I found two good options using the lenses we already have in the lab.

Version 2:

1000 mm @ z = 0.517 m (distance from PBS)

750 mm @ z = 0.618 m

Version 3:

750 mm @ z = 0.42 m

750 mm @ z = 0.673 m

As we can see in fig 2 and fig 4, the robustness is not as good as the one of entry #1365 (fig 5), but we are still under 10% of mismatch.

In fig 6 there is the scheme on the bench of version 2 (PURPLE) and version 3 (BLUE).

After the improvement of homodyne matching entry #1354, the parameters of the beam entering the injecion telescope have changed (results of measurement in entry #1363).

Parameters changes: 

w0

126um --> 94um

z0 (with respect to PBS)

15 cm --> 8.9 cm

Focal lenghts needed for the new injection telescope are:

600 mm (instead of -1000mm)

750 mm (instead of 500mm)

Robustness is still really good (fig 2).

As I mentioned in the entry, I moved the first lens after OPO. So the beam parameter going to filter cavity is changed. To have a better design for the telescope matching OPO transmission to filter cavity. I made the beam measurement again.

The way that I did measurement is illustrated in the attached figure 1. 

By measuring seven points along a long distance, I fit the beam. The result is shown in the attached figure 2.

The beam waist is even smaller than the last measurement.

I also matched BAB into OPO. The spectrum is shown in the attached figure.

The matching of main laser optical fiber is also changed after the cleaning of some mirrors.

The good thing is we see the main laser is very stable after the installation of this second FI. The situation is definitely much better than the situation before.

Chien-Ming, Yu-Hang

We installed the new Faraday Isolator (FI) on the main laser beam. The optical intensity transmission efficiency we measured was >98% although the polarizer at the entrance of this FI was accidentally scratched yesterday.

The mode matching statuses before and after inserting this new FI are shown in the attached figure. It got a little worse after inserting FI.

Now the SHG can provide 218mW when input 580mW IR. We cleaned some optical component, then we can get more IR power (up to 880 mW) to the SHG if needed.

Other mode matching status are also shown.

Since I have been changing name of the channels and modified the RTmodel quit a lot I put here the screenshots of the filter/model current configuration of photon for BS and PR, to recover it in case it is lost.

For each d.o.f of the two suspensions there is the model and the contoller (called damp).  The d.o.f and suspension is indentifed by the "module"  field and the ZPK configuration is relative the filter selected in the "section" field.

Fig1: BS PITCH  DAMP

Fig2: BS PITCH  MODEL

Fig3: BS YAW  DAMP

Fig4: BS YAW  MODEL

Fig5: PR PITCH  DAMP

Fig6: PR PITCH  MODEL

Fig7: PR YAW  DAMP

Fig8: PR YAW  MODEL

There are two minor issues I couldn't solve so far:

1) Despite many suggestion from Oshino-san, I could not make work the medm screen botton which saves and restores the EPICS channels values (so called screenshot). It seems there is no error in the coding but it doesn't work. So I have done two simple scripts (located in /home/controls) to save and restore the snapshot from terminal. The commands are respectively

./takesnap.sh  and ./restoresnap.sh

2) I also tried to implement the "wave rotator" function to rotate the oplev signal in order to compensate a possible inclination of the PSD but I couldn't find the name of the "angle channel" to be written in the medm. I asked the help of Shoda-san and we tried to implement the function on a test model in the ATC DGS but we failed as well. We could find some EPICS channels created into the fuction on the dataviewer but when we tried to read or write them on the terminal (commands caget, caput) it said they do not exist. They cannot be shown in a "text monitor object" either. The workaround for the moment is to use a matrix and put in the already computed cos/sin values corresponding to the desired rotation.

What has been done:

1) The area around the work station of the new DGS has been cleaned and tidied up.

2) The cabling toward and from the new ADC and DAC has been realized.

3) The real time model has been completed to include all the suspensions (pic 1). It is mainly fine even if I could not implement some more sofisticated features (see dedicated entry #1359).

4) The damp on BS has been implemented. See TFs of pitch and yaw (pic2-3) and specra with open and closed loop (pic4-5).  They can be compraed with the "labview ones", measured in november 2016, reported in entry #337.

Some comments on BS CONTROL

1) TF are both one order of magnitude smaller wrt labview ones, this seems a bit strange to me. I would say It shouldn't depend on the digital system. But note that in KAGRA DGS there is a factor 2 btw ADC and DAC count calibration (entry #1315).

2) Pitch TF is less clean wrt labview one:  there is noise and lack of coherence in correspondece of the zero of the TF. Not that the maximum output of DAC is +/-5 V, while in labview was +/- 10 V. The max  white noise  amplitude I could send to avoid any saturation is 6500 count witch according to the count/ volt calibration corresponds to about 2 V.

3) Spectra levels look similar. A rough computation of the count -> rad calibration (using the information from entry #337 and  #1315) gives roughly 1.9e-7 rad/count.

4) There are more structures at high frequency then before. I don't think this corresponds to a real motion of the mirror but rather some oplev mount resonances. It is clearly visible in the time doman where it masks the low frequency motion. This excess of noise at high frequency wrt before showed up also in PR. The gain should be low enough not to feed this noise back to the mirror.

5) There is a high Q resonance at 10 Hz  which is effectively damped by the control.This was already observed in labview. Another line is present at 11.5 Hz both in pitch and yaw.  I don't think it is from the mirror and I fear the gain is not low enough at that frequency. To be investigated.

In the future, we could turn off ceiling light only to avoid obvious 50/100Hz noise on PD. And keep wall light on to avoid dark.

The characterization of the DDS output was done long time ago. You can find the graph here.

As clear from the schematics there is a high order low pass filter with corner frequency around 200MHz, therefore the cut is expected.

I made the optimization of the telescope from OPO transmission to AMC. The entry is here. From my impression, I feel I move both lenses quite a lot. I just checked the position difference before and after optimization. I also compared the final implementation scheme with the initial design. The result is attached as the following form.

1. The position of the lens is relative to 'the dichroic mirror after OPO'

2. I assume that the beam waist inside OPO is 2 holes away from 'the dichroic mirror after OPO'

I am sorry that all the design work of Eleonora Polini should be done again because I moved this lens.

Chien-Ming, Yu-Hang, and Aritomi

We replace the flipping mirror of BAB and CC to a 2" BS today.

As for the p-pol PLL, Yu-hang saw a 0.1Hz slow fluctuation signal at the output of the monitor mixer. The input to the mixer were 252 MHz laser beat frequency and 252MHz DDS channel 2 output.  Today, we try to figure it out but fail to see the fluctuation at 252MHz but at 150MHz (We adjust the beat frequency to close to 150MHz). 
We use the spectrum analyzer to check the output of the DDS directly. We see a drop in DDS amplitude after 200MHz as shown. The drop after 240MHz is serious and there is another peak appearing. However, we assume that the DDS should be able to provide up to 500MHz.

We feel that the 0.1Hz slow fluctuation may be due to the phase (or frequency) difference from DDS after frequency division. (For locking to 252MHz, the PLL needs to be divided by 3).

[Aritomi, Yuhang, Chien-Ming]

We replaced a flipping mirror for BAB and CC with 95:5 BS since repeatability of the flipping mirror is not good. 95:5 BS is CVI PR1-1064-95-IF-2037-C. We put this BS so that 95% of BAB reflects and 5% of CC transmits. Instead of increasing the CC power, we removed ND1 and ND0.4 in CC path before the flipping mirror.

Then we tried to align CC and BAB to OPO. We found the resonance of CC, but the alignment of CC is not finished. We'll finish the alignment of CC and BAB tomorrow.

Chien-ming, Yu-hang, and Aritomi

Today we found power fluctuation after we lock IRMC. Then we did lots of investigation. Especially we put power meter at some position to monitor laser power coming from the main laser. However, as we found before, we didn't see power fluctuation from it.

However, later we used photodetector directly monitor the part of the beam from the main laser. The signal from PD is shown in the attached figure. We could see a clear signal fluctuation.

Then we blocked the light going to SHG, and this power fluctuation disappears.

This proves that it is really necessary to install the second FI for the main laser.

To see the effect of coherent control beam on homodyne noise, I performed the measurement of the noise spectrum with LO or LO+CC on homodyne.

The result is attached as a figure from 10Hz to 51.2kHz. Coherent control beam brings a large 280Hz peak and several small peaks. Also, it rises up the noise level between 10Hz and 40Hz.

There are also videos show spectrum without average. Please refer to LO https://drive.google.com/open?id=12jaWRG_3HjDOSp6thXOixYxd7Y17WkIx, CC+LOhttps://drive.google.com/open?id=1OjRokr1ly4zR8Tf0TltP-oGDvwoo92RT

Notice that the signal of homodyne SUB-DC low-frequency noise spectrum has fluctuation. It means there are noise sometimes large and sometimes small. If we could float bench, it will be better.

Today we used the oscilloscope to monitor the homodyne SUB-DC channel. We can see clearly that the unlock of the IR phase part is related to the jump of the homodyne signal. This jump will rise up the whole spectrum level. The video can be checked from the following link. (blue line: IR coherent control error signal. yellow line: homodyne sub dc signal)

https://drive.google.com/open?id=1RpOabqKvQRVOS4uYonU8hqClGpDq8HDu

We can see from this video that every time homodyne signal jumps when the IR coherent control error signal (blue line) becomes thin.

The thickness of the thin line is comparable with the situation when there is no coherent control signal. So I checked the PLL and the lock of OPO. All of them are fine. The strange thing is this line doesn't drift away but get thinner. 

Also:

3. The spectrum is plotted with poor resolution, so another reason could be that at low frequency nearby lines are "grouped" and the baseline seems higher.

Eleonora, Irene, Yuhang, Federico

We made the same excercise as yesterday, but now with squeezing ON

fig.1 is a whole spectrum from 10Hz to 100kHz, comparing the squeezing ON with the squeezing OFF conditions

fig.2 is a zoom from 10Hz to 200Hz

fig.3 is a zoom from 100Hz to 2kHz

(note that an offset has been added to the data with no squeezing in order to have the level of noise 4dB above the squeezing curve)

We see and recognize many peaks with seismic origin, and we know for sure that there is also a large noise associated with acoustics.

13.5Hz is associated with a bench resonance; 24.62Hz is a "scroll" vacuum pump; 33.88Hz is a "moving line" that jumps from 34 to 37 Hz with a cycle of about 20 seconds; perhaps it is connected to an air conditioning machine that "modulates" an air flow to maintain the controlled temperature; 603Hz comes from all Turbo Molecular vacuum pumps in operation at that time. The large noise from 400 Hz to 1200 Hz could be of acoustic origin and could be associated with air flows. This is the region where are the resonances of the various mounts, it is not surprising that they are excited by ambient seismic and acoustic noise.

The possible solutions are the suspension of the bench (seismic isolation) and its complete seal with aluminum and rubber panels 1 cm thick; you should get a reduction from these actions.

I think there are mainly two points:

1. Turn off fan of clean room. Especially it brings noise around 20Hz. It is roughly the corner frequency where our spectrum starts to go up(from 20Hz to 0Hz, spectrum goes up). And also some other noise around low frequency region.

2. Maybe common noise rejection is also better. Everytime we make measurement, we make sure it is good.

Thank you a lot for your work. 

I want to know the reason why the homodyne noise is improved from before.

Alignment of homodyne is improved? or something environment is changed?