More details of locking

In the first picture, I showed the component on the optical bench now, and the red line represents the infrared and green line for green. As it shows in the picture, the transmit beam of the cavity divided by the BS mirror, the reflection part goes into the PD and the transmission part goes into the CCD, so I can see the shape of the beam through the screen connect to it.

And the second picture shows the loop I used to lock the cavity,the red line stands for the beam, I only list the optical component which connected to this loop. The black lines and the orange lines show the wire, but the wires in orange only used for the first step.

As I mentioned in my last report, the Function Generator on the left provides a 15.2MHz sin wave, which is sent to EOM to do the modulation, and also there is the other line come out from this, goes into the 'LO'(Local Oscillate) port of the demodulation board.

Then from the output of PD also goes to two different ways, one goes to the 'RF'( radio frequency) port of the demodulation board and the other goes to the channel 1 of the oscilloscope which shows in yellow on the screen of it.

The signal of 'RF' and 'LO' mixed together in the demodulation board and the output of it will go through the low pass filter of the board and then connected to the channel 2 of the oscilloscope, it shows the error signal and it is blue.

At the first beginning, I used the Function Generator on the right to send a triangle wave to the PZT driver to scan it and find the mode matching. Then when I got the error signal(the output of the demodulation board), firstly, this signal was sent to Stanford who acts as a low pass filter and then the output of Stanford goes to the PZT driver also.Now the loop has been closed.

And then I stopped scanning the cavity(plug out the orange lines), but just change the voltage of the PZT driver by myself. Until from the CCD screen, I am around the maximum of the TEM00 modes. Then I checked, the cavity is locked and the green beam is very powerful.

That's how I did until now, but actually the lock can not keep for very long, only last about 20-30 mins. So since I cannot switch off the loop(there is no switch I can use), I just need to plug out one off the wire and change the voltage of the PZT driver near the resonance and recover the loop. Try to do it this way and find out if I can get better result or not.

Comment to SHG cavity locked (Click here to view original report: 362)

Add the picture, yellow is from the PD and the blue one is the error signal.

SHG cavity locked

Today we found out the reason why there is some flat part in the spectrum, it is because the PZT driver reached its limitation, so we reduced the offset of the PZT driver and got a normal spectrum. Then except the TEM00(the highest peak) we found a vertical first-order mode and a laguerre mode which can not be killed together. So we chose to reduce the TEM01 as lower as possible and keep the laguerre mode and try to lock the cavity.
Before start to lock the cavity, I set the temperature to the phase matching temperature which we found quiet a while ago. The readjust the mirror a little to bit to get the best mode matching I can. Then I set the the sinusoidal wave with frequency of 15.2MHz and amplitude of 400mV, sent it to the EOM to do the modulation,and this is also used to be the 'LO' signal in the demodulation process. Then used the PD to get the transmitted beam of the cavity, which is the 'RF' signal of the demodulation. Then with the demodulation board, the 'LO' and 'RF' signal mixed together, the output( error signal ） sent to the oscilloscope.Also the error signal will go through the low pass filter and send to the input of the PZT driver. The other part of the PZT input is a triangle signal(parameter: 28Hz, amplitude: 2.5V, offset:2.5V) As soon as I got the error signal I stopped scan the cavity with triangle wave and adjust the PZT by changing the offset of PZT driver until the cavity is locked. After the cavity locked, the green light become much more powerful, and the reflection of the green light goes back into the Faraday Isolator, and the reflection from the Faraday point to the height around people's eyes which is really dangerous.

Optical Table move

I temporarily moved another optical table inside the clean booth because I need some extra space in a clean environment to test the new assembly of the translation stage.

TEM00 and the space for new laser

Yesterday I finished the alignment. Today when we tried to clean up the central room, found the beam splitter in one box, so I divided the transmitted beam from the cavity into two part, one received by the PD to see the spectrum and the other received by the camera to see the mode shape. For now from the first picture, the highest peak is the TEM00. But the problem is the height of the peak is very sensitive. I connected the cavity to the thermal control to stabilize to some extent, but if I touch the table or the mirror, it changed a lot. And it seems also a slow change with time exist.
Today we clean up the unused part of the optical bench for the two lasers, and we found a rack for the power supply. The arrangement shows in the second picture.

New parts for the translation stage

In order to decrease the height of the sample on the translation stage, I designed a new configuration of the assembly of the translation stage and some new parts. With the help of Hirata san, I made these drawings and submitted at the mechanical shop of ATC.

Progress of Jan 20th,23rd

I tried to increase the height of the beam, but every time I increased beam height, it was far more than I need. Finally I found out the difference of the beam height and the height of the cavity is very tiny, before the beam is lower is because a pretty slow reduction of the beam height. So I adjusted the beam path again and made the beam goes into the cavity from the center in vertical way, but this time the situation went back to before, there were two beams in the horizontal direction, but it seems better since there are not very strange behavior of the two beams, and I can see the mode is exactly some higher mode. The alignment will kept doing tomorrow.

Clean up some multiple sockets

I tidied up the power supply of the absorption measurement system because it was too messy. There were a lot of unnecessary multiple sockets connected in series (which is bad for the electrical grounding), especially under the stairs (all covered by dust). So, I rearranged the multiple sockets and put the unneeded ones on a shelf in the first floor, see the picture.

Try to adjust the beam height

I rotated the dichroic mirror in order not to block the green beam for next step. For easy understanding, the first picture shows how I did the rotation. Before I did the rotation, we tried for very long time to do the mode matching, but there are always two beams, and no matter how we changed the two mirrors and the position of the cavity, it did not get better, and the misalignment seems in the horizontal direction. But after I rotated the dichroic mirror for 90 degree, which I think will not effect the transmitted beam, the mode I saw now are all vertical modes. I found a mode with round shape which is likely to be the TEM00, but I am not pretty sure that is the real fundamental mode or just some coincidence that the two beams overlap.I found its peak from the spectrum, and tried to make it larger with adjustment of the two mirrors, but it did not have an obvious change. For now the misalignment is in the vertical direction and it is true that now the beam is a little bit lower than the cavity, it maybe better to higher the beam, since we have four mirrors before the cavity, it seems not very hard to higher the beam and recover it parallel to the table. Then I will check again if I got the TEM00.

Progress of 16th and 17th

We found out the beam we are looking at is not clear enough, it will be harder to do the alignment with that, so I adjusted the camera and also the beam path. From the first picture, it is the beam path that if we want to look at the beam directly with CCD, I lower the camera instead of bending the holder which will make the situation more complicated. The second picture shows what we have now with CCD. It seems the higher order beam is in horizontal direction but the mode is a liitle bit strange. And before if we use the camera with the camera lens, and use the camera to look at the card, the beam is quiet small. So I changed the lens to make the beam larger and have more details of the beam. But then I found out that instead one beam, there were two separate spots on the card.( I didn't get very good picture to show this.) And first I thought it may be the TEM01 mode, but if it is, the pattern should be like one round spot cutting into two, but what I saw it two round spots and there is some distance between them. The other guess is that it is the reflection in the cavity, which means that the beam path inside the cavity is not overlap with the line between the center of miniscope and crystal.But when I drove the PZT, the distance between the two spot seems not change. So I am not sure where the two spots came from.

Alignment of the SHG

I did the alignment SHG cavity for some days. For now, the first picture in the attachment shows the spectrum from the PD. At the beginning, the mode I got are all Laguerre modes. So I moved the position of the cavity to get better waist position matching. After that, I removed the mirror, let the transmitted beam from the cavity go directly to the CCD, in the second picture shows the situation of the optical bench now. Then from the screen that connected to the screen, I got the mode visually. Since the height of the camera mount is too high, so I bend the holder for 90 degree, so from the third picture although it seems like the misalignment is in the vertical direction, actually it is in the horizontal direction. The third picture only shows one of the mode I saw, but the others are all like this, and there are also some others too weak to see.During the adjustment I did on Friday, with adjusting the two mirror before the cavity in horizontal direction, did not make the situation better. I'll keep trying.

Current version of the VIs to control the large translation stage

I upload the front panels and diagrams of the current version of the VIs and subVIs that I made with the help of Sakai-san.

The main VIs are "Translation Stage Map.vi" and "Translation Stage Scan.vi"

I don't upload the diagrams of subVIs that belong the SR830's and the Zaber motors' libraries.

Adjustment of the telescope and other progress

According to the conclusion we got before holiday (the detailed report of the work we did before the holiday will be uploaded later), I did some adjustment of the telescope today to get the beam waist we need as about 52 micrometer. At first, I changed the position of the lens little by little, but since the starting point is the beam waist less than 40, so after three times changing, the size was still far from the goal. So I recalculate the parameters of the telescope, found one of the result with the same lens we used before. According to this result and many times tiny adjustment, I got the needed waist size. The position of each lens list below( still the origin is at the output of the EOM):

L1: 37 cm
L2: 46 cm
L3: 55 cm

the final beam waist is 52.7 micrometer and the position is at 82.2 cm.
The first picture shows what we have on the optical bench before holiday, it is only for doing the measurement, so the dichroic mirror is not in the right gesture. So today I flipped the dichroic mirror, and put the SHG housing in the right position and direction, according to the result showed in the last paragraph. Aligned the inject beam and the reflection beam, got the transmission beam after the SHG and the reflection beam at the back side of the mirror after EOM. I tried to find some mode but did not get any. I will try to do it tomorrow.
Another problem is that I thought the stage we are using now is broken...in one of the direction, it can only move one way, but it can still be used.

Cavity locked and got the beam size

Member: Matteo,Yuefan
This week we increased the height of the crystal instead of lower the miniscope. But after the increase, we still cannot get rid of the higher modes. We guessed it is because of the mismatch of the beam waist size. Since the increase of the crystal was reached with putting a spacer under the 'L' part which used to fix the crystal, the wire of the thermal sensor and the peltier are also connected to this part, we cannot push too much on the crystal,so now when we touch the wire outside the housing, the position of the crystal will change slightly. Another problem is that we found out the first turning mirror is too closed to the beam waist focused by the first lens of the telescope, and this caused the optical damage of the mirror coating, since the power density at the beam waist is too high when we use the full power of the laser. So we decided to move the the easily damaged turning mirror one hole closed the first lens, and do the same change with all the other component of the telescope without changing the distance between them. Then according to the new version optical scheme, we changed the dichroic mirror into the one transmit green and reflect red. In this case, we can use this dichroic mirror and the last mirror of the telescope to adjust the beam, things became easier. As I mentioned before, we guess the reason of the higher mode existence is because of the mismatch of the beam size. So we turned the cavity again,inject the beam from the back side of the housing, tried to lock the cavity and get the exact beam waist size we need. Give the EOM 15.235MHz modulation and use the PD to get the error signal. With the demodulation board found in TAMA, and the Stanford as a low pass filter, we locked the cavity, but cannot get rid of the offset. The lock last for more than two hours until we swtiched off everything and left. During the lock, we measure the beam size and got the beam waist size the cavity need is 54.1 micrometer. So the next step is the adjust the telescope again to get the beam waist size given by this locking.

Picomotors status

Picomotors of the 4 suspended mirrors (2 of filter cavity and 2 of telescope) need to be put in operation.

 

The cables that connect the picomotor to the the chamber flange inside the chamber should be already in place for the 4 suspension (to be checked)

 

Takahashi-san provided driver for the picomotor (fig 1) to be connected to a cable (fig 2) to the extern part of the flange. This device were used to control picomotors of TAMA SAS.

 

The drivers can be controlled by a joystick and by remote using a labview software that Takahashi-san will provide

 

Some more information:

 

1- Picomotors drivers are 3 (IM1 IM2 NM3), NM2 could not be found. We checked in both end rooms without success.

 

2- We need 4 cable to connect them too the  4 chambers. For the moment we have only 2. (Again we checked in both the end rooms).  According to Takahashi-san the missing two cable can be built using spare connectors like that in figure 2 and spare suitable cables I already found in TAMA. Takahashi-San can also provide the suitable tool to connect pins in the connector.

 

3- All this material is temporarily stored in a box under my desk in TAMA

NIM-modules to filter and amplify optical lever signals

In the past days with the valuable help of Matteo L. we realised 4 NIM-modules able to filter an input signal with a lowpass filter of 2 order (cut frequency at 100 Hz) and to amplify it of a factor 100.

 

The scheme of the circuit, using a multiple feedback (MFB) filter, drawn with the software LTSpace is shown picture 1 and its transfer function in picture 2. 

 

Each module can process two channels, so in total we have 8 available filters+amplifiers.

 

A picture of the circuits and the module are shown in attached files 3 and 4.

 

They will be used to filter and amplify the output signal of the PSD  used for the filter cavity optical levers, replacing the stanfords that have been doing this job up to now.

Progress in SHG

Since we found a spare PSD in TAMA, we used that PSD to make the alignment of the crystal optical axis with the beam. We changed the stage under the SHG housing, for two reason: take less space, let the center of the crystal to be the exact the same height with the beam. With the available component in TAMA, in the attachment I put the picture of the present stage we made. At first step, we use the PSD to get the total power and the power in two directions. Then put the SHG housing on the new stage, but with 180-degree turning compare to the final position of the housing, in this way we can make sure that the beam really goes along the optical axis of the crystal. After getting almost the same power value compared to the first step, which means the crystal has been put into the right position, we installed the miniscope, try to find some modes. But finally we found out we need to lower the miniscope to get better result,the reduce of the height probably can be finished in NAOJ. So the question for now is how much we want the engineer to make the reduction. For now we have two kinds of spacer,one is 50 micrometer, the other is 150 micrometer.

Optical scheme changed

We finished the alignment of the beam, kept the reflection beam and transmission beam at almost the same height as the input(76cm) and got the green. According to the new optical scheme, we need to change to a different kind of dichroic mirror(reflect infrared and transmit green), which we don't have in lab now. And also the present transfer stage we are using for SHG housing take too much space. If we do not change it, it will be hard to realize the new optical scheme. In the attachment, I put the picture of the stage.

Alignment of the beam hight

Member: Matteo, Eleonora, Yuefan
Today we restart the alignment from the first beginning. Firstly, with the power meter, measure the input and output of the EOM, adjust the transfer stage until almost the same power value. Then for the component, try to adjust them until the beam shine on the middle of everyone. With the tilt angle of the mirror, adjust the beam height as 76cm at both the near field and far field. Finally, we found out the transfer stage under the SHG housing is too high for present beam height.So we remove the bottom layer, got the transfer beam and the reflection beam, but the green is weak. The possible reason may be:1.Low input power 2.During the adjust process, the position of the lens may change,leading to the change of the waist position, so the input beam size of SHG crystal is too large 3.The polarization 4. The misalignment of the beam height and the crystal height. We will check all the possible reason tomorrow.

Cavity alignment

Member: Matteo, Eleonora, Yuefan
Last Friday, we installed the mirror part of the cavity,tried to align it. For doing this, firstly we add 20 kiloohm resistance to the present photo diode to increase the sensitivity at the cost of reducing the bandwidth(200 kHz). At the first try,we got multiple modes.With two mirrors and the lens, we tried to rule out higher modes and get the TEM00 one, but found out that there is misalignment in the vertical direction. After adding some piece under the mirror block, this problem has been solved, but still we cannot get rid of the effect of TEM02(TEM20) mode. We will restart the alignment again today, try to make some progress.