When I turned them on, the control signal in Pitch from WFS2 started going up with no stop. It was like the integrator in the loop was fed with a DC bias. The effect of that was to misalign the MC cavity from the good state in which it was with the only length control on (that is, transmission ~2.7, reflection ~ 0.4).
I don't know why that is happening. To exclude that it was due to a computer problem I first burtrestored C1IOO to July the 18th, but since that did not help, I even restarted it. Also that didn't solve the problem.
At least one problem is the mis-centering of the resonant spot on MC2, which can be viewed with the video monitors. It's very far from the center of the optic, which causes length-to-angle coupling that makes the mulitple servos which actuate on MC2 (MCL, WFS, local damping) fight each other and go unstable.
I played with the MC alignment for the beam centering. After that, I restored the alignment values.
In principle, one can select the MC2 spot as one likes, while the transmitted beam axis to the IFO is not changed
as far as you are at the best alignment. This principle is almost trivial because the beam axis matches
to the input beam axis at the best alignment.
The alignment solution is not unique for a triangle cavity if we don't fix the end spot position.
In practice, this cruising of the MC2 spot is accomplished by the following procedure:
0) Assume that you are initially at the best alignment (=max transmission).
1) Slightly tilt the MC2.
2) Adjust MC1/MC3 so that the best transmission is restored.
I started from the following initial state of the alignment sliders:
After many iterations, the spot was centered in some extent. (See the picture)
The instability looked cured somewhat.
Further adjustment caused a high freq (10Hz at the camera) instability and the IMCR shift issue.
So I returned to the last stable setting.
Of course, if you move MC1, the reflected spot got shifted.
The spot has been apparently off-centered from the IMCR camera. (up and right)
At this stage, I could not determine what is the good state.
So, I restored the alignment of the MC as it was.
But now Alberto can see which mirror do we have to move in which direction and how much.
I set the MC back to its good alignment (June 21st) using this procedure. The trend of the OSEM values over the last 40 days and 40 nights is attached.
Then I aligned the periscope to that beam. This took some serious periscope knob action. Without WFS, the transmission went to 2.7 V and the reflection down to 0.6V.
Then I re-aligned the MC_REFL path as usual. The beam was far enough off that I had to also re-align onto the MC LSC PD as well as the MC REFL camera (~2 beam radii).
Beams are now close to their historical positions on Faraday and MC2. I then restored the PZT sliders to their April snapshot and the X-arm locked.
Steve - please recenter the iris which is on the periscope. It has been way off for a long time.
So it looks OK now. The main point here is that we can trust the MC OSEMs.
Afterwards I rebooted c1susvme1 and c1susvme2 because they were skewed.
It is really surprising that we now have again the data from the MC OSEMs since up to two days ago the record looked corrupted (see the attachments in my entry 1774).
The reason I ended up severely misaligning the the MC is exactly that there wasn't anymore a reference position that I could go back to and I had to use the camera looking a the Faraday.
I found two ThorLabs PDA55 Si photodetectors that says detect visible light from DC to 10MHz that I'm going to use from now on. I don't know how low of a frequency they will actually be good to.
ETMY oplev is still out of order. Hopefully I'll get it under control by tomorrow.
I mapped out the corresponding pins on both ends of the Guralp seismometer cable. Here is the diagram:
The circular 26-pin end of the cable (that plugs into the seismometer) is labeled as above. The other end (the 39-pin end) is not physically numbered, so I just came up with a numbering system. They are both pictured on the non-cable end of the connector. The colored circles indicate the pin pairs.
FROM JENNE, 30JULY2009: the Dsub end is 37 pin, not 39.
Peter and Koji,
We are constructing a setup for the new 40m CDS using Realtime Code Generator (RCG).
We are trying to put simulated suspensions and test suspension controllers on a different processors of megatron
in order to create a virtual control feedback loop. Those CDS processes are communicating
each other via a shared memory, not via a reflective memory for now.
After some struggles with tremendous helps of Alex, we succeeded to have the communication between the two processes.
Also we succeeded to make the ADC/DAC cards recognized by megatoron, using the PCI express extension card replaced by Jay.
(This card runs multi PCI-X cards on the I/O chasis.)
- Establish a firewall between the 40m network and megatron (Remember this)
- Make DTT and other tools available at megatron
- Try virtual feedback control loops and characterize the performance
- Enable reflective memory functionalities on megatron
- Construct a hybrid system by the old/new CDSs
- Controllability tests using an interferometer
Note on MATLAB/SIMULINK
o Each cdsIPC should have a correct shared memory address spaced by 8 bytes. (i.e. 0x1000, 0x1008, 0x1010, ...)
Note on MEDM
o At the initial state, garbage (e.g. NaN) can be running all around the feedback loops. They are invisible as MEDM shows them as "0.0000".
To escape from this state, we needed to disconnect all the feedback, say, by turning off the filters.
Note on I/O chasis
o We needed to pull all of the power plugs from megatron and the I/O chasis once so that we can activate
the PCI-e - PCI-X extension card. When it is succeeded, all (~30) LEDs turn to green.
This morning I found the Mode Cleaner unlocked.
I check the sliders for the mirrors bias and they have not changed. Also the OSEMs readbacks show no change in the optics positions.
I don't understand what's wrong because in the previous days, in this state of alignmanet, it could lock.
I tried to tweak a little bit the periscope to check whether it was a problem of beam matching but that didn't help the cavity to lock.
I don't want to change the periscpe alignment to much becasue I believe it is still good and I suspect that there is something else going on.
For the past couple of days I have been trying to understand and perform Koji's method for impedance measurement using the Agilent 4395A Network Analyzer (without the impedance testing kit). I have made some headway, but I don't completely understand what's going on; here's what I've done so far.
I have made several transfer function measurements using the attached physical setup (ImpedanceTestingPhysicalSetup.png), after calibrating the setup by placing a 50 Ohm resistor in the place of the Z in the diagram. The responses of the various impedances I've measured are shown in the attached plot (ImpResponses.png). However, I'm having trouble figuring out how to convert these responses to impedances, so I tried to derive the relationship between the measured transfer function and the impedance by simplifying the diagram Koji drew on the board for me (attached, ImpedanceTestingSetup.png) to the attached circuit diagram (ImpedanceTestingCktDiagram.png), and finding the expected value of VA/VR. For the circuit diagram shown, the equation should be VA/VR = 2Z/(50+Z). 50 Ohms is good to use for calibration because the expected value of the transfer function for this impedance is 1 (0 dB).
So I used this relationship to find the expected response for the various impedances, and I also calculated the impedance from the actual measured responses. I've attached a plot of the measured (red) and expected (black) response (top) and impedance (bottom) for a 1 nF capacitor (1nF.png). The expected and measured plots don't really match up very well; if I add extra inductance (7.6 nH, plots shown in blue), the two plots match up a little better, but still don't match very well. I suspect that the difference may come from the fact that for my analysis, I treated the power splitter as if it were a simple node, and I think that's probably not very accurate.
Anyway, the point of all this is to eventually measure the impedance of the circuit I created on Friday, but I don't think I can really do that until I understand what is going on a little better.
Yesterday, Jay brought over the IO box for megatron, and got it working. We plan to firewall megatron this afternoon, with the help of Jay and Alex, so we can set up GDS there and play without worrying about breaking things. In the meantime, we went to Wilson House to get some breakout boards so we can take transfer functions with the 785, for an ETMX controller. We put in a sine wave, and all looks good on the auto-generated epics screens, with an "empty" system (no filters on). Next we'll load in filters and take transfer functions.
Unfortunately we promised to return the breakout boards by 1pm today. This is because, according to denizens of Wilson House, Osamu "borrowed" all their breakout boards and these were the last two! If we can't locate Osamu's cache, they expect to have more in a day or two.
Here is the transfer function of the through filter working at 16KHz sampling. It looks fine except for the fact that the dc gain is ~0.8. Koji is going to characterize the digital down sampling filter in order to try to compare with the generated code and the filter coefficients.
I was in the lab last night accelerometerizing and noticed some dents on the tubes that stick out horizontally from the MC2 optical chamber (sorry, I don't know what they're called or what they do). One of them is pretty big... I don't know if this is a problem, but it probably isn't a good thing. Photos below:
This last one is a little hard to see... I was having trouble getting a good angle on it, but it's there. Not quite as significant as the first one though. (The first two pictures are of the same dent.)
This week, Joe and I have been setting up the laser and optics. The mephisto laser is emitting a very ugly beam that we can hopefully remedy using an iris and a lens. After scanning the beam width at a few different distances from the laser, I am currently trying to determine the appropriate lenses to use.
The construction people next door seem to be getting pretty excited about pounding things lately. At my desk the floor was shaking like a mini-earthquake, and all of the accelerometers were pretty much railed. Clara has the Guralp box out right now, so the Guralp is unplugged, but the Ranger didn't seem to be railed.
This either (a) is part of the reason the MC is being wonky lately, or (b) has nothing whatsoever to do with it. The MC watchdogs haven't been tripping all the time, so maybe this isn't a primary cause of the wonky-ness.
In looking at a many-days/months trend to see how far back this has been going, it looks like the accelerometers are hitting their rails pretty much all day every day. This may be significantly hindering Clara's Wiener filtering work. I think the gain on the accelerometer's controler panel is already set to 1, but if it's set to 10, we may want to reduce that. Alternatively, we may want to put in attenuators just as the signal is entering the PEM ADCU, to help reduce the amount of rail-hitting that's going on. I don't remember this from a couple of months ago, so this may be a problem that will go away once the construction / landscaping is done next door.
I tilted the periscope beam and aligned the MC. Now the spot at the Faraday entrance is near the center of the aperture in up/down space. The arm powers are only going up to ~0.8, though. Maybe we should try a little bit of left/right.
I looked at the IP POS spot with a viewer card, and it looked round, so no obvious egregious clipping in the Faraday. Someone might take a picture with one of the GigE camera and get us a beam profile there.
We no longer have an MC1 and MC3 camera view.
I can see a bright scatterer that can be seen from the east viewport of the BSC, but I can't tell what it is. It could be a ghost beam.
It would be nice to get an image looking into the north viewport of the IOO chamber. I can't see in there because the BS oplev table is in the way.
I checked the setup and found RF reflection at the load was the cause of the unreasonable response in the impedance measurement.
So, I have put a total 22dB attenuation (10+6+6 dB) between the power splitter and the load to be measured. See the picture.
This kind of attenuators, called as PADs, is generally used in order to improve the impedance matching, sacrificing the signal amplitude at the load.
Then, It looks the measurements got reasonable up to 100MHz (at least) and |Z|<1kOhm.
For the measurements, I just followed the procedure that Stephanie described.
Stephanie has measured the impedance of her resonant circuit.
As a test of the method, I measured impedances of various discrete devices. i.e. 50Ohm, 10-1000pF Cap, Inductances, circuit opened.
a) 50Ohm (marine-blue) was calibrated to be recognized as 50Ohm.
b) The mica capacitances (orange 10pF, yellow 100pF, green 1000pF) appeared as the impedances f^-1 in the low freq region. It's nice.
At high frequency, the impedances deviate from f^-1, which could be caused by the lead inductance. (Self Resonance)
So 1000pF mica is not capacitance at 50MHz already.
c) Open BNC connector (Red) looks have something like 5pF. (i.e. 300Ohm at 100MHz)
d) I could not get good measurements with the inductors as I had 200nH (and some C of ~5pF) for a Pomona clip (blue).
This is just because of my laziness such that I avoid soldering the Ls to an RF connector!
I was able to make an impedance measurement of the flying-component circuit using Koji's method for impedance measurement. I first measured the impedance of the circuit with a 10 pF capacitor in the place of the EOM (as shown in the circuit diagram). This impedance plot is attached. I then added resistance to adjust the impedance slightly, attached the circuit to a New Focus KTP 4064 EOM, and took another impedance measurement (circuit diagram and impedance plot attached). The peaks are relatively close to 50 Ohms; they are at least the same order of magnitude.
I put the flying-component circuit in a box; a photo is attached. I also measured the impedance; it looks exactly the same as it looked before I put the circuit in the box.
Friday afternoon the mode cleaner got unlocked. Then some adjustment of the MC1 bias sliders locked it again. The driftmon showed the excursion for pitch and yaw of MC1 becasue it wasn't updated after the change.
Tonight Rana found the MC unlocked and simply touched the sliders to bring the OSEMs back to the driftmon values.
MC1 Yaw remains different from the driftmon. If brught back to htat value, the MC would get unlocked.
More investigation is needed to understand why the MC lock hasn't been stable for the last few days.
The mode cleaner is still unlocked. I played with the cable at the MC2 satellite to enusre they were all plugged in.
Then I tweaked the the mirrors alignment by the sliders and eventually I could get it locked stably with 1.3 reflection. Then I rebooted C1IOO because the WFS wouldn't engage. After that the cavity wasn't locked anymore. Trying to adjust the mirrors around their position didn't restore the lock.
More work is necessary.
I'll be back on it in a while.
Alex has firewalled megatron. We have started a framebuilder there and added testpoints. Now it is possible to take transfer functions with the shared memory MDC+MDP sandbox system. I have also copied filters into MDC (the controller) and made a really ugly medm master screen for the system, which I will show to no one.
I am (was) able to get the mode cleaner mostly locked, but because WFS2 wasn't centered, the MC would drift, then lose lock. I recentered both the WFS (after unlocking the MC and the MZ), and am now about to commence relocking both of those.
Note to self: WFS get centered based on the direct reflection from MC1. Once the MC is close enough, the WFS are enabled, and they twiddle all 3 MC mirrors to minimize their error signal. Moral of the story: make sure the WFS are centered.
The mode cleaner seems to be locking again. I've manually unlocked it a few times in the past 20min, and most of the time it catches lock again (maybe about 80% of the time). Other times, it starts to lock in a bad mode, and can't fix itself, so I unlock it, and let it restart and it usually does fine the second time around.
I'd like it to be a little more robust, but I'm having a bit of trouble zeroing in on the optimal alignment for quickest, most durable lock aquisition of the MC. Right now I'm going to leave it for a little while to make sure it doesn't fall apart.
While aligning the optics, we tried to start up the CCD. Although nothing should have changed since the last time I used it, the code claimed it could not find the camera. All the right leds are lit up. The only indication that something is awry is that the yellow led on the camera isn't blinking as it does when there is ethernet activity.
Jenne, Koji, Rana
After fixing up the Mode Cleaner a bit more (fiddling more with the MC_align sliders to get the alignment before locking, making sure that it is able to lock), we noticed that the MC Trans path could use some help. To align the MC, we put MC1 and MC3 back into the position where Rob left it on Thursday and then maximized the transmission with MC2. Then we went back and maximized with MC1/3 keeping in mind the Faraday. We got a good transmission and the X-arm had a transmission of 0.8 without us touching its alignment.
Upon looking at the AP table portion of the MC_trans path, we decided that it was all pretty bad. The light travels around the edge of the AP table, then out the corner of the table toward the PSL table. A periscope brings it down to the level of the PSL table, and then it travels through a few optics to the MC_trans QPD.
The light was clipping on the way through the periscope, and so the MC_trans QPD was totally unreliable as a method of fine-tuning the alignment of the Mode Cleaner. Ideally we'd like to be able to maximize MC_trans, and say that that's a good MC alignment, but that doesn't work when the beam is clipped.
1. The first turning mirror on the AP table after the beam comes out of the vacuum was changed from a 1" optic to a 2" optic, because the spot size is ~4-6mm. We were careful to avoid clipping the OMCT beam, by using a nifty U200 mount (C-shaped instead of ring-shaped).
2. We placed a lens with a RoC of 1m (focal length for 1064nm is ~2m), a 2" optic, between the first two mirrors, to help keep the beam small-ish when it gets to the periscope, to help avoid clipping.
3. Rana adjusted the angle of the upper periscope mirror, because even when the beam was centered on the steering mirror directly in front of the periscope and the spot was centered on the first periscope mirror, the beam wouldn't hit the bottom periscope mirror.
4. We noticed that the bottom periscope mirror was mounted much too low. It was mounted as if the optics after it were 3" high, which is true for all of the input optics on the PSL table. However, for the MC_trans stuff, all the optics are 4". We moved the periscope up one hole, which made it the correct height.
5. We removed the skinny beam tube which guided/protected the beam coming off the periscope after a steering mirror since it (a) wasn't necessary and (b) was clipping the beam. We cannot use such skinny tubes anymore Steve.
6. There was a lens just before the 2nd steering mirror on the PSL table portion, which we removed since we had placed the other lens earlier in the path. 2 lenses made the beam too skinny at the QPD.
7. After this 2nd steering mirror, there had been a pickoff, to send a bit of beam at a crazy angle over to the RFAM mon, which we removed. This results in a much brighter beam at the MC_trans QPD, and at the camera. The QPDs readouts are now a factor of ~3.5 higher than they used to be. These (especially the camera) could use some ND-filtering action.
8. The steering optic directly in front of the MC_trans QPD is a beamsplitter, and instead of dumping the light which doesn't go to the MC_trans QPD, we used this to go over to the RFAM mon (instead of the pickoff which we had removed).
9. Koji fixed up the optics directly in front of the RFAM mon, accomodating the new position of the input light (now at a much more reasonable angle, and about 15cm farther back from the PD). Note the beam dump which is preventing the cables from the FSS board from entering the beam path. This included removing an ND filter wheel, so the RFAM mon values will all be higher now. Koji also has the beam going to the PD going at a slight angle, so that the beam isn't directly reflected on itself, so that it can be dumped.
10. We aligned the beam onto the MC_trans QPD using the first steering mirror on the PSL table.
11. We also removed the giant wall of beam dumps separating the squeezing section of the table from the rest of the table.
Alberto will elog things about the RFAM mon, including different values of the PD output, etc.
Still on the to-do list:
A. Replace the second steering mirror on the AP table after the MC_trans light leaves the vacuum with a 2" optic, since the lens we placed isn't tight enough to make the spot small there yet. Us a U200A mount if possible, because they are really nice mounts.
B. Put an ND filter in front of the MC_trans camera, because the image is too bright.
C. Normalize the MC_trans QPD - the horz and vert are pretty much direct voltage readouts, with no normalization. They should be divided by the DC value. This lack of normalization results in higher sensitivity to input pointing.
D. Long term, next time someone wants to optimize the MC_trans path, move all the optics, including the MC_trans QPD and the camera closer to the periscope on the PSL table. There's no reason for the beam to be traveling nearly the full width of the PSL table when we're not manuvering around squeezing stuff.
E. Never, ever purchase these horrible U100 or U200 mounts with the full ring and the little plastic clips. They are the "AC28" version. Bad, bad, bad.
Image 1: The new setup of the AP table, Mc_trans portion.
Image 2: New setup of the MC_trans part of the PSL table.
The camera wasn't working because the router has no built-in dhcp server. We had to manually start the server after rebooting the computer.
The MC_trans QPD Pitch and Yaw readout on the Lock_MC screen are now normalized by the trans_sum. I used the method described in my entry elog 1488.
/caltech/target/c1iool0/ioo.db now includes:
field(SCAN, ".1 second")
field(SCAN, ".1 second")
The Lock_MC screen was changed to show these new P and Y channels.
Yesterday we found that the channel C1:MDP-POS_EXC looked distorted and had what appeared to be doubled frequency componenets, in the dataviewer. This was because the dcu_rate in the file /caltech/target/fb/daqdrc was set to 16K while the adl file was set to 32K. When daqdrc was corrected it was fixed. I am going to recompile and run all these models at 16K. Once the 40 m moves over to the new front end system, we may find it advantageous to take advantage of the faster speeds, but maybe it's a good idea to get everything working at 16K first.
Last night Rana noticed that the overflows on the ITM and ETM coils were a crazy huge number. Today I rebooted c1dcuepics, c1iovme, c1sosvme, c1susvme1 and c1susvme2 (in that order). Rob helped me burt restore losepics and iscepics, which needs to be done whenever you reboot the epics computer.
Unfortunately this didn't help the overflow problem at all. I don't know what to do about that.
Just start by re-setting them to zero. Then you have to figure out what's causing them to saturate by watching time series and looking at spectra.
We put a simple pendulum into the MDP model, and everything communicates. We're still having some kind of TP or daq problem, so we're still in debugging mode. We went back to 32K in the .adl's, and when driving MDP, the MDC-ETMX_POS_OUT is nasty, it follows the sine wave envelope but goes to zero 16 times per second.
The breakout boards have arrived. The plan is to fix this daq problem, then demonstrate the model MDC/MDP system. Then we'll switch to the "external" system (called SAM) and match control TF to the model. Then we'd like to hook up ETMX, and run the system isolated from the rest of the IFO. Finally we'd like to tie it into the IFO using reflective memory.
After the mini boot fest that Jenne did today, I checked whether that fixed the overflow issues we yesterday prevented the alignemnt of the arms.
I ran the alignment script for the arms getting 0.85 for TRX and 0.75 for TRY: low values.
After I ran the script ,C1SUSVME1 and C1SUSVME2 started having problems with the FE SYNC (counter at 16378). I rebooted those two and fix the sync problem but the transmitted powers didn't improve.
Are we still having problem due to MC misalignment?
I have spent the past couple of days gathering optics and mounts so that I can observe the modulation of the EOM attached to the circuit I built using the optical spectrum analyzer (OSA). A rough diagram of the planned layout is attached.
I also built a short SMA cable so that the EOM did not have to be connected directly to the circuit box. The cable is shown attached to the EOM and circuit box in the attached photo. After checking to make sure that all of the connections in the cable were sound, I remeasured the input impedance of the circuit; the impedance measurement (black) is shown in the attached plot with the impedance before the SMA cable was added with and without the box (green and blue, respectively--these two are almost identical). The new impedance has a strange shape compared to the original measurements; I'd like to understand this a little better, since adding extra inductance in LTSpice doesn't seem to have that effect. Since I had already taken apart the setup used for the previous impedance measurements, I had to rebuild and recalibrate the setup; I guess the difference could be something about the new calibration, but I don't really think that that's the case.
I have spent the past couple of days gathering optics and mounts so that I can observe the modulation of the EOM attached to the circuit I built using the optical spectrum analyzer (OSA). A rough diagram of the planned layout is attached.
After investigating this a bit further, I discovered that some of the components in the circuit were pressed firmly up against the inside of the box, and when they were moved, the impedance plot changed shape dramatically. I think that originally, the components were not pressed against the box, but the box's SMA joint was rather loose; when I connected this to the SMA cable, I tightened it, and this seems to have twisted the circuit around inside the box, pushing the components up against the side. I have fixed the twisting, and since the SMA joint is now tight, the circuit should no longer have any twisting problems.
A new plot is attached, showing the impedance of the circuit with nothing attached (blue), with the SMA cable and EOM attached (green), and with the EOM attached directly to it taken last friday with the old calibration of the setup (red). All three curves look roughly the same; the center peak is shifted slightly between the three curves, but the circuit with SMA and EOM is the version we'll be using, and it's central peak is close to the correct value.
The daq on megatron was nuts. Alex and I discovered that there was no gds installation for site_letter=C (i.e. Caltech) so the default M was being used (for MIT). Apparently we are the first Caltech installation. We added the appropriate line to the RCG Makefile and recompiled and reinstalled (at 16K). Now DV looks good on MDP and MDC, and I made a transfer function that replicates bounce-roll filter. So DTT works too.
The 40m-IFO vacuum envelope doors are sealed with dual viton O-rings and they are pumped through the annulos lines.
This allows easy access into the chambers. The compression of the o-rings are controlled by the o-ring grooves.
The OOC (output optic chamber)'s west side door has no such groove and it is sealed by just one single O-ring.
We have to protect this O-ring from total compression by 3 shims as shown below.
There were control shims in place before and they disappeared.
Let's remember that these shims are essential to keep our vacuum system in good condition.
I've now also trended the MOPA output power for the last 200 days to check a possible correlation with the FSS reflected power. See attachment.
The trend shows that the laser power has decayed but it seems that the FSS reflected power has done it even faster: 30% drop in the FSS vs 7% for the MOPA in the last 60 days (attachment n.2).
Here we trended also the PMC and the MZ. The drop in the PMC happens at the same rate as the MOPA's.
That let us think that the FSS transmitteed power has gone down because of the reference cavity progressive misalignment to the laser beam.
We need to adjust that alignment sometime.
The drop in the NPRO output power (upper row, 3rd plot: Ch10 C1:PSL_126MOPA_126MON) accompained an increase of "fuzziness" in PMCTRANSPD and both coincided in time with the day we tempoarirly removed the flap from the laser chiller's chiller (July 14 2009).
Stephanie has needed the doors to the PSL open all day, and still has them open, so I just turned the HEPAs on high.
For the past week Dmass and I have been ordering parts and getting ready to construct our own modified version of EUCLID (figure). Changes to the EUCLID design could include the removal of the first lens, the replacement of the cat's eye retroreflector with a lens focusing the beam waist on a mirror in that arm of the Michelson, and the removal of the linear polarizers. A beam dump was added above the first polarizing beam splitter and the beam at Photodetector 2 was attenuated with an additional polarizing beam splitter and beam dump. Another proposed alteration is to change the non-polarizing beam splitter from 50/50 to 33/66. By changing the reflectivity to 66\%, less power coming into the non-polarizing beam splitter would be ``lost" at the reference detector (1/3 instead of 1/2), and on the return trip less power would be lost at the polarizing beam splitter (1/6 instead of 1/4). Also, here's a noise plot comparing a few displacement sensors that are used to the shot noise levels for the three designs I've been looking at.
I thought slightly harder and I think that the beamsplitter stays. We will lose too much power on the first PD if we do that:
33/66: Pwr @ PD2 = 2/3*1/3*1/2 = 1/9 Pin
Pwr @ PD3 = 2/3*2/3*1/2 = 2/9 Pin
50:50 Pwr @ PD2 = PWR @ PD3 = 1/8 Pin
balancing them is probably better.
We aligned both the reference cavity and the PMC, each by looking at their Trans PD on Davaviewer, and adjusting the two steering mirrors to maximize the transmission power. We got a pretty good amount of improvement for the ref cav, but since the PMC hasn't decayed a whole lot, we got a much smaller amount of improvement.
I turned the HEPAs back down to ~50.
Stephanie and Koji
We left two carts near the PSL table.
We are using them for characterization of the tripple resonant EOM.
After Alberto and I worked on aligning the reference cavity, Rob asked the important and useful question: what is the visibility of the reference cavity. This helps tell us if we're optimally aligned or not even close.
I did a scan of the ref cav temperature, using /scripts/PSL/FSS/SLOWscan, but there seems to be no real signal is C1:PSL-FSS_RFPDDC. As shown in Alberto's 200-day plot, it does change sometimes, but if you zoom in on the flat parts, it seems like it's not really reading anything meaningful. I did a cursory check-out of it, but I'm not 100% sure where to go from here: There are (as with all of these gold-box PDs) 3 outputs: a ribbon cable (for ADC purposes I think), an SMA for the RF signal, and a BNC for the DC signal. The photodiode is clearly working, since if you stick the Lollypop in front of the PD, the cavity unlocks. I plugged a 'scope into the DC BNC, and it also behaves as expected: block the beam and the signal goes down; unblock the beam and the signal goes up. Something of note is that this readout gives a positive voltage, which decreases when the beam is blocked. However, looking at the dataviewer channel, nothing at all seems to happen when the beam is blocked/unblocked. So the problem lies somewhere in the get-signal-to-DAQ path. I unplugged and replugged in the ribbon cable, and the value at which the channel has been stuck changed. Many days ago, the value was -0.5, for the last few days it's been -1.5, and after my unplug/replug, it's now back to ~ -0.5 . The other day Alberto mentioned, and made the point again today that it's a little weird that the PD reads out a negative voltage. Hmm.
we have a tester cable, but you don't want it. Instead the problem is probably at the cross-connect. The D-cable goes to a cross-connect and you can probe there with a voltmeter. If the signal is good there, trace it to the ADC. Also trend for several years to see when this happened - Yoichi may know the history better.
Also, we still need to complete the FSS RFPD task list from last year.
For some reason a few minutes ago the FB DAQ crashed and I had to restarted.