[Yuhang, Eleonora]

While waiting for the quibig EOM to be repaired, we have tried to estimate the ampituede of the PDH signal expected both in reflection and trasmission of the OPO.

We used the following paramenters:

R1:  99.975%  (crystal)

R2:  92%         (incoupling mirror)

INPUT POWER: 6 mW

MOD. DEPTH: 0.3 

PD GAIN tot: 16e2 [V/W]  (photosensitivity at 1064:  0.1 [A/W], amplification:16e3 [V/A])

BS before OPO:  R = 4% T= 87% (p-pol)

We did a simulation with Finesse (see attached plot) and confirm it with analytical computation. (We will upload the code on the wiki ) 

Conclusions:

1) The PDH signal is larger in transmission (due to the fact that in this configuration the cavity is undercoupled) 

2) The PDH signal is anyway very small (less than 10 mV pp) with the current values. 

It is for sure convenient to lock the cavity in transmission (as done in Virgo1500 and GEO) but we should also consider how to increse the signal. Some possibilities:

1) Increase the input power (change EOM telescope to increase allowed power? How much power is used in GEO and Virgo?)

2) Increase the PD gain (the current one has low photosensitivity for IR) 

3) Increase the modulation depth (is it feasable?) 

Note that the simulation has been done considering nominal reflectivity values for OPO but we observed that the transmission is lower than expected (0.18% instead of 1%), this may result in a discrepancy between the  simulation and the real signal.

[Yuhang, Eleonora]

We have locked the infrared mode cleaner. 

Some details:

1) P_in = 10mW  P_tra = 7.5mW  => Transmissvity: 75%   less then expected (90%) but enough for the homodine detection.

2) we locked it in reflection using a TAMA PD. See attached pic 1 and 2.

3) Since we use 15 MHz sidebands (same EOM as SHG) we could use TAMA demodulation board

4) We use a SR560 as temporary servo (1st order low pass, cut of frequency at 3 Hz, gain 10 after lock is engaged and 1 to acquire it) 

5) An attenuation of 40 dB had to be put on the demodulated signal before the SR560,  in order to avoid overload and be able to acquire the lock.

6) We measured the optical-mechanical TF (cavity + piezo). See attached plot (Fig 3). It Note that the high pass filter to compensate the low pass of the piezo driver has not be used. 

We have uploaded the data and the plot on the TFs wiki section.

I did some simulation of the green reflection path from the filter cavity input mirror. The detail is in the attached file.

1. I clean up the IRMC, fortunatly, the dirty is outside so we can clean it.

2. For the RF signal, I used the TAMA PD which was used for FC reflection.

3. Since we found that we cannot see the modulation produced by OPO EOM, we decided to test this EOM. This EOM is from TAMA(length = 5.5cm, Diameter = 3mm, power density = 4W/mm2, resonant frequency = 40MHz). Since I have already had a good alignment for IRMC, I didn't want to destory it. I just want to add lenses and then put EOM. Besides, I have a lot of space to put lenses and make the beam smaller than 500um of radius with a range larger than 5.5cm. So I don't want to care a lot about mode matching. Note here, the mode matching affected by additional lenses and EOM. Besides, we can compare the 15MHz sideband's power to know how different interference affect the sideband power. Because we know the power of 15MHz sideband before puting the additional lenses as a reference. The set-up of this test is attached as Fig.1. (Note here the beam power is 10mW, even for the concentrated beam, power density is 1.8W/mm2)

For the test of EOM of collimated beam inside, the 15 MHz sideband power is around -26dBm. The 40MHz is around -49dBm. (see attached Fig.2)

For the test of EOM of concentrated beam inside, the 15MHz sideband power is around -32dBm. The 40MHz is around -56dBm. (see attached Fig.3)

Here the 15 and 40MHz sideband disappeared after I blocked the beam.

From these two results, we found although the 40MHz sideband power decreases, it seems come from the worse interference(worse interference verfied by 15MHz reference). (See attached Fig.4 for bad interference)

So the conclusion is: The TAMA EOM works well and the collimation of beam doesn't matter.

-----------------------------------------------------------

15MHz sideband comes from the TAMA EOM, which is just after the main laser and before the first BS on the bench. 

A very unfortunate possible explanation of the calibration problem could be a double mistake on the true absorption value bulk reference sample from the company. The Schott glass #12.
So I measured it again with a laser and a power meter.
Incident power         =   76.7 mW
Transmitted power  =    42.5 mW    55%
Reflected power      =   5.5 mW       7.2%

Therefore the absorbed portion is  37.8%
The absorption rate 37.8% / 0.36cm = 105%/cm

This confirms the measurement done 3 years ago by Tatsumi-san elog entry 88

According to the simulation I did, I installed the lens and align the infrared mode cleaner today. After the alignment, I did the scanning of mode cleaner and monitor the mode cleaner transmission by PD. Since I have this scanning on the oscillscope, I used the data to do the fit. The result is Finesse =  192(according to entry 566, expect value should be 300-500). I will do some power budget statics after the lock of it.

Today after I installed the first lens, I check the beam parameter. It is quite far from the simulation. Then I found the reason maybe I didn't use a correct distance before. (Also here I found 200mm lens disappears)

Then I measured the beam again and used the lens we have to design the telescope. However I cannot get any result.

So I decide to remove the first along the west edge. Then I peformed the measurement. Before do that I aligned the beams to make them flat and go through the center of the lens and mirror as well as possible. Then I measured the beam agian. The result is w0 = 850um, z0 = 3.05m(relative to the 0th hole of west edge of the bench).

Then I use the result and mode matching tool in Jammt(by using the lens we have, I just update the lens situation today). The target beam parameter is calculated last time. It is w0 = 390um, z0 = 0.8875(relative to the 0th hole of the west edge of the bench). 

The simulation result is f = 250mm @ z = 0.484m, f = 75mm @ z = 0.795m. I also check this result with the optical layout we have. It doesn't overlap with the mirror we have in the optical layout.

[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.