[Eleonora, Yuhang]

Last week, after removing the qubig EOM, we took the chance to test two resonant EOMs from TAMA, in order to see if we could see a error signal at least with them. Unfortunately even with these EOMs we could not get any PDH error signal from OPO.

Some details:

1) The EOMs from TAMA are Newport 4003(damage threshold 4W/mm2, aperture D = 3mm, length = 5.5cm) resonant at 40 MHz and 76.18 MHz resplactively.

2) We did small modifications on the p-pol path to get a reasonable matching into the OPO. (We mainly moved the lens after EOM). The reflected beam from OPO looks quite astigmatic but it was supposed to be just a quick test so we didn't spent much time to optimize matching and alignment.

3) On the newport EOM manual we read that the beam into the EOM ideally should be collimated. This in not the case for us, as we have it focused into the EOM, but it shouldn't prevent to see some modulation effect.

As a general remark, we are puzzled by the fact that even if the setup is not optimized, we cannot see any trace of error signal. Of course TAMA EOMs are ten years old and there is no guarantee they are still working but it makes us to suspect that there could be something else wrong in the set up. 

Small sapphire sample.
As I did for the LMA measurement reported in elog entry 745,
I calculated the ratio overlapping the map taken with my setup in February with the map taken by Alexandrovski.

I shifted the maps to find the minimum of the standard deviation of the ratio on the map.

The ratio is 2.55+/-0.3

Tama -size Sapphire Sample2. I plot together the data taken:

 - with the original setup 2 years ago (red line);
 - with the current setup, which should have all the same parameters as the original setup, according to the company instructions (blue line)
 - at LMA (black dots)

It looks like we are back to the factor of ~2 we had at the beginning.

(note: the phase is not calibrated, so it is different in the 2 plots mainly because of the different position of the chopper)

[Yuhang, Eleonora]

In the past days we observed that the SHG lock was quite unstable and difficult to acquire. Yesterday we find out that the demodulation phase was very badly tuned thus the error signal was extrimely small. We have optimized it and now the lock seems more robust.

[Yuhang, Eleonora]

We have continued the investigating about the cause of the missing PDH signal for the OPO.

1) We tested the OPO photodiode on the SHG and we were able to see a clear PDH error signal (with a IR beam with a power of about 80 uW). See  pic 1. 

2) We checked again (with a large bandwidth oscilloscope) the amplitude of the RF signal sent to the EOM. It seems to be about 18 dBm (5V pp)  which corresponds to a modualtion depth of 0.5 rad, that, for sure, is not too small.

3) FInally we removed the EOM from the bench and we ispected it. Indeed the crystal looks damaged. see pic 2. We sent the picture to Qubig and  they also confirmed that the crystal is not healty and need to be changed. They suggest us to ship the EOM back to proceed with the reparation. We are now organizing the shipping.

[Yuhang, Eleonora, Matteo]

The goal is to lock the OPO using P-pol beam 

Preliminar information: The p-pol beam is modulated at 88 MHz (with resonant Qubig EOM). Because of the lack of space, the beam had to be focuesed inside the EOM and thus the power sent to it has to be reduced. Currently we have 5.5 mW reach the OPO and this power cannot be increased without changing the EOM telescope configuration.

After some difficulties due to a wrog setting of the gain of the photodiode in trasmission, we managed to align also the p-pol beam. (see pic1. left)

S-pol is also reasonably well aligned. (see pic1. right)

The trasmitted power for p-pol is about 10 uW while the input is power is 5.5 mW. The transmissivity for p-pole is about 0.18%.  For the s-pol we found 0.25% and the nominal is 1.2%.

[Note that since the cavity has not be locked, we made the measurent by manually driving the piezo in order to bring the cavity on the top of 00 resonance.  It may be not very accurate.]

After that we installed the locking set up but we couldn't find any PDH signal from the PD in reflection.

Some tests we did:

1) We ruled out that it is a malfunctioning of the PD, as we checked that it was able to sense PDH signal from the green mode cleaner. (But note that responsivity for IR is more than a factor two smaller)

2) We double check the alignement of the PD.

3) We lowpassed the signal with a SR560 to get rid of some high frequency noise, but it didn't help

4) We check the signal coming from the PD(before demodulation) with a spectrum analyzer but we couldn't find any line at 88 MHz. So we know that the problem doesn't come from the demodulation, but it's present before.

5) We checked that the driving signal was reaching EOM. 

6) We checked that the EOM orientation was the one required by the input beam polarization

7) We tried to reduce the modulation depth but we coudn't see any change in the amplitude of the TEM00 peaks in trasmission. 

Some hypotesis:

1) The EOM may not work. How to check it?

2) The signal on the lock PD may be too weak?  We send it 200uW ( which should be fine) but the responsivity at 1064nm is small (see attached PD datasheet) 

Note: we uploaded the datsheets from Qubig components on the FC wiki at this link https://gwpo.nao.ac.jp/wiki/FilterCavity/Datasheets

The scans in the comparison reported in elog entry 973 were shifted to make the comparison more clear, but if we plot the original data together (see first plot), we see that the crossing point has shifted after we changed the probe size and pump size.

The shift is between 2.3mm and 3mm, therefore to compare a measurement with the map taken at 50mm (reported in elog entry 678 ) I took a map at 53mm and another one at 52.5mm.

I calculate the ratio between the large beams maps and the small beams maps. See last plot

Today I count the clean suit and put them in the box. I wrote 'gwpo' on each suit's lable. I also used the NAOJ washing machine to wash all the gloves we collected. It is quite a lot, I dry them in my office.

Today cube BS arrived, I put it in a desirable position. I found some points to say

Good:

1. It is easy to align cc beam and make it resonate in OPO and produce green.

Bad:

2. After p-pol, I can see very very small peak(around several mV)(sorry I didn't take picture) on the oscilloscope. This may mean we will have problem for locking OPO? Probably this is because of the low power of p-pol.

3. The p-pol reflection from OPO goes to the Qubig PD I soildered. However, I cannot find that PD is sensing light. The signal of PD is attached as Fig.1. It seems there is a floor from 0Hz to around 100MHz. I checked light is going into PD properly. I checked even when there is no light going into PD, there is that signal. I also check the soilder I did(in attached figure 2), it looks really the same with another Qubig soilder. 

Not good not bad:

4. I found the beam is not going through the hole centrally. This may come from the crystal is not well centered. I also check the SHG's incident beam is not going through SHG's hole centrally. So maybe this is fine.

The frequency shift between cc and p-pol is used for the coherent control in the future. At the same time, we need to control this frequency at a certain point with a proper value. This is pretty similar with the double control of infrared and green resonanting inside the filter cavity. In FC case, we use AOM. Here we use PLL to control this fixed frequency shift.

Today I set up the PLL and it works well.(see attached picture) Note here the singal is 1/10 of the signal. However, since we use PBS while we are aligning OPO, we need to take it out to make p-pol and cc both go to OPO. The reason is we will use cube BS in the future. We use cube PBS to simulate the optical path change by the substrate. Taking out cube PBS change the phase of beam quite a lot. It makes beam not resonate inside OPO. So I stopped today's experiment. We will wait the arriving of cube BS.

In order to reduce the size of the HeNe probe I used Jammt to design the optical path.
First I measured the profile without lenses. First plot. The axis is in the translation stage reference. The waist of the HeNe is right at the output of the laser tube.
After some attempts, designs on Jammt and profile measurements, I found a good set of lenses to have the waist about 3 times larger than the pump.
The lenses are a f = -50mm lens and a f = 75mm lens at about 47mm from each other. The second plot shows the final probe profile. The probe size is now 2.8 times larger than the pump.

I aligned the pump and the imaging unit to maximize the signal on the surface reference sample (usual procedure). Then I scanned the bulk calibration sample.

When moving the Imaging Unit with the micometric screw to finely maximize the AC signal, I noticed that there was not a clear maximum in the range. So I unclamped and moved the whole IU much closer to the sample. At about the same position as it was in the very original setup. Now all the conditions are the same as the original setup, or at least as the specs say.
Now I'm wondering if/why it is not possible to have the same size of the image on the detector when we move the telescope further. This is to be cleared.

Then I measured the tama-size sapphire sample again. First with 5W of pump power and then with 10W (max). Noise from chopper (constant phase)  is again a bit high, but let's consider this later.

In the last plot I compare the last sapphire measurement with the one of last week, when the probe was larger. Remider: reducing the pump size without reducing the probe size didn't change the signal.
Now the absorption value is smaller. It's not straightforward to tell a precise ratio, but let's say it is between 1 and 2. It is certainly not a factor of 3, as we would be very happy to have.

I have the feeling that the calibration factor between materials (3.34 according to STPS between sapphire and Schott glass) depends on all the parameters I changed.
What I really don't understand is how the imaging affects the calibration. I thought that the image size depends on the focal lengths of the telescope lenses only, not on the distance of the telescope, but maybe I'm wrong.

The question is: let's call d1 the distance from the sample to the first lens of the telescope and d2 the distance between the two lenses. Is there a d2 for each d1 so that the image on the PD is exactly the same (in sharpness and size)? 

Yesterday I did beam characterization of infrared beam along west and south edge of bench. The first attached figure is along the south edge, which is also the beam directly goes to PR chamber. Actually we did a similar characterization before, see elog634. This time the result is similar with before.

The second attached figure is along the west edge, which is after the first lens on the rail. We will put a combination of lenses to make the beam meet the requirement of IR mode cleaner. According to the measurement result, we have the initial beam information. We also know the target beam information, which comes from calculation of IR MC configuration. Then I use these information did the simulation in JamMt by using mode matching assistant. I choose one I prefer showing as attached figure 3 and 4. I checked they are seating at place between lens and first steering mirror. This means it should be a reasonable solution. Also this solution doesn't use small focal length lens.

The detailed calculation is also attached in PDF file.

Yesterday, I got more information from Matteo. The reflectivity of crystal should be 99.975%. And for in-coupling mirror, it is 92%. By using formula, T=T1*T2/(1-r1*r2)^2, the updated transmission should be 1.192% without considering losses. Now what we found, transmission of 0.25%, seems resonable.

Today I also soildered the Qubig photodiode.(see attached picture 1) According to the specfication, the resonant frequency of EOM is 88.1244MHz. I set up corresponding frequency in the DDS board.(see attached picture 2) Then I did the test of Qubig PD. However I didn't put the PD in the final position. The reason is we don't have the cube BS. If I use plate BS, I will shift beam a little bit and may destory the alignment of OPO. Now Matteo is trying to order it. Hope it will arrive soon.

So I put it at the reflection of PBS, since we have some residual s pol(around 140uW which is luckly a good value for PD) in the p pol beam. I measure this modulation in this perspect. As expected, the level should be low. But we can use this small effect to test EOM and PD. And I found both of them are working well. (see attached picture 3)

COMMENT: I talked with Matteo, he pointed out what I saw should be the comunication between two channels. I checked today that that peak should be the communication. We will do the test after cube BS arrive.

Last entry about OPO alignment is done with the transmission of OPO seperated by normal 50:50 BS. Today I replaced it with a BSY11(dichroic mirror reflects infrared). The first time we align OPO with a temperature of 6.740kOm, but now I found I cannot find green with this tempereture. So I did a scan of temperature, I found green transmission in another temperature. The temperature now is 7.198kOm. (See attached picture 1)This is a roughly fine value, we will find the best value after the lock of OPO. See attached picture 2 of the green I found. And picture 3 of it on the screen. I check the value of infrared transmission again today, which is around 0.4mW. It is similar with last week's condition.

Participants: Yuhang, Eleonora

After achieving the OPO internal alignement (entry #963) we removed the set up we used for this activity (periscope etc..) and we placed the OPO in the final configuration on the bench. (See optical scheme on the wiki  updated by Yuhang ) 

The first rough aligment was done with low power p-pole beam, placing the OPO in order to have its reflection superposed to the incoming beam and then using the last two steering mirrors for the fine alignment.

At the begining we had some trouble with the piezo:  since we were only able to get some slowly drifting mode of the camera in trasmission, we suspected that the piezo was not working.

First we check the piezo driver (which was fine), then we check the small box containing electrical connection between BNC from piezo driver and the piezo wires. As soon as we opened the box the piezo start working again. We observed that it was very sensitive to touch. To be checked further.

In the end we could achieve a good aligment, using CC beam (s-pol). See Pic.1. Anyway the trasmission seems much lower than what we expect.

We had only 0.5 mW trasmitted power over 218 mW of the input beam. That is less than 0.25%.

Assuming R1 = 92% and R2 = 99.75%, we should find T =  T1*T2 / (1-r1*r2)^2 = 10%

Are the values of R1 and R2 correct? Is the computation of T correct? 

This measurement has been done with no temperature control, anyway once we set it on the maximum of the TEM00, transmission seems to be resonably stable to perform the measurement.

After align OPO with CC beam, I did the alignment with p-pol beam in this morning. As soon as I launch p-pol beam into OPO, I got the attached figure 1.

We know the p-pol power we sent to OPO is only 5.9mW. The OPO choose s-pol to resonant inside the crystal, so we cannot have green if use this p-pol. But I can see signal for infrared on the photodiode. I use this photodiode signal to align the p-pol beam. The steering mirrors I used are the two steering mirrors we put before combinging p-pol and cc. The alignemt result is attached with scan frequency of 60Hz and amplitude of 3Vpp, the signal is shown on the oscilloscope.

After align them, I measured the power I got for transmission, which is 2.5uW. This is very small so I can use camera directly to see it. I put the camera just in front of photodiode. While scanning the PZT with frequency of 50mHz and 3Vpp ramp signal, I can see the modes I have now. Most of them are pitch higher order modes.

This is the best I can do by moving that two steering mirrors. I still don't know why I cannot improve it anymore.

Participants: Matteo, Yuhang, Eleonora

After aligning the crystal along the beam axis, as reported in entry #959, we put the incoupling mirror and align the whole cavity. The aligment has been optimized by maximazing the TEM00 mode from AUX1(Choerent control laser), while the cavity was scanned driving the piezo of the incoupling mirror. The final result is quite good as it can be seen from Fig 1. At the beginning we observed that two different TEM00 mode were resoanting in the same FSR.  We found that this was caused by a laser mode hop and was solved by slighly changing the laser temperature.

We also set the crystal temperature control in order to produce green light. The best value was found to be about 6.740 kOhm, but a further optimization will be done once the cavity is locked. The green light produced was transmitted by the dichroic after the cavity and observed with a camera.

We set the PID gains of the temperature controller to the same values of those used for SHG and the control seems to work fine.

Since my result is different from Marc's result, I did calculation again. I found a mistake in my calculation of ABCD matrixs.

From the calculation point of view, Marc's result is correct.

I will check in actual case to see if the calculation aggres with calculation or not. As I have already mentioned in the meeting, we can see the reflected beam is shaking while the filter cavity is locking. So if they don't agree with the actual case, I think the discrepancy comes from the beam shaking.