Better green beam

I tried to change the cavity temperature to see if the beam gets better. And then at some point I got the green beam shows in the picture, it looks good and much more bright then before. And I also tried to put some filters to see if there is any detail I can not find with this brightness, but even when it's very week it still looks good.

Before when we tried to get the relationship between the temperature(resistance) and the green beam power we did not install the miniscope ,so that's maybe the reason why now we need to change the temperature a little bit to get better phase matching.

I will take some data points to get the exact new best temperature.

Translation stage: final assembly test

I got the parts I draw (elog entry 359) from ATC machining shop. Hirata-san washed them and  I assembled the translation stage putting the parts to make the mirror height as low as possible.

Check the infrared beam with CCD

I checked the infrared beam with the CCD camera.

The first picture is set to check the beam in front of the dichoric mirror, and the second picture is what I got. It seems a really normal beam.

The third picture is set to check before the cavity, since I tried many different ways but cannot put another mirror in the middle of the dichoric mirror and the cavity without moving them, so I used the filter to reflect the beam. The fourth is what I got, it does not look like a round one.

So I change the mirror in first picture to filter to see if it is the reason of beam shape change, and I got the fifth picture.After checking the infrared beam many many times, I think the strange green beam probably not caused by the infrared beam.

GigE Camera

I bought a GigE camera.

http://www.baslerweb.com/en/products/cameras/area-scan-cameras/ace/aca2040-25gmnir

 

You can find information on KAGRA camera system.

http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=4237

 

To use this camera, we need

* Giga-bit Ethernet cable

* Power supply : http://www.baslerweb.com/en/products/accessories/ios-power/power-supply-12v-hrs-6pin

* PoE (Power over Ethernet) Injector:  http://www2.elecom.co.jp/network/injector/lan-gsw01es1/

and

* Pylon software: http://www.baslerweb.com/en/support/downloads/software-downloads

Mirror Glueing JIGs

Together with Yuefan and Manuel,

I confirmed that two mirror glueing jigs are storaged in small clean booth.

One is for 100mm diameter mirrror, and the other is for 150mm diameter mirror.

 

Related parts are packed into one large container box.

This large box is placed at large shelf. It is around the desiccator.

Inside this large box, magnets (phi 2mm x 10 mm) and stand-off (wire breaker)

are storaged in each small containers.

Check the infrared beam

I checked the infrared beam at different position: before the telescope, before the dichoric mirror and before the cavity.

The former two positions, I just put a mirror to reflect the beam and look at it at far field, since the space before the cavity is narrow, so I used the filter to reflect the beam.

For now, the infrared beam looks normal even in far field, but I cannot manage to look at with the CCD.

And I changed the BS mirror after the cavity to another dichoric mirror(CVI 1 inch one) as the picture shows, which reflect infrared and transmit green.

According to Raffaele's advice, I used the signal generator to send a sin wave at modulation frequency but with 100mV amplitude to the oscilloscope, and with a T shape lemo connector plug in and out the RF in port of the demodulation board to see the change of the amplitude. When the RF in port did not connect, the amplitude showed on the oscilloscope is A, when the RF in port plug in, it changes to nearly a quarter of A.

And Manuel found these two PD while clean-up, it can be used in infrared.

Optical Signal Transfer from End-room to Center room

There is two systems.

When we replaced the system to new one, I wrote the attached report.

New signal transfer system has low latency for monitoring the transmitted light of 300 meter FP cavity.

 

Today I found two receiver modules in the center-room.

Both of old and new system modules are placed behind a LCD monitor for digital control system of optical lever.

Maybe signal sender modules can be found at both end rooms.

Because optical cables are unplugged for new system, please check the cable assignments

by sending some signal from the end-room.

Vacuum compatible adhesive: MasterBond EP30-2

I confirmed that a set of MasterBond EP30-2 is storaged in desiccator (dehumidified storage shelf).

Repair of JASMINE scatterometer

I started repairing the scatterometer in the JASMINE lab today.

Before doing the actual setup of the new laser, I cleaned up the desk and put the PC, the lock-in, and the motor-power supply on the desk beside the optical table. This gives now much more space to anybody working there.

I also tested the switch-box that has been build by Tanioka-san with the new laser. So far, everything works well.

Improved assembly of translation stage

I added some aluminum beams, bought from Misumi company, to increase the rigidity of the translation stage

The picture of the resonance peak while power changing

According to Aso-san's advice that the asymmetric may be caused by the laser power heat up the cavity when it nears to resonance leading to the increase of the cavity length, so it takes longer time to reach the maximum, I tried to see the resonance peak at different input power, the four pictures are took at laser power about: 2W, 1.5W, 1W and 0.5W.(When the laser power is 2W, the input power of the cavity is 820mW). It seems a little bit better with lower power.

Some pictures for better understanding

The first picture shows the shape of the green beam when the cavity is locked at the right temperature.

For the second picture, I tried to scan the cavity with pretty small amplitude triangle wave, and then change the PZT driver by hand to get near to the resonance.

From the picture, there are two problems:

The first one is the what I described before, the slopes from two side of the peak is different, take the first peak as an example, the reduce process is faster than the increase part.

The second is that the two different peaks are actually the same mode but they did not look the same. I discussed with Manuel and we thought it maybe because of the hysteresis, there are some loss in the PZT.

Description of the problem now

There are a few problems with the locking now:

1.The power of the TEM00 transmission drops suddenly

When we try to scan the cavity by changing the output voltage of PZT driver by hands slowly, we can observe the slowly increasing part of the TEM00 power both in the PD and CCD, but then when it reaches its maximum(or near) it drops suddenly. On the oscilloscope connected to the PD, we can see that the power suddenly drop from several tens of mV to several mV, and from the CCD, the diameter of the beam and also the brightness suddenly get smaller.

When I try to lock the cavity, I did as what I will describe below, which I am not sure it's the right way or not. The power of the TEM00 will change from weak to strong and go back to weak when I scan the cavity, which means there is a range of the voltage that gives to the PZT, like from 15V to 17V, I tried to lock the cavity at different voltage. If I chose to lock the cavity at the maximum power, the lock will last for less than 5 mins, and from the CCD I can see it get weaker and move to next modes. And if I locked the cavity at the range where the power is slowly increasing, the lock will last longer, like more than one hour, but the power still slowly increase to the maximum and start to reduce, finally it moves to the same 'next' modes as in last situation. And if I start in the 'suddenly drop' range, it seems much more stable then the other situations, I wait for more than one hour, the power increase to some point at the first beginning then it stops there and does not have any obvious change after. I am not very clear about what this means, and I am also not sure about this result can be repeated since I tried the last situation only once. So maybe if it has some meaning,I can try to do it tomorrow again.

2.The strange shape of the green beam

If we look at the green beam when the cavity is locked(the green is much more powerful and easy to see the detail)and the temperature is at the 'good' temperature, the beam is not a spot, there are many irregular dark lines in it which cut the green like into many strange shape pieces. At the beginning of this week, when we see the green light under room temperature, it almost a spot with a circle which is caused by the Laguerre mode. But now even when we turn the temperature back to the room temperature, it is not as good as what we saw before, but because it's very weak so we can not see it very clearly what exactly the pattern is.

3.The TEM00 is 20 times less in cavity-locking compare to mode-matching-finding and the error signal is only several mV

In my Tuesday logbook entry I gave the picture how I lock the cavity, the PD is connected to the 'RF' port of the demodulation board and also the CH1 of oscilloscope. Also in this picture, from the small picture of the oscilloscope screen,(the yellow is from the PD), it shows that the magnitude of TEM00 (the highest peak of the CH1) is about 50mV. But when we did the mode matching(CH1: PD, CH2: the triangle wave), the TEM00 is more than 1V. Then we found out if disconnect the demodulation board 'RF' port and the PD, the TEM00 goes back to it used to be (>1V). It may caused by the input resistance of the demodulation board,so I will check the PD output resistance and the demodulation board input resistance, to make sure this problem is caused by them or not.

4. The output voltage of the PZT driver does not change

When the loop is closed, if there is some fluctuation, then the feedback should work and the PZT driver will give out the voltage to push or pull the PZT, make it back to the resonance. And in my situation there is a screen on the PZT driver shows the output voltage of it, but it does not change during any lock process until now. My guess is the error signal is too small (couple of mV), the magnification of the PZT driver is 30 times, so even after the signal is amplified, it's couple of tens mV, which cannot show on the display of PZT driver whose digit limit is 0.1V.

I will check the following things later:

1. The properties of the PD and the demodulation board (resistances, the cutting frequency of low pass filter on the demodulation board)

2. Which PD we used in last locking with Matteo

3. The properties of the dichroic mirror, since it should be reflect infrared and transmit green, and now the reflection green beam is really powerful.

4. The exact power sent to the cavity when using the maximum output of the laser

5. The polarization we are using now

Two questions in locking

As what I showed in yesterday's report, through the CCD camera I can see the mode shape and power while I changed the output voltage of the PZT driver. So today I try to lock the cavity with different power, and found out if lock the cavity with the maximum of the TEM00 mode, I will lose the lock very soon, like in 5-10 mins. If I set the start point at some point near the maximum, the cavity will lock for longer period.

But it seems if I call the two modes near TEM00 as the previous mode and the next mode, the unlock always happened when the TEM00 move to the next mode, I think this means the PZT only let the mirror goes to one direction but I am not sure.

The other problems, the screen on the PZT driver shows the voltage it gives to the PZT, but during the lock process, it does not change. I think it is strange, because when there is fluctuation and the PZT will move to recover it, at this time,the number should change somehow.

Comment to More details of locking (Click here to view original report: 365)

Attchment

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.