11/12 Participation: Chien-Ming, Shu-Rong, and Yuhang

Following the SHG telescope set up last Friday, we try to improve its mode matching by adjusting the position of Lens 1 (that is the distance to the input waist which is located at the output end of the EOM). The focal length of Lens1 (L1) =200 mm and L2 = 125 mm (see Fig. 1)

First, we change the position of M1's clamping fork to increase the moving space of L1. However, this cause the alignment of M1 slightly deviated. We spend a lot of time recovering the alignment, but it is worse until we find a 3-Adjuster mirror mount to replace the original M1 mount (2-Adjuster).

Then we install the L1 on the optical rail to optimize the mode matching. Last Friday's L1 position was about 138 mm from the EOM output port, and L2 was about 675 mm from EOM output port. After optimization today, the new position of L1 from EOM is 115 mm and L2 is 674 mm. The SHG scanning spectrum is attached in Figure 3. You can see the mode matching is improved to 95.8%.Although the TEM03 mode shown on the scope is almost disappeared after optimization, and the peak of TEM02 mode also drops compared to last Friday. However, the conversion efficiency of SHG output power is still the same as the result of last Friday.

We also slightly change the temperature of the SHG crystal, but can't find a better result. So we decide to stop here and start to set up another telescope of the light leading to the OPO(bright alignment beam).

We also tested with a LASER current of 1.34A. We can have maximum green power of 237+/- 6 mW.

Problem: Now the green production has a power fluctuation from 137~147mW when SHG's incident IR power is 420mW.

Fig. 1: The modified position of the SHG telescope.

Fig. 2: The pattern of the remained TEM02 mode obtained by the CCD camera, even its peak of transmitted signal showing on the Scope is lower than that of last Friday.

Fig. 3: SHG scanning spectrum

Fig. 4: The simulation of SHG telescope in the update position(now the waist zise of this simulation agrees with Chienming's calculation result)

After checking the alignment of both the probes, I tried to measure a LMA coating (that absorbs a few ppm).
I increased the pump power up to 1W (980mW) rotating the HWP in the IPC (so without changing the laser current the power is immediately stable).

The looking at the scan on the screenshot attached we can see that there is a large constant-phase signal.

After removing the sample it was clear that it is stray light from the pump because the phase is -22deg.

In front of the PD there is a long-pass filter 1250nm that has OD 5.5 @1064, but it is not enough.  The transmission at 1310nm is 85%.
Probably the fastest solution could be to put 2 filters together in the same SM1 attached at the PD, but I'm afraid of internal reflections effects.

I made the profile of the pump, the HeNe and the 1310nm laser.

I used the blade on the translation stage to cut the beam and measured the transmitted power with the power meter connected to the labview software that makes scans. Then I fitted each scan with the erf function and then I fitted the profile for each laser beam.

The blade 0mm position is at 75mm from the optical board, and at 172mm from the mount of the last focusing lens of the pump.

I upload the previous measurements of the profiles (first 2 plots) and the new one (third plot), after replacing the 1310nm laser. After changing the fiber, the waist size didn't change but the waist position moved a few cm.

Today I tried to swap the input of fiber for coherent control PLL, and did the alignment. I achieved coupling ratio of more than 50%. So this means they are not broken.

Participaint: Chienming, Shurong and Yuhang

Situation before changing telescope: there is a higher order Laguerre-Gaussian mode appeared in the spectrum while we were scanning SHG. The conversion efficiency of SHG is 13%.

Motivation: Have more green production for the measurement of non-linear gain measurement of OPO. Decide to improve mode-matching situation. Certainly, more coupling of infrared into SHG will produce more green.

So we removed all the lens set by yuefan and implement the telescope we designed before. I am so sorry that I didn't ask Matteo to buy some new rails for telescope, so we put only one lens on the rail. Now the situation is we put 200mm at hole between (26, 7) and (26,8)(closer to 7) while put 125mm at hole (18,17). The lens on the rail is 125mm one. See attached figure 1 and 2.

Situation after changing telescope: The mode matching now is shown in the attached figure 3. We can see from the plot that mode-matching now is 94%. So there is still possibility to increase mode matching and further increase conversion efficiency. We need to note here that the movement of the second lens mainly change the position of waist while the movement of the first lens change mainly the waist size. And since we know the measurement result of yuefan, the waist size should be 54um. We can improve the mode matching further easily if we have another rail. Then we can easily move two lenses together and achieve a good beam waist size and position together. After did the mode matching we measure the power of infrared going inside SHG, which is 419.5mW(shown in attached figure 4). Then we locked SHG, while the invert is set as 'on'(shown in attached figure 5) and the gain is set as full gain(shown in attached figure 6). Then we measured the green production, now the green power is 101mW(shown in attached figure 7).

Situation after changing temerature: As suggested by Chienming, the mode-matching change will change phase matching situation. We increased temperature and measured the green power generation. The result is shown in attached figure 8. We found the best phase matching temperature is between 3.151 and 3.147kOm, which is smaller than before. This mode-matching difference of 20% bring optimal temperature difference of 0.2K. Now the best temperature is  around 331.4K. Now the green power can reach 147mW(as shown in attached figure 9) Then we lock SHG again, we found now the alignment is quite sensitive. Even we touch a little bit mirror mount, we will degrade the green power by several mW. This means we may need a better mirror mount(so this can be somthing be improved in the future). And now the tranmission voltage is 1.46V(as shown in the attached figure 10). This is a little bit higher than the peak value of scanning as we expected. And also now we change temperature to 3.151kOm(as shown in attached figure 11). This means conversion effciency of 35% now. As pointed out by manufacture, it can reach 45%. So we still can improve it anyway. The good thing is we have enough green as we want.

Problem we found and solved:

1. One of the lenses is with a wrong coating. This can expalin the strange ratio of power we found before. But anyway we removed it.

2. The telescope for matching SHG's transmission into PD. The beam size was very large and collimated with using a lens of 100mm. We replaced it with a 75mm lens as shown in attached figure 12 . But now the pd saturates, we put a ND filter (ND = 1).

Problem found but not solved:

1. stray light: the stray light hit on the mount of mirrors or lenses. Maybe this is something we should consider in the future.

2. alignment after SHG is changed as shown in attached figure 13.

Additional check and work needs to be done:

1. check the beam shape before EOM(for filter cavity)

2. Buy a new rail and improve alignment further more.

3. Replace mirror mount for the two steering mirrors in front of SHG by two very stable mirrors.

4. Align the path after SHG.

I engaged a control loop on the S1FC1310PM laser.

I used the modulation input of the laser controller. I used the SR560 as a servo. I set the low pass filter at 30Hz and gain 2000.

First I measured the noise of a 50 Ohm terminator to check the spectrum analyzer noise floor.
Then I measured the spectra of the 2 PDs (in-loop and out-of-loop), without laser (dark noise) and with the laser on.
The other day there were some structures on the out-of-loop PD that we didn't understand at the time, then I found that the beam was not well centered on the PD, so after I centered (maximizing the DC) the structures disappeared.
Then I closed the loop and measured the spectra again. The signal at 380Hz in the out of loop PD reduces by 10dB (about a factor of 3).

I confirmed the noise reduction by checking the lockin output with and without control loop. The 2 plots have the same axis scale, so the reduction is more clear.
The noise now is 1.3 ppm*W

I measured the actuator TF (plant) and fitted it with a zpk model: 2 single poles at 7kHz and 30kHz.
Then I modeled a servo TF and plotted the measured open loop TF. There is a factor of 2 of discrepancy with the model because the oscilloscope was connected to the modulation input, since they have the same input inpedance, the measured TF dropped by a factor of 2. But when I closed the loop the oscilloscope was not connected, so the actual OLTF is the dashed blue line on the plot.

A reference procedure for the filter cavity alignment:

power ratio of newly replaced BS, R:T = 39.98:10.7 = 78.89:21.11. (s-pol)

I used the calculation result from Chienming and shurong. The beam waist size is 49.3um located inside SHG mirror 2.8cm. Here I attached some possible solutions for SHG redesign.

And today I talked with yuefan, we conclude that the higher order Largerre Gaussian mode should come from the movement of main laser box. 

As reported entry 769 while working on the green MZ and MC it was found out that the not quite gaussian shape of the beam might come from the SHG.

It might be useful to check if the new telescope can correct this.

We decided to change the telescope for SHG. For three reasons:

1. the mode matching for SHG needs to be improved.

2. the beam is too small around one of the mirrors, which may bring probability of damaging mirror

3. the telescope changing will influence the bright alignment beam. So it's better to change it before having telescope for bright alignment beam

I did the measurement in the region shown in the attached figure(in the black block). The fit result is shown in attached figure. The waist is located around the end of EOM for SHG. With a size of 120um.

So if we want to increase power, we also need to investigate the damage threshold of this EOM for SHG. The 50mm lens is located in 30cm after the zero of this plot.

Since this characterization is before 50mm lens, I use Jammt to put this 50mm lens. And compared with the result we got yesterday, they agree with each other. The result is shown in attached figure 3.

Since we received the mirror from Thorlabs BST11. We replaced this mirror with the mirror we replaced the day before.

1. We recovered the alignment of SHG. We increased the laser current and recovered the green production. Now the power after EOM is 53mW. This recover is also done by a better alignment. We found a mode hop between 1.2A and 1.34A. We also recovered the lock of filter cavity.

2. We did the characterization of transmission beam of BST11. The measurement of beam size is quite collamited with a beam dimension of around 1950um in diameter(we put a lens of 50mm). I did the simulation, the initial beam should be 17.5um in radius. (See attached picture). 

We also found astigmatism and we solved this by rotating lens. However, now the tilt of 50mm lens is very large, like 30 degrees.

According to Matteo, we need 50mW of green to be injected into OPO. For example, Marco Vardaro used 57mW for this gain measurement and Chua used 84mW. If we want 50mW, we need to know how much Laser current we should give. This can be done according to many characterization work we did before. And we also cared about the after during the whole path, which may concern about the damage threshold.

1. From OPO back to GRMC. We assume we loss 10% while propagation since we have one dichroic(transmit more than 90%) and three green mirror(NB07-K12 has R = 99.5% for S-Polarization). So the transmission of GRMC should be 55.6mW.

2. GRMC(according to entry, we know T = 65% for s-pol and T = 79% for p-pol). If we use s-pol we need 86mW(71mW for p-pol) before GRMC. Let's assume we use s-pol for the derivation after.

3. MZ(according to entry, we should lock MZ around 70% transmission level). Then before MZ, we should have 123mW.

4. MZ to EOM(just before BS). Since we will have a 90:10 BS during this path and we take 90% for squeezing, we should have 137mW in front of EOM.

5. Green production. By using the relationship we got from tomura-san measurement, we should have 830mW of infrared just in front of SHG.

6. Infrared power and laser current. By considering the relationship we got from entry, and the BS(70:30) we just put for getting bright alignment beam. So we need 1.85A of current. This corresponds to 1.37W of laser power infront of the BS we just replaced. We are considering this power may damage this BS.

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.