According to entry of telescope change of p-pol EOM, the smallest beam diameter is 0.6mm. And the datasheet tells us the power density threshold is 1000W/cm2.  Then I calculated the maximum power I can send is 1.4W. Then I found the AUX2 laser clamp current is 1.554A, which corresponds to 516mW of infrared beam. This is well lower than the damage laser power. However, the change of the AUX2 laser power also affect the laser power sent to fiber detector(DET01CFC/M). So we should measure the power transmitted by fiber. It should be less than 5.5mW(if the wavelength is 1550nm). The damage power of it is 70mW.

The Green measurement before GRMC is shown in attached figure 1. It is 424mV. The transmission while scanning is shown in attached figure 2. The dark noise is 133mV and TEM00 is 302mV. Actually, I checked the manual of thorlab biased PD, the dark noise comes from the dark current. So in principle, this dark current should give only an offset. If this is correct, we can just remove from all the detection value this offset. That means the green we detected before GRMC should be 293(424-133)mV. And the TEM00 transmitted by GRMC should be 169(302-133)mV. So the ratio is 169/293 = 58%. This result agrees with the measurement we did before of GRMC.

Besides, if the dark current just give an offset. The higher order mode we saw on the camera maybe really low. And this means the mode matching now should be fine enough. Anyway, we checked in the case we set 70dB gain of PDA10CS and highest resolution of oscilloscope. And then look at only higher order modes part of GRMC scanning spectrum. This is shown in attached figure 3. We can see the higher order mode becomes a little bit clearer. We can use this to improve matching better. Also a video is attached here. https://drive.google.com/open?id=1sDFy5q8tSbb1VEuDanusD3TIvpaR2KVj

Finally, I found the unknown peak appeared in last entry is because of yaw misalignment.

Participiant: Enomoto and Yuhang

We want to know how much green we can produce according to the current and also the laser power. We did the measurement. Since we care about the power we send to SHG, we need to detectec the power just in front of SHG. However, the power meter is small enough to put in correct place but it can only detect up to 500mW. And the power meter(S145C) can detect up to 3W is too large in volume. So we measure power in point one with S145C and point two with the same current. Then the relationship is shown in the attached figure.

The conclusion is laser power is proportional to laser current. And the power ratio between point one and two is 0.75.

The relationship between laser current and power infront of SHG is P = 980*I-775

I closed the control loop, I tried many configurations. It doesn't work.
As usual, it reduces the noise only in the in-loop PD. In the out-of-loop PD it barely reduces it by 5dB.

I have wasted more than enough time with this laser. I will buy a new one.

Participaint: Enomoto, Eleonora and Yuhang

According to the design of telescope, we installed it to match the beam between OPO and GRMC. The method is to use the green light generated by OPO and send it through telescope to GRMC. Then make this green light resonant inside GRMC. In this case, the transmission of GRMC will also resonant inside OPO.

The procedure is like this:

1. Install the telescope in the designed position.

2. Make both OPO and SHG produce green. And then make the transmission of GRMC overlap with the green produced by OPO.

3. Swith off SHG. Put a camera in the transmission of GRMC to monitor the transmission signal. See attached photo 1. At that time, the mode matching is very bad. See here, the scan of GRMC shows also the mode-mismatch situation.https://drive.google.com/open?id=1QDXRWudVucsCfB4_BUoiSu5zzuikkT8l

4. Do rough mode matching improvement by looking at the camera, and then make the pattern on the camera has less Laguerre-Gaussian mode. After this, we have on camera this. https://drive.google.com/open?id=1C7Pt_8Wltid6sxGtgC08nJ0exEal21ln   Now we can see the beam by eye, then center it on PD. We can see the GRMC transmission while scanning the GRMC. At the same time, we tried to close the light. By comparing the signal on oscilloscope of attached figure 2 and 3, we found the ambient light just increase the offset but not cover the real signal.

5. Now we can improve mode matching and alignment again. We misalign pitch and yaw to identify the higher order modes(See attached figure 4 and 5). As you can see in the attached figure, the peak we have in the best case doesn't change no matter how we misalign the beam. So we decide to remove that peak by improving the mode matching. However, we also found the higher order modes increase only a little bit although TEM00 is reduced. This is just evaluated by summing up the height of the higher order modes and reduced TEM00, then compare this sum with the previous TEM00 height. We found the case of misalignment is smaller than the good case. This maybe mean that we loss some energy to soemwhere else. And we cannot see most of other higher order modes. We guess they are covered by PD noise.

6. But anyway, the information we have now on the GRMC scan can only help us to remove the mode matching peak. We did that and the result is shown in attached figure 6. The video is here.https://drive.google.com/open?id=17Uko8FaUJYfKaxx3gB6HdcTGxy4EefRI If you look at this video, you will see there are still  a lot of higher modes. However, these higher order modes don't appear on the scan signal of GRMC. This also means the PD's noise cover these modes？

I repeated the measurement reported in entry 1055. I did the whole turn of the rotational mount.

The noise doesn't show a clear dependence on the  polarization angle.

I also plot the DC, and we can see a periodical (4 periods in 360deg) fluctuation of the laser intensity of 3% peak to peak. This shows that even if the optics are nominally non-polarizing, they still have some small polarizing effects.

I took 5min of noise as in entry 1054. Pump OFF, Probe ON, Chopper ON.
I repeated it for several angles of the HWP.
I plot the noise mean as a function of the HWP angle.

I made a noise check of the 2 probes, with the pump OFF.

the attached image shows 6 plots in 2 rows.
For each plot, I acquired 5 minutes of the AC signal from the lockin at 100ms of sampling period (3000points).
The first row is the 633nm probe, the second row is the 1310nm probe.
The first column is with the probe OFF, the second and the third columns are with the probe ON.
The first and second columns are with the chopper ON, the third column is with the chopper OFF.

The circle is centered in the mean value of the 3000 points, and the radius is the standard deviation.
The blue line connects the center of the circle with the zero of the axis.

I converted the size of the circle in ppm*W (ppm valid for 1W of pump power) using the calibration I measured for both the probes.
R=19.4 1/W for the 633nm probe (from entry 1033)
R=5.25 1/W for the 1310nm probe (from entry 1045)

The noise with the 633nm probe looks to quite small, 0.3ppm*W, but the specs of the original setup said better than 0.25ppm*W.
The noise with the 1310nm probe is far too high, 13ppm*W not enough low to measure the crystalline coatings (<1ppm).

The enclosures of the setup are all open, so the noise could reduce a bit after I cover everything from the wind.

The chopper doesn't seem to contribute much to the noise.

All these measurements where don with the pump OFF, so the stray light from the pump may increase a bit the noise.

I don't know the polarization orientation after the HWP, so I'm going to check the noise for different rotation angles of the HWP.

[Enomoto, Yuhang, Matteo, Eleonora]

In the past days we observed an oscillation in the SHG lock at about 20 kHz.  This is likely to be due to the change of the loop shape as we removed both a low pass fiter and a high pass filter in the control servo.

Before finding out that the LP filter in the High Voltage Piezo Driver could be disabled (see entry #1016), Matteo had designed a HP filter with the goal of componsating its effect and allow for a larger loop bandwidth.

The TFs of this two filters and their combination are shown in the attached picture (taken from entry #585).

As suggested also by Raffaele, the LP and the HP where non exactly compensating above 10 kHz and this could avoid the oscillation at 20 kHz (likely due to a Piezo resonance) that we see when we remove both of them. 

As a temporary solution we decided to keep both the filters on. We will study a better filter to be implement in a new version of the control board.

[Enomoto, Yuhang, Eleonora]

Yesteday, just after openening the gate valve between the pipe and the end chamber we were able to lock the filter cavity again (after more than 2 months!) 

The alignment was recovered in the past days and we were able to see some dim flashes even with the closed gate valve.

The lock acqusition was smooth with the 1/f filter shape but we got an oscillation at 155 kHz when swhiching to the 1/f^4.

The gain setting was:  PIEZO GAIN = 5.   INPUT ATTENUATION = 9.5 (almost maximum)

The oscillation is likely to be caused by a piezo resonance. (See main laser piezo resonance characterization in entry #859). We couldn't get rid of the oscillation by changing the piezo gain but we noticed that for some gain values, the oscillation is at lower frequancy, about 75 kHz (probably another piezo resonance.)

In the end we found a good setting of the gains (we had to change the input amplification also): PIEZO GAIN = 4.   INPUT ATTENUATION = 1.6 (much lower than before)

In this configuration the UGF is about 16 kHz with phase margin 62 deg (see pic 1). By eye, the spectrum of the error signal seems even lower than before but we need to check again the calibration to confirm this.

Note that:

- the locking photodiode is now the qubig one without DC (as the one with DC is used for OPO) but the gain of the RF channel shoud be the same.

- the change of the PIEZO GAIN affects the piezo dynamics, as reported by Pierre in entry #747. 

Participaint: Enomoto, Eleonora and Yuhang

Today we did the filter cavity green reflection characterization again after achieved its lock. Here I want to put some information we found for the green reflection from filter cavity.

Firstly let's review the set up. The configuration is we put a BS for filter cavity green reflection extracted from Faraday isolator. Small part of green goes to FC locking. Another part is used for our characterization and it is sent to a good height by using a periscope. Then let's look at some information:

1. The reflection seems to be cutted by something if you look at our green at a decent distance. See attached figure 1. It is taken several months ago by me and Marc. As you can see, there is a very clear boundary around the green light. Although the brightest part is smaller than this boundary, it is essential to know where it is cutted. And we confirmed that it is cutted by one side of Faraday isolator whose cover is not dismounted.(See attached figure 2)

2. The reflected beam shape is quite bad. I think you have already noticed that in the attached figure 1, the beam seems to be flatted by astigmatism. This effect becomes quite obvious if you look at the beam detected by the beam profiler. See attached figure 3, the beam shape is really horizental oriented ellipse. However, the axises of our beam profiler detection is accidently aligned to two direction that have the same dimension. That means we cannot have a numerical estimation of this astigmatism by chance. But this brings also an advantage, it is roughly the average of the long axis and short axis of this ellipse. So it is reasonable to continue the measurement even in this case.

3. We did the measurement and fit of this beam directly although it is quite collimated. Besides, this beam is quite unstable. So you can see the points we took are quite scattered. Then we did the fit and the result is shown in attached figure 4. As you can see, the beam waist size is quite different in these two cases(900 and 500 um with an error of roughly 10 percent). Also the waist position has a quite large difference(-300 and -260 cm with an error of roughly 10 percent). 

4. The last method we tried is to put a lens and do beam characterization after lens. By this characterization result, we propagate back to the beam before lens by using JaMmt and ABCD matix. However, this time we set up the average inside beam profiler as 20 while last time it is 5. Now the number we can read becomes more stable. Then we took this more stable data and did the fit. The result is shown in attached figure 5. 

The lens we used is 150mm, and the measured result is quite reasonble now. As you can see in figure 5, z0 is both roughly 150mm. Then we used JaMmt propagate the beam back, the result is shown in attached figure 6. From this result, we can see the beam waist size should be 957um, while its position is after the lens about 2m. This means the waist is located not inside the beam going to filter cavity. Besides, there maybe a measurement shift of several millimeters of waist position of the beam after lens. And this can influence quite a lot the waist position. Also the focal lens of our 150mm lens can also be smaller or larger than this nominal 150mm value. Then influence this waist position quite a lot. But anyway, the waist position is not so important for a collamited beam. So it should be fine.

We also verify this result by using ABCD matrix. The method I used is taking the q factor of gaussian beam. Then the free space propagation is just an addation of this distance to this q. The lens is just a modification of the invert of q by 1/f. Since we use a converging lens, the f is positive. We used the detection beam wait size and waist position to reconstruct the original waist size and its position. Then make real and imaginary part equal with each other to have two equations and solve two unkown valuables. The result is shown in attached figure 7. The averange of these two result is 1.2m before the lens and wasit size is 1008um. It complies with the result of JaMmt, and this is reasonable because they are using the same principle. (Actually I want to propagate the error of fit result, but the python code of this error propagation cannot deal with imaginary number and solve equation, so I give up the error estimate in the end.)

I used OD filters to reduce the probe power.
I avoided using polarizing optics to limit the polarization fluctuations effects on the intensity.
I made the beam pass through the pinhole in 2 positions.

I used the surface reference to maximize the signal and made a scan. See screenshot 1.

The beam was not exactly passing through the center of the lens, so I think it made it astigmatic on the PD. 

I started over the alignment of the imaging unit. I moved the sphere with respect to the lens to find the sharp image of the blade.

In order to not saturate the PD, I changed the laser current from 300mA to  200mA, and the OD 2 to OD3. 

I found a larger signal and I maximized it moving the whole imaging unit, I made a scan for 3 positions of the imaging unit 20,25,30mm, and the maximum is at 30mm. See last 3 screenshots.

Participiant: Enomoto, Yuhang

Since the mirror to do the coherent control is necessary for the measurement of power threshold of OPO gain amplification. We did the mirror replacement the day before yesterday. Also recovered the alignment of green mode cleaner.

Since I found we used to use high voltage deriver not in a proper way. The reason is demonstrated in the e-log entry before. Since this information is crucial for the filter design, I measured these transfer functions for which the high voltage drivers we didn't use properly. The result is uploaded to our wiki page with the name as listed below. However, I don't know if this coherence level is enough for our user.

For the data, please go to the following page to download. https://gwpo.nao.ac.jp/wiki/FilterCavity/Transfer%20Function

Participiant: Enomoto, Eleonora and Yuhang

The week before last week we found the problem of high voltage driver, and this is also the reason why we found there is a low pass filter inside our high voltage driver. So we decided to use it in a proper way, which means switch off the sensor-in switch. However, we found the lock of SHG becomes unstable after did that. So we did the characterization of open loop transfer function again.

We found the unity gain frequency of previous set-up is 795Hz while the phase margin is 89 degree which is fine.

However, the unity gain frequency of revised set-up is 376Hz while the phase margin is 66 degree which is worse. Besides, there is also a peak going above 20k Hz and brings oscillation.

Since Pier is developing servo in APC and we also want to use high voltage driver in a proper way, so we need to measure a list of open-loop-transfer-function again with the correct set-up of high voltage driver.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Participaint: Eleonora and Yuhang

Besides, the remote control of 290m targrt is recovered based on yuefan's elog. However, its control requires the change of IP address and netmask. For the convience of remote control, we kept that computer inside our clean booth and we used another ether cable to achieve remote control.

[Takahashi, Matteo, Eleonora, Yuhang, Enomoto]

With the help of Takahashi-san, we have opened the vacuum chamber in the end room (only the top part).

[Note that even if the venting was started last Friday the vacuum level was still not low enough so we had to inject some air before opening the chamber.]

The goal was to recover the alignment of the end mirror and solve the issue with the yaw picomotor reported in the entry #954.

By visual ispection, we could confirmed that the picomotor was stuck, as it reached the end of its range, and the intermediate mass was touching the damping magnets.

We have manually put back the yaw picomotor at half range and moved the suspension with the help of a traslation stage (pic1) and also by hand, to make the reflected beam superpose on the incoiming one. The superposition was checked by letting the incoming beam pass through the hole of the second target and check the reflection from the end mirror on the rear side of the second target.

The chamber has been closed and the punping down has been restarted. The gate valve between the chamber and the pipe is still close. According to Takahashi-san, the vacuum level to be reached before opening it is 2e-7 Torr.

Even if the alignment has been recovered we were not able to see any flashes, probably beacuse the beam distortion induced by the gate valve window is too large.

Some comments:

1) The good direction for recovering the alignment was not the one in which the picomotors reach the end of the range, meaning that the suspension position and the overall alignment hadn't moved too much. In any case we prefered to move the picomotor to exactly mid range (and have an optimally centerd intermediate mass), and then move the whole suspension to recover the alignement

2) We confirm that the yaw picomotor is quite critical as by moving it it's easy to make the intermidiate mass touch the magnets. For this reason also in KAGRA during the in-air alignment procedure, the whole suspension is usually moved with the help of some pusher (see Fig. 2)

3) We checked that the beam was well centered on the end mirror and slightly adjusted the camera in the end bench in order to have the beam also centered on it.

4) Some pictures of the current position of picomotors and intemidiate mass are shown in Figs 3, 4, 5.

[Eleonora, Matteo, Yuhang]

On Mon 22 we found out that the supervisor PC used for Labview control system was not able to connect to the CPU (NM2 for TM: 133.40.121.78), which manages the ADC and DAC for the telescope suspended mirrors (PR and BS).

We checked this CPU (which is in the central area) and find out that it was not possible to swich it on anymore. Futher investigations suggested that the problem was in  the power supply unit.

I noticed that slightly touching the mount of the first mirror on the HeNe probe path makes the beam deflect a lot. So I changed it with a more rigid mirror mount.
I also wanted to clean the mirror (Thorlabs 1/2" Protected silver mirror PF05-03-P01), but I broke it, so I replaced it with a Newport 5103 General Purpose Silver Coated Mirror that I found in Tama.

Then I aligned the HeNe probe using the pinhole. The beam passes through the following positions of the pinhole:

I noticed that the reflection of the first lens on the IU was going up to the chopper. This is very likely to generate noise in phase, so I tilted the lens of the IU.
I also noticed another reflection from the sample that is going to the chopper. I can't tilt the sample, so I solved it putting a simple aperture near the chopper. See picture.

I used the surface reference to align the pump and maximize the AC signal.

I used the new power meter for the pump:
pump power: 74mW , chopper on: 37mW, with surf ref sample: 27mW

I moved the imaging unit to find the maximum of the signal on the surf ref.

The maximum is not in the range of the IU stage. Probably because tilting the lens to move the probe reflection from the chopper, I missed  the focus of the IU telescope.

Therefore I started again the IU alignment. I put the IU stage at 60mm. I found the sharp image of the blade. I looked for the maximum of absorption signal again:

So I put the IU at 70mm.

I made a scan of the surf reference.

I noticed that the DC is not linear with the power of the probe. I measured 6.4V for 1.71mW, and 6.5V for 2.58mW. Then I replaced the LDS9 power supply with the 12V battery and the 2.58mW gave 9V of DC. So I decided to keep the DC below 6V with a OD wheel, and continue using  the LDS9 power supply.

After putting the wheel, I missed the alignment pump/probe, so I used again the  pinhole to center the probe.

Then I made a scan of the surf reference again.

And a scan of the bulk reference. The transmitted power is 20mW (of 37mW incident)

[Yuhang, Eleonora, Matteo]

In order to match the green beam transmitted from the green MC into the OPO we have characterized the green beam produced by OPO when it is locked with s-pol beam. Its paramenters will be the target ones for the mode matching telescope we have to design.

Since the beam is very weak (about 0.2 uW) we couldn't use the beam profile and we used a blade to do the characterization, instead.

Procedure: we attached a cutter blade to a traslation stage (See Fig.1) in order to be able to progressively cut the beam, by small and known steps.  For each step we recorded the power of the beam with a PD placed after the blade. Since the beam in gaussian the shape of this function is expected to be a error function (erf):

P (x) =  a*erf(sqrt(2)*(x-x0)/w)+o 

By fitting the data with this function we could extract the beam radius w for each position z at which the measurement was performed.

The beam size and the corresponding waist (in meter) are reported below and plotted in Fig 2.  The zero of the z-axis is taken at the incoupling mirror side of the OPO.

According to the measurement result of entry about green mode cleaner output beam characterization and OPO green output beam characterization, we can design the telescope to match them. From the above entries, we know the source waist is 287um while the target waist is 24um. Based on the revised version of optical layout which I just modified today(the position of green mode cleaner and distance between bench and chamber are modified)(https://gwpo.nao.ac.jp/wiki/FilterCavity/OpticalLayout). I also attached the part of bench we care, see attached figure 1. In this picture we know the distance between waists should be 82.5cm. Besides, we should avoid putting lens around 10cm, 27.5cm, 55cm and 75cm. And putting lens between 5cm and 72cm. 

By using all the information listed above, I used JaMmt did the simulation. See attached figure 2, which shows the initial condition I gave. Besides, I upload the green lens we have in our clean booth on our wiki page(https://gwpo.nao.ac.jp/wiki/FilterCavity/Optics). See attached figure 3, it shows the lenses I used. Actually I didn't put lens with very small focal lens, although we have lots of small focal length lenses. The fitting shoes many good results. I attached three here. Some of them even tell me the mode matching is 100%.

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Since we found the mirror to reflect green after dichroic (outside OPO's incoupling mirror) should be farther than last version of optical layout. I changed it and did the simulation again. The result is attached in 7th and 8th figure.