Simon, Pengbo

we finished the birefringence characterization of the shinkosha s5, as can be seen from the attachment(the first four figures are the result this time , the last two figures  are the results last year.), it seems the offset is even larger than before.

Simon, Pengbo

Today we finished the charaterization of the birefringence properties on Shinkosha S1 substrate. For the s-pol, we got 87.77 +/- 0.25 deg, which is slightly better than last year's result, 87.1+/- 0.05 deg. For the p-pol, we got 1.93+/- 0.68 deg, because we didn't do this measurment last year, there are no comparision. From the maps, it seems this time the distribution is more homogenous.

Here the absorption map of the second surface.

The cryostat was pumped down againg and the pressure became less than 10^-3 Pa.
This level is enough for the measurement at room temperature.
So I vented the chamber and opened in order to do the alignment of the cavity.

Background

Since I modified the mode matching lenses, I had to also modify the beam path.
Furthermore, in order to avoid putting the mode matching lenses between the steering mirrors, the layout should be reconsidered.

What I Did

I determied positions of input optics around the viewport.
They are a littele bit complicated as shown in the attached picture due to the limited space.
Then I aligned the mirrors in TEM00 path.

Next Step

• Alignment of the TEM00 path.
• Alignment of the input and output fused silica mirrors.
• Need to purchase some mirror holders and pedestals for the HOM's paths (next FY).

Pengbo, Simon

Yesterday and today we repeated basically the measurements we did on S5 for S1. The results can be seen in the attachement.

Also for this one, it became obvious that there is a large absorption excess on the surface, This time, however, it seems that this excess is existent on both surfaces. Therefore, I had run the surface-absorption measurements on both (as the time I write this, the second one did not yet finish).
The center-map shows a distribution which is somewhat comparable to the measurements we did last year. However, the mean absorption coefficient has been increased by a factor of ~2.

The next step will be to do the birefringence measurements on both samples.

Preface

As the viewport attached the cryostat was not AR coated at the wavelength of 1550 nm but 1064 nm, there was a 10% loss for beam power at 1550 nm.
So I decided to replace the viewport for 1550 nm AR coated.
The cost, however, was a little bit expensive.
Therefore, I bought a AR coated window and a viewport without a window separately and they were supposed to be assembled.

What I did

I assembled the viewport as attached pic. 1.
Then I connected to the conversion flange though I was not confident to use M8 bolts and washers stored in the ATC.
After that I installed to the chamber and vented as a test.
Before venting, I measured the beam power before the viewport and after.

Result

First, the transmitted beam power was 2.73 mW against 2.74 mW input.
So the total loss was about 0.5% which is dramatically reduced compared to the previous one.
Moreover, the viewport does not have any serious leakages and the pressure reached less than 10^-2 Pa.

What I Did

• Tweaking the lens and mirror position in a double-pass AOM path
• Mode matching for TEM00 mode

Details

As I replaced a lens in double-pass AOM path, I tweaked the position of the lens and the mirror.
Current lens is f=70mm one.

Since the transmitted flash could not see due to the poor mode matching, I tried to improve the mode matching.
The lenses were replaced and f=-75mm and f=300mm ones are put now.
Their positions are about 0.226m and 0.659m from the beam waist, respectively.
After that I could get beam size about 65um at its waist where it is reasonable position for the apex mirror of folded cavity.

Next Step

• Install some mirrors to inject the TEM00 beam into the chamber.
• Alignment of the input and output mirrors.
• Tweaking.

Pengbo, Simon

On the weekend, Pengbo finished the measurements on the annealed S5 sample.
Already last week, we recognized by doing the Z-scans that a large absorption excess must exist on one side of the sample (actually the side which has NO damages). So we decided to take actually 3 maps: in the center, on the suspicious surface and along the Z-axis. The results can be seen in the attachement.

As can be clearly seen, there is a very prominent structure on the suspicious surface which leads to an excess in the absoption data. This structure is partly visible also in the center-map, probably due to interference effects coming from the absorption excess (at least judging from the phase map). Please note that the sample for the center-map has been flipped so that the suspicious surface is on the out-going side because we had large problems in obtaining a meanigful result when the beam got influenced by the excess before reaching the targeted position.

As can be also seen from the map along the Z-axis, this excess is apparently not limited to the single surface area but has a depth which is hard to quantify (given the strong disturbance in phase and AC) but we estimate the affected depth to be several millimeter.

As a reference, the histogram for the absorption coefficient taken at the center is also given. The actual mean-value is not so far away from the measurements last year (however biased by the mentioned structure), which is at least one good news.

Pengbo, Simon

We received the samples S1 and S5 back from France after their annealing.
Before we started measuring the absorption coefficient, we inspected the samples and we discovered some damages on the edges of both samples. Interestingly, the damages are only on one side (see attached photos).

The measuremetns started with S5 and are ongoing. However, before setting the sample into the sample holder, we cleaned it with FC on both sides.

I tried to see the transmitted flash but I could not on Monday.
Indeed, the beam size was at its waist was much larger than the design value i.e., 100 um though 50 um is required.

I decided to change the mode matching lenses in order to achieve the designed spatial mode.
To be honest, the obtained minimun size was about 80 um radius.

I gonna try improving the mode matching until end of this week.

Note

The lenses are biconvex or biconcave in order to reduce the aberration of reflected beam.

We performed the measurement which is close to the real case when we use QPD. The difference is only light is adjusted into only one segment of QPD.

We could see the modulation frequency we are interested in can be seen.

I tried to see the transmitted flash with scanning the laser frequency, but I could see nothing.
I decided to remove the input and output mirrors and do the alignment work again.

I also modified the PDH servo.
The schematic of the servo will be uploaded on wiki.

Pengbo and Yuhang

We performed the measurement although the mirror is not very stable. As you can see from the attached figure 2. This makes the low frequency measurement of shot noise(with the corruption of backscattering) much worse.

The measurement of squeezing level is only 2.5dB.

We measured again with 5mW green power, but this time we make the beam smaller to see if it will decrase or not.

The result is that shot noise will decrease if the power density is higher than 47mW/mm2.

There is a discrepancy between this measurement and the one measured last time(entry 2067). I think the reason is that, the beam size is highly related to position. And this time we removed QPD and we may put it back to a slightly different position. Then it leads to this discrepancy.

We need to use a more robust telescope if we want to have a more precise measurement.

Pengbo and Yuhang

We measured shot noise spectrum with 5, 15, 26mW seperately. For each power, we measured also with different beam size ranging from to.

The set-up is using the second segement of QPD2. We checked DC voltage with oscillscope. We also checked RF channel after an amplification of 32dB with spectrum analyzer operates in 1MHz RBW(this time we average for 10 times so that the noise vurve is smooth). The noise spectrum of RF signal is plot for each case.

1. 5mW case(attached as the first picture): the noise floor is the same for all the beam size.

2. 15mW case(attached as the second picture): the noise floor reaches maximum when the power density is below ~25 mW/mm2

3. 26mW case(attached as the third picture): the noise floor reaches maximum when the power density is below ~22mW/mm2

We didn't measure the changing point for 5mW, however, from the beam density we measred, all the measurement we did for 5mW has low power density relative to the threshold (roughly between 22 and 25mW/mm2). We should see the noise floor decrease when the beam size is smaller than 500um.

Conclusion: The QPD response will saturate and decrease if the power density exceeds around 23mW/mm2.

(Notice: the beam size in this entry is diameter)

Simon, Pengbo
 

Attach to this report I show the result on shinkosha #7 sample with different polarization input beam(S-pol and P-pol).

We can see a 1 percent difference from these two maps, which is even smaller than the difference for TAMA-size sample.

Simon, Pengbo

After the birefringence measurement, we change to the absorption system with a controllable polarization of the laser. First, we did two XY-plane absorption measurements on TAMA#1 with different polarization, which is P-pol and S-pol. Then we choose a small area of the mirror and did another measurement under the S-pol incident beam. We can see a very clear structure in the map. Then we did another YZ-plane absorption measurement with an S-pol incident beam. The distribution is quite homogenous.

We change the sample to Shinkosha #7, then follow what we did before, checking whether the polarization might have some influence on the absorption. We already have the result of the absorption map with the S-pol incident beam. The result is almost the same compare with the former result.

We did the same measurement as in logbook entry 1875, the QPD response to different size of the green beam

The measurement was done by using the green beam reflected by the green mode cleaner of the Virgo squeezer(1500W). 

First, we measured the green beam size without any lenses and saved the data file from the beam profiler.  The beam profiler has 1928*1448 pixels corresponding to active are of 7.1mm*5.3mm, we got power value at each data point, and we did a 2-d Gaussian fit to these data points. One of the fit shows in figure 1. From the beam plotted in this figure, it is clear that the beam has astigmatism, so the final beam waist size and position are quite different in two axes. Below is the number (position zero is just a random point we chose easy for the measurement) 

By checking the 2d plot of the raw data, we found out the beam profiler is saturated (fig 2 shows the top cut shape). But in the 2d fit, we were not able to remove these points by substituting them into 'NaN' while using 'lsqcurvefit' function in Matlab, because this function needs 'double' format input.  Then to check the quality of the fit, we plot the difference between the fit the original data, result shows in figure 3. It seems the fit is fine. Since the data is very noisy, I was guessing maybe we actually got the peak of the Gaussian, and the saturation part is just the noise. 

Anyway, after measuring the beam size, we put a 50mm lens and measured the beam with the QPD in different positions. We did two groups of measurements with different power by changing the density. the results show below.

Eleonora and Yuhang

We measured the filter cavity reflection beam height. It is 74.5mm, which is 1mm lower than injection beam measured three months ago.