TEST_ADC1 is acquiring the Thorlabs photodiode, and TEST_DAC1 is attached to the laser power input.
I added in the model a TEST_CTRL that connects the output of TEST_ADC1 to the input of TEST_DAC1. The plant transfer function has a very large delay (about 600 us) and so it's difficult to make a loop with high gain. Nevertheless, I implemented a simple digital controller to reduce the intensity noise at 10 Hz of about 100 and at 100 Hz of about 8 times. Lot of gain peaking at 300-400 Hz.
Although the loop is working, it makes the intensity noise at the Michelson PD worse. Indeed, with the loop open, the Thorlabs PD see an intensity noise which is incoherent with what is seen by the Michelson PDs. It's not a problem of electronic noise. No explanation for the moment being.
All coordinates are relative to the front surface of the breadboard, origin is on the lower right corner (looking at the optics side). X increases from right to left, Y increases moving away from the board toward the observer, Z increases going up. All dimensions in millimeters. Uncertaints are of few millimiters.
* Estimated from the suspension point position and the roll resonant frequency f0 = 0.57 Hz. From Solidworks the total mass is about 27 kg and the moment of inertia is 0.88 kg m^3. So the suspension point is 43 mm above the center of mass.
Block suspension wires 106 mm
From board to Roll Decoupling Stage (RDS) 77 mm
From RDS to Blades: Left 96 mm, Right 83 mm
From Intermediate stage (IS) to blades: Front 122 mm, Back 129 mm, Right 128 mm, Left 125 mm
The attached PDF reports the distances of all posts to the closer reference point. This is similar to the set of distances we took to assess the installed positions. We'll use them tomorrow to move all components to their new nominal positions.
Today the DOPO v0 got disassembled to make way for the optical table swap. Most components have been stored in the white cabinet's bottom panel.
There might have been some vagueness in the way I talked about the difference in displacement amplitude. What worries me is the difference in the two highC steel blades displacement amplitude with the common force excitation, where with 16000 uN driving amplitude, pk2pk(Z1) ~ 30 um, pk2pk(Z2) ~ 43 um. The similar amount of ~ 10 um difference were seen in maraging steel blades common driving.
Nevertheless, Gabriele's comment reminds me of one wierd observation: for maraging steel blades we input 6000 uN for ~ 30 um pk2pk displacement amplitude, while for highC blades we input 16000 uN for similar displacement amplitude. We designed the blades with close resonance frequency ~ 2 Hz for both maraging and highC blades, but highC blades are loaded with much less mass, which means k_highC has to be much smaller. In turn, we would expect to drive the more compliant highC blades with much less force for similar amount displacement amplitude, but the fact came out as a contrary. I will measure the blades transfer function soon.
I am wondering why there is large difference in displacement amplitude with same force amplitude, considering the resonance frequency of the two cantilivers are close enough. One possible explanation is the asymmetry in the two Z-OSEMs alignement?
Displacement at low frequency
So same frequency, but different mass and spring constant imply different displacement for the same force
After at least 9 hours, the disk cloning finished. The output of the 'dd' command is shown below, there is some 'input/output error' but it looked like all sectors were copied.
I installed the new disk and rebooted, and bummer: it's not working:
So, I installed back the old disk, booted the cymac2, and restarted all processes. Again, all gains were screwed up! How can we set a safe snapshot for when cymac restarts?
The sweeps looked strange to me, as if there was something broken. Zach assured me that the solder/paste is sound. He tested the voltage continuity of the ESD just before pumping down.
I used the HV knob to change the ESD's DC voltage from -2 kV to +2 kV to look for arcing, etc. There is indeed some arcing (the next version should have less pointy edges).
However, the big discovery for me is that any voltage beyond +/- 500 V is enough to stick the optic to the ESD by the electrostatic force. All of our runs for the past few days have been at +1-2 kVdc, so the optic was stuck the whole time.
I have now set the DC voltage to +200 V and confirmed by eye that the optic is swinging (its obvious with a flashlight): the red laser beam swings around in the chamber with a several second period.
I have set up a sweep with a 0.1 Vpp with a 200 Hz span around 5030 Hz and with a 5 sec settling time and a 55 s integration time per point. Also the 'auto resolution' feature of the sweep is on. Let's see what happens.
Next time around, we should set up active damping or make the yaw frequency higher by a factor of 5-10.
We have sent version 2 noisemon boards (modifications from version 1 noted in 1835) to Livingston and Hanford. Chris noticed that there might be some upconversion problems under 20Hz (attachment 1 and 2). These plots from Chris are noisemon output with drive subtracted. Attachment 1 is the spectrum after replacement of noisemon board (version 2 board used), compared to attachment 2 which is before the replacement (version 1 board used). There is something going on near 15Hz. We think it might be due to upconversions under 15Hz.
We still have a spare board here. I modified it, tested it and looked at this issue, trying to reproduce. I sent a 10Hz 100 count amplitude signal to the board and compared it to the output with no input. This produces attachment 3. We see that once the 10Hz sine signal is sent, there are lines above 10Hz, which is a sign of distortion.
I changed the input frequency to 5Hz and got attachment 4.
Attachment 5 is a linear plot to see where exactly those lines are. It seems they are indeed harmonics of 5Hz when the input signal is 5Hz.
I also tried higher frequencies up to 100Hz and saw similar harmonics.
it might be the low input impedance of the board which the coil driver cannot drive..
I suggest you use probes to see where in the noisemon circuit the distortion is starting
I'm attaching a script to download data from the LIGO sites with python.
I recommend using it in your anaconda3 ENV:
conda install -c conda-forge nds2-client python-nds2-client
and then before running the script you have to initialize your Kerberos token:
then you run the script:
python getData.py --ifo=L1 --fs=1024
as usual, run with the -O or -OO flags to silence the debug messages.
# this function gets some data (from the 40m) and saves it as
# a .mat file for the matlabs
# Ex. python -O getData.py
import scipy.io as sio
import scipy.signal as sig
from astropy.time import Time
Unlike in the earlier post, the configuration (specifically, the PZTs....we will switch tot the actual blade experiment soon) were driving the circuit.
Below are the resulting power spectrum density plots. Each represents the same basic configuration, but at different conditions (both driving frequency f and voltage amplitude Vamp could be adjusted).
Three conditions were tested:
For the first two plots, the same conversion factor to get from V/SQRT(Hz) to the desired units of meters/SQRT(Hz). Conversion=2.578199052E6 Volts/meter. Since the voltage amplitude was changed during the third test, the conversion factor had to be adjusted to conversion=3.892575039E6 Volts/meter. [If there is any confusion on how these were calculated, reference the post here]
The fourth plot superimposes all three previous plots.
One night of data taken with drive at 600 counts and 125 mHz. No signal of modulated noise:
I'm not sure about the absolute value of the confidence intervals. I'm running a check on simulated data to see if they are properly calibrated.
I did EBSD for Stainless Steel 340:
I also did a grain size analysis in their manager-data software, which directly gives us histogram of grain sizes. It should be noted that different statistics would lead to different results, but still it should allow us to compare the grain information quantitatively between different materials.
I did an Electron Backscatter Diffraction (EBSD, please refer to http://www.ebsd.com for more background information) analysis for the maraging steel (18% Ni Grade 250) blade sample.
To generate this map, BCC and FCC structures were used as the model. It was found that BCC structure is the best fit. Then the high resolution map with pixel size of 0.03 um was taken.
We should use the pole figures as a color key to interpret the map -- it tells us how the grains are orientated in the sample. Since the sample is thin and flat we care only about the Z0 direction, which is the direction normal to the blade surface. For example the greens correspond to grains with zone axis (101) facing up along Z0.
The next step is to choose a good grain with single slip geometry, within which to make a pillar with at least 1 um diameter for compression test.
As the small grain size (max ~ 5 um) measured in the last maraging steel sample was skeptical for well annealed steel, I annealed the other non-annealed maraging steel sample under the condition described in E0900023-v12: 450 C for 100 hrs, however in air but not in inert gas atmosphere. I did another EBSD analysis for the self-annealed sample:
Although the data quality is not good mainly because of relatively poor polishing, we should still see that the grain size is actually really large (max ~ 50 um). It indicates that the last sample is not well annealed as it's claimed to be. I plan to double-check by taking a look at the grains of the original non-annealed sample.
On the side I characterized the carbon steel sample:
It should be noted that the Z inverse pole figure (color key) stays the same for all measurements (green 101, blue 111, red 001).
Seems like a rather qualitative analysis. Is there any way you can make a 2D FFT of this so that we can see what the distribution of grain sizes are? What are typical sorts of grain size analysis people do in order to get quantitative comparisons?
The gratings and aspheric lenses glued on the mounts were delivered to the lab on Thu.
The powermeter controler + S401C head was lent from OMC Lab. Returned to OMC Jul 15, 2020 KA
Worked for a few hours to get the aspheric properly aligned. The procedure is quite finnicky, as the four 2-56 flexure screws have too much game and the fine thread setscrew that adds tension is too constrained. Anyways, it generally goes like this:
After this, I installed a second amplified InGaAs detector, hooked up the unbalanced MZ beamsplitter output into the two PDs, adjusted the gains to equalize the output voltages and then hooked the two signals to the A and B inputs of an SR560 in "A-B" mode. The output (gain 1) was good enough to feed back in the HV PZT amplifier input modulation which allowed the MZ to lock mid-fringe. The lock is rough, as the balanced homodyne signal retains a tiny offset due to imperfect balancing... Attachment 1 shows the setup, including a typical scope trace after coarse current tuning (Ch1 and Ch2 in yellow and blue represent the photocurrents in the two MZ ports in the absence of feedback).
Indeed, scanning the nominal PZT voltage broke the lock, potentially after crossing a mode hopping region.
Tasks to be done:
Next, as was suggested during yesterday's group meeting, we will transition into a self-heterodyne setup (with an AOM which I have yet to check out in the QIL).
Attempted two methods to soften EP30-2, the results are summarized below.
(a) Heat -- Used the heat gun set to 200 F (~ 93 C) and held it near the back of the part so that the grating surface was never in direct exposure. The airflow was kept constant for a period of ~ 10 min, while I periodically checked to see if there were any signs of bond softening. After no signs of softening, I stopped and moved to method (b).
(b) Solvent -- After brief investigation and referring to T1400711, I got some acetone from CTN, and set up a ~ 50 ml bath. The part was not completely submerged and was arranged such that the grating face was always exposed to air, which I left for ~14 hours. The drawback of this method is that some of the acetone evaporated and at some point the EP30 bond stopped being in contact with the solvent. A picture for reference is attached, with the light blue line indicating the highest acetone level, and the red line indicating the EP30 bond level at the beginning of the bath.
Quoting Rana: "The cable capacitance (measured from the driver side of the SHV cable) is 200 pF. This must include the cable capacitance + the ESD capacitance.".
The ESD Capacitance was measured 3 separate times using an LCR Meter at 1kHz:
LCR Meter - the exact meter I used to measure the capacitance of the ESD
ESD - The arrows point to where we put the meter probes to measure the capacitance across
The Cable capacitance needs to be measured still.
Alastair removed the tank on Wednesday afternoon. I went ahead and grabbed the ESD and it seemed fine - and screwed it completely into the vacuum chamber, plugged into the HV. For some reason, the joint that's being soldered keeps breaking off as the wire is super fragile - even if the cold solder is also coating the wire shell. I'm not totally sure why it's breaking off after a while, since I put a test pair of wires and soldered those together at the same time - and those didn't break when I tried pulling them apart. I'm pretty sure it has something to do with the thickness of the wire and my caution in not overdoing it (since we don't want to accidentally connect to the front side of the ESD). For now, we will do the pick-up measurements without the ESD in the vacuum chamber.
Alastair put the tank back on and the UHV is currently on. It'll be set for tomorrow morning when I go in to do measurements. Here's the tentative plan for the measurements:
Get a spectrum analyzer, and look at the noise before any changes, and after I make changes to figure out the optimal set up. Changes include:
- grounding and fixing ground loops
- wrapping each cable around each other
- move everything away from the HV supply (last resort)
- faraday cage things with the foil
Responses/Comments from Zach:
As we were testing the newly adjusted setup yesterday, we noticed a faint blue glow within part of the ESD upon application of the DC bias voltage of ~2-3 kV. This are bad.
I removed the ESD from the chamber last night and inspected it. It appears to have some obvious damage in the are where we saw the glow:
It appears that we have exceeded the dielectric strength of the Rogers 4350 substrate. I (somewhat conservatively) calculated a maximum voltage of ~5 kV DC between the electrodes, and I think this could still be accurate. The problem is that we have inadvertently exceeded this on at least one occasion while I was trying to set up the feedback damping (i.e., I set the gain too high and the control + bias signal together exceeded 5 kV.
I am going to prep another ESD (which must be silver epoxied to the HV wires) ASAP so that we can get back down to business.
We tried some more today to actually get the 25-kHz mode to ring up significantly. We were unsuccessful.
We also toyed with the idea of using the differential output (band-pass filtered) to feed back to the ESD and actively damp the low-frequency pendular motion. This, too, was unsuccessful. I think the problem here is that we only have one actuation mode, whereas the sample is moving at low frequencies in three different ways (pendulum, twisting, and tilting). So, no matter what we set the DC bias and AC gain to, it's impossible to make the thing stable; it is either too weak to combat gravity or it just lists until it physically contacts the ESD. We actually saw some arcing from when the mirror shorts between two ESD electrodes. The HV amp current-limits itself, but we were able to see some slight browning of the ESD substrate in the location where this happened. All still seems to work fine .
We resolved to vent the chamber and translate the ESD sideways with respect to the sample. The idea is that since the modes we are looking at have some radial---and thus bilateral---symmetry, then applying a uniform force across the X and Y axes does no good. So, we moved the ESD over ~1-2 cm so that there is a force imbalance from side to side. Here is the best shot I could get of the new configuration:
An added benefit to the change is that one of the beam holes is now nearer to the center of the sample. This is good because, as you can see in Giordon's nice COMSOL plots, most of the modes we are examining have antinodes at the center.
We re-sealed the chamber and began pumpdown around 2pm. I came back down to the lab around 5 or 6pm and the pressure was ~1 mTorr, so I engaged the chamber-mount turbo. By the time I went home around midnight, the pressure was back down to 2 x 10-7 Torr.
I did a few minutes of playing around with it in the new configuration, to see if it had any real effect on the measurement. It didn't seem to. The SNR of the peak on the analyzer was roughly the same, and I measured roughly the same ~2-Hz linewidth. I was still unable to close a stable damping loop, and I also discovered that the apparent driving-up we have seen using a swept-sine drive is in fact just EM coupling . For that, I just turned off the HeNe and turned on the HV drive, and I demodulated signal at ~100 Hz that was just as strong as the one we see with the laser on.
In conclusion, we are still completely unable to drive the mode up to an acceptable level. Considering how reliably the sample will twist and contact the ESD on one end, it doesn't seem practical to try and move it any closer. Thinking cap time...
I have made a first draft of the design for the ESD (see PDF below). The electrode comb spacing is 0.2", which should be roughly what we want for objects ~1-3" in size.
Sunstone offers boards printed on Rogers 4350 material as part of their fast and cheap QuickTurn service. This is a glass- and ceramic-based material that is designed for RF applications, but I have seen some examples of it being used at UHV in some ion trapping experiments. Since our vacuum doesn't have to be outrageously good (and the lab isn't clean enough for that anyway), this ought to work fine.
The finish will be silver (gold would be preferable for oxidation purposes, but Sunstone only offers silver with this service---otherwise we'd have to submit a much costlier and time-delaying custom order).
I have designed the ESD such that there are two holes offset horizontally from the center of the plate. These are for the passage of the measurement beam. I chose off-center because most modes' signals should be weaker at the center from symmetry. I chose to put two in for no particular reason other than symmetry, again. NOTE: we will have to drill these holes ourselves.
The electrodes will be connected to the HV supply by soldering to relatively large plates on the back side. One will be connected to the (positive, single-sided) HV amp output, while the other will most likely be connected to a wire that is bolted to the chamber (earth) at the other end.
It was not clear from the quote page, but apparently the RO4350 material is a little pricier than the standard FR4. For two of these boards, the cost is $493.35 ($246.68 /ea.). I think this is reasonable---assuming it works---considering how fast we can get it.
If no one has any objections or comments, I will put the order in.
With only minor changes to the actual electrode pattern, I have made the ESD design much more compact, which will reduce the cost by over 33%. The total cost for two boards is now $327. I am going to purchase them with Steve's card tomorrow.
As Zach writes, after we took the can off the top, I repositioned the ESD by one screw position so as to center it on the optic. The idea is that then the DC bias will generate less DC yaw torque.
After we got to ~! torr, we took the bias up to 100 V and the optic still seems to be parallel to the ESD. The dominant signal in the PD signals seems to be the pendulum mode (at ~1 Hz) rather than the yaw. The presence of the pendular modes indicates that the thing is not touching.
After the pressure goes below 1 mTorr, we can up the bias and try again.
We also measured the pickup into the individual PDs and the difference signal. Seems to be much less now; Giordon has the notes which give the quantitative numbers.
I've uploaded photos of the optic + ESD before movement to our shared Picasa; forgot to take photos afterwards.
I went to the supply shop in CES and bought some more Teflon that we'll need for supporting the ESD within the vacuum chamber. The leftmost one is one that Alastair had from before, and the other 4 are the ones I got today. They are a tad wider, but that's not a problem.
Here is a picture of the suspension setup as it exists today (you can see the intermediate mass and then some fiber hanging below it if you look real close), and then a diagram showing a bird's eye view of how the ESD will be supported (assuming the bird is actually a bat hanging upside down from the steel disc at the top):
Things to do before mounting the sample, etc.:
The teflon bars were drilled out at the far ends to fit 1/4-20 screws. In two of them (which are attached to the ESD support bar) - there's a 1/4-20 hole drilled lengthwise in the middle of the bar to allow for fine adjustments.
These were all smoothed down and cleaned with acetone and methanol. They're currently wrapped in UHV foil in the lab right now.
The laser is leveled out to be 44.5 cm from the top of the suspension. The ESD will be placed so that the laser passes through the vertical middle of it.
Each teflon bar has a length (vertical) of roughly 1.5 centimeters. The ESD itself is 7.5 cm long, so half of that is 3.75cm. This means the top of the ESD needs to be at 44.5 - 3.75 = 40.75cm. The teflon bars at 3cm more, so they were placed such that the bottom of the bottom teflon bar is at 43.75cm. Which places the top of the ESD at 40.75cm. Which places the vertical center of the ESD at roughly 44.5 cm (where the laser is).
All parts were cleaned, the foil was placed back on the suspension mount. And we're done.
I received the ESDs just now. They look pretty good, although the silver finish seems a bit worn in places.
I will drill the beam holes today, and with any luck get one mounted into our setup with Giordon.
I made some final changes to the ESD design and then purchased them. The only changes from yesterday were:
After talking with Alastair about it, I also decided to order 4 boards instead of only two. The price goes down from $163/ea to $102/ea, and Alastair figured we might burn one out in testing and/or want to modify one later, etc. So, the total shipped cost was $410.
Here is the final layout:
The first noisemon board was installed at the ETMY station. It was a prototype board that we brought to LLO and installed there since then. I checked the data today (11/14) and its LL channels is not working.
I checked the time series in the attachment 2 and 3. There are some problem causing the circuit to saturate.
I checked the drive signal below 10Hz in attachment 4. The drive signals across all channels are the same.
The formula used to calculate the noise is
Noise = Sqrt[1 - coherence] * drive
ETMY is about the same as ITMX.
Will compare ITMX with ETMY.
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This afternoon we openend the chamber and installed two eddy current dampers on the roll-decoupling stage. The goal is to try to reduce the ~9 Hz peaks.
The dampers are made of two parts. The first part consist of a strip of copper glued to the roll-decoupling stage. The second part is a U shaped copper bar with two magnets glued to the extremes: the two magnets sits on the sides of the strip of copper attached to the roll-decoupling stage. The U shapes are attached to aluminum blocks, glued to the suspension intermediate stage: the U shapes are bolted to the block, and there is enough clearance in the through hole to adjust a bit their position, if needed. The U shapes have set screws that can be used to asdjust the opening.
We installed two such systems, with the copper strips pointing verticallly and orthogonal to each other. This should give us enough damping for all degrees of freedom.
After installing the dampers the suspension configuration changed quite a bit so several OSEMs were touching. After recentering all OSEMs, I I did a quick investigation for the eddy current damping.
I compared the Z motions (presenting Z1 only to avoid a messy figure) with two different distance between the magnets: d_mag ~ 0.5 cm, and d_mag = 1 cm in air. With closer magnets, which means a larger magnetic field, we observe less motion at ~ 4.5 Hz and ~ 9.5 Hz. I am also able to lock the mich in these two configurations. Please note the colors in the mich spectrum is swapped (sorry!). The ~ 9.5 Hz peak, which we believe is related to the highQ motion of the roll-decouple frame, is suppressed by a factor of ~ 4 with d_mag ~ 0.5 cm, in comparison to d_mag ~ 1 cm. This is as expected: the damping gain should have a quadratic relationship with the distance between the magnetic field. G ~ d^2.
I closed the chamber and pumped down to ~ 300 mTorr.
It looks like the best OSEM signal combination to see the 9 Hz peaks is X1-X2. So here's a comparison of this signal last night (with the eddy current damper in place) and a couple of nights before, without dampers. It's a 10mHz resolution spectrum, averaged over one hour of data.
The peaks are reduced in amplitude by about a factor 2. I would suggest to beef up the magnets to further improve damping. Also, the frequency is slighlty lower, compatible with the additional mass we added to the stage.
This morning the suspension OSEM were slighly off center, as an effect of the night drift. I repumped the legs up to 6 atm (some of them were as low as 5). As visible in the attached plot, this helped improving the centering, even though it wasn't enough to get back to the optimal position.
Here are the fits for the new shadow sensor and coil driver boards. Everything looks ok, except for S07: one of the whitening poles is at lower frequency than all the others. It might be due to a wrong component. I'll check later, however even if I can't fix it it's not a big deal. The listed transfer functions are already inverted, i.e. they are what you need to plug in into foton.
Today i finished soldering all the components to the second set of OSEM electronics, and mounted the boards inside the box. All power supplies are connected, but there are still some cable missing between the boards. I haven't tested the circuits yet.
Late elog entry... Last Thursday I moved all the electronic boards for the bread board OSEMs into the final box and connected all the input and outputs to the BNC connectors on the front panel. The boards are not yet attached to the box with final standoffs, since the present ones are too tall and we can't close the box top panel.
Unfortunately a fw connections broke during the rearrangment, and i didn't have time to fix them, so I couldn't test that everything is still working.
I move the electronic boxes from the floor to the rack, using th falt cables to connect them to the feedthrough.
Everything has been recabled and working. The bundle of BNC cables we were previously using has been removed.
For future reference, here are the connections to ADC and DAC
To make the story brief, after one day of work I was able to move the rack to the other side of the room with respect to the optical table, and everything seems to be in working order. The accelerometer mounted on the table shows an improvement of about a factor 2 everywhere. I was hoping for something more, but I'll take it.
Some details follow:
The following table reports the cable number, the corresponding DAC or ADC channel, and what is connected.
Orders have been placed for the improved version of the electronics. The basic ideas are described in T1500539 and detailed schematics are available in D1500402.
The second OSEM electronics box has been partially tested:
I fixed the connections broken during the rearrangment, and tested the electronics. At first the PD's were all working properly but DAC channels were outputing 0 or 2V voltage no matter what commands were sent, but this problem was solved after rebooting CyMAC. Everything is now in place, working as normal. The boards are good to go for suspension!
[Rana, Zach, Giordon, and more Giordon]
Using all changes from before - I know that the pick-up noise is drastically reduced when I put the photodetectors on a 20 dB gain (gain of 10 compared to 0 dB gain) and the preamp gain is set to 100. After I spent time watching Rana's magic brown fingers on the spectrum analyzer and the sine sweep - I went ahead and did one (at a higher resolution). Let me slightly elaborate on the set-up:
The laser is on the side of the vacuum chamber, the wobbly periscopes are being used, both photodiodes (20dB gain) are unblocked, using a differential output from a preamplifier with a gain of 100, bandwidth of 1kHz to 100kHz. 20 minutes into the scan - I notice a really strange feature:
When I mean strange, I mean interesting strange - not... creepy or weird. I think it's interesting because it's not a "peak" but more broad as if someone flipped a switch and the transfer function increased for a short period, and then flipped the switch off and it slowly went away... This isn't due to any dancing antics of yours truly - as I took this picture as soon as I walked over there to check it after like 30 minutes after starting it... I might feign a guess and say, that given that it's a broad peak - it may just be something like the vacuum being crazy... So, after this scan is done, I'll save it and run it one more time before actually calling it a night.
Update: it appears that this peak shows up again with the second scan and the same conditions duplicated. It is definitely not time-dependent (because something happened during the time period it scanned it) and definitely exists in our system. More analysis needs to be done to figure out what it is. A suggestion would be to turn off the vacuum, run the scan, and see if it still persists (and since we're sealing the chamber up tomorrow - that'll be convenient).
Update: the plots are made - for both sweeps (they are identical, just taken one right after the other):
Again, most of the commentary about the plots is within the obvious peak in the magnitude of the transfer function while the phase appears to be "noisy" . Peak is centered at 4997 Hz with a 4 Hz span (roughly).
In the morning, I'll swap out the periscopes and sweep at 21.408 kHz.
To be updated...
[Rana, Giordon, Zach]
This serves as an update of the last few days of coating Q work. This is terrible elog practice on my part; all underlings should do as I say and not as I do.
Giordon was sick last week, so on Friday Rana and I went in to try and ring up a mode. At first we used a fixed sinusoidal drive out of the SR830 lock-in amplifier to characterize the EM pickup with the laser blocked. It was large, so we took some steps to mitigate it:
Both of these were done so that components mentioned were not so close to the HV amplifier (they previously shared the rack).
This pushed the laser-off EM pickup into the noise. There still seemed to be a pickup on the laser light at the drive frequency (as measured using a single PD), but there was strong enough common-mode rejection at the differential readout the the peak was in still in the noise with the laser on. At this point, we decided we wanted a finer frequency resolution, so we began making transfer function measurements using the internal drive of the Agilent spectrum analyzer (i.e., drive the ESD with the source through the HV amp, then measure the ratio of the SR560 output to the source). The HVDC bias was 2 kV, and the AC amplitude was 1 kVpk after the HV amp.
Using a span of ~70 Hz (24.15-24.22 kHz) and 1600 points (i.e., bandwidth ~0.043 Hz), with 1-s settle and 25-s integration time, we measured a very flat response in magnitude and phase:
Rana recorded this data on Sunday, after which he did something to the averaging settings(?) and left it running for another long measurement, which was also uninteresting:
It appears that the span was changed to 90 Hz (24.16-24.25 kHz), giving df = 0.056 Hz with 1600 pts, but the averaging remained unchanged. I am not sure why the measurement is so much nastier.
Yesterday (Monday), Giordon moved the laser head itself away from the rack with the HV amp in it. It is now next to the vacuum chamber, along the long edge of the table, closer to the PDs:
Following this change, we took another transfer function using the same frequency settings as in the last measurement, but with 5-s settle and 100-s integration time. The data was taken in a more sophisticated way, and it appeared that there might be some sort of resonance feature near 24.19 kHz:
To get a better look, I changed the span to 10 Hz (24.185-24.195 kHz), using the same averaging settings. This sweep shows no obvious features:
This morning I finished sealing the enclosure with silicone rubber. I also cleaned most of it. I tried to fit the front wall into the setup, and I had to move the laser board back by few centimeters to accomodate the panel between the chamber and the board. So the input alignment is no more good.