NAOJ GW Elog Logbook 3.2
We observe that the PDH filter cavity signal has an offset of ~ 170 mV. See picture.
The offset is present even when the 78 MHz signal sent to the EO modulator is swithed off (and the 78 MHz sent to the local oscillator is ON). When both signals are OFF, we see a slowly varying offset between 200 mV and -200 mV, which also have an higher frequency oscillation. To be investigated.
We have measured the spectrum of the PZT correction signal sent to the laser when the cavity is locked, using the output PZT_mon (1/100 of the PZT correction signal). The spectrum is in the attached plot.Since in this region the gain of the loop is very high, the signal is proportional to the cavity length/frequency noise.
The calibration is 1 MHz/V (given by the manufacturer).
at 100 Hz we have ~ 700 nV/sqrt(Hz) corresponding to 70 Hz/sqrt(Hz), at 1 kHZ we have 100 nV/sqrt(Hz) corresponding to 10 Hz/sqrt(Hz)
The shape of the spectrum is compatible with the free running laser noise ~ 7-10 kHz /f Hz/sqrt(Hz) up to a few kHz. According to aother measurement, after ~4 kHz the spectrum is limited by a flat noise, which is compatible with the noise of the 100 kOhm resistor at the output of the PZT_moni signal. For f<10 Hz probably the mirror control noise and the seismic noise are limiting the spectrum.
We also see several 50 Hz harmonics. It is not clear if this harmonics can be reduced rearranging the grounds and if they have an impact on the RMS of the error signal of the filter cavity locked. To be investigated.
Summary of yesterday night work (thu 29-->fri 30). The goal was to make a characterization campaign for the cavity lock, in order to make it more stable.
1) Beam stability
In the past we observed an evident jitter of the beam. From a comparison of the spectra we were convinced that this was caused by the residual motion of BS and PR. In the past days we where able to improve the stability by improving the local control filters (a dedicated entry will follow).
We observed that the beam direction (observed by misaligning the input mirror) was drifting and we decided to test a new strategy to keep the mirror position. We change the local control filters in order to avoid to gain at low frequency (we changed a pole at 0.1 Hz with a double zero at 0.1 Hz and we controlled the mirror position not by adding an offset of the loop but simply sending a DC signal to the coils.
We coudn't see a major improvement in the performances.
We also observed the intermittence presence of spikes in the error signals from BS and PR which makes difficult to keep the cavity alignment.
Eventually the old controls (with integrators at low frequency) were restored.
2) Laser servo gain transfer function
We have set the gain of the servo in order to have ~10 kHz bandwidth. See the transfer function in fig.1. (in 1/f^4 mode)
At a first look, the TF behaves as expected. The data have been stored in the floppy disk and they will be compared with the model. The phase margin at ~10 kHz is about 40 degrees.
The transfer function has been measured with the Agilent 35670A spectrum analyser, with a swept sine with 50 mV ptp.
3) Servo parameters
- modulation depth = 1 V pp at 78 MHz (reduced with respect to before). This should correspond to a modulation depth of m= 0.185 rad.
- LO = 8.5 Vpp at 78 MHz (increased with respect to before)
- Demodulation phase = 111 deg
--> With this data the error signal is 3-4 V ptp, for a transmited signal of ~ 3-4 V depending on the alignment of the cavity (note that we did not checked the green laser power yesterday night)
- attenuation of the input signal =9.1
- PZT gain = 0.7
- thermal control gain = 3
- Threshold on the transmitted signal ~ 2 V
4) Auto-relock
With this configuration the cavity automatically locks when the transmitted power crosses the resonance. When the cavity unlocks, it relocks automatically. Note that the servo is always in the 1/f^4 configuration. The video shows the cavity locked, then the input mirror is on purpose misaligned, then it is re-aligned and the cavity re-locks.
5) Stability
During yesterday night lock the cavity was very stable. The plots 2 and 3 show the transmitted power (in cyan) and the error signal (in yellow) for 500 s. No actions were performed to realign the cavity on the second plot. Max transmitted power was ~ 4 V.
On Tuesday 27th june we managed to lock the laser on the filter cavity length.
In the first attachment there is a plot of the transmitted power during the lock acquisition, in the second there is a picture of the transmitted beam when the cavity is locked. A short movie of the the lock acquisition can be seen here.
Friday, June 23, 2017
Following the following procedure, I aligned the Imaging Unit for the HeNe probe beam.
1) move IU micrometer closer to the end of translation which would give you enough translation range to move the whole IU farther away in case you test thick objects;
That will complete rough alignment of the IU. The fine tuning is done by maximizing AC signal coming from the surface calibration piece. For that, try different micrometer positions around one you started with. For every position you have to center the probe (maximize the DC), maximize AC if needed. The maximum R should be close to the original R for the surface calibration. Then make scan with the bulk calibration piece.
According to the theory the signal is maximum when the detector at the Rayleigh length of the perturbation, experimentally we can check this changing the position of the blade and aligning again the imaging unit and measuring the signal. So, in order to maximize the signal, I repeated the procedure changing the position of the blade from 18mm to 12mm and 6mm but I got a lower signal, so I aligned it back to 18mm.
The absorption signal of the reference is similar to original value (the one we had since we bought the system) even if I changed by the 20% the waists of pump and probe,
Parameters: LD current = 0.8A, power without sample = 33mW
We changed the layout of the end bench, added one more mirror after the beam splitter to divide the beam into two, one is received by a screen and a camera was set to look at this screen, the other goes directly into the CCD. The mirror we used has the maximum reflection when it is putting in 45 degree, so we turned it a bit in order to have large enough power also for the transmission. The one received by the screen we took it as a reference for the alignment of the cavity and the CCD is to find a good mode matching. In this configuration we will need three images sending back from the end room at the same time(two on the end bench and one for the second target), the electrical board we are using now for the video only has two channels. So we took the same board from the west end, but it seems one channel of that board is broken, but it is enough for our current need. The fiber used to send the video signal now is '1-13,1-14,1-15,1-16', each board need two fibers. Also each monitor only can receive two channels, so we also took the monitor at the second target of west end to the central room.
2.Analog signal board
On the reflection path of the beam splitter on the end bench we put a PD before and use the receiver box to send this signal to central room for locking the cavity. But since the the aperture of the PD is limited, the spectrum we saw will also be effected by the alignment of the beam. So yesterday we changed the PD to PSD in order to have the information of the beam position. But in this way, we need to send back three analog signals together. We found two board for this and one of them have four channels. We tested the board with a sine wave sent from the end room board, and received it in the central room. The fiber they used for this board is too short to connect into the rack('1-11,1-12'), but the fiber system of TAMA is too complicated, so we just simply changed with longer spare fibers, now we are using '3-13,3-14' for this board.
From the board it seems we can change the gain and offset of each channel, but when we sent a 4V peak to peak signal, it pretty hard to see if it is changed or not, so maybe we can only changed in very small scale. Also it seems four channels all have different setting offset and gains, but we need further check about this.
So we decided to increase laser current to have higher power in infrared and also in green, the current we used before is 1.040A which gave a 8mW-green power at the end of the bench, now we increased the current to 1.2A which gives out a three times green power around 27mW at the end of the bench.
With this power we easily found the beam at the second target and also received it at the end bench.By looking through the window of the end chamber, we better adjust the beam on the end mirror and let it pass more or less the center of it, luckily we got the flash of the cavity with the first try.
The other thing we did before closing the chamber is that we sent the beam out of the chamber to the corridor again after the BS, tried to superpose the green and infrared both in the near field and the far field with the last two infrared steering mirror on the bench. Although considering the air fluctuation in the corridor, two beams moved a lot, but we did our best to make them overlap at 300m.
2-inch mirror: f=-30cm z=4.51m
PR suspension: f=3m z=7.235m
Input mirror: f=-218.35m z=12.135m
End mirror: f=-218.35m z=312.135m
With these values, the simulation result shows that the beam will keep diverging in the arm which is not what we want(pic1). So I did some calculation, we need to move the 2-inch 3.9cm backward to get the beam waist around 150m inside the arm. (pic2)
The other reason why we should move 2-inch mirror is after we installed the real input mirror, we got green reflect back from the from surface of that mirror. But it seems the reflected beam is much larger the original one.
Before moving the mirror, we set two reference point, one is the position of the beam in front of the input mirror, the other is the beam reflected back to the bench. Also after the input mirror we put two aluminum mirror to reflect the beam into the corridor to check the beam propagation. At first we only tried to move the picomotor of the 2-inch, but even we when we finished all the range, nothing changed. So we moved by hand little by little. Every time we moved, we recovered the beam with pitch and yaw of the 2-inch on two reference point and check the beam size at the entrance of the corridor and bench. After moving about 3cm, the reflected beam size seems fine, so we followed the beam inside the corridor, found it has beam waist around 150m, and the size is from 1cm to 2cm which is acceptable.
Then we took the reference of the BS chamber like what we did to the PR, with two aluminum mirror, the beam transmitted by the 2-inch and the BS suspension have been reflect out of the chamber.
The filter's fan part was fine, so it was enough to buy only the filter part. New HEPA filters were delivered to Tama last friday. I washed the prefilters with water, I cleaned the fans, replaced the filters and placed them back to the top of the absorption bench clean booth.
After 30'-45' the pressure was at 100 mTor so I started the turbo pump.
On Friday evening (June 17th) the pressure in the EM2 chamber reached 3.1e-7 Torr.
On the other side of the gate valve the pressure was 1.1 e-7 Torr.
So I open the large gate valve and immediately after the valve between the turbo pump and the tube
(the valve between the turbo pump and the EM2 chamber was already open).
The pressure went up a little bit in the 1e-6 Torr range and then came down again.
This morning (Monday June the 19th) the pressures are:
EM2 chamber = 1.6e-7 Torr
EM2 tube = 2.3e-7 Torr
Middle of the tube: 4.7e-8 Torr
Middle of the tube: 2.7e-8 Torr
NM2 tube = 8.6e-8 Torr (this sensor was not working but is working again now)
NM2 chamber = in air
Overall it looks reasonable to me even if there is probably some out-gassing coming
from the EM2 chamber.
Setting the limits at the translation stage generated a bug in the part of the VI where it sends the positioning commands: if the position exceeds the axis range, the VI goes in loop. I fixed it making a subVI "Read limits axis.vi" that reads the limits from the translation stage controller and set the minimum and maximum positions in the property node of the position controls of the VI.
The origin was set at the front surface of the beam splitter. The beam waist size is w0=51.2um and waist position is z=-9.5cm.
Then I took this as the initial value to calculate the lens we are going to use for the telescope. So the condition is, the MC input mirror is 60cm away from the origin, and the beam waist should be around the position of the input mirror and has a size of 277um.
I let the program chose the lenses with focal length we have now and got this result.
L1: f1=100mm z1=5cm
L2: f2=200mm z2=37.5cm
L3: f3=200mm z3=55cm
So with this design the final beam waist is w1=279um, z1=60cm, which is the result we want.
The other thing is before we were using a 200mm lens to focalize the green beam we distracted from the Faraday to the PD. But yesterday we found out, if we want to give power supply and the signal output, we need more space for cable. But the position of the PD before was too close to the post of the steering mirror. So today I changed the lens into a 150mm one, and moved the PD 5cm forward.
I assembled the imaging unit for the HeNe probe laser. I placed it on a 50mm translation stage.
I had to adjust a bit the position of some parts of the translation stage, sothe new pin-hole position at the cross point is
@01-axis: 2646049 2646049
@02-axis: 279284 279284
Yesterday, we finished the installation of the optical components to extract the signal from the end mirror on a photodiode and a CCD camera.
The optical bench is composed of one lens (focal length = 1m) inside the end chamber, one periscope, another lens (focal length = 0,150 m), one dichroic mirror, one beam splitter, one CCD camera and a photodiode.
Instead of using a periscope fixed on an elevated optical frame, we built a higher periscope.
On this periscope, 2 Aluminium mirrors (TFA-50-C08-10) were mounted. We also checked that they were reflective for infrared : 92,08% at 45°
The 2 lenses are used to get a smaller beam size onto the photodiode and the CCD camera. Currently, the beam hitting these two components is 6,67 times smaller that the beam hitting the end mirror. We might have to change the second lens as we expect a drastic change in the beam shape when we will remove the tube windows.
The beamsplitter will also be changed as it is supposed to be working for green (instead of infrared for the moment). It is still able to split the beam in two, so we were able to detect the beam in the photodiode and send it back to the central room.
Today I tried to measure the spectrum of the beam motion on the optical end bench, after the end mirror. The main problem is that the beam shape is pretty irregular and very elongated in the orizontal direction (almost comparable with the sensor size). In the attached plot is shown the spectrum I found for the vertical motion (both with open and closed PR and BS loops). It is just a very qualitative measurements and I'm not sure about how much it makes sense.
Some remarks
1) I used a calibration measured here for a PSD of the same kind, with red light. I'm not sure if and how it scales with the wavelenght (to be measured..)
2) The total rms (using this calibration) seems underestimating the real motion of the beam which by eye should be of the order of few millimiters.
3) The peaks are clearly coming from the PR and BS resonances. In particular the one at 10 Hz desappears when the BS pitch loop is closed as observed on BS spectrum
4) The PSD was not covered and the measurement seems affected by acustic noise.
I'll try do more quantitative measurement (better calibration, smaller beam, box on PSD) as soon as possible.
First thing is to turn the target into the remote mode, since on the label there are only two modes(remote and local), but actually when you try to move the handle, there are three levels.Remote is the one on the left.
I connected the power control box of the target (1st picture) to the '133.40.121.***'port, so when you want to control the target, you need connect to the laptop to the same IP address.
The first thing is to set the IP address and the netmask of your own laptop to 192.168.10.2 and 255.255.255.0.
Then go the address 192.168.10.1, you will see the webpage in pic 2. Enter your username(admin) and the password(magic). The next page (pic 3)shows the condition of every outlet now.
Then press the button on the left side with the red rectangle out of it, you will enter the page where you can control the power in and out.(pic 4)
Every column in the chart means:
1st: The number of the port which written on the top of the control box, from right to left is from 1 to 4.
2nd: The name of the outlet, this depend on the order you plug in everything, for now we did like what shows in the picture.
3rd: This column is used to control the power, the three buttons in each row are "power on","power off" and "reboot".
4th: The condition of the power supply now.
Since we are going to use the green beam reflect back from the end mirror to lock the cavity. We need to send the reflected beam into a PD, so we need to extract that beam out from the FI. But when we tried to do it, we found out the beam we need is reflected to the table direction, which is very hard to extract. Also the FI we are using, only the output polarizer can be turned to tune the frequency. Finally we found a way to do it, the final setup shows in the third and fourth pictures.
Today I tried to send the beam transmitted from the BS mirror out of the chamber. At first I used a one inch mirror, but the beam is almost the same size as the mirror, so it has been a little bit cut on the edge. So then I changed it into a two inch aluminium mirror. The beam is coming out from the spare window beside the tube which connected the PR chamber and the BS chamber.Then I put a aperture out side the window, center the beam on it and another beam damper after it.
There was another problem we found today. The beam looks good everywhere until it enter the tube, then when we check at the first target, it was cut.There is a bright spot in the middle and a large ring outside, the cut part is the outside part. When we tried to move the telescope mirror, the beam on the first target moved in a very strange pattern which we still cannot understand. Finally we decided to send the beam out after the BS mirror, sent into the corridor, then we checked it further but did not see either the outside part or the cut. Now we suspected that this large cut beam may come from the reflection between the target and the window. We will try to move each telescope mirror again tomorrow for better understanding the situation.
I moved the large sample to a z position such that the first surface is about 1 cm beyond the pump-probe cross point.
Using the Zaber Console, I read the position with the command "/02 0 tools storepos 1 current". Position is 1039045 1039045. Then set this position as the max translation limit for each axis with the commands "/02 1 set limit.away.pos 1039045" and "/02 2 set limit.away.pos 1039045".
I also noticed that there is a screw of the mount that risks to hit the HeNe probe mirror mount, so I also set the min limit on axis z to 50000 steps (~6.2mm) with the commands "/02 1 set limit.home.pos 50000" and "/02 2 set limit.home.pos 50000" see picture.
In the past days I have been working in order to restore end mirror local controls. I had a hard time finding a satisfying diagonalization both for the sensing and the driving. The current mechanical TFs, openloop TF and the comparison between open loop and closed loop spectra are shown in the attached pdf. See for comparison what I found for the dummy mirror last july.