Katsuki, Marc

Recently we found out that there are large stripes visibles in the birefringence measurements.

Furthermore, if we check either the raw or normalized s and p polarizations power, their sum is not at all constant (see figure 1).

We installed a power meter at the beginning of the imaging unit and measured the pump power without sample (fig2) with sample (fig 3) and with sample moving as a birefringence measurement (fig 4).

The power is not stable at all...

While we could see something like +/- 1 % fluctuations during the polarization calibration, we can see that the long term peak to peak fluctuations seems to be around +/- 10 % during 30min and +/- 20 % during a birefringence measurement.

This explains why we see the stripes and why the sum of s and p polarizations power is not constant during a birefringence measurement.

Note that this large power fluctuations does not affect drastically the birefringence measurements as we don't see the stripes in them.

Furthermore, it seems that these power fluctuations are far more important at low power compared to high power (few watts for absorption measurements) because we can not see the stripes in these measurements.

Another by-product of this measurement is that we could estimate the background level of each PSD to be about 1uV.

After the evacuation of input chamber, I aligned the input oplev by tweaking the steering mirror just after the oplev laser source.

Then I measured the input oplev spectra as shown in the attached figure. The input pitch and yaw oplev spectra look very similar now.

I checked the oplev beam height at the injection, readout viewports. The oplev beam height at the injection, readout viewports were 110mm, 106mm, respectively. I tweaked the injection steering mirror and made the both beam height 110mm, but the pitch and yaw oplev spectra still look very similar.

Before I opened the gate valves between input/BS and input/arm, the pressures in input, BS, arm was 1.3e-6 mbar, 9.9e-10 mbar, 3.1e-8 mbar, respectively.

First I opened the gate valve between input/BS. After 1 hour, the pressures in input and BS chambers became 5.5e-7 mbar and 1.9e-8 mbar, respectively.

Then I opened the small/large gate valves between input/arm. The pressures in input, arm became 5.4e-7 mbar,4.5e-7 mbar, respectively.

Michael and Yuhang

18.2% power is not coupled.

Michael and Yuhang

We checked the mode matching condition between BAB and OPO. 13.2% power is not coupled.

I started the evacuation of input chamber. First I used rotary pump until the pressure in the input chamber reaches below 0.1mbar. After I removed the rotary pump, I closed the small gate valve close to arm and opened the small gate valve close to the input chamber. I will wait until the pressure in the input chamber reaches the similar one in the BS chamber.

Yuhang and Michael

We found an issue with the Mokulab unit used in the ATC cleanroom

We used 100 Hz 10 dBm 1Vpp signal from another function generator. Putting the signal into Mokulab IN2 and using the oscilloscope function shows no frequency signal and 4 mV offset. IN1 reads 9Vpp (+5.4, -3.6) at 118 MHz even with no connection.

Using 1 MHz signal into the spectrum analyzer function likewise shows no signal, just -131 dBm floor. Using a T connector from the function generator to oscilloscope and Mokulab, we see that the signal on the oscilloscope is reduced when switching from Moku IN2 to IN1, but no change when switching IN1 input impedance between 50 Ohm and 1 MOhm.

Mokulab function generator output works fine though, we have been using it to scan the cavity.

I have now taken 3 measurements with s polarization at the input that saturated the lockin amplifier..

It seems that we can really easily saturates the p polarization.

Indeed, it is connected to the old lockin amplifier where the range (0 to 1 V) is changed depending on the sensitivity setting we are using.

Now I'm injecting about 160 uW of power but when doing the measurement with s polarization it seems that the p polarization power changes by more than a factor 14...

I'm starting hopefully the last measurement with s polarization at the input where the sensitivity of the old lockin is set to 100 mV despite the value at the center being 0.2 mV.

It would be convenient to have a new lockin amplifier to avoid this issue...

For easier comparison with direct measurement with PCI (see elog 2755), I show here the absolute value of delta n flipped both horizontally and vertically .

It seems that the larger delta n area have somehow a close triangular shape.

I had a look at the normalized Is and Ip data.

As seen in figures 1 and 2 which show respectively the s and p polarizations normalized intensities the stripes are mainly present in s polarization.

This was not the case for the previous measurements (see figures 3 and 4) were stripes were actually visibles in both s and p polarizations...

Also it seems that p polarization is saturating..

Recently I've been trying to compute birefringence from TWE measurements of spare ETMY based on Aso-san's computation.

I'm using the S1thruS2 measurements with roll angle of the mirror of 45, 90, 135, 225, 270 and 315 deg with 20 measurements averaged.

The code I wrote does the following :

1) Find the real center and roll angle of each TWE maps

Hirose-san who did the TWE measurements at Caltech placed 3 markers on the mirrors.

First I overlapped 3 circles on the markers of the map with roll angle of 90 deg as it is the same orientation as the PCI measurement.

Then, I changed the centering of all other maps + rotated them to match the circles positions of the 90 deg one.

See figures 1 to 6 for the TWE maps (piston, tilt and curvature removed) without rotation and 7 to 11 for the rotated maps.

2) Sanity check of the RoC

OSCAR is used to removed the piston, tilt and focus of the TWE measurements but states that the removed RoC is about -716 m..

This is actually due to the Fizeau interferometer setup. Computing the RoC taking into account the clear aperature and position of the reference sphere gives a mean RoC = 1.9 km (as expected).

Especially, we recovered the same RoC as measured by Hirose-san with 270 deg roll angle.

3) Combining TWE maps into birefringence

Using 4 rotated maps, it is then possible to compute the spare ETMY birefringence (delta n and theta the fast axis orientation) as in figure 12.

Actually, because there is an arctan(2*theta) used, theta is only defined between -pi/4 and pi/4 that causes some wrapping of theta (and therefore delta n as well).

So in figure 13 I used an unwrapping algorithm to try to remove this wrapping. Results are reported in figure 13.

We can recognize so similar patterns to the measurements with the PCI setup. Final conclusion should be done after a new direct measurement with the beam at normal incidence).

4) Next steps

- Tune the unwrapping algorithm -> still some issue at the bottom of the map

- Check the orientation of the mirror during PCI and TWE measurements to understand why there seems to be both a vertical and horizontal flip with respect to each other

- Finalize the direct measurement at normal incidence

Picture of the mirror without magnets.

Pictures of gluing.

Since the two magnets of end mirror fell down, this measurement does not make sense. I brought back the rotation angle for END oplev to 0 deg.

[Takahashi, Aritomi]

Since there is a new 6.35Hz peak in END oplev spectra as reported in elog2808, we opened the end chamber and checked the suspension. We found that two magnets of end mirror fell down! This is the reason why the Z correction does not work recently. Takahashi-san will glue the magnets next Monday.

Takahashi-san glued the magnet to the input mirror and closed the input chamber. Since it takes one day for the glue to be fixed, we will evacuate the input chamber after tomorrow.

The attached figure shows the CCFC error signal on 20210622 and 20211227. The elogs on 20210622 and 20211227 are elog2597 and elog2770, respectively.

I checked the first measurement of the shinkosha 7 with s polarization at the input,

As shown in figure 1 there are some vertical stripes present in the measurement.

I tuned both the hwp and qwp in the injection and found out that when injecting s polarization, the p polarization readout is minimized for hwp = 0.1 deg instead of previously 1.3 deg.

Then, I also added beam dump to catch the PSD reflections and a small wall to isolate the imaging unit from the injection part.

The beam dump were removed because they are supposed to be used for visible light...

In any case the good news is that we can recognize similar patterns to the previous measurement with tilted beam.

Finally I restarted the s polarization measurement.

Recently, a 6.35Hz peak is visible in END pitch and yaw oplev spectra (Fig 1). For END oplev diagonalization, I changed the rotation angle for END oplev so that the 6.35Hz peak disappears in oplev yaw spectrum, assuming that the 6.35Hz peak is a natural pitch peak. I changed the oplev rotation angle from 0 deg to 1 deg. The END oplev spectra before/after this rotation are shown in Fig 2 (red, blue: after rotation, green, brown: before rotation).

To check END actuation, I put an excitation (12 Hz, amplitude 1000, sine wave) in END length (K1:FDS-END_LEN_ex2). The 12 Hz peak is visible in both pitch and yaw oplev spectra (Fig 3). The 12 Hz peak height was 13.5 and 20.8 for pitch and yaw, respectively. The current EUL2COIL matrix is shown in Fig 4. This matrix might not be optimized for L2P, L2Y coupling.

In the case of a rigid cavity, we can estimate internal losses without locking. We just need to scan cavity and look for the reflection signal as shows in Fig.1.

1. We scan cavity and know that about gamma = 5% power is not coupled to cavity for TEM00 resonance.

2. We take the reflected power when cavity is on resonance for TEM00 (33.2mV) and off resonance (36.6mV)

3. We get Rcav = ((33.2/36.6)-gamma)/(1-gamma)

4. RTL = T1*(1-Rcav)/2/(1+Rcav) = 0.206%, here T1 = 0.08. Note: the polishing/coating company (LASEROPTIK) specifies AR coating reflectivity is less than 0.1%.

5. escape efficiency is T1/(T1+L)

In this way, we get escape efficiency of 97.5% for the new OPO.