Participaint: Chienming, Shurong and Yuhang

Today Chienming and Shurong aligned the SHG, GRMC and OPO. There are some coments from them.

1. The alignment of SHG was always degraded by the constantly shifting of the second Faraday isolator.

2. There are two higher order modes of SHG cannot be removed. We guess these two modes come from some inherent mismatch inside SHG. We also found the bad shape of green beam comes from the cut of this green beam by the edge of SHG housing. However, as Chienming suggested, the side of beam is trivial compared with center. And we also found it is fine for filter cavity locking. So we guess it is fine.

3. Chienming put small spacers for the FI just after SHG. The purpose is to move the point(diffracting the light) to the edge of the beam.

4. GRMC and OPO were aligned to the best situation by Chienming and Shurong.

After that we found the beam was shifted. I guess this is because of the align of beam done by Chieming. Anyway, we recover the green beam direction by using BS before 100mm lens. Then we changed the AOM driving amplitude, the value now is -6.4dBm. It used to be -10dBm. This change of driving amplitude was suggested by Chienming. We increase driving amplitude and look at AOM first order power. We stop when we found the maximum. At that time, it became -6.4dBm. We can see from the attached figure. The zero order now is not round.

However, we still can see the beam seems to be cut by AOM. But Chienming suggested to check this beam with a much lower power(like what we have before, 8mW). But anyway, we cannot see this structure when there is only zero order going inside AOM.

So tomorrow we can do

1. Check the first order of AOM with lower power

2. Try to align filter cavity to see flash

Figure: (1and2: BAB inside OPO. 3and4: SHG. 5and6: GRMC 7:AOM situation of beam shape when amplitude maximized 8:AOM RF driving amplitude)

Participaint: Eleonora and Yuhang.

After the measurement of parametric ampkification, we considered to recover our filter cavity. But we found the beam was cut by EOM(GRMC/FC) and AOM. At the begining, we think we can recover the beam if we recover it for GRMC. However, it seems not like that.

Then we tried to align the beam better for EOM(GRMC/FC). We found the situation becomes better.

The entry 893 shows the spectrum of GRMC trasmission. I compared it with the GRMC situation now. I found there is a higher order mode is a little bit higher than before. Maybe this is the reason why we found the problem of beam cutting.

I am thinking maybe we can try to move the first lens(the lens just after SHG) to remove that peak.

Or tried to move 100mm lens just after the BS which seperates the beam going to GRMC and filter cavity.

Or we can further try to do the alignment of beam into AOM.

I summarize here some information that can be useful in order to design the telescope to match the squeezing beam to the filter cavity.

1) The distances from the last hole of the bench to the 2 inch telescope mirror (and the optics on this path) are reported in pic1. They have been measured by Yuefan in entry #441.

2) The target beam for the telescope is reported in the second attachment. The beam waist should be about 1 mm and it should be located close to the 2 inch telescope mirror (named M1 in the attached table and scheme). See also entry #442 for the former telescope design.

I aligned again the HeNe probe and made the calibration on the surface sample and on the bulk sample. Incident power= 35mW

I repeated the measurement on the small sapphire sample: a scan on the center, and a map at half thickness (according to the scan). Incident power 10W.

Then I made the ratio between my map and the one measured at SPTS as I did in entry 985. The ratio is now 1.17+/-0.2.

Participaint: Chienming, Shurong and Yuhang

Our measurement method: Injecting bright alignment beam(BAB) and pump beam inside OPO. Scanning phase with 500Hz but scanning OPO's PZT with only 5Hz. Then we use small amplitude of OPO's PZT to see the parametric gain effect inside the scanning peak. As you can see from the first picture.

Then by using this oscillated peak, we did the alignment of the pump beam into OPO. The improvement of alignment will bring more amplification for the peak of scanning. So we aligned the two steering mirrors between GRMC and OPO. After alignment, we get that peak as figure 2.

Then we tried to do this for pump power from 10mW to 79mW. For looking at parametric gain more clearly, we stop the scanning of OPO. On contrast, we manually tune OPO around resonance and use cursor to mark down the maximum and minimum of the oscillation. The result of these values is shown in tha attached figure 3.

Then we fit this result with the formula

gain = (1+B/Bth)^2/(1-B^2/Bth^2)^2   where B is the pumping field amplitude:  P = B^2

We found a threshold of 80mW. We also clearly see the lasing when we provide 79mW of green pump.

[Shurong, Yuhang, Eleonora]

We have completed the assembly of the auxiliary IR mode cleaner, which will be used for the matching of the beams on the homodyne detector.

Fot the input/output mirros we used two mirrors from the same batch (6 pieces) used for the other IR modecleaner. 

We double checked that all the mirrors were installed with the side arrow pointing towards the cavity

We put the white orings only on the input/output mirror, which should be fine.

Participaint: Aritomi, Chienming, Eleonora, Shurong and Yuhang

At the beginning, we want to do like this. Scanning green beam phase with a 1Hz, 1.5Vpp ramp signal. Then adjusting MZ offset to have different power going to OPO. Lock p-pol and adjust PLL to make both p and s-pol resonant. Locking OPO by p-pol. Sending green. Measuring OPO s-pol(BAB) transmission DC to see the parametric amplification and de-amplification. 

1. Today's change: For making sure we are locking OPO at resonance. We set up a DC/RF photo detector for OPO s-pol transmission.

2. Lock OPO with p-pol, found demodulated s-pol transmission RF oscillating at 1Hz. The RF signal before demodulation oscillates at 15MHz with an amplitude modulated at 1Hz.

3. The green injection changes the co-resonant condition. We guess this is because of temperature change. See attached picture 1.

We did like this finally. Manually put the transmission of the seed beam on maximum pk-pk. Don't use p-pol lock and PLL. For different pump power, we measured the amplification and deamplification. See attached figure 2. We did the fit and found the threshold now should be around 140mW.

Since we know the green changed the temperature of OPO crystal, we also checked the temperature influence. For a green power of 60mW, we changed temperature from 6.9kOm to 7.2kOm. Now the optimal temperature seems to be 7.06kOm, which is 0.04kOm far from previous optimal temperature 7.02kOm. However, the influence of temperature change caused by green power seems not crucial. In any case, the non-linear crystal is still working in the broad optimal region.

However, we decided to check the matching.

Since we could not find any spare half-wave plate for 532 nm,  we have temporarily removed that placed just after the last green faraday isolator, before the filter cavity.

We use it to operated the green mode cleaner in p-pol.  The value of the angle in the original configuration is shown in pic 2. 

We will need to buy (or find) some new ones.

We did more detailed test to the board last week.

Since last week when we powered up the board with NIM rack, it heated up very soon and had some bad smell. Just in case the NIM rack we have doesn't give the right power and may cause some damage, this time we powered up the board with external power supply. The first problem we found out, it is the 8V power supply of this board which give power to drive the galvo, has a much large current than it is supposed to be, that is also the reason why they board heat up so soon we guess.

At the beginning, we just powered up the 20V part of the circuit, and sent a sine wave instead of the quadrant signal to A,B,C,D port one by one. By checking each check-point inside the board, we could see clear sine wave. 

Then we powered up also the 8V part, the signal at the check-point started to have high frequency oscillation in MHz range. We traced it back, found out the oscillation was already there since the beginning of this path.  

When we wanted to do more investigation of the problem, we were looking for some checking point on the board according to the design, but we could not find them. So we checked the date written on the board, it has been done in Aug 1999, but the design was finshed later than that, so we guess there may be newer version of this board in TAMA and they could have better performance than the one we have in Nikhef now.

[Yuhang, Eleonora]

We found four more boards for the galvo control.  Two of them were in the shelves of TAMA entrance, the other two (those on the right in pic1) were installed in one of the TAMA injection rack 

All the board are dated Aug 1999 (see pic 3) but one reports the writing "corrected by Koji Arai & Sa 2002/12/10 "  (see pic 2)

See entry #831 for more details about galvo and boards.

Participaint: Matteo and Yuhang

By considering the space and alignment of homodyne detector(HMD). We designed the telescope for both beam coming from local oscillator and bright alignment beam(BAB).

The attached figure 1 and 2 show the telescope. The attached figure 3 shows the overall situation in the future implementation.

Participaint: Aritomi, Chienming, Eleonora, Shurong and Yuhang

Today we achieved the lock of OPO on both p and s pol. The method is to lock OPO with p-pol (PDH signal goes to PZT of OPO) and then use PLL to lock s-pol to p-pol. Actually, before locking PLL we made sure the p and s both resonant inside OPO with peak overlaping.

The tricky part is to lock PLL as following:

1. We found that the frequency diffrerence of p and s is ~360MHz while the main laser current is 1.4A. However, this frequency increased ~420MHz after we increased main laser current to 1.8A.

2. Locking PLL for 420MHz is not possible. According to datasheet, we can provide VCO frequency only below 400MHz. At this moment, we decided to not use optimal temperature(optimal is 7.2kOm) but use 7.171kOm.

3. We investigated quite a long time and found we can use the built in divider to lock beatnote smaller than 400MHz. Since the phase detector can hold frequency only below 104MHz(also from datasheet), so we can divide signal and make it smaller than 104MHz. Then we can use reference frequency to compare with it.

4. As we have already mentioned in the last entry, we can only lock one side of PLL. We further found that we can change the sign of correction signal inside PLL board. We need to find the good comnbination of correction signal sign and the original frequency side(lower or higher than locking frequency). However, we were think if there is a way to change the offset of correction signal and make it have both positive and negitive signal.

After locking PLL, it is easy to lock OPO. The locking parameters are single-pol low-pass at 300Hz, gain of 100 and invert for SR560.

The locking parameters are single-pol low-pass at 30Hz, gain of 2 for SR560.(for GRMC)

I did the simulation about how much green we can produce now. The temperature we lock is 7.171kOm. The green we produce should be 90% of the peak value. However, the green production will be even smaller if we consider the overlap of p and s is not perfect.

The design of the QPD telescope has been finished.

The idea is to seperate the reflected beam from the cavity into two path, and put one QPD on each path, in a way that the guoy phase on the two quadrants has around 90 degree difference. So one of the quadrant could sense the shift and one could sense the tilt.

The target beam is decided by the size of the quadrant sensor :

1. The size of each quadrant element is 11 mm²
2. The gap between the elements is 70um

So I chose the target beam size at QPD should be around 1~1.5mm

The lenses I used in the simulation are the green lenses I could found on Thorlabs website.

In both of the case, the origin is set at the beam extracting port of the FI. 

1.

2.

The guoy phase difference at two QPD is around 98 degree. So I think we could go on with this design.

Participaint: Aritomi, Eleonora, Chienming, Shurong and Yuhang

1. We checked the spectrum analyzers by signal generator. We found out

HP E4411B (the new one): Has a wrong amplitude value but higher bandwidth up to 1.5GHz.

HP 8563E(the old one which was used by Marco): Has a correct amplitude but the bandwidth is limited to 170MHz.

So we decide to check firstly by the new one and then tune beatnote frequency smaller than 170MHz. FIncally check amplitude by the old one.

2. The lesson we got from PLL locking

We need to load Marco set-up and remember to write this set-up into chip. How to change this set-up and implement it for the second board still needs to be investigated.

We checked the correction of PLL, we found now it is only positive. So we can bring initial frequency locked only when it is lower than reference frequency.

To do list:

1. investigate the amplification factor of each channel.

2. investigate the locking peformance of PLL(the highest locking frequency).

3. lock PLL to make both p-pol and s-pol resonant.

4. investigate how to apply lock for PLL board two.

DDS1:

DDS2:

I made a scan of the calibration sample with the 1310nm. Attached first screenshot. AC=0.115V; DC=2.14; phase=-85; power=34mW -> R=7.9W^-1.

Then I set the power at 10W and made a scan of the crystalline coating in the center, with and without loop. The loop doesn't make a lot of difference.

the peak signal is 380microV, with DC=1.65, the calibrated value is 2.9ppm. A lot more than expected.

Then I made a map of radius 4mm around the center, resolution 100micron. There are a few peaks where the signal saturated the lockin scale, but most of the map is uniform at around 3.4ppm.

Chienming suggested to check the bandwidth of both SHG and GRMC. To make sure the BW of SHG should be smaller than the BW of GRMC.

From Eleonora's thesis, I found the FSR of SHG is 4GHz(infrared) and finesse is 75. So

BW(green of SHG) = 4000*2/75 = 106MHz

The scale information of GRMC can be found from entry before. So

FSR = c/L = 533MHz

finesse = pi*sqrt(r1*r2)/(1-r1r2) = 391

BW(GRMC) = 533/391 = 1.36 MHz

I did the calculation of Airy function(of SHG) intergration around maximum over 1.36MHz and 106MHz. The ratio is 1.5%.

Note: the bandwidth of mephisto laser is 1kHz.

With the pump open at the max power of 10W (after the chopper), I noticed that the control loop introduces a noise at a constant phase.

Without the loop, the cloud of points is 40microV large (radius) and centered at 20microV (phase around 150deg)
With the loop, the cloud of points is 20microV large (radius) and centered at 40microV.
This means that some residual stray light from the pump is still entering through the OD11 filters in the in-loop PD.

So I moved the in-loop PD to another position and now the noise is reduced from 40microV to 20microV and the cloud center keeps staying at 20microV.

I scanned again the 0.65ppm-absorbing LMA sample. I Plot the AC signal scan and, with dashed lines the AC with phase filter applied (expected phase: -85deg, from the reference scan).
The phase filter removes the component orthogonal to the absorption signal.
The central peak of the dashed orange line is 110microV, which corresponds, with DC=2V, R=7.7W-1, and pump power=10W, Abs = 110/2/7.7/10 = 0.7ppm. In very agreement with LMA measurement (0.65ppm)

I assembled the filters on the PDs, now each PD has a FEL1250 and a FELH1250, for a total OD=11. On the spectrum analyzer the pump peak disappeared.

I moved the imaging unit closer to the sample to increase responsivity. I looked for the sharp image of the beam and maximized the signal with the surface reference sample.
I maximized the signal changing the imaging unit position.
I attach a screenshot of the best calibration scan I got. R = 7.7 W^-1, similar to the expected half of the HeNe one. (R = 18 W^-1)
I also plot all the scans in the same 3d plot. I chose to set the IU at 27mm.

I measured the LMA sample that absorbs about 6ppm using 10W of pump power. I made some scans and a map.

I measured the LMA sample that absorbs 0.65ppm, with open loop and with closed loop.

I plot all the scans together