NAOJ GW Elog Logbook 3.2
This entry is log on Mar. 2nd.
What I did
I aligned the output mirror of the folded cavity so that the reflected beam can be picked off at the FI.
The procedure is as follows.
- Install the output mirror.
- Check the reflected beam postion by a sensor card.
- Insert the shim sheet between the mirror holders to adjust the direction of mirror.
Next Step
Align the input mirror as the same procedure.
Then scan the laser frequency and monitor the transmitted beam.
Note that back scattering on Saturday was particulary bad. It seemed the mirrors were more excited than usual.
By the way I remember that before Christmas, whitout the additional Faraday, we managed to measure less then 2 dB above shot noise at 30 Hz. See here.
What I did
- Replaced two pedestals used in the chamber
- Removed the output fused silica mirror and re-aligned the beam. At this moment, the cavity direction was not good as the beam reflected by the apex mirror could not reach the center of the output mirror. So I tweaked the cavity direction.
- Re-installed the output fused silica mirror and checked the transmitted laser power.
Some results
The transmitted laser power was about 1.03 uW with 6.4 mW input power.
Assuming no loss in the apex mirror , the transmittance of the fused silica mirror is estimated about 0.016%.
In practice, there are some loss in the apex mirror and the transmittance of the fused silica mirror is slightly larger than 0.016%.
According to the spec sheet, the transmittance of the fused silica mirror is 0.017%.
So this result is reasonable and the loss in the fused silica mirror is not too large, about 30ppm with the assumption that the reflectivity of the fused silica is same as shown in the spec sheet (99.98%)
In any case, I need to compose the cavity and see the flash.
Next step
- Tweak the alignment of the output mirror to pick off the reflected beam. In fact, I could see the reflected beam on PD by adjusting the direction of the output mirror. Shim sheet, however, is indispensable to maintain the optimal direction.
[Aritomi, Yuhang, Eleonora]
We measured shot noise and squeezing with additional faraday before homodyne to reduce back scattering (attached picture).
Difference between black and blue is back scattering with faraday which is much smaller than before. Difference between blue and red curve might be effect of CC.
Note that back scattering on Saturday was particulary bad. It seemed the mirrors were more excited than usual.
By the way I remember that before Christmas, whitout the additional Faraday, we managed to measure less then 2 dB above shot noise at 30 Hz. See here.
I re-installed a folded cavity inside the chamber with one apex silicon mirror.
Then I aligned the 2 steering mirrors to pick off the reflected beam and I could pick off the beam from the chamber.
A PD was put the transmitted side to check the transmitted power when the cavity is composed.
After that, I installed a fused silica mirror as the output mirror and tried to see the transmitted beam power.
Some amount of stray light, however, prevented me to check the transmittance of the fused silica mirror.
I will install an input fused silica mirror and try to see the flash by scanning the laser frequency.
[note]
There is an optical loss at the viewport, about 10%.
The temperature change inside and outside the bench is monitored. At the same time, the suspended mirrors position is also monitored.
The comparison is attached.
Also the seperate measurement is attached.
In order to mitigate back reflection noise we plan to install an additional Faraday isolator (FI-1060-5SC-HP) on the squeezing path in reflection from FC, at the edge of the optical bench. See pic 1 and 2.
To finalize the istallation we need to modify a bit the custom support: drill holes for the screws and reduce the hight of about 2.5 mm. (ATC will do it by next Wednesday afternoon)
The Faraday should bring additional 3% of propagation losses, the associated squeezing degradation (simulated in pic 3) doesn't seem too bad.
The drawback is that the faraday will prevent to close one side of the bench cover. But so far we have never closed it.
The beam waist is ~1mm and FI aperture is 5 mm in diameter, so clipping losses shoud be negligible.
Aritomi and Yuhang
We measured backscattering noise with different kinds of filters added to the servo we had. The added filters contain corner frequency ranging from 0.03Hz to 10Hz. The gain is also adjusted for different corner frequencies. All the measurement is plot together in the attached figure.
According to the measurement done in last Friday, we have a variation region of back scattering noise. It is shown in the attached figure as a grey area.
We could see no matter how we set the filters, the back scattering noise is always in the variation region.
I measured backscattering noise of homodyne in two configurations. The first case is that BHD's signal port opens to filter cavity with the path between filter cavity and OPO blocked. The second case is measuring squeezing case.
The measurement is done with network anylizer with bandwidth of 400Hz, averaage is 100 times. The measurement starting time is put as legend.
The result is attached and we could see that this noise varies between -127 to -120 dBVrms/sqrtHz at 10Hz.
As we reported in elog 2046 and 2049, we had issue of filter cavity correction singal drifts. According to the experience of Raffaele, this is due to the tide and can be corrected if we send portion of correction signal to the filter cavity end mirror.
Today I tried to do this, I fed portion of correction signal back to filter cavity end mirror at UTC time 2020-02-20-05:23 and then the correction signal becomes very stable. Up to now, the lock has already lasted for one hour.
Aritomi and Yuhang
We set up three temperature monitors and one humidity monitor around our bench. The position for the moment is shown in the attached first three pictures.(One is localted on the wall of cover and is beisde AMC, another is under the step and on the ground inside clean room, the last one is just beside PR chamber)
We have monitored these parameter for two days already. The conclusion is
Temeprature inside the bench cover is changing between 25deg to 26deg. Temperature on the ground(inside clean room) is changing between 24.3deg to 25.3 deg. Temperature beside PR vacuum chamber is changing between 23.3deg to 24.3deg. All of these temperature measurement go up and down with daytime and nighttime. The minimum of temperature is around 8am and the maximum is around 6 or 9pm. The difference of maximum inside and outside the cleanroom maybe related to manual work.
The humidity changes as well but it doesn't follow the day or night time.
Pengbo, Simon
After having found a good calibration for the upgraded system, Shinkosha#7 has been characterized again in terms of absorption (input P-polarization). The results can be seen in the attached figures (Z = 76), compared to the ones before the upgrade (Z = 74.1).
The basic shape as well as the mean aborption coefficient are almost the same, which is a very good achievement.
However, there are also small differences. It seems that the center absorption, which is at maximum in both measurements, is higher than before the upgrade. Also, there are basically two peaks in the absorption-coefficient distribution for yesterday's measurement. The reason is not clear. Maybe the change in polarization has some effect on the results. We are going to further analyze this phenomenon by using a polarization control on the pump-beam.
Today BS was less noisy, however, as reported in elog 2046, filter cavity was unlocked by the filter cavity length correction signal drift.
Actually I was thinking maybe it is related to I just turned on laser. But even after more than one hour of laser-on, the length correction signal is still drifting. Anyway, in the end, the signals including FC_GR_tra, FC_GR_correction, FC_IR_tra and PDH_detuning were monitored for one hour.
We could see that during this one hour, without any re-alignment, the detuning never jumps. The detuning changing range is roughly from 40Hz to 80Hz.
There was one period of time, green is quite stable. This is different from elog 2047, which has green misalignment. We could see during this green stable time, IR detuning drift is still happening.
Simon, Pengbo
Today we started the IU adjustment with the bulk reference trying to find the maximum of AC/DC value.
We start setting the IU back to initial position ( -> 70 mm) , decreaing the distance then find the maximum at 54 mm with a value of 0.0158. We stopped here since the maximum position is too far away from the achievable distance towards measuring KAGRA-size samples.
Then we moved the first lens of the telescope 2 mm away the second lens and adjust the crossing-point with pin-hole and surface reference sample(IU back to 70 mm). We fastly found it and adjust the AC to be maximized with second lens -> R_surf = 16 1/W.
After that we did another adjustment of IU to find the AC/DC maximum, and found it was at 69 mm. We make a bulk reference scan and get R_bluk = 0.713 cm/W. Then we start the absorpiton measurement on KAGRA-size #7 sample.
(This is the report of yesterday20200217) Aritomi, Eleonora and Yuhang
We found BS moved quite a lot, this was figured out later that it is because of the saturating of BS coil drivers. Then we tried to remove the DC offset of BS, which didn't work very well since we found BS moved more than before. In the end, we open the oplev local control for BS and did the experiment.
We locked filter cavity with GR and closed the pitch dithering. The BAB is sent to filter cavity and we demodulated the filter cavity reflection to have PDH signal. We detuned IR by 60Hz and leave it for one hour. During this time, the filter cavity was unlocked several times. We relocked three times, once we just relock and twice we re-aligned filter cavity.
The filter cavity GR/IR transmission and GR filter cavity length correction and IR detuning was monitored.
The result is attached. We can see different alignment causes different detuning value.
There was a zoom-in part, which we can see the correction singal doesn't change but alignment and detuning keep changing.
Eleonora and Yuhang
Today we tried to monitor detuning drift but filter cavity always got unlock(almost every 5 minutes). And we noticed that the length correction signal was always drifting. Considering the change of 3000counts, we can convert it to the voltage we send to laser PZT is drifting by about 3.66V(3000count/1000count*0.61V*(100/50)). Maybe it drifts out of range and made filter cavity unlock. However, from the manual of laser, the PZT driving coefficient is 1MHz/V. And the PZT driving range is +/-65MHz. It seems they are not consistent.
Besides, the reason for this correction drift is till under investigation.
Since the parts for fixing the cavity have been delivered, I started the installation of a folded cavity.
- Installed a stage and put a spacer on it.
- Fix the position of the spacer.
- Install a mirror and injected a beam.
- Adjusted the alignment roughly.
Now the "transmitted" beam can be picked off with two steering mirrors.
Pengbo, Simon
after the successful polarization test, we started to do also the absorption tests in order to see whether we can reproduce our results from before the upgrade.
Actually, the first test was performed with the TAMA#1 sample but we repeated the test with Shinkosha#7 since we don't have any direct reference for TAMA#1.
We can say that the actual results in terms of the absorption coefficient are well reproducible (see attached figures from before [Z = 74.1] and after [Z = 80] the upgrade).
However, there is an issue with the "effectivity of absorption", meaning that we get a factor of 2-3 smaller AC-values than before. The reason is still not quite clear to us. It may be that the size of the beam-waist, which is 10% larger than before, has a bigger influence on the AC-value than anticipated. We will try to further reduce the waist and see what may happen.
At least we can exclude that the polarization of the beam has any significant influence on the absorption. We have tested this yesterday as well.
I installed 2 lenses for mode matching of TEM00 mode beam.
Their positions can be optimized by the stage.
The beam is collimated less than 100 um dia.
The requirement is about 50 um and I need optimization.
Tomorrow, the rest parts for folded cavity installation will be delivered and I will install the cavity with these parts.
This is my fitting result.
