[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]

I checked the voltage and curent in the SIPs today.

I started the SIPs between the NM2 and the mid point in the south arm. The power supply #1 (DIGITEL MPC) drived "N-S P1" and "N-S P2", and the power suply #2 (DIGITEL 1500) drived "N-S P3"(photo) and "N-S P4". Applied voltag and current were changed as follows.

[Just after starting]

 [Ater 2 hours]

Yohei,

What I did:

I have measured the beam profiles of the Mephisto laser. I have used a f250 lens to focus the beam, then estimated its original q-parameters of the laser.

Result:

The measured beam parameters (Fig1).

* z=0 is set to 100 mm away from the laser head. See the yellow tape in the attached picture (Fig2).

The estimated original parameters of the laser.

The weist is 60-70 mm inside the laser head. The weist position is supposed to be 90 mm inside the laser head, but the estimated values suggest it's almost near the laser head. I don't know why it happens.

** Sorry but there is some error on the first figure.

What I did: estimated Q factor

I tryed to measure Q factor of Roberts Linkages that resonant frequency is 0.67Hz by ring down curve.
When I estimated Q factor, I used the data from 500s to 1400s (just Fig 2).
Estimated Q factor is 5.97×10^3.

Blue points are measurement and red points are fitting.
Vertical axis is read out of photo sensor 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.
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 600s to 620s.

Yohei,

I started to built a new set up of the speed meter experiment. 

What I did:

I put a QWP and HWP in front of the source laser (Mephisto, Innolight), mazimizing p (holizontal) polarization.

Then, I put a Faraday Isolator (FI), optimizing its extinction ratio.

I measured the transmissivity of the FI:

Input = 10.3 mW,

Ouput = 9.3 mW,

Transmissivity = 90.3 %.

When measuring the beam power by a power meter, I attatched a ND filter, ND=2.0. From this value, the source lase power can be extimated as ~ 1W, keeping its original value.