The following components were removed from mounts and kept in their boxes.
1. BS014

2. LA1131-C

3. 1064-HWP

4. mounted zero-order QWP 1064nm

5. green sapphire

6. blue sapphire

7. Sapphire

8. 50mm lens 1064nm

All boxes are labeleled

The power meter was returned to FC clean room.

only the part where the controller was switching off was removed. The part where we set the current of the controller at the time of overshooting still remains. Also, the controller is designed to remain switched off in case the current overshoots. So, the reenabling part remains as well.

For temperature control of LC we had implemented the PID control. But, before PID implementation, there was a feedback setup. Here, we were switching off the temperature control for 2s if the current applied was overshooting. But, this was creating an issue that the labview would consequently not save data or create a nan value in file.

We should have removed this after PID was implemented. In any case, it has been removed. Indeed the nan values or empty data don't appear anymore in labview graphical plots. 

We will save retardation of LC tomorrow and finalise this conclusion. 

[Shalika, Katsuki]

In the cross polarizer configuration, we calibrated both the liquid crystals by placing the liquid crystal in between the two polarizers.

The liquid crystals were rotated and the voltage scan was done at each position. The voltage scans were done from 0 to 25V with 0.6 Hz sweep frequency of the saw tooth voltage.

The fast axis of LC1 is at 44.15. Hence it is kept at 89.15, which is 45° with its fast axis. see LC1 Fast axis plot.

The fast axis of LC2 is at  74.86. This LC is kept at its fast axis position. see LC2 Fast axis plot and retardance plot at fast axis position.

The retardance with respect to voltage was also measured for both the LC.

folder_name = r'C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\LC1_calibration data\20242901_LCcal\fast axis'
folder_name = r'C:\Users\atama\OneDrive\LC-Experiment\Measurement Data\LC2_calibration data\20240129_LCcal\fast axis'

I made a mistake in calculating the isolation ratio

mistake: -10*np.log(3.6e-6/15e-3) = 83.35 dB

correct: -10*np.log(270e-6/15e-3) = 40.17 dB

The isolation from faraday isolator is 40.17 dB

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.