The laser demo was delivered yesterday.

I replaced the fiber without moving the collimator.

I realigned the beam moving the last lens before the sample, and I maximized the signal on the surface reference.

In order to have a DC of 2V on the imaging unit PD, I set the laser power to 428uW (the laser display shows 0.7mW).

Then I noticed that I had a better signal moving the PD further on the IU, and the DC dropped to 1.3V.

Then I increased the power to 980uW (the laser display shows 1.55mW) to have 3V of DC on the PD.

I attach the last calibration scan.

The pump power without the sample measured with the new power meter is 33mW 

I measured the noise on the reference sample with and without chopper and pump

When everything is off, the noise is 2uV (0.14ppm*W), which is the dark noise of the PD plus the environmental light.

The noise of the new laser is 3ppm*W, without any filter, nor control loop. Much better that the previous laser, but still above the expected signal level.
It could be worth to try a control loop and see if the noise reduces more.

Participaint: Shurong, Jianming, Yuhang and Eleonora

Today we replaced one mirror shown in the attached figure 1. The replaced one is pointed out by a black circle. This work broke the alignment of SHG, but we recovered it in the end.

Here I attached the simulation result of optocad, which tells us the beam going to this mirror with a beam waist of only 10um. As shown in the attached figure 2.

Future work:

We will increase infrared power to have at least the same green power as before.

Characterize the beam transmitted by this mirror. 

In order to assess the impact of losses on the low transmissivity of the green mode cleaner (see entries #850, #892), we asked Laurent Pinard (at LMA) to characterize one mirror from the same batch of mirrors used in input/output.

Here the results he found at 44° at 1064 nm (S-pol):

- absorption: 0.7 ppm

- average scattering: 15-20 ppm

We tried to recovered the fiber for PLL today. The status for each fiber port is like this(now main laser has power of around 700mW)

BS fiber one:

1. Main laser pick off for AUX1 PLL laser power (before fiber) is 3.8mW. This mean roughly the power ration is 0.55%. Then we tried to couple light into fiber, we achieved the couple around 53%.

2. AUX1 pick off laser power (before fiber) is 8.11mW. This means the power ratio is 1.6%. The couple we recovered is 57%.

BS fiber two:

1. Main laser pick off for AUX2 PLL laser power (before fiber) is 3.25mW. The couple for this fiber we achieved only 25%.

2. AUX2 pick off laser power (before fiber) is 1.7mW. However, this couple no matter how we tried we achieved almost nothing.

So we guess this second fiber may be broken. There is still possibility that we need more alignment work.

Today, we wanted to charatrize the green produced by SHG. But we found the power changes very fast. Our metphor laser becomes unstable after turn-on and change-current. But after roughly 1hour this fluctuation will becoms stable. We characterize this fluctuation by the coefficient of variation.

The power fluctuation after turn-on is 0.01 with a period of around 15min. This range of power change is comparable with the current change of 0.03A. This means the power change of 0.03A corresponds to roughly 25mW of infrared produced by main laser.

The power fluctuation after 3 hours is 0.001 and doesn't have a clear frequency.

The power fluctuation after change current is 0.0045 with a very long period. For this period, I didn't have enough time to monitor it.

So for a better measurement of green laser power produced by SHG, we should operate main laser after a long enough time to stabilize. Then measure infrared just the moment after the measurement of green.

[Eleonora, Matteo, Enomoto, Yuhang, Yuefan (remotely)]

A fiber system is in place in TAMA to send and receive signals between the central area and end rooms. Since we considered the possibility to use such a system (to avoid a timing system in the digital control system), we report some details about its organization and performances.

"STANDARD" BOARDS

In the central area and in the south end room there are respectively 2 boards, named both A and B. The two A boards (that in the central building and that in the end)  are connected between each others with 2 fibers each (one to send signal in one direction, the other in the other direction).  The same happens for the two B boards.

Each fiber can transmit 4 channels, so that each board has 4 input channel and 4 output channel.

VIDEO BOARDS

There are also 2 video board in the central and in the end room, named respectively video board 1 and video board 2. The two video board 1  (that iin the central building  and that in the end)  are connected between each others with 2 fibers each.  The same happens for the two video board 2.

In the case of the video boards each fiber corresponds to one channel and they are only used to send signals from the end room to the central building.

DELAY

In order to estimate the delay of the trasmission we sent a signal through the fiber to the end room and we sent it back to the central building and measure the TF beetween the two. See attached pic 1.

The phase delay at 500 Hz is 49.15 deg, corresponding to a delay of 0.27 ms (round trip)

The delay due to the finite speed of light (which is 2us)  is a negligible contribution.

SIGNAL QUANTIZATION

By looking at the signal after a round trip it is clear that there is a quantization effect from the board ADC. See attached pic 2, 3.  The sampling frequency is 12 kHz.

Recap of the fibers disposition:

board A (central)                    board A (end)

TX:   1-9                                 TX:   1-10

RX:  1-10                                RX:  1-9

board B  (central)                   board B (end)

TX:   1-11                                 TX: 1-12

RX:  1-12                                 RX: 1-11

Video board 1 (central)          Video board 1 (end )

CH1:  1-15                               CH1: 1-15

CH2:  1-16                               CH1: 1-16

Video board 2 (central)           Video board 2 (end )

CH1: 1-13                                 CH1: 1-13

CH2: 1-14                                 CH1: 1-14

Current channel  use:

video board 1                           video board 2                board  B

CH1 : GREEN  CAMERA          CH1: IR CAMERA          CH2 (from end to central) : FC_IR_TRA_DC

CH2:  SECOND TARGET    

Other infomation:

1) The fibers numbered from 1-9 to 1-16 are arriving from the south end,  close to the east input vacuum chamber (see pic 4, 5). From there, some extensions are used to connect them to the boards. In the past ( see entry #444 and #518) some of these cables have been exchanged because they were too short. I took the time to redo all the labels of the extension cables to make them match the fiber number.

2) The fibers for the west end  are numerd as 3-XX.  Fibers numered 2-XX and 4-XX comes from the 150 m station respectively of the south and west arm. 

3) The fiber from 1-1 to 1-4 arrive in the up-right corner of the storage room.

4) There is another fiber which has a dedicated reciver and sender box, it is currently used to send the signal FC_GREEN_TRA_DC from the end room. See entry #524. Its performances should be better than the other fibers.

I attach also a pictures of the boards. (pic 6)

Here I attached some photos maybe yuefan can check to arrange the space for auto-alignment system. They are taken from different directions.

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.