I changed optics in the ETMX transmon path to remove clipping (which made a false QPD signal).
During the weekend I found that there was an offset in X arm c1ass pitch servo, which derives the signal by demodulating the arm cavity power, coming from the beam clipping in the transmon path.
The clipping was on the pair of the 1" mirrors that steer the beam after the 2" lens (see attached picture). The beam is about 5-6 mm in diameter at this distance from the lens and was not well centered.
I moved the steering mirrors downstream by about 8" where the beam is about 2-3 mm (the attached picture shows the mirrors in the new location). The Y arm layout is different from X arm and I didn't find any obvious clipping in transmon path.
The max X arm buildup went up from 1.3 to 1.5. I changed the TRX gain from -0.003 to -0.002 to obtain the normalized X arm power of 1 in this state. The MC refl DC is 1.6 out of 4.9 V and the Y arm buildup is ~0.9 so the TRX(Y) gains will have to be adjusted once the MC visibility is maximized.
GET CAMERA IMAGES OF EVERYTHING
Check all OpLevs centered, in and out of vacuum
IPANG needed to be re-aligned today. Heavy doors are in place and bolts tight (torque 25 & 45).
Steve! We are ready for pump down!
I will check the IFO alignment once again early tomorrow morning before Steve starts pumping down.
I tried a bunch of times to reduce my CARM offset so I could jump to REFLDC digitally, but I think I'm maybe being a little ambitious with the arm power I'm trying to get to.
I have modified the carm_cm_up script so that it does my new procedure. Everything is the same through locking the PRCL and MICH on 3f. Then it reduces the CARM offset to 1.5 nm. This is where we *used* to transition to sqrtInvTrans. Now I have it going a bit farther to 0.5 nm, and arm powers of about 1 before doing that transition. Also, before it transitions it lowers the CARM gain and engages the 1kHz lowpass in FM9. A gain of about 4 is fine to keep the gain peaking in the CARM loop to only about 10dB, and sets a UGF of 100Hz which is the peak of the phase bubble with the lowpass engaged.
Once I got to this point (several times tonight), I turned on CARM and DARM oscillations and looked at the transfer functions between (CARM and REFLDC) and (DARM and AS55Q). I have 2 DTT templates setup for this, in /users/Templates/PRFPMI. These templates assume that you have your new DARM signal (AS55) going to SRCL_IN1 and your new CARM signal (REFLDC, which is actually REFL11I coming through the CM board) going to MC_IN1.
I'm not sure why I'm losing lock. I don't see anything terribly telling on the time series plots, in particular none of the loops look like they are oscillating. Here is one of the better examples from this evening:
* I realigned the Xgreen on the PSL table (again) to maximize the beatnote amplitude. Y was fine, but X was very poorly overlapped on the camera.
* I put the SR785 back by the LSC rack and plugged it into the CM board for transfer functions. Didn't take any tonight.
* We have a small wishlist for scripting things: (1) DRMI restore script should reset REFL11 to "normal" REFL11. (2) CARM/DARM acquisition restore script should reset REFL11 to REFLDC. (3) CARM/DARM acquisition restore should also set PRMI parameters (as Q noted last week).
System-wide CIT LDAS cluster maintenance may cause disruptions to summary pages today.
LDAS has not recovered from maintenance causing the pages to remain unavailable until further notice.
> System-wide CIT LDAS cluster maintenance may cause disruptions to summary pages today.
FYI this issue has still not been solved, but the pages are working because I got the software running on an
alternative headnode (pcdev2). This may cause unexpected behavior (or not).
> LDAS has not recovered from maintenance causing the pages to remain unavailable until further notice.
> > System-wide CIT LDAS cluster maintenance may cause disruptions to summary pages today.
Adjusting the thickness of the cylindrical hole for the mirror on the 2" optic sleeve, from .6875" to .61" thick, moves the CoM to 0.0003" out of plane from the suspension wire. This is with the dumbell at its neutral point.
How close to zero do we need this to be? More fine tuning of that thickness can get it to zero, but this would require much tighter machining tolerance on that hole depth.
Moving the dumbell towards the back of the SOS assembly (noted as negative direction, with origin at the plane formed by the wires), moves the CoM to -0.002" from the plane.
Moving the dumbell towards the front of the SoS assmebly (positive direction wrt the plane formed by the suspension wire), moves the CoM to +0.0022" from the plane.
So the total adjustment range with the dumbell is -0.002"to 0.0022", with the plane formed by the wires as the origin.
We want to move the CoM with the adjustment range so that the residual deviation is adjusted by the bottom dumbbell. 0.0003" is well within the range and good enough.
I briefly talked with Jordan about this. This suspension will have OSEMs right? With 400ohm series resistance for the coil drivers, we will have ~+/-20mrad actuation range. Of course we'd like to use as much of this for interferometry and not static pitch alignment correction (possibly even increase the series resistance to relax the dewhitening requirements). But what is the target adjustability range in mrad with the dumbell/screw config? My target in the linked elog is 500urad (not any systematic optimum, but will allow us to use most of the DAC range for interferometry). Are these numbers in inches commensurate with this 500urad?
On a related note - are there grooves for the wires to sit in on the side of the sleeve? We looked at the solidworks drawing, and noticed that the groove doesn't extend all the way to the top of the clamp. Also, the material of both the clamping piece and the piece onto which the wire is pressed onto is SS. Don't we want them to be Aluminium (or something softer than the wire) so that the wire makes a groove when the clamp is tightened?
Jordan's screenshot actually shows that the vertical distance (Y) is 0.0000". We want to have the vertical distance of CoM from the wire clamping point to be 0.9mm in the nominal SOS design (this might need to be adjusted to have a similar pitch resonant freq for the different inertia of moment). Let's say it is ~mm ish.
The full range of the bottom dumbbell adjustment gives us the CoM adjustment range of +/-0.002” = +/-50um. This corresponds to an alignment range of +/-50mrad. And we want to set it within +/-500urad.
So we need to adjust the dumbbell position with the precision of 1/100 of the full range (precision of 0.5um).
The groove does not extend to the top of the clamp. The groove shallower than the wire diameter cause the hysteresis of the alignment. Also, the material of the pieces should be stainless steel. Al clamp is softer than the wire and will cause the groove to be dug on the material, causing increased bending friction and hysteresis again.
Saying, all of our suspended masses with Al stand-offs are suffering this issue to some extent. That was the reason to buy the ruby standoffs.
As it currently stands the Center of Mass of the Adapter Ring/Optic assembly is 0.0175" out of the plane formed by the suspension wire. See Attachments. The side plate, along with the EQ stops are hidden to show the CoM and the plane.
Note: The changes discussed in the meeting with Calum have not been added and are a work in progress. These changes include:
- Adding a 45 deg chamfer to the both parallel faces of the adapter ring. This along with a modified bracket for the EQ stops will allow for easier adjustment of the screws.
- Potentially changing material of adapter ring to stainless stell to more accurately emulate the mass of a 3" optic.
- Different adjustment mechanism of the "dumbell" at bottom of adapter ring to something similar to the VOPO suspension (will need to consult Calum further)
Note from today's meeting:
1. Can we adjust the thickness of the cylindrical hole for the mirror to move the COM in the plane of the wires. (We should be able to do that)
2. Please check how much we can displace the COM by the bottom dumbbell.
Adding the chamfer around the edge of the optic ring did not change the center of mass relative to the plane from the suspension wires.
The CoM was .0003" away from the plane. Adding the chamfer moved it closer by .0001". See the attached photo.
I've also attached the list of the Moments of Inertia of the SOS Assembly.
The current vertical distance between the CoM and the wire clamping point on the 3" Ring assembly is 0.33mm. That is the CoM is .33 mm below the clamping point of the wire. I took the clamping point to be the top edge of the wire clamp piece. see the below attachments.
I am now modifying the dumbell mechanism at the bottom of the ring to move the CoM to the target distance of 1.1mm.
After changing the material of the Balance Mass from 6061 Al to 304 Steel, and changing the thickness to 0.21" from 0.25". The CoM is now 1.11mm below the clamping point.
Koji expected a mass change of ~ 4g to move the mass to 1.1mm. The 6061 mass weighed ~1.31g and the 304 mass weighs 4.1g.
A potential issue with this is the screw used the adjust the position of these balance masses, threads through both the aluminum ring and this now 304 steel mass. A non silver plated screw could cold weld at the mass, but a silver plated screw will gall in the aluminum threads.
How about to use the non-Ag coated threaded shaft + the end SS masses with helicoils inserted? Does this save the masses to get stuck?
I have managed to orient the PZT in the mount such that its axes are approximately aligned with the vertical and the horizontal.
In the process, I discovered that the 4 screws on the back face of the PZT correspond to the location of the piezoelectric stacks beneath the tip-tilt platform. The PZT can therefore be oriented during the mounting process itself, before the mirror is glued onto the tip-tilt platform.
In order to verify that the pitch and yaw motion of the mirror have indeed been roughly decoupled, I centred the spot on the QPD, fed to the 'pitch' input of the PZT driver board (connected to channel 1 of the PZT) a 10 Vpp, 1 Hz sine wave from the SR function generator (having turned all the other relevant electronics, HV power supply etc ON. The oscilloscope trace of the output observed on the QPD is shown. The residual fluctuation in the Y-coordinate (blue trace) is I believe due to the tilt in the QPD, and also due to the fact that the PZT isnt perfectly oriented in the mount.
It looks like moving the tip-tilt through its full range of motion takes us outside the linear regime of the QPD calibration. I may have to rethink the calibration setup to keep the spot on the QPD in the linear range if the full range is to be calibrated, possibly decrease the distance between the mirror and the QPD. Also, in the current orientation, CH1 on the PZT controls YAW motion, while CH2 controls pitch.
[Attachment #1]: Computed spectral power transmissivity (according to my model) for the coating design at a few angles of incidence. Behavior lines up well with what FNO measured, although I get a transmission that is slightly lower than measured at 45 degrees. I suspect this is because of slight changes in the dispersion relation assumed and what was used for the coating in reality.
[Attachment #2]: Similar information as Attachment #1, but with the angle of incidence as the independent parameter in a continuous sweep.
Conclusion: The coating behaves in a way that is in reasonable agreement with our model. At 41.1 degrees, which is what the PR3 angle of incidence will be, T<50 ppm, which was what we specified. The larger range of angles was included because originally, we thought of using this optic as a substitute for SR3 as well. But I claim that for the shorter SRC (signal recycling as opposed to RSE), we will not end up using the new optic, but rather go for the G&H mirror. In any case, as Koji pointed out, ~50 ppm extra loss in the RC will not severely limit the recycling gain. Such large variation was not seen in the MC analysis because we only varied the angle of incidence by +/- 0.5 degrees about the nominal design value of 41.1 degrees.
I cleaned up my directory (/users/masha) today. A lot of the files are just code that I experimented with, but the important files for training the classification neural network are in "neural_network_classification". The "EarthquakeData" subdirectory contains my entire dataset. Files of the form "GenerateRNNInput" are used to create input vector sets to the network, while files of the "*NeuralNetworkClassification* actually run the code that generates the neural network vectors for the classification code block in the c1pem model.
Also, the folder "feed_c", which can also be found in Den's directory, contains the neural network controller code we played around with.
Target: Phase locking can be acheived by giving a scan to the oscilator frequency. This frequency is now controlled using the knobe on the AM/FM signal generator 2023B. But we need to control it remotely by giving the inputs of start frequency, end frequency and the steps.
The frequency oscilator and the computer is connected with the help of GPIB Ethernet converter. The IP address of the converter I used is '192.168.113.109' and its GPIB address is 10.
I could change the oscilator frequency by changing the input frequency with the help of the code I made (Inorder to check this code, I have changed the oscilator frequency multiple times. I hope it didn't create trouble to anyone). Now I am trying to make this code better by adding certain features like numpy, argument parse etc, which I will be able complete by next week. I am also considering to develop the code to have a sliding system to control the oscillatory frequency.
For record: The maximum limit of frequency which i changed upto is 100MHz.
For a few days now, the "code status" page has been telling us that the summary pages are DEAD, even though the pages themselves seemed to be generating plots. I logged into the 40m shared account on the cluster and checked the status of the condor job (with condor_q), and did not find anything odd there. I decided to consult Max, who pointed out that the script that checks the code status (/home/40m/DetectorChar/bin/checkstatus) was looking for a particular string in the log files ("gw_daily_summary"), while the recent change in the default output of condor_q meant that the string actually being written to the log files was "gw_daily_summa". This script has now been modified to look for instances of "gw_daily" instead, and so the code status indicator seems to be working again...
The execution of the summary page scripts has also been moved back to pcdev1 (from pcdev2, where it was moved to temporarily because of some technical problems with pcdev1).
Looking into the coherence between the seismometers and IMC length (MC_F):
FIrst with the seismometers only AC filtered at around 0.003 Hz and AA30Hz:
Ignore the increase in coherence at very low frequencies. That is an artefact.
Then with an additional filter single complex pole @1Hz Q=1000 (giving 20dB per decade in attenuation above 1Hz) , only for GUR1X:
Following is the coherence plot obtained when Guralp1 and STS2(Bacardi, Serial NR 100151) are placed very close to each other (but they aren't touching each other):
The seismometers were placed as shown in the picture below:
They are placed below the center of the mode cleaner vacuum tube.
We moved the seismometer STS2(Bacardi, Serial NR 100151) as we told in this Elog Entry, so the distance between Guralp1 and STS2 is 31.1m. Following is the coherence plot for this case:
then we also moved the Guralp1 under the BS and plugged it with the Guralp2 cable (at 7:35pm PDT), so now the distance between the two seismometers is 38.5m. Following is the coherence plot for this case:
Jessica and I took 45 mins (GPS times from 1122099200 to 1122101950) worth of data from the following channels:
C1:IOO-MC_L_DQ (mode cleaner)
C1:LSC-XARM_IN1_DQ (X arm length)
C1:LSC-YARM_IN1_DQ (Y arm length)
and for the STS, GUR1, and GUR2 seismometer signals.
The PSD for MCL and the arm length signals is shown below,
I looked at the coherence between the arm length and each of the three seismometers, plot overload incoming below,
For the coherence between STS and XARM and YARM,
Finally for GUR2,
A few remarks:
1) From the coherence plots, we can see that the arm length signals are coherent with the seismometer signals the most from 0.5 - 50 Hz. This is most evident in the coherence with STS. I think subtraction will be most useful in this range. This agrees with what we see in the PSD of the arm length signals, the magnitude of the PSD starts increasing from 1 Hz and reaches a maximum at about 30 Hz. This is indicative of which frequencies most of the noise is present.
2) Eric did not remember which of GUR1 and GUR2 corresponded to the ends of XARM and YARM. So, I went to the end of XARM, and jumped for a couple seconds to disturb whatever Gurald was in there. Using dataviewer I determined it was GUR1. Anyways, my point is, why is GUR1 less coherent with both arms and not just XARM? Since it is at the end of XARM, I was expecting GUR1 to be more coherent with XARM. Is it because, though different arms, the PSD's of both arms are roughly the same?
3) Similarly, GUR2 shows about the same levels of coherence for both arms, but it is more coherent. Is GUR2 noisier because of its location?
After setting up the microphones last week, I modified the Wiener filtering programs so as to include the microphone signals. They didn't seem to do much of anything to reduce the MC_L signal, so I looked at coherences. The microphones don't seem to have much coherence with the MC_L signal at all. I tried moving Bonnie to near the optical table next to the PSL (which isn't in a vacuum, and thus would, presumably, be more affected by acoustic noise), but that didn't seem to make much of a difference. Eventually, I'd like to put a mic in the PSL itself, but I need to work out how to mount it first.
Bonnie's new location.
You can see in bonnie_butch.pdf that none of the mic signals are giving very good coherence, although they all seem to have a peak at 24 Hz. (In fact, everything seems to have a peak there. Must be a resonant frequency of something in the mode cleaner.)
I've also attached plots of the coherences for all six accelerometers and the three Guralp seismometer axes. I plotted the most coherent traces together in the last pdf: the y-axes of the MC2 accelerometer and the two seismometers (the Ranger measures ONLY y) and, interestingly, the z-axis of the MC2 accelerometer. Unsurprisingly, the seismometers are most coherent at the low frequencies, and the MC2-Y accelerometer seems to be coherent at very similar frequencies. The MC2-Z accelerometer, on the other hand, seems to be coherent at the higher frequencies, and is highly complementary to the others. I am not really sure why this would be...
Finally, I was curious about how the noise varies throughout the day, because I didn't want to mistakenly decide that some particular configuration of accelerometers/seismometers/whatever was better than another b/c I picked the wrong time of day to collect the data. So, here is a plot of Wiener filters (using only accelerometer data) taken over 2-hour intervals throughout the entirety of July 6, 2009 (midnight-midnight local).
It's a little bit confusing, and I should probably try to select some representative curves and eliminate the rest to simplify things, but I don't have time to do that before the meeting, so this will have to suffice for now.
I liked to know quantitatively where the spot is on a mirror.
With an interferometer and A2L scripts, one can make the balance of the coil actuators
so that the angle actuation does not couple to the longitudinal motion.
i.e. node of the rotation is on the spot
Suppose you have actuator balancing (1+α) f and (1-α) f.
=> d = 0.016 x α [m]
Full Imbalance α = 1 -> d = 15 [mm]
10% Imbalance α = 0.1 -> d = 1.5 [mm]
1% Imbalance α = 0.01 -> d = 0.15 [mm]
Eq of Motion:
I ω2 θ = 2 R f
(correction) - I ω2 θ = D f cos(arctan(L/2/D))
(re-correction on Sep 26, 2017) - I ω2 θ = D f
m ω2 x = 2 α f ,
(correction) - m ω2 x = 2 α f ,
where R is the radius of the mirror, and D is the distance of the magnets. (kinda D=sqrt(2) R)
d, position of the node distant from the center, is given by
d = x/θ = α I / (m R) = 2 α β / D,
where β is the ratio of I and m. Putting R=37.5 [mm], L=25 [mm], β = 4.04 10-4 [m2], D~R Sqrt(2)
i.e. d = 0.015 α [m]
Oh, but it gets even better: in order to trust the A2L script in this regard you have to know that the coil driver - coil - magnet gain is the same for each channel. Which you can't.
But we have these handy f2pRatio scripts that Vuk and Dan Busby worked on. They use the optical levers to balance the actuators at high frequency so that the A2L gives you a true spot readout.
But wait! We have 4 coils and the optical lever only gives us 2 signal readouts...
Yes, of course. But so far I am trusting that the coils are inheretly balanced.
Probably you are talking about the dependence of the nodal position on the frequency...I need to check if 18Hz is sufficiently high or not for 0.1mm precision.
Also I am practicing myself to understand how I can adjust them by which screws as we probably have to do this adjustement many times.
(i.e. removal of the MZ, move of the table, PSL renewal and so on)
For the actuator calibration, we may be able to calibrate actuator responses by shaking them one by one while reading the OPLEV P/Y signals.
Oh, but it gets even better: in order to trust the A2L script in this regard you have to know that the coil driver - coil - magnet gain is the same for each channel. Which you can't.
18 (9 pairs) Coil Drivers have been modified. Namely ETMX/ITMX/ITMY/BS/PRM/SRM/MC1/MC2/MC3.
ETMX Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100624 ETMX Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100631
ITMX Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100620 IMTX Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100633
ITMY Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100623 ITMY Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100632
BS Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100625 BS Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100649
PRM Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100627 PRM Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100650
SRM Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100626 SRM Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100648
MC1 Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100628 MC1 Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100651
MC2 Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100629 MC2 Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100652
MC3 Coil Driver 1 (UL/LL/UR)now has R=100 // 1.2k ~ 92Ohm for CH1/2/3 S2100630 MC3 Coil Driver 2 (LR/SD)now has R=100 // 1.2k ~ 92Ohm for CH3 S2100653
Will be updating this linking each coil driver to the DCC
The DCC has been updated, along with the modified schematic. Links have been attached.
Per Gautam's request, I've checked the coil resistances and inductances.
A DSUB25 breakout was directly connected to the flange (Attachment 1).
The impedance meter was nulled every time the measurement range and type (R or L) were changed.
Feedthru connector: PRM1
Pin1 - flange: R = 0.8Ω
Pin11-23 / R = 1.79Ω / L=3.21mH
Pin 7-19 / R = 1.82Ω / L=3.22mH
Pin 3-15 / R = 1.71Ω / L=3.20mH
Feedthru connector: BS1
Pin1 - flange: R = 0.5Ω
Pin11-23 / R = 1.78Ω / L=3.26mH
Pin 7-19 / R = 1.63Ω / L=3.30mH
Pin 3-15 / R = 1.61Ω / L=3.29mH
Feedthru connector: SRM1
Pin1 - flange: R = 0.5Ω
Pin11-24 / R = 18.1Ω / L=3.22mH
Pin 7-20 / R = 18.8Ω / L=3.25mH
Pin 3-16 / R = 20.3Ω / L=3.25mH
Feedthru connector: PRM2
Pin1 - flange: R = 0.6Ω
Pin11-23 / R = 1.82Ω / L=3.20mH
Pin 7-19 / R = 1.53Ω / L=3.20mH
Pin 3-15 / R = N/A
Feedthru connector: BS2
Pin1 - flange: R = 0.6Ω
Pin11-23 / R = 1.46Ω / L=3.27mH
Pin 7-19 / R = 1.54Ω / L=3.24mH
Pin 3-15 / R = N/A
Feedthru connector: SRM2
Pin1 - flange: R = 0.7Ω
Pin11-24 / R = N/A
Pin 7-20 / R = 18.5Ω / L=3.21mH
Pin 3-16 / R = 19.1Ω / L=3.25mH
The SRM pinouts seem mirrored compared to the others. In fact, these two connectors are equipped with mirror cables (although they are unshielded ribbons) (Attachment 2).
The SRM sus is located on the ITMY table. There is a long in vacuum DSUB25 cable between the ITMY and BS tables. I suspect that the cable mirrors the pinout and this needs to be corrected by the in-air mirror cables.
I went around the lab and did not find any other suspensions which have the mirror cable.
WIth the BHD configuration, we will move the feedthru for the SRM to the one on the ITMY chamber. So I believe the situation is going to be improved.
For consistency, today, I measured both the BS and PRM actuator balancing using the same technique and don't find as serious an imbalance for the BS as in the PRM case. The Oplev laser source is common for both BS and PRM, but the QPDs are of course distinct.
BTW, I thought the expected resistance of the coil windings of the OSEM is ~13 ohms, while the BS/PRM OSEMs report ~1-2 ohms. Is this okay?
ugh. sounds bad - maybe a short. I suggest measuring the inductance; thats usually a clearer measurement of coil health
I didn't repeat Koji's measurement, but he reports the expected ~3.2mH per coil on all the BS and PRM coils.
So it would seem that there is some other noise which has a 1/f^2 shape and is at the same level we expected the DAC noise to be at. Rana suggested checking coherence with MC transmission to see if this could be laser intensity noise.
I also want to re-do the actuator calibrations for the vertex optics again before re-posting the revised noise budget.
I downloaded a segment of data from the time when the DRMI was locked with the BS and ITM coil driver de-whitening switched on, and looked at coherence between MC transmission and the MICH error signal. Attachment #1 doesn't show any broadband high coherence between 60-300Hz, so it cannot explain the noise in the full range between 60-300Hz.
The DQ channel for the MC transmission is recorded at 1024 kHz, so to calculate the coherence, I had to decimate the 16K MICH data.
Since we have the AOM installed, I suppose we can actually measure the intensity noise coupling to MICH by driving a line in the AOM.
I also checked for coherence in the 60-300Hz band between MICH/PRCL and MICH/SRCL, and didn't see any appreciable coherence. Need to think about this more.
Rana suggested checking coherence with MC transmission to see if this could be laser intensity noise.
The absence of evidence is not evidence of absence.
This measurement has been troublesome - I was plagued by large 60Hz harmonics (see Attachment #1), the cause of which was unknown. I powered all electronics used in the measurement set up from the same power strip (one of the new surge-protecting ones Steve recently acquired for us), but these remained present. Yesterday, Koji helped me troubleshoot this issue. We did the various things, I try to put them here in the order we did them:
Today, I tried to repeat the measurement, with the newly made twisted ribbon cable, but the large 60Hz harmonics were back. Then I realized we had also disconnected the WiFi extender and GPIB box yesterday.
Turns out that connecting the Prologix box to the SR785 (even with no power) is the culprit! Disconnecting the Prologix box makes these harmonics go away. I was using the box labelled "Santuzza.martian" (192.168.113.109), but I double-checked with the box labelled "vanna.martian" (192.168.113.105, also a different DC power supply adapter for the box), the effect is the same. I checked various combinations like
but it looks like connecting the GPIB box to the analyzer is what causes the problem. This was reproducible on both SR785s in the lab. So to make this measurement, I had to do things the painful way - acquire the spectrum by manually pushing buttons with the GPIB box disconnected, then re-connect the box and download the data using SRmeasure --getdata. I don't fully understand what is going on, especially since if the input connector is directly terminated using a 50ohm BNC terminator, there are no harmonics, regardless of whether the GPIB box is connected or not. But it is worth keeping this problem in mind for future low-noise measurements. My elog searches did not reveal past reports of similar problems, has anyone seen something like this before?
It also looks like my previous measurement of the de-whitening board noises was plagued by the same problem (I took all those spectra with the GPIB boxes connected). I will repeat this measurement.
At the meeting this week, it was decided that
I also think it would be a good idea to up the 100-ohm resistors in the bias path on the ITM coil driver boards to 1kohm wire-wound. Since the dominant noise on the coil-driver boards is from the voltage noise of the Op-Amps in the bias path, this would definitely be an improvement. Looking at the current values of the bias MEDM sliders, a 10x increase in the resistance for ITMX will not be possible (the yaw bias is ~-1.5V), but perhaps we can go for a 4x increase?
The plan is to then re-install the boards, and see if we can
We can then take a call on how much to up the series resistance in the DAC signal path.
Now that I have figured out the cause of the harmonics, I will also try and measure the combined electronics noise of de-whitening board + coil driver board and compare it to the model.
I've given Steve a list of the thin-film resistors we need to implement the changes discussed in the preceeding elogs - but I figured it would be good to see if we can realize the projected improvement in MICH displacement noise just by fixing the BS Oplev loop shape and turning the existing whitening on. Before re-installing them however, I did make a few changes:
Photos of all the boards were taken prior to re-installation, and have been uploaded to the 40m Google Photos page - I will update schematics + photos on the DCC page once other planned changes are implemented.
I also measured the transfer functions on the de-whitened signal paths on all the boards before re-installing them. I then fit everything using LISO, and updated the filter banks in Foton to match these measurements - the original filters were copied over from FM9 and FM10 to FM7 and FM8. The new filters are appended with the suffix "_0517", and live in FM9 and FM10 of the coil output filter banks. The measured TFs (for ITMs and BS) are summarized in Attachment #1, while Attachment #2 contains the data and LISO file used to do the fits (path to the .bod files in the .fil file will have to be changed appropriately). I used 2 complex pole pairs at ~10 Hz, two complex zero pairs at ~100Hz, real poles at ~15Hz and ~3kHz, and real zeros at ~100Hz and ~550Hz for the fits. The fits line up well with the measured data, and are close enough to the "expected" values (as calculated from component values) to be explained by tolerances on the installed components - I omit the plots here.
After re-installing the boards in the Eurocrate, restoring rough alignment, and updating the filter banks with the most recent measured values, I wanted to see if I could turn the whitening on for one of the optics (ITMY) smoothly before trying to do so in the full DRMI - switching off the "SimDW_0517" filter (FM9) should switch the signal path on the de-whitening board from bypass to de-whitened, and I had confirmed last week with an extender board that the voltage at the appropriate backplane connector pin does change as expected when the FM9 MEDM button is toggled (for both ITMs, BS and SRM). But today I was not able to engage this transition smoothly, the optic seems to be getting kicked around when I engage the whitening. I will need to investigate this further.
Unrelated to this work: the ETMY Oplev HeNe is dead (see Attachment #3). I thought we had just replaced this laser a couple of months ago - what is the expected lifetime of these? Perhaps the power supply at the Y-end is wonky and somehow damaging the HeNe heads?
I think the reason I am unable to engage the de-whitening is that the OL loop is injecting a ton of control noise - see Attachment #1. With the OL loop off (i.e. just local damping loops engaged for the ITMs), the RMS control signal at 100Hz is ~6 orders of magnitude (!) lower than with the OL loop on. So turning on the whitening was just railing the DAC I guess (since the whitening has something like 60dB gain at 100Hz).
The Oplev loops for the ITMs use an "Ellip15" low-pass filter to do the roll-off (2nd order Elliptic low pass filter with 15dB stopband atten and 2dB ripple). I confirmed that if I disable the OL loops, I was able to turn on the whitening for ITMY smoothly.
Now that the ETMY OL HeNe has been replaced, I restored alignment of the IFO. Both arms lock fine (I was also able to engage the ITMY Coil Driver whitening smoothly with the arm locked). However, something funny is going on with ASS - running the dither seems to inject huge offsets into the ITMY pit and yaw such that it almost immediately breaks the lock. This probably has to do with some EPICS values not being reset correctly since the recent slow-machine restarts (for instance, the c1iscaux restart caused all the LSC RFPD whitening gains to be reset to random values, I had to burt-restore the POX11 and POY11 values before I could get the arms to lock), I will have to investigate further.
GV edit 2pm 31 May: After talking to Koji at the meeting, I realized I did not specify what channel the attached spectra are for - it is C1:SUS-ITMY_ULCOIL_OUT.
But today I was not able to engage this transition smoothly, the optic seems to be getting kicked around when I engage the whitening. I will need to investigate this further.
We tried to debug the mysterious sudden failure of ASS - here is a summary of what we did tonight. These are just notes for now, so I don't forget tomorrow.
What are the problems/symptoms?
What are the (known) changes since the servos were last working?
Hypotheses plus checks (indented bullets) to test them:
For whatever reasons, it appears that dithering the cavity mirrors at frequencies with amplitudes that worked ~3 weeks ago is no longer giving us the correct error signals for dither alignment. We are out of ideas for tonight, TBC tomorrow...
In light of the discussion at today's meeting, Guantanamo and I looked at how the series resistance for the test mass coil drivers limits the amount of squeezing we could detect.
The parameters used for the following calculations are:
Since we need to operate very close to signal recycling, instead of the current signal extraction setup, we will need to change the macroscopic length of the SRC. This will change the mode matching requirements such that the current SRM does not have the correct radius of curvature. One solution is to use the spare PRM which has the correct radius of curvature but a transmissivity of 0.05 instead of 0.1. So using this spare PRM for the SRM and changing the length of the SRC to be the same as the PRC we can get
This lower transmissivity for the SRM also reduces the achievable squeezing from the current transmissivity of 0.1. For an SRM with a transmissivity of 0.15 (which is roughly the optimal) we can get
The minimum achievable squeezing moves up from around 205 Hz at 1 W to 255 Hz at 5 W because the extra power increases the radiation pressure at lower frequencies.
I've been thinking about what we need to do to the de-whitening boards for the ITMs and ETMs, in order to have low noise actuators. Noting down what I have so far, so that people can comment / point out things I've overlooked.
Attachment #1: Block diagram schematic of the de-whitened signal path on D000183 as it currently exists. I've omitted the unity gain buffer stage at the output, though this is important for noise considerations.
Some considerations, in rough order of priority:
I will experiment with a few different shapes and investigate noise and de-whitened digital signal levels based on these considerations. At the very least, I guess we should remove the x3 gain on the ETM boards, they have already been bypassed for the ITMs.
We measured the MC coil driver noise at the output monitors of the coil driver board with an SR785 in order to further diagnose the excess IMC frequency noise.
Attachments 1 and 2 show the noise for the UL coils of MC3 and MC2 with various combinations of output filters engaged. When the 28 Hz elliptic filter is on, the analog dewhitening filter is off, and vice versa. The effect of the analog low pass filter is visible in MC3, but the effect of the digital low pass filter is swamped by the DAC noise.
We locked the arms and measured the ALS beatnote in each of these filter combinations, but which filters were on did not effect the excess IMC frequency noise. This suggests that the coil drivers are not responsible for the excess noise.
Attachment 2 shows the noise for all five coils on MC1, MC2, and MC3 as well as for ITMY, which is on a different DAC card from the MCs. All filters were on for these measurements.
Why is MC2 LR so different from the others???
The previous measurements were made from the coil driver output monitors. To investigate why the MC2 LR coil has less noise than the other coils, I also measured the noise at the output to the coils.
The circuit diagram for the coil driver board is given in D010001 and a picture of the rack is on the 40m wiki here. The coil driver boards are in the upper left quadrant of the rack. The input to the board is the column of LEMOs which are the "Coil Test In" inputs on the schematic. The output monitors are the row of LEMOs to the right of the input LEMOs and are the "FP Coil Volt Mon" outputs on the schematic. The output to the coils "Coil Out" in the schematic are carried through a DB15 connector.
The attachment shows the voltage noise for the MC2 LR coil as well as the UL coil which is similar to all of the other coils measured in the previous measurement. While the LR coil is less noisy than the UL coil as measured at the output monitor, they have the same noise spectrum as measured at the output to the coils themselves. So there must be something wrong with the buffer circuit for the MC2 LR voltage monitor, but the output to the coils themselves is the same as for the other coils.
I was preparing a short write-up / test procedure for the custom HV coil driver, when I thought of something I can't resolve. I'm probably missing some really basic physics here - but why do we not account for the shot noise from DC current flowing through the series resistor? For a 4kohm resistor, the Johnson current noise is ~2pA/rtHz. This is the target we were trying to beat with our custom designed HV bias circuit. But if there is a 1 mA DC current flowing through this resistor, the shot noise of this current is 18pA/rtHz, which is ~9 times larger than the Johnson noise of the same resistor. One could question the applicability of this formula to calculate the shot noise of a DC current through a wire-wound resistor - e.g. maybe the electron transport is not really "ballistic", and so the assumption that the electrons transported through it are independent and non-interacting isn't valid. There are some modified formulae for the shot noise through a metal resistor, which evaluates to 10pA/rtHz for the same 4kohm resistor, which is still ~5x the Johnson noise.
In the case of the HV coil driver circuit, the passive filtering stage I added at the output to filter out the excess PA95 noise unwittingly helps us - the pole at ~0.7 Hz filters the shot noise (but not the Johnson noise) such that at ~10 Hz, the Johnson noise does indeed dominate the total contribution. So, for this circuit, I think we don't have to worry about some un-budgeted noise. However, I am concerned about the fast actuation path - we were all along assuming that this path would be dominated by the Johnson noise of the 4kohm series resistor. But if we need even 1mA of current to null some DC DARM drift, then we'd have the shot noise contribution become comparable, or even dominant?
I looked through the iLIGO literature, where single-stage suspensions were being used, e.g. Rana's manifesto, but I cannot find any mention of shot noise due to DC current, so probably there is a simple explanation why - but it eludes me, at least for the moment. The iLIGO coil drivers did not have a passive filter at the output of the coil driver circuit (at least, not till this work), and there isn't any feedback gain for the DARM loop at >100 Hz (where we hope to measure squeezing) to significantly squash this noise.
Attachment #1 shows schematic topologies of the iLIGO and proposed 40m configs. It may be that I have completely misunderstood the iLIGO config and what I've drawn there is wrong. Since we are mainly interested in the noise from the resistor, I've assumed everything upstream of the final op-amp is noiseless (equivalently, we assume we can sufficiently pre-filter these noises).
Attachment #2 shows the relative magnitudes of shot noise due to a DC current, and thermal noise of the series resistor, as a function of frequency, for a few representative currents, for the slow bias path assuming a 0.7Hz corner from the 4kohm/3uF RC filter at the output of the PA95.
Some lit review suggests that it's actually pretty hard to measure shot noise in a resistor - so I'm guessing that's what it is, the mean free path of electrons is short compared to the length of the resistor such that the assumption that electrons arrive independently and randomly isn't valid. So Ohm's law dictates and that's what sets the current noise. See, for example, pg 432 of Horowitz and Hill.