To measure the fast axis orientation we shuold have the 2 polarizers parallels and not crossed.

[Shalika, Marc]

1. The extinction ratio of our second LC was measured using the method mentioned in elog 3241. The power at transmission was observed to obtain the maximum and minimum power for every voltage by correspondingly rotating the LC. 

2. The error bars are coming from the power fluctuations we measured.

3. The extinction ratio observed was at about 1.0332+/-0.0013 for all voltages. [see Fig 1]

4. Also the setup was moved to accomodate the new vertical slider and box because we have overhead table too. [see Fig 2]

[We also measured the fast axis orientation using the method in elog 3242 but the results were a bit confusing so its work in progress]

[Marc, Yuhang {remote)]

We checked the reason why healthcheck failed.

The test.sh code calls a .xml file for each coil of every suspension. All these files are located in the check_after_earthquake folder.

These .xml files are used to set up the measurement, from excitation to measurement.

However, no measurement nor excitation channels were specified aswell as reference measurements.

Looking at various folders, we found the correct files.

They were restored to the proper folder and we confirmed that the suspension health-check is recovered.

[Hirata, Marc, Shalika]

In order to ease the installation of optics for birefringence measurement we added a vertical slidder to the box.

We use  an unused rack slider fixed to an unused part of the old scattering frame.

To stop the box at a higher position it is good to use 2 keys for safety.

[Marc, Michael]

To prepare the shipping of the AOM function generator back to APC, we installed our new one (WF1968).

We used same settings as before (continuous mode, freqeuncy = 109.036035615 MHz, 1.124Vpk=5dBm).

We had quite low green power after the IRIS after the AOM.

Power budget was :

SHG reflection ~270mW

before AOM ~41.2mW

after AOM ~37.6 mW

after iris ~5.7mW

before FC ~5.26 mW.

We tuned a bit the steering mirror between the AOM and iris. Especially, there seemed to be a little clipping in the vertical direction but it did not help to recover the usual power.

We checked the function generator output on spectrum analyzer and found a -3.28 dBm amplitude. Therefore, we increased the amplitude to recover the usual 5dBM : 2.95VPk gives 5.1 dBm.

In this configuration we recovered the usual AOM transmission.

We had to realign the last steering mirror on the bench to recover the PR targets.

The beam was then really far of the BS target. We tried to move PR mirror bu it did not seem to move. The health check has some error message ('No measurement channel defined') that we need to investigate.

The overhead sheld (ATS6) was delivered and assembled by Japan Laser people.

It is possible to move the hanging shelf and the cable management tray by unscrewing the bolts.

Be careful that there are other bolts inside the frame that need to be moved as well.

There are also extra bolts and smaller tube for the hanging shelf in a plastic bag taped to the optical table foot.

[Marc, Takahashi, Yoshizumi]

We removed the failling tape and replaced them with plastic ones.

The pre-clean room and clean room wall are now repaired.

[Marc, Munetake, Shalika]

We reconfigured the PCI for birefringence measurement.

First, we put back all required cables with the 6dB attenuators at the output of the PSD sum.

We tuned the laser power to 1.6mW (laser current = 1A and HWP rotated to 348deg.

We installed the razor blades and tuned the vertical and horizontal angle of incidence to about 0.006deg.

We realigned the readout part.

We tuned the QWP and HWP to minimize p pol in transmission with HWP = 351.2 deg.

We took 10mn of s and p polarization and got the attached calibration coefficients.

[Marc, Shalika]

Following the several months with the too large scattering box, the wall of the PCI clean room are quite damaged...

We should definitely replace them.

We removed the box and several of the optics used for the scattering measurement.

Because of the damaged wall, the 'clean room' was quite dirty and we spent some time cleaning it up.

We install a new shelf in front of the PCI clean room entrance to store the scattering components

I started the SIP "N-S P8" near the south end. Applied voltag and current were changed as follows.

[Just after starting]

[After 3 hours]

[Marc, Shalika]

When we measured the polarization state with the polarization camera directly after the LC we could see that the azimuth angle seems to change a lot around the half-wave retardation voltage.

We tried to measure the fast axis direction by minimizing the transmitted power of the LC after a polarizer but the results were quite strange and not so consistent when repeated..

One possible explanation is that the power minimization was not precise enough to do by hand.

We decided to follow the procedure of [1] where we installed the output polarizer to be in cross-polarizer configuration and measured the transmitted power while appling a sawtooth voltage and rotating the LC from 0 to 360 deg with 10deg increment.

The resulting function is fitted for a given voltage by a sum of cosine with different orders, all as a function of (x-x0) where x is the rotation angle of the LC and x0 a possibly voltage-dependent offset that should correspond to the LC fast axis rotation as a function of the applied voltage.

By repeating this for every voltage we can get the attached figure where we found that the fast axis orientation is almost voltage independent (within 1deg).

[1] : Measurements of linear diattenuation and linear retardance spectra with a rotating sample spectropolarimeter David B. Chenault and Russell A. Chipman

[Marc, Shalika]

To measure the extinction ratio of our LC, we removed the polarizer and rotated the LC to measure the maximum and minimum of the transmitted power.

To be more precise, the transmitted power is normalized by the input power.

We repeated this measurement for several voltages applied to the LC as in the attached figure.

The errorbars are coming from the power fluctuations we measured.

The extinction ratio seems quite constant at all voltages at about 1.009+/-0.002

Waht I did: estimated Q factor after attached coil magnet actuator.

I estimated Q factor of Roberts linkage after attached coil magent actuator.
This Robert linkages' resonant frequency is 0.67Hz.
When I estimated Q factor, I used the ring down curve from 2000s to 2500s(just Fig 2).
Estimated Q factor of Roberts linkages is 3.91×10^3.

Blue points are mesurement and red points are fitting.
Vertical axis is read out of photo seosor that can detect displacement, horizontal axis is time.
Fig 1 is over view of the ring down curve.
Fig 2 is the data that is used for estimating Q factor(2000s to 2500s).
Fig 3 is over view and fitting results.
Fig 4 is the ring down curve that is used for estimating Q factor and fitting results.
Fig 5 is also the ring down curve and fitting results from 2000s to 2020s.

What I did: measure the transfer function from coil magnet actuator to photo sensor.

I measured the Roberts Linkage's transfer function from coil magnet actuator to photo sensor.
First of all, I attached magnets on the suspended mass, and also set a coil.
Fig 1, 2, 3, 4 is a setup of coil magnet actuator.

When I measured the transfer function, the data was measured from 0.1Hz to 10Hz and also measured form 10Hz to 0.1Hz.
The reason is that a resonant osillation of Roberts linkage remained for a while, so I can't measured the trasfer function appropriately after passing throught the resonant frequency.

Fig 5 is the transfer function when I measured it from 0.1Hz to 10Hz.
Fig 6 is the transfer function when I measured it from 10Hz to 0.1Hz.
Fig 7 is combined one. 

[Marc, Shalika]

As our calibration and vi are now finalized we are now setting up our final calibration before starting birefringence measurement.

We made a black cover box from 2 un-used optical lever covers. We drilled one small hole for the laser input and another one for the several cables we need.

The ambient light from the power meter or the camera were reduced by a factor 100.

[Marc, Shalika]

Following the speed improvement of the LC voltage control we implemented a sine modulation of the voltage.

However, we found that the resulting retardance is different if the voltage is increasing or decreasing.

We decreased a lot both the sine freq and sampling freq and could resolve this issue.

This seems related to the LC different switching frequency when applying increasing or decreasing voltage with decreasing voltage being faster.

To mitigate this effect, we also implemented a decreasing sawtooth function and plan to mainly use this one for our future calibration and measurements.

I checked the voltage and curent in the SIPs today.

[Shalika, Marc]

Overview: The speed of saving data/characterization is 80 Hz. 

Details:

We changed a "lot" of stuff. Techniquely we were removing everything one by one and seeing how they affected the speed. We did this with every single component in our VI. We were simultaneously optimizing speed by removing VIs which were not so important. Previously our speed was 8Hz, so we were acquiring 8 points per second. Now we have around 80 points per second, i.e 80Hz. 

Refer to Fig 1 for more details, but below are the most essential parts which helped optimize the speed. 

1. Temperature controller was extremely heavy. It doubled the speed when we removed it. We brought the control outside the main loop. 

2. We did the same with the Power meter and Polarization camera. 

3. And, we are now using global variables to access and save data in the main loop.  

I started the SIPs between the EM2 and the mid point in the south arm. The power supply #3 (DIGITEL 1500) drived "N-S P5" and "N-S P6", and the power suply #4 (DIGITEL MPC) drived "N-S P7". Applied voltag and current were changed as follows.

[Just after starting]

[After 3 hours for P5&P6 or 1.3 hours for P7]