11/19 Participants: Chien-Ming, Shu-Rong, and Yuhang

Today, we finished the telescope of the bright alignment beam, the focal length of L3 is 100 mm and L4 is 150 mm (see Fig.1).
Comparing to the results on 11/14, the beam shape now becomes elliptical after using the Faraday Isolator 2 as shown in Fig. 2 (remove the M4 mirror in this case).
Fig 3. shows the beam shape of the Aux laser 1 coherent control beam (CC beam for short)

Fig 4. shows the transmission spectrum of the OPO cavity by injecting the CC beam. Here we use the Thorlabs InGaAs photodetector to measure the spectrum.

We use the M4 mirror placed on the translation stage to swap the CC beam or the bright alignment beam sending into the OPO.
Fig 5. shows the transmission spectrum of the OPO by injecting the bright alignment beam at the same optical power (60.3 mW) as the CC beam using in Fig. 4
The peak height of TEM00 in Fig.5 bright alignment beam is 8.1 V higher than in Fig.4 CC beam (7.6 V). However, when scaling up the vertical axis, we can see the peaks of other high order modes in the spectrum of bright alignment beam (see Fig. 6) is a little bit worse than in CC beam (see Fig. 7).

Fig. 1 The schematic of the telescope for the bright alignment beam.
Fig. 2 Beam measurement of bright alignment beam on 11/15.
Fig. 3 Beam measurement of CC beam on 11/14.
Fig. 4 The transmission spectrum of injecting the CC beam into OPO.
Fig. 5 The transmission spectrum of injecting the bright alignment beam into OPO.
Fig. 6 Scaling up the transmission spectrum of the bright alignment beam to see the high order modes.
Fig. 7 Scaling up the transmission spectrum of the CC beam to see the high order modes.

Participaint: Yuhang and Matteo

Last Friday, we reveived the compnent of alignment infrared mode cleaner. With the help of Yano-san, we cleaned it in ATC by using aceton sonic bath. Then we did the assemble in our clean booth but we found we lost the orings. We will continue this work after we get some orings.

11/15 Participants Chien-Ming, Shu-Rong, and Yuhang

Today we insert the second Faraday Isolator IO-2-YAG (FI 2) into the main laser light path and modify the telescope for SHG.(see Fig. 1)
The transmission of this small aperture (<2mm) isolator is around 85% even if the beam radius inside the isolator is less than 500 um.

We achieve a good mode matching (see Fig. 2) for SHG by using the lens L1 f = 75 mm and lens L2 f= 100 mm.
The output power of 532nm is 148 mW when injecting 426mW 1064 nm light into SHG. This efficiency is consistent with the previous results on 11/9 and 11/12.

However, we did find that the laser power stability of both SHG and Main laser have improved a lot after adding the isolator FI 2.
The SHG 532 nm output power have the ratio of Max/Min is 1.007:1 (see Fig. 3) which is much better than the result 1.088:1 measured yesterday.
The ratios Max/Min of the main laser are both improved as 1.010:1 and 1.012:1 when operating in a higher power and low power. (see Fig. 4 and 5). The power meter is placed behind the BS1.

We believe that such stability should be enough even though it is still somewhat different from the situation of blocking the reflected light from SHG with the ratio of Max/Min is 1.002 :1.(see Fig. 6)

Since the output polarization of the FI 2 is 45 degrees, we use a half waveplate (HWP1) to ensure the s-polarization for BS2 with the split ratio of T/R is 20:80. Otherwise, the BS2 split ratio of T/R is 38:62 when using the p-pol light. The second half waveplate (HWP2) rotates the polarization to p-pol. for serving the SHG.

Fig. 1 A schematic of adding the second isolator FI 2 and modify the telescope for SHG
Fig. 2 The mode matching result of SHG
Fig. 3 SHG output power stability
Fig. 4 Main laser power stability at higher power scale. 
Fig. 5 Main laser power stability at lower power. 
Fig. 6 Main laser power stability when blocking the reflected beam from SHG

Today we tried to test the boards and galvo. 

Since at this moment we don't have a quadrant yet, we tried to send signals through the check point and to see if the galvo give any response. 

We got two galvo and two boards, their situations are below:

Board 1: It is able to give both x and y direction correction signal to the galvo, but the monitor ports for the currents send to galvo both have high frequency oscillation, especially in x direction, the amplitude of this oscilation is very large.

Board 2: Only the y output can drive the coil, for the x direction when we switched on the board, we could see the mirror on the galvo has a sudden move and then goes back slowly to the original position. The monitor signals for both x and y ports are at their maxmum, so they don't change when we send the signals through check point. 

We are going to tune the offset of the board to see if the situation can be changed.

Galvo 1: Working fine. We could hear the vibration sounds from both the motors.

Galvo 2: One of the mirrors is working fine, the other one is a bit loose from the motor, I think after we fasten it, it could be fine.

11/14 Participants: Chien-Ming, Shu-Rong, and Yuhang This morning we set up a telescope for the bright alignment beam. (see Fig. 1)
The focal length of M3 is 150mm. It is located at 65 mm from BS2.
The focal length of M4 is 200mm. It is located at 337.5 mm from M3.

By using the Beam Profiler (Placed on the edge of the optical table), we measure the beam size of the bright alignment beam. (See Fig. 2, a clearer picture will be added soon)
The beam shape is close to circular symmetry and beam size is 2539 um on W-plane and 2342 um on V-plane.
We also check the beam size of Aux laser 1 (CC) by inserting the Mirror M4 placed on the translation stage. (see Fig. 3)
Its shape is ellipse and beam size is 2542 um on W-plane and 2233 um on V-plane.
The size of these two beams is similar.

In order to check the isolation of the Faraday Isolator (FI: IO-5-1064-HP), We introduce the light from Aux laser 1 to FI. (See Fig. 4)
By using the power meter, we measure and optimize the isolation of FI is 39.4dB which is confirmed to the spec. (38 ~ 44dB) on Thorlabs website.

After optimizing the FI, we block the Aux 1 beam and reinstate the main laser beam to the SHG. (see Fig. 5)
By placing the power meter behind the BS1, we can measure the power fluctuation of the main laser beam.
First, we tune the PZT of the SHG to non-resonance state and obtain the ratio Max/Min is 1.054:1 (see Fig. 6)
which is larger than the ratio of 1.020:1 when SHG is on-resonance (see Fig. 7).
According to this result, we suspect that the power instability is mostly caused by the 1064 nm feedback light from the SHG cavity.

We also measure the ratio of 1.049:1 when increasing the input IR power by 2.8 times ( about 200mW) to SHG. (see Fig. 8).
The generated power of 532 nm is 28.4 mW and the ratio Max/Min is 1.088:1 which is the most serious. (see Fig. 9)

Since we have roughly determined that the interference is coming from the 1064 feedback light, we plan to insert another isolator to the main laser light path.
Fortunately, we found another isolator (IO-2-YAG made by OFR) that is not in use. However, the aperture of this isolator is less than or equal to 2 mm while it can work in high power 750 W/cm².
This means that it can bear the case of the input power is 1 w at 1064 nm with the waist of 250 um inside the isolator where the laser fluence is 509.3 W/cm².

Fig. 1 A schematic of the telescope for the bright alignment beam.
Fig. 2 Beam measurement of bright alignment beam.
Fig. 3 Beam measurement of Aux laser 1 beam.
Fig. 4 A schematic of measuring the isolation of FI
Fig. 5 A schematic of measuring the laser power fluctuation.
Fig. 6 Laser Power fluctuation: after optimizing the FI, the SHG cavity is off-resonance
Fig. 7 Laser Power fluctuation: the SHG cavity is on-resonance
Fig. 8 Laser Power fluctuation: the SHG cavity is on-resonance and increasing 1064 nm laser power
Fig. 9 Laser power fluctuation of the SHG 532 nm output

Participaint: Chienming, Shurong and Yuhang

As we reported, we found the green production has an fluctuation. Following that, we found the similar fluctuation in the power of bright alignment beam and even the beam going to IR mode cleaner. Here we present the result of power monitoring at two different points and in three different situation. The position of monitoring is shown in the attached figure 1. We kept monitoring each case for around 4min.

Montioring point :

Situation 1: Block the beam going to SHG, almost no light going back.

          power fluctuation level is 0.29%

Situation 2: Lock SHG, both infrared and green going back.

          power fluctuation level is 3.9%

Situation 3: Tune SHG off-resonance, only infrared going back.

          power fluctuation level is 4.8%

Monitoring point 2:

situation 1: Lock SHG, both infrared and green going back.

          power fluctuation level is 1.6%

situation 2: Block the beam going to SHG, almost no light going back.

          power fluctuation level is 0.055%

Scanning cavity:

          In the last attached figure, we put the power of point 1 while scanning cavity. As you can see, Cavity resonance causes the drop of main laser power. However, there is one thing strange, when cavity is not resonance, the mainly laser power should be the same since the reflection of SHG is the same. But in the attached figure, we can see the feature caused not only by cavity resonance.  There is also a signal in-phase with the ramp signal. Maybe this is caused by the phase change of light. Need more investigation.

Conclusion: As we can see from the result, the monitoring point is only related to main laser. So we are sure the fluctuation comes from main laser. And we found the fluctuation will disappear only when there is no reflection. We also found the fluctuation will be even larger if there is more infrared reflection. And since even when there is no green relfection, fluctuation exists. We guess green should not be the main reason of this fluctuation.

As a confirmation for what I reported in entry 1090, I measured the spectrum of the in-loop PD at different powers of the pump. In the plot, we can see the peak at the chopper frequency.

This confirms that there is stray light going to the PD. We are processing the purchase of new filters: additional long-pass at 1250nm

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.