NAOJ GW Elog Logbook 3.2
Participaint: Aritomi and Yuhang
We decide to change the position of OPO transmission PBS because of the space limitation. We also put the ''flip'' mirror on the translation stage. By moving this mirror, we will switch its direction to homodyne or to filter cavity. The scheme now is to use the two steering mirror before this ''filp'' mirror to do alignment for filter cavity. And adjust this ''flip'' mirror to align the beam into homodyne. Because this ''filp'' mirror has three adjustable knobs.
According to the design shown in the first attached figure, we aligned the BAB into AMC. However, the matching is not as good as LO.
From all the peak values, we can compute the matching is (0.51-0.0065)/((0.0115+0.0153+0.0173-0.0065*3)+0.51-0.0065) = 95.34%
We didn't see the visibility and found out the reason is we forgot to convert BAB into P-pol.
Participaint: Aritomi and Yuhang
Since we found PDA05CF2 (thorlabs InGAS PD) is more suitable for infrared signal. We decided to use it for OPO locking. Before we took the filter cavity locking PD for OPO locking because it has a DC channel. Since we have a better one, we decide to put this qubigPD (with DC) back for filter cavity locking.
Besides, according to simulation, the OPO reflection error signal is 5 times smaller than transmission. And we improve the error signal by a factor of 10. So in principle, now the error signal from OPO reflection should be also enough to lock OPO.
This PD has only one output channel. RF and DC components are separated by using a minicircuit Bias-Tee ZFBT-4R2G+ (datasheet uploaded on the wiki).
The RF part is sent to the mixer for PDH demodulation, the DC is used to monitor the lock status.
Actually we did this replacement long time ago. But I would like to put here some reference value of this PD. We should have error signal pk-pk value more than 120mV. And DC value more than 3.3V. Note here the impedence is .
Besides, we can also see that this PD has a better performace than the Qubig we had. There is a result of previous measurement of Qubig PD. Compared with that, we improve the error signal by a factor of 10.
Actually, there is only a factor of 3 or 4 difference between Qubig PD and thorlabs PD's resoponsivity.
This PD has only one output channel. RF and DC components are separated by using a minicircuit Bias-Tee ZFBT-4R2G+ (datasheet uploaded on the wiki).
The RF part is sent to the mixer for PDH demodulation, the DC is used to monitor the lock status.
Participaint: Aritomi and Yuhang
This beam splitter will be used for homodyne. And the balance of these two power value is very important. So we decide to measure this value.
We bought two BSW41-1064, according to specfication. The overall performance is T=50+/-5% while R=50+/-5%. So R/T should be between 0.905 and 1.105.
This power ratio depends also on polarization. According to thorlab website, p-pol should be closer to 50:50. We measured the power ratio relationship with polarization for one mirror. The result is shown in the attached figure. It seems there is relationship between polarization and power ratio. But it is small dependence. We can also see this mirror is still within the error range of power ratio provided by thorlab. But it is not as Matteo suggested.
Then we changed it to the second mirror. Then the power ratio becomes R:T=607.5/615.4=0.987. This is much better.
Matteo and Aritomi recovered the vacuum pump for filter cavity. The procedure is to first bring back rotative pump and after it reachs a certain level. Turn on the molecular pump. There are some valves to seperate pump. According to the sequence of the pump on, open them.
Manuel, Victor
1310nm probe
we aligned the pump beam to maximize the AC signal of the surface reference sample, and we made a calibration scan with pump power 35mW.
Then we increased the power at the maximum and measured the crystalline coating with three different probe power: DC=1.01, DC=1.62, DC=2.54.
Then we plot the three scans together and the result is that the AC/DC overlaps.
We also repeated a scan with higher resolution in z.
Participaint: Aritomi and Yuhang
We aligned the local oscillator into the alignent mode cleaner(AMC). The method is to remove input and output mirror and then make sure the beam going through MC through the center. And the beam should be flat horizentally and vertically. The reason why we aligned LO first is it is easier because it is much brighter than the OPO transmission. Then we can use this as a reference for the alignment of the second beam.
The resule of this match is shown in the first and second attached figure. The matching is (1.75-0.012-(0.02-0.012))/(1.75-0.012)=99.5%
Participaint: Aritomi and Yuhang
According to the design, we implement the telescope which is very close to OPO. However, I found I can improve the mode matching by moving the first lens close to OPO. However, after movement of 1cm closer, I cannot go on because of the limitation of space. Then I did the simulation by Jammt, it proves that the waist position should be farther away the first lens.
So I did simulation again(with waist position 1.5cm back) and also leave more space for the lens to be moved. We will implement this design tomorrow.
[Aritomi, Yuhang]
Here is information of lock of IRMC.
Input power: 2 mW, output power: 1.56mW (transmissivity: 78%)
Filter for lock of IRMC:
SR560 with low-pass 30 Hz, gain 10
40dB attenuation
We put an half-wave plate on the path before homodyne BS to get p-pol.
Then we measured reflection and transmission from BS for homodyne detector.
Reflection: 0.74mW, Transmission: 0.82mW
BS (BSW41-1064) somehow does not split the beam half and half now.
[Yuhang, Eleonora]
We have tried to recover the alignment and the lock of the filter cavity.
The alignment was easily recovered even if we could not use the local control for the telecope. See entry #1139. (We just tweeked a bit the BS position wiith picomotors)
We optimized the aligment with the input and output mirrors local control and it seems it is quite fine (flashes movie here:https://drive.google.com/open?id=1045raQj_n84CjnnUuYfYBT6Kqngv-id8) nevertheless we could not lock the cavity. We didn't have much time to investigate. Some possibilties:
1) The power is too large (a factor 3 higher than before), so the gain of the loop could be too high (we tried to change the gain value and also put some attenuation but it didn't work)
2) The beam fluctuation it too large since the BS and PR are not controlled (but this should be visible in the flashes).
3) Since the power is higher the camera in transmission could saturate more than before and mask the presence of HOM flashes. (We suspect there coud be a higher mismatching due to change of the beam from SHG)
The initial value of the gain were: input attenuaion = 5 and PZT gain = 4. As mentioned before we tried to change them but still we coudn't achieve the lock.
We will try to go back to the previouse power value and setting configuration to see if we can lock in this condition.
[Eleonora Yuhang]
While trying to recover the filter cavity alignement we found that the target PC used for the telescope local control had died.
We already had this issue which was solved by changing the power box. See entry #1035. We tried it again with another power box. It seemed to work fine and the supervisor PC could connect to it but we were not able to read and write any signal. We suspect a problem with the ADC/DAC unit connection. We will try another power box.
[Takahashi-san, Yuhang, Eleonora]
With the help of Takahashi-san, we have switched off all the instrumentation in TAMA in view of the power shutdown planned for tomorrow.
In particular we switched off the vacuum system in the central area and south arm and the compressor in the elecshop. We also closed the gate valves between the TM towers of the south arm and the pipe.
Some details of the swiching off precedure are reported in the attached file.
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.
