Nishino

Laser-lock box function of Mokulabs is very useful to built a good loop for cavity locking. An error signal is splitted into two paths, fast and slow paths, and one can create two independent filters to two actuations, PZT and laser crystal temperature, for example.

Filters are configured in PID controller. Intergartion and Derivative have their saturation limits, denoted as IS and DS, and you can also set unity-gain frequency.

This log is about how to derive mathmatical forms of the PID filters. It will be useful when you want to reconstruct something in the controling loop, optical transfer function of cavities, for example.

Definition:

g_I, f_I : Intergrater saturation limit, unity-gain frequency (w_I = 2*pi*f_I)

g_D, f_D: Derivative saturation limit, unity gain frequency (w_D = 2*pi*f_D)

Actually, it takes an envelope of two functions: complete integration (derivative) and constant gain. The overall gain, G_I(s), G_D(D) can be written as:

G_I(s) = g_I / (1 + g_I *s/w_I)

G_D(s) = (s/w_D)/(1+s/g_D/w_D)

Of course propotional gain (denoted as P) is just a frequency-independent gain. You can constract the overall filter function as:

G_sum(s) = P + G_I(s) + G_D(s)

See this if you want to check derivation.

Nishino

I put Faraday isolator and EOM in the green path of Prometheus.

Faraday: Thorlabs, IO-3-532-LP (2.7 mm)

Resonant EOM: Thorlabs EO-PM-R-20-C1, 20MHz (2 mm)

Before them, two lenses with focus length of 100 and 150 mm are placed at z=125 and 262.5 mm, respectively. Schematics are shown here.

Beam widths around input and output ports of Faraday and EOM are measured as below:

These are not well precise, but just to check that the beam size is smaller enough than apertures of Faraday and EOM.

They are all less than 1.5 mm for 6 sigmas. It approved that beam will not be clipped.

[Shalika, Marc]

Our polarization camera's aperture is 3 mm and required divergence of beam is 2°. We investigated that the telescope had beam divergence of 0.038°. But, the issue was that beam waist was exceeding (almost) the 3 mm aperture. See plot 1 to see evolution of beam after the lens. The position of lens was 0.21 and 0.253 m from laser. 

For modification, BSN11 was removed from the path, and the beam was measured after the lens using beam profiler, both before and after modification. 

The lens position was modified to tune the beam waist at a far distance of around 1.3 m after the telescope. See plot 2 of beam evolution after the telescope modification. For optimal modification a reference point was set around 1 m after the lens. The beam waist before modification was around 3.2 mm. The lens position were tuned to reduce the waist. The position was finalised when the waist was around 8.5 mm at 1 m. After this modification, the beam profiler was used to obtain the plot 2. The lens of f = -50mm is kept at almost 0.3 cm after the faraday isolator. The lens of f = 75mm is kept within 25mm after the 1st lens. 

After this, we placed back the BSN11 and the position was tuned to obatin the beam back on the polarization camera. 

Also, the characterisitcs of BSN11 were evaluated(The arrow on the optics points toward the coating). The reflection of this had two beams.

incident: 39 +/-0.1 mW

reflected: 4+/-0.1 mW

reflection without 2nd beam: 3.7 mW

transmission: 34.5+-/0.1 mW

With optimal beam parameters we will proceed to use the setup for polarization generation and other future experiments. 

[Marc, Shalika, Takahashi (remote)]

Today from around 14:00 there was a strange sound from BS pump. We found out that the STP control unit had error message 'motor overheating'.

Following Takahashi-san recommendation we closed gate valves between BS chamber and turbo pump and between the 2 pumps around 19:30.

Attatched links are not valid anymore.

Please see this folder. You can find how the data is handled in jupyter notebook, filter_gain.ipynb.

Nishino

This is a report on 22th Janualy 2024.

I measure the badwidth of the main cavity again by modulating the laser-crystal temperature to scan its frequency. 

To calibrate frequency from scanning velocity, phase modulation sideband at 15.24 MHz was used. Vector data, plots, and jupyter code are available here.

Measured bandwidth are (N=13, unit: kHz):

325.7 366.6 428.4 347.9 350.1 321.2 353.0 388.9 415.3 361.8 377.3 367.0 334.0 330.4

Mean: 362.0,

Standard deviation: 31.0

From these values, total loss of this cavity is derived as 4549 (\pm 390) ppm. Input transmissivity is ~ 4000 pppm, therefore the estimated absorption and scattering loss is ~549 (\pm 390) ppm.

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