160 MHz and doesn't change very much when the green injection power is changed slightly. But we kept on with taking data. During each green injection value I also made sure to check that the scan still covered squeezing and antisqueezing. The scan range also had to be reduced slightly for higher green injection due to the CC2 loop unlocking more easily.

After 30 minutes for each data point, I then took 5 minutes each for homodyne shot noise and homodyne dark noise at each SR560 gain level used in the measurement.

Yuhang, Michael

From 2023-12-29

We felt the squeezing (CC1 error signal when scanned) was too low, so we decided to realign alignment mode cleaner (i.e. realign infrared to homodyne). The alignment mode cleaner can have some backlash if adjusted over a large range too quickly, i.e. if the system is out of use for a long time. The LO was a bit misaligned, and BAB was very misaligned with about a 10% mode mismatch appearing since the last alignment a few days ago.

We then saw that CC2 has some issue with a quite fast oscillation in the error signal (when scanning, it should be going slowly up and down on a timescale of about 0.5s). It turns out I accidentally put the AMC scan signal (1V 10Hz) into the CC2 phase scan for data taking (should be 10 mV 0.01 Hz). Apparently this misaligned the green phase shifter (before GRMC) quite a lot, and the GRPS PZT had some hysterisis and didn't recover properly.

Otherwise, LO and BAB alignment were individually fine afterwards.

From 2023-12-28

Yuhang, Hsien-Yi, Michael

The goal of this day was to properly characterize and set up the squeezing/antisqueezing scan for taking the data for Taiwan's machine learning project. From the squeezing vs power data that was taken previously, it seems we can only go up to about 47.5 mW injection before the data becomes less meaningful.

We send a scan signal to CC2 Perturb in but the error signal becomes quite noisy. It was sent to an SR560 (although it's just in low noise mode, DC coupling, 1 gain - no filtering is selected). While inspecting the output of the homodyne time series in DGS, we suspected there was some DC power being amplified and deamplified inside the OPO. Perhaps it is backscattering of the LO from the homodyne detector into the OPO (a recurring issue). We wanted to look at bit further back but get 'synchronization error' in dataviewer. This must be improved with the installation of new DGS. Anyway, we saw that the possible scan range of CC2 phase without unlock is somewhat low, so we decided to check visually if it covers squeezing and antisqueezing. We sent a constant voltage at several points from -10 to 10 mV to CC2 Perturb In and checked the homodyne spectrum in diagguie, where the electronic and shot noise levels were saved last time. We managed to set the homodyne angle to a good point where we can start just before peak antisqueezing and end just after peak squeezing. Also in DDS 40 degrees homodyne corresponds to squeezing while 90 degrees corresponds to antisqueezing. A bit odd.

So we confirmed the feasibility of the squeeze anti-squeeze scan and were ready to leave it and take data. Considering that the fitting previously also showed large optical loss, we decided it would be good to optimize alignment of BAB (squeezing) and LO to alignment mode cleaner (i.e. homodyne). But we ended up taking data at 25 mW green injection for about half an hour.