If ITMX already has another side magnet, we can migrate the side OSEM of ITMX to the other side. This way, the interference of the OSEMs can be avoided.

[Paco, Anchal]

BS Chamber work

• ASL was positioned in nominal place.
• PR3 was moved to its nominal place from temprorary position.
• BS Table was rebalanced
• Earthquake stops were removed from all SOS from BS table (LO2, SR2, BS, PR3)

ITMY Chamber work

• AS2, AS3, LO3, LO4, and BHDBS were positioned in the nominal place.
• AS1 was moved to its nominal place from temporary position.
• ITMY tbale was rebalanced
• Earthquake stops were removed from all SOS from ITMY table (AS1, AS4, ITMY)

• The arm powers could be stabilized somewhat once the CM_SLOW path to MC2 was engaged.
• However, I was never able to get the AO path to do anything good.
• Took a bunch of CM board TFs, need to think about what I need to do differently to get this next bit to work.
• An SR785 is sitting next to the LSC rack hooked up to the CM board. I also borrowed the GPIB unit from the AG4395 to grab data from said SR785.
• One thing I noticed that the CARM_B (=CM_SLOW) and DARM_B (=AS55_Q) signals both had a DC offset, so maybe this is indicative of some DC offset in the PRMI 3f signals? Right now, I lock the PRMI without any offsets, and as I reduce the CARM offset, I can see the DC value of REFL11_I and AS55_Q changing significantly. To be investigated in tonight's locking.

The transients are likely due to doppler interference due to the input laser frequency sloshing due to errant control signals after the IMC unlock. I performed a few "partial" ringdowns by reducing the power by about 80% while keeping the IMC servo locked. (Function generator at 0.5Vpp square wave, 0.25V offet. Turned IMC boosts off to increase the stable range of the servo).

I need to work out how to extract the loss from this, I think having a partial ringdown may change the calculations somewhat; the time constants in the trans and refl signals are not identical.

Thanks to Gautams nice setup, it was very easy to take these measurements. Thanks! Code and data attached.

Mach Zucker on howto do Ringdowns:  https://dcc.ligo.org/LIGO-T900007

Summary:
Since we reduced the integration time of the particle counter by a factor of 10, we had to add a gain of 10
to the EPICS channels C1:PEM-count_full and C1:PEM-count_half.
I asked Alex to change it and he did it. I forgot to ask him to change the gain of C1:PEM-count_half. So now only
C1:PEM-count_full has x10 gain.

Detail:
C1:PEM-count_full and C1:PEM-count_half are 'Soft Channel' records in the database (Pcount.db). The values are actually
written into the VAL fields directly by an SNL code Particle.o.
Particle.o reads data from the RS-232C port, to which the particle counter is connected. Then it parses the data and put values
into relevant EPICS channels using channel access. This means we cannot change the gain of the channels by modifying the
database file. For example, ASLO field does not have any effect when the value is directly written into the VAL field.
We had to modify the SNL code. Alex modified Particle.st and the new SNL object file is Particle_x10.o sitting in 
/cvs/cds/caltech/target/c1psl/. I modified seq.load so that c1psl loads Particle_x10.o when rebooted.
The source code for the old Particle.st can be found on lesath.ligo.caltech.edu in
/export/CDS/d/epics/apple/Caltech/40mVac/40mVacScipe/dev/src
I asked Alex to disclose the location of the source of the new code.
In order to compile the SNL code into an object file for Motorola CPU by ourselves, we have to call Dave Barker at LHO.

I have placed a GT321 particle counter on top of the MC1/MC3 chamber next to the BS chamber. The serial cable is connected to c1psl computer on 1X2 using 2 usb extenders (blue in color) over the PSL enclosure and over the 1X1 rack.

The main serial communication script for this counter by Radhika is present in 40m/labutils/serial_com/gt321.py.

A 40m specific application script is present in the new git repo for 40m scripts, in 40m/scripts/PEM/particleCounter.py. Our plan is to slowly migrate the legacy scripts directory to this repo overtime. I've cloned this repo in the nfs shared directory at /opt/rtcds/caltech/c1/Git/40m/scripts which makes the scripts available at all computers and keep them upto date in all computers.

The particle counter script is running on c1psl through a systemd service, using service file 40m/scripts/PEM/particleCounter.service. Locally in c1psl, /etc/systemd/system/particleCounter.service is symbollically linked to the file in the file.

Following channels for particle counter needed to be created as I could not find any existing particle counter channels.

[C1:PEM-BS_PAR_CTS_0p3_UM]
[C1:PEM-BS_PAR_CTS_0p5_UM]
[C1:PEM-BS_PAR_CTS_1_UM]
[C1:PEM-BS_PAR_CTS_2_UM]
[C1:PEM-BS_PAR_CTS_5_UM]

These are created from 40m/softChansModbus/particleCountChans.db database file. Computer optimus is running a docker container to serve as EPICS server for such soft channels. To add or edit channels, one just need to add new database file or edit database files in thsi repo and on optimus do:

controls@optimus|~> sudo docker container restart softchansmodbus_SoftChans_1
softchansmodbus_SoftChans_1

that's it.

I've added the above channels to /opt/rtcds/caltech/c1/chans/daq/C0EDCU.ini to record them in framebuilder. Starting from 11:20 am Oct 20, 2021 PDT, the data on these channels is from BS chamber area. Currently the script is running continuosly, which means 0.3u particles are sampled every minute, 0.5u twice in 5 minutes and 1u, 2u, and 5u particles are sampled once in 5 minutes. We can reduce the sampling rate if this seems unncessary to us.

The particle count channel names were changes yesterday to follow naming conventions used at the sites. Following are the new names:

C1:PEM-BS_DUST_300NM
C1:PEM-BS_DUST_500NM
C1:PEM-BS_DUST_1000NM
C1:PEM-BS_DUST_2000NM
C1:PEM-BS_DUST_5000NM
 

The legacy count channels are kept alive with C1:PEM-count_full copying C1:PEM-BS_DUST_1000NM channel and C1:PEM-count_half copying C1:PEM-BS_DUST_500NM channel.

Attachment one is the particle counter trend since 8:30 am morning today when the HVAC wokr started. Seems like there was some peak particle presence around 11 am. The particle counter even counted 8 counts of particles size above 5um!

SVG doesn't work in my browser(s). Can we use PDF as our standard for all graphics other than photos (PNG/JPG) ?

rethinking what I said on Wednesday - its not a good idea to put the particle counter on a vac chamber with optics inside. The rumble from the air pump shows up in the acoustic noise of the interferometer. Let's look for a way to mount it near the BS chamber, but attached to something other than vacuum chambers and optical tables.

I have done some reading about where would be the best place to put the particle counter. The ISO standard (14644-1:2015) for cleanrooms is one every 1000 m^2 so one for every 30m x 30m space. We should have the particle counter reasonably close to the open chamber and all the manufactures that I read about suggest a little more than 1 every 30x30m. We will have it much closer than this so it is nice to know that it should still get a good reading. They also suggest keeping it in the open and not tucked away which is a little obvious. I think the best spot is attached to the cable tray that is right above the door to the control room. This should put it out of the way and within about 5m of where we are working. I ordered some cables to route it over there last night so when they come in I can put it up there. 

I mounted the particle counter over the BS chamber attached to the cable tray as seen in Attachment 1. The signal cable runs through an active 30ft cable to the 1x2 rack. the wire is labeled and runs properly through the cable tray. The particle counter is plugged in at the power strip attached near the cable tray. The power cord is also labeled. 

I restarted the particle counter service in the c1psl computer in the /etc/systemd/system/ folder using the commands

I cannged the usb hub assigned in the service file to ttyUSB0 which is what we saw the computer had named it.

Checking the channels from this elog show the same particle count as when testing with the buttons and checking the screen. It seems that the channels had been down but are now restarted.

nice - please update the particle counter page in the 40m wiki. Its probably years out of date.

 The east end particle count is 155K for 0.3 micron and 15K for 0.5 micron  counts/cf min.  In order to minimize diffusion of dirt into the IFO we set up the mobile HEPA with CP Stat 100 tent.

This set up gives us practically ZERO inside the tent

[Jenne, Kiwamu]

While Kiwamu was finalizing the X green alignment, I started to prepare to remove the ETMY door, and begin checking out its OSEMs, etc, so we could start moving it to it's new place, and figure out why it's been wonky for a while.  I ran the particle counter, and we have a factor of ~5 more particles than normal.  Kiwamu and I agreed not to open ETMY.  Since we had briefly opened the IOO and Output Optics chambers to check the X green's position on the PSL table, we immediately shut those doors.  They were probably open for ~15 minutes or so.  (Yes Steve, we should have checked before opening any doors, but at least we remembered to check at all, and the doors were only open for a few minutes rather than for a few hours.)

I attach a 24hrs trend of the particle counts, for reference.  It looks like it's been a little high for a while, but today it's really dirty in the air.

I'm copying and pasting Nikhil's email here as he was unable to login to the elog (but should now be able to in order to reply to any comments, and add more details about this test, motivation, methodology etc).

I've started putting together a list of things we'll need to buy to do BHD readout. I'm still messing around with more detailed optics layouts, but wanted to get a list started here so people can let me know if I'm missing any big, obvious categories of goods.

My current plan makes minimal changes to the signal path going to the OMC, and tries to just get the LO beam into the OMC with minimal optics. I'm not thinking of any of the optics as suspended, and it requires several reflections of the LO beam, so probably this is not an excellent configuration, but it's a start for getting the parts list:

1. My current thought is to use the MC reflection, the beam that heads from MC1 to MCR1, as the LO beam
   1. From MCR1, send the LO to a BS that directs it into an MMT, oriented along x (and lets us keep the MC refl PO)
   2. After the two MMT optics, the beam will be traveling along -x, and can be directed to a mirror that sends it towards -y to two steering mirrors that send it along -x then +x near the top of the table (perpendicular to the signal MMT.
   3. Then, send it to a PBS, which replaces the mirror directly after the signal MMT. This is where it combines 
2. Beam is then sent to the steering mirrors to bring it into the OMC
3. In parallel, the signal beam is going through the same path it has now, including the pickoff beam, with the one change that we need a HWP somewhere before the PBS, and the PBS replaces the mirror directly after the MMT (and needs to move a bit closer to have the beam properly directed)

I started making a layout of this scheme, but it's probably not going to work so I'm going to make a quick layout of this more major modification instead:

1. Both the MCR beam and the AS beam come in about parallel. Send each to a PO mirror.
2. The PO mirror directs both beams into parallel MMT aligned along x
3. From the MMT, each is directed to a pair of steering mirrors located where the OMC MMT is located now
4. From the steering mirrors go to the PBS that combines the signal and LO
5. Then to two more steering mirrors to get into the OMC, which may be moved towards +x
6. From the OMC go to the BHD PBS

What we need

Optics

• HWP for just before the LO combines with the signal
• HWP for just before the signal combines with the LO (is this necessary?)
• PBS to replace OM5 (combines the LO and the signal)
• Two MMT optics
• Two piezo-driven TT optics for steering the LO to the PBS
• One additional non-piezo optic for between the LOMMT and the LO-TTs
• One additional BS to get the LO into the MMT (and to let us still have the PO)
• -1 optic—we pick up one mirror that we replace with the PBS

Optomechanics

• 2x HWP mounts
• 1x PBS mount
• 2x mounts for piezo-driven TT
• 2x MMT optic mounts—I didn’t take a close enough look at these during the vent to know what we need here
• 2x mounts for ordinary optics
• 9x clamps for optics mounts (maybe fewer if some are on blocks)
• 9x posts for optics mounts

Electronics

• Additional TT driver
• HV supply for the new TTs
• Are the HWP actively controlled? We might need something to drive those?
• Do we have enough DAC/ADC channels?

Questions

These are mostly just miscellaneous

1. What of these optics need to be suspended? If we need suspensions on all of the LO optics, including the MMT, I’m not sure we’re going to be able to fit all of this on the table as I envision it…..
2. What if anything can we put out of vacuum (HWP for example)?
3. Do we actually need two MMT?

Can we use the leakage beam from MMT2 on the OMC table as the LO beam? I can't find the spec for this optic, but the leakage beam was clearly visible on an IR card even with the IMC locked with 100 mW input power so presumably there's enough light there, and this is a cavity transmission beam which presumably has some HOM content filtered out.

That seems fine, I wasn't thinking of that beam. in that case could we just have a PBS directly behind MMT2 and send both beams to the same OMMT?

Alternatively we can move OM5 and the beam path OMPO-OMMTSM towards -y, then put the LO-OMMT parallel to the existing OMMT but displaced in +x... we'd have to move the existing OMC and BHD towards +x as well. 

I've attached the diagram of what I mean.

There are a couple caveats and changes that would have to be made that are not included in this diagram, because they would be made on different tables.

1. I moved MMT2, which means the other MMT optics would have to be adjusted to accomodate this. I checked out the configuration on the BS table and this seems doable with a small rotation of MMT1 and maybe PJ2.
2. I don't know the best way to get the OMC REFL beam out... thoughts?
3. This diagram is kind of crappy after my edits, someone should tell me how to avoid collapsing all layers when I open these layout diagrams in inkscape, because I ended up editing the layout in Acrobat instead, where the lack of object grouping caused a bunch of the optics to lose one or two lines along the way.
4. I didn't include all beam paths explicitly but can if this looks like a good configuration.
5. The optic that picks off the transmission through MMT2 will need to move a bit, but there was a clamp in the way; this should be a minor change.
6. The optic just before the OMC needs to be moved up a bit.
7. The optic after the signal OMMT should be changed to a PBS and translated a bit; this is where the LO and signal beams will combine

Gautam also had some questions about the BHD/OMC timeline and plan. I feel somewhat on shaky ground with the answers, but figured I'd post them so I can be corrected once and for all.

1. Is the plan really to use the current OMC setup to make a homodyne measurement? 
   1. ​I'm not sure where on the timeline the new OMC and BHD switchover are relative to each other. I have been imagining doing the swap to BHD before having a new OMC.
2. I thought the current OMC resurrection plan was to do DC readout and not homodyne?
   1. ​I think the OMC resurrection plan is leading to DC readout, but when we switch over to BHD we'll be able to operate at the dark fringe. Is that right?
3. Is it really possible to use our single OMC to clean both the LO and dark port beams? Isn't this the whole raging debate for A+?
   1. ​My understanding is yes, with the LO and DP in orthogonal polarizations. It's not clear to me why we expect to be able to do this while there is a debate for A+, perhaps our requirements are weaker. It is something I should calculate, I'll talk to Koji.
4. A layout diagram would be really useful.
   1. ​Attached now.
5. Where in the priority list does this come in?
   1. ​I am a leaf in the wind. I think this comes well after we have the OMC resurrected, we just want to get a sense for what parts we need so we can order them before the fiscal year closes.

I ran the following command to find which models/parts are not under version control, or have modifications not commited:

grep "mdl" $(cat models.txt) | awk '{print $NF}' | sort | uniq | xargs svn status

models.txt includes lines like "/opt/rtcds/caltech/c1/rtbuild/c1ass.log" for each running model. These are the build logs that detail every file being sourced. 

The resultant list is:

?       /opt/rtcds/userapps/release/cds/common/models/BLRMS_HIGHFREQ.mdl
C       /opt/rtcds/userapps/release/cds/common/models/rtdemod.mdl
M       /opt/rtcds/userapps/release/cds/common/models/SCHMITTTRIGGER.mdl
?       /opt/rtcds/userapps/release/isc/c1/models/blrms.mdl
?       /opt/rtcds/userapps/release/isc/c1/models/IQLOCK.mdl
?       /opt/rtcds/userapps/release/isc/c1/models/PHASEROT.mdl
?       /opt/rtcds/userapps/release/sus/c1/models/QPD_WHITE_CTRL_MUX.mdl

I will commit the uncommited c1 parts, and think about what to do about the rtdemod and SCHMITTRIGGER parts. 

Once I check out the latest revision of the userapps repo (in a seperate location), I will do something similar to look for models that have changed since the svn revision that is checked out in our running system. 

In response to Steve's elog entry, and for 40m posterity, I provide the Paschen Curve.

I spent some time the other day trying to diagnose the problem with the Y Arm universal PDH box (S/N 17), which Katrin has been unable to use for locking the green beam. As far as I can tell, there is nothing wrong with the box itself (though the weird TF behavior that Katrin noticed was not initially reproducible, so its cause may still be there).

I did notice that I was unable to generate a PDH error signal using the universal box. In this configuration, a summing circuit is needed to add the PZT modulation signal (f_mod = 178875 Hz) in along with the feedback signal. To do this, Katrin was using a slightly different version of the passive summing box that Kiwamu built for the X Arm. I read this entry to understand how it is supposed to work and noticed that the "expected transfer functions" were not what the circuit actually does. I have talked to Kiwamu about it and he found that he posted the wrong TFs (he has the right ones on his computer). As you can see from the plot below, there is extra low passing that severely attenuates the modulation path to the PZT. In addition, there is a phase shift of ~-60 degrees, which is bad.

To combat this, I propose we simply change the resistor in the modulation path from 1M to 10k. This leaves the feedback path TF unchanged, and changes the mod path into a sort of bandpass filter for the modulation frequency. The fact that the phase is near zero at f_mod means we don't have to come up with some way to phase shift the signal for demodulation. The attenuation level of ~-36 dB is also convenient: The ZAD-8 mixer wants 7 dBm, so, 10 dBm (FG) - 3 dB (splitter) - 36 dB (sumbox) = -25 dBm ~ 12 mV. This is roughly the desired PZT voltage level.

Larry W said that some security issues were flagged on nodus. So I ran

on nodus. The exclude flag is because there were some conflicts related to that particular package. Hopefully this has fixed the problem. It's been a while since the last update, which was in January of this year.

Steve and Koji

We aligned the peeping mirrors to look at the surface of the ITMs.
They had been misligned as we move the positions of the ITMs, but now they are fine.

The periscope design for beam elevation of the green beams is posted. The design for the 90 deg steering version is also coming.

(2010-03-29: update drawings by daisuke)

90deg version: http://nodus.ligo.caltech.edu:8080/40m/2725

Here the design of the periscope for the 90 deg steering version is posted.

straight version http://nodus.ligo.caltech.edu:8080/40m/2709

 I had a think about the algorithm we might use for the phase camera measurement. MATLAB has an fft function that will allow us to extract the data that we need with a single command.

We record a series of images from a camera and put them into a 3D array or movie, image_arr, where the array parameters are [x-position, y-position, time], i.e. a 2D slice is a single frame from the camera. Then we can do an FFT on that object with the syntax, f3D = fft(image_arr, [ ], 3), which only does the FFT on the temporal components. The resulting object is a 3D array where each 2D slice is an 2D array of amplitude and phase information across the image for a single temporal frequency of the movie.

So if we recorded a movie for 1s where the sample rate is 58Hz, then the 1st frame of f3D is just a DC image of the movie, the 2nd frame are the complex 1Hz components of the movie, etc all the way up to 29Hz. 

Suppose then that we have a image, part of which is being modulated, e.g. a chopper wheel rotating at 20 or 24Hz, or a laser beam profile which contains a 1kHz beat between a sideband and a reference beam. All we have to do is sample at at least twice that modulation frequency, run the command in MATLAB, and then we immediately get an image which contains the phase and magnitude information that we're interested in (in the appropriate 2D slice o the FFT).

As an example, I recorded 58 frames of data from a camera, sampling at 58Hz, which was looking at a spinning chopper wheel. There was a white sheet of paper behind the wheel which was illuminated from behind by a flashlight. The outer ring was chopping at 24Hz and the inner ring was chopping at 20Hz. I stuck all the images into the 3D array in MATLAB, did the transformation and picked out the DC, 20Hz and 24Hz signals. The results are shown in the attached PDFs which are:

1. phase_camera_DC_comp.pdf - a single image from the camera and the DC component (zoomed in) of the FFT
2. phase_camera_F1_comp.pdf - the magnitude and phase information of the 20Hz component of the FFT
3. phase_camera_F2_comp.pdf - the magnitude and phase information of the 24Hz component of the FFT (this PDF contains a typo that says 25Hz).
4. load_raw_data.m - the MATLAB routine that loads the saved data from the camera and does the FFT

You can, and I have, run the MATLAB engine from C directly. This will allow you to transfer the data from the camera to MATLAB directly in memory, rather than via the disk, but it does need proper memory allocation to avoid segmentation faults - that was too frustrating for me in the short term. In this case, the 58 frames were recorded to a file as a contiguous block of data which I then loaded into MATLAB, so it was slower than it might've otherwise been. Also the computer I was running this on was a bit of a clunker so it took a bit of time to do the FFT.

The data rate from the camera was 58fps x (1024 x 1024) pixels per frame x 2 bytes per pixel = 116MB per second. If we were to use this technique in a LIGO phase camera, where we want to measure a modulation which is around 1kHz, then we'd need a sample rate of at least 2kHz, so we're looking at at least a 30x reduction in the resolution. This is okay though - the original phase camera had only ~4000 spatial samples. So we could use, for instance, the Dalsa Falcon VGA300 HG which can give 2000 frames per second when the region of interest is limited to 64 pixels high.

In the middle of the last month, Kiwamu and I went to Garilynn's lab to measure the phase maps of the new ITMs and SRMs.

Analysis of the phase map data were posted on the svn directory:
https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/cocdocs/PhaseMaps/

The screen shots and the plots were summarized in a PDF file. You can find it here:
http://lhocds.ligo-wa.caltech.edu:8000/40m/Upgrade_09/Main_Optics_Phase_Maps

The RoCs for all of the PRMs are turned out to be ~155m. This is out of the spec (142m+/-5m) although the actual effect is not understand well yet..

These RoCs are almost independent from the radus of the assumed gaussian beam.
In deed, I have checked the dependence of the RoC on the beam spot position, and it turned out that the RoCs vary only little.
(In the SRMU01 case, for example, it varies from 153.5m to 154.9m.)
The beam radius of 3mm was assumed. The RoCs were calculated 20x20mm region around the center of the mirror with a 2mm mesh.
 

I took the two 'flat' 2" mirrors over to Downs and Garilynn showed me how to measure them with the old Wyko machine.

The files are now loaded onto our Dropbox folder - analysis in process. From eyeball, it seems as if the RoCs are in the neighborhood of ~5 km, with the local perturbations giving ~10-15 km of curvature depending upon position (few nm of sage over ~1 cm scales)

Koji's matlab code should be able to give somewhat more quantitative answers...

Ed: Here you are. "0966" looks good. It has RoC of ~4km. "0997" has a big structure at the middle. The bump is 10nmPV (KA)

Here is the result for the measurement of the phase noise.  We used the marconi function generator and compared it with an Isomet AOM driver (model 232A-1), so this is really a measure of the relative phase between them.  We used a 5x gain and a frequency response of 13 Hz/V for the modulation.  In all the attached plots, the blue is the data and the red is the measurement limit (as determined by the noise in the SRS785).  Also note that the units on the yaxis of the Phase noise plot are incorrect, they should be dB/Sqrt(Hz), I will fix this and edit as soon as possible.

We've set up a beat note measurement between the VCO driver and the Marconi (see Suresh's elog).

Here's the 'unWhiten' filter for compensating the SR560 TF.

It has poles = 1 mHz, 5 kHz, 5 kHz

and  zeros = 30 mHz, 1 kHz

The gain is set to be ~0.001 in the 1-100 Hz band to compensate the G=1000 of the SR560.

I have gotten the hang of the procedure for measuring phase noise on the AOMs. 

Koji suggested I right up a short guide (wiki page?) on how to do this. 

I will finish up here, then go measure the AOMs at the other lab (may have to be tomorrow, after laser safety), and then write up the instructions.

We want both the X and Y phase trackers to have the same UGF, so that the X and Y ALS signals are subject to the same phase characteristics and can be nicely decoupled into CARM/DARM. 

I've started implementing a simple normalization scheme that Koji suggested, namely, dividing the I output of the phase tracker by a low passed version of the Q output. (Since the I is servoed to zero, the radius of the error signal in the IQ plane is essentially equal to the Q value) I put some simulink logic into the IQLOCK library part that BEAT[XY]_FINE are instances of to switch the normalization on/off, and to protect from divide-by-zeros. I also exposed the switching and FM on the ALS screen.

I then tried using it, to mediocre results. I put a 10mHz LP in the filter module, found a Y-Arm beat, set the phase tracker gain to give me a 2kHz UGF, and then set the gain of the UGH normalization FM to turn the current average Q to unity. 

I then moved the laser temperature around to get different beatnote locations/amplitudes, hoping that the phase tracker UGF would stay the same when the UGH normalization was on.

It did not.

It did, however, correct it in the right direction... more work will be done with this, to try and make it useful. There's also the unfortunate effect that locking/unlocking the green causes erratic phase tracker output, which messes with the input to the normalizing LP filter, so if one were to leave it switched on, wonky stuff would come out. I don't want to go overboard with triggering shenanigans before I even get it working in the first place, though.  

The measured open loop TF of the ALS Phase Tracker loop for each arm was characterized by an empirical model on LISO.

The model for the open loop TF has pole 1m instead of the one at DC as LISO has a difficulty to model it.
Digital time delay and the sampling effect seem to be well represented by a zero at ~8kHz and delay of  ~60us.
(cf 16kHz sampling => 61us)

The XARM phase tracker has the UGF of 1.5kHz. This is too low because
1) The phase rotation at 100Hz is visible in the plot.
2) We don't much care about the closed loop bump in the phase tracker as long as the phase tracker keeps its continuity.

So I suggest to increase the gain so that we have the UGF of 3kHz. (phase margin: 24deg)

The red curve in the plot is the closed loop response calculated by CLTF =  - OLTF / (1-OLTF).

The model results are used in the ALS servo models.

Summary:

After replacement of the fiber delivering the LO beam to the airBHD setup (some photos here), I repeated the measurement outlined here. There may be some improvement, but overall, conclusions don't change much.

Details:

The main addition I made was to implement a digital phase tracker servo (a la ALS), to make sure my arctan2 usage wasn't completely bonkers (the CDS block can be deleted later, or maybe it's useful to keep it, we will see). I didn't measure it today, but the UGF of said servo should be >100 Hz so the attached spectrum should be valid below that (loop has not been done, so above the UGF, the control signal isn't a valid representative of the free running noise). Attachment #1 shows the result. The 1 Hz and 3 Hz suspension resonances are well resolved. Anyways, what this means is that the earlier result was not crazy. I don't know what to make of the high frequency lines, but my guess is that they are electronic pickup from the Sorensens - I'm using clip-mini-grabbers to digitize these signals, and other electronics in that rack (e.g. ALS signals) also show these lines.

It is pretty easy to keep the simple Michelson locked for several minutes. Attachment #2 shows the phase-tracker servo output over several minutes. The y-axis units are degrees. If this is to be believed, the relative phase between the two fields is drifting by 12um ove an hour. This is significantly lower than my previous measurement, while the noise in the ~0.5-10 Hz band is similar, so maybe the shorter fiber patch cable did some good?

I think there is also correlation between the PSL table temperature, but of course, the evidence is weak, and there are certainly other effects at play. At first, I thought the abrupt jumps are artefacts, but they don't actually represent jumps >360 degrees over successive samples, so maybe they are indicative of some real jump in the relative phase? Either fiber slippage or TT suspension jumps? I'll double check with the offline data to make sure it's not some artefact of the phase tracker servo. If you disagree with these conclusions and think there is some meaurement/analysis/interpretation error, I'd love to hear about it.

Next steps:

1. Budget the offline inferred phase noise spectrum, overlay a seismic noise model, to see if we can disentangle the contributions from the suspensions and that from the LO fiber.
2. I'll see if I can setup an LO pickup with some RF sidebands on it in parallel to this setup so we can try some of the ideas discussed on the call this week. There are several beams available, but the question is whether I can get this into a fiber without 1 week of optical layout work.

I have left the heterodyne electronics setup at the LSC rack, but it is not powered (because there are some exposed wires). Please leave it as is.

I've noticed that there is some phase loss in the POX/POY locking loops - see Attachment #1, live traces are from a recent measurement while the references are from Nov 4 2018. Hard to imagine a true delay being responsible to cause so much phase loss at 100 Hz. Attachment #2 shows my best effort loop modeling, I think I've got all the pieces, but maybe I missed something (I assume the analog whitening / digital anti-whitening are perfectly balanced, anyway this wasn't messed with anytime recently)? The fitter wants to add 560 us (!) of delay, which is almost 10 clock cycles on the RTS, and even so, the fit is poor (I constrain the fitter to a maximum of 600 us delay so maybe this isn't the best diagnostic). Anyway, how can this change be explained? The recent works I can think of that could have affected the LSC sensing were (i) RF source box re-working, and (ii) vent. But I can't imagine how either of these would introduce phase loss in the LSC sensing. Note that the digital demod phase has been tuned to put all the PDH signal in the "I" quadrature, which is the condition in which the measurement was taken.

Probably this isn't gonna affect locking efforts (unless it's symptomatic of some other larger problem).

Summary:

Calibration of the phase map interferometer was calculated for the data on Oct 8th, 2010.
The calibration value is 0.1905 mm/pixel.

This is slightly smaller than the assumed value in the machine that is 0.192mm/pixel.
This means that the measured radii of curvature must be scaled down with this ratio.
(i.e. RoC(new) = RoC(old) / 0.192² * 0.1905²)

Motivation:

Our tagets of the phasemap measurement are:

1. Measure the figure errors of the mirrors
2. Measure the curvature of the mirrors

The depth of the mirror figure is calibrated by the wavelength of the laser (1064nm).
However, the lateral scale of the image is not calibrated.
Although Zygo provides the initial calibration as 0.192 mm/pixel, we should measure the calibration by ourselves.

Method:

We found an aperture mask with a grid of holes with 2mm diameter and 3mm spacing (center-to-center).
Take the picture of this aperture and calibrate the pixel size. The aperture is made of stainless and makes not interference
with the reference beam. Thus we put a dummy mirror behind the aperture. (UPPER LEFT plot)

As the holes are aligned as a grid, the FFT of the aperture image shows peaks at the corresponding pitches. (UPPER MIDDLE plot)
As the aperture was slightly rotated, the grids of the peaks are also slanted.

We can obtain the spacing of the peak grids. How can we can that values precisely? I decided to make an artificial mask image.
The artificial mask (LOWER LEFT plot) has the similar FFT pattern (LOWER MIDDLE plot). The inner product of the two
FFT results (i.e. Sum[abs(fft1) x abs(fft2)]), quite a large value is obtained if the grid pitch and the aperture angle agrees between those images.
Note that the phase information was discarded. The estimated grid spacing of the artificial mask can be mathematically obtained.

Result:

The grid pitch and the angle were manually set as initial values. Then the parameters to give the local maximum was obtained by fminsearch of Matlab.
UPPER RIGHT and LOWER RIGHT plots show the scan of the evaluation function by changing the angle and the pitch. They behave quite normal.

The obtained values are

Grid pitch: 15.74 pixel
Angle: 1.935 deg

As the grid pitch is 3mm, the calibration is

3 mm / 15.74 pixel = 0.1905 mm/pixel

Discussion:

A spherical surface can be expressed as the following formula:

z = R - R Sqrt(1-r²/R²)      (note: this can be expanded as r²/(2 R)+O(r³) )

Here R is the RoC and r is the distance from the center. This means that the calibration of r quadratically changes the curvature.
We have measured the RoC of the spare SRM. We can compare the RoCs measured by this new metrology IFO and the old one,
as well as the one by Coastline optics. 

 Raji took the optics over. They were all measured at 0 deg incidence angle, although we will use them at the angles required for the recycling folding mirrors.  Here's the summary from GariLynn:

In general all six pieces have a radius of curvature of around -700 meters.

Quote:

Raji took the optics over. They were all measured at 0 deg incidence angle, although we will use them at the angles required for the recycling folding mirrors.  Here's the summary from GariLynn: In general all six pieces have a radius of curvature of around -700 meters. They all fall off rapidly past 40 mm diameter.  Within the 40 mm diameter the rms is ~10 nm for most.  I can get finer analysis if you have something specific that you want to know.    All data are saved in Wyko format at the following location: http://www.ligo.caltech.edu/~coreopt/40MCOC/Oct24-2012/ Gari

 After a long search, I've found a way to finally read and analyze(?)  the Wyko opd format data using Image SXM, an image analysis software working only on mac osx.

I am attaching the images (in tiff) and profile plot of all the 6 mirrors.

 Great, however, unless you can save the images in FITS format, we still need another reader for the opd images.

Previous phasemap data and analysis for the new 40m COC are summarized on the following page

https://nodus.ligo.caltech.edu:30889/40m_phasemap/

(Use traditional LVC authentication (not albert.einstein))

The actual instance of the files can also be found on nodus below the following directory:

/cvs/cds/caltech/users/public_html/40m_phasemap

The programs for the analysis are found in

/cvs/cds/caltech/users/public_html/40m_phasemap/40m_PRM/mat

The main program is RunThis.m

Basically this program takes ascii files converted from opd by Vision32.
(i.e. You need to go to Downs)
Then the matlab program takes care of the plots and curvature analyses.

[yutaro, gautam]

Summary:

Having obtained a working FS725 Rubidium standard and syncing it to out GPS timing unit, I wanted to have one more pass at calibrating the phase tracker output, with the RF signal generator calibrated relative to an 'absolute' source. I also extended the range of frequencies swept over to 15MHz to 110MHz. We found that the phase tracker output appears linear over the entire range scanned, but taking a closer look at the residuals suggested some quadratic structure. Restricting the fitted range to [31MHz 89MHz] yields the following calibration constants for the X and Y arm respectively: 0.9904 +/- 0.0008 and 0.9984 +/- 0.0005. This suggests that out previous calibration was pretty accurate, and that it is valid over a wider range of frequencies, so we could plausibly fit in more FSRs in future scans if necessary. I have not updated these values on the EPICS screens (though judging by how close they are to 1, I wonder if this is even necessary)...

Details:

The principle change in the setup compared to that used to collect the data presented in elog 11738 was the addition of the FS725 rubidium standard. As detailed here, I synced the Rubidium standard to our GPS timing unit (this took a while - the manual suggests it should only take minutes, but it took about 10 hours - the two photos in Attachment #1 show the status of the front panel before and after it synced to the external 1PPS input). I then took 10 MHz outputs from the FS725, and ran one to the Fluke 6061A, and the other to the AG4395A. The Fluke 6061 A has a small switch at the back which has to be set to "EXT" in order for it to use the external reference (it has now been returned to the "INT" state). We then connected the output of the signal generator via a 3-way minicircuits splitter to the AG4395A, and the two beat channels. 

I cleared the phase history on the MEDM screen, and set the phase tracker UGF. We then swept through frequencies from 15MHz to 110MHz (using the AG4395 to verify the frequency at each step). I used the following command to record the average value (over 10 seconds) and the standard deviation: z avg 10 -s C1:ALS-BEATX_FINE_PHASE_OUT_HZ >> 20151113_PT_X.dat and so on.. The amplitude of the signal generated (i.e. before the splitter) was -18dBm (chosen such that the Q outputs of either phase tracker was between 1000 and 3000), while the gains were ~100 (X) and 50 (Y). I then downloaded the data and fitted it.

Fitting details:

The output of the phase tracker looks roughly linear over the entire range of frequencies scanned - but looking at the residuals, one could say there was some quadratic structure to it (see residual plots in Attachment #2). By looking at the shapes of the residuals, I judged that if we fit in the range [31MHz   89MHz] (for both X and Y), we should see negligible structure in the residuals. Attachment #3 contains the fits and residuals for these fits. One could argue that there is still some structure in the residuals, but is markedly less than over the entire range, and, I think, small enough to be neglected. The calibration constants quoted at the beginning of the elog are from the fits over this range. In principle, we could always break this down into smaller pieces and do a linear fit over that range. But this should allow us to scan through >5 FSRs.

Other remarks:

Since the beat signal also goes to the frequency counter via the couplers, I was also collecting the readouts of the frequency counter. Attachment #5 contains the data collected. It is interesting to note that the FCs fail at ~101 MHz (corresponding to ~6146 Hz after the dividers).

Also, we had taken another dataset last night, but found that there was an anomalous kink in the X phase tracker output at (coincidentally?) 89 MHz (I've attached the data in Attachment #6). I'm not sure why this happened, but this is what led me to take another dataset earlier today (Attachment #4).

Summary of Attachments:

1. Attachment #1: Photos showing the front panel of the FS725 before and after syncing to the external 1PPS input.
2. Attachment #2: Fits and residuals over the entire range scanned.
3. Attachment #3: Fits and residuals over restricted range [31 89] MHz
4. Attachment #4: Data used for phase tracker calibration.
5. Attachment #5: Frequency counter data.

 [Alex, Koji]

We characterized Koji's BBPD MOD for REFL165 (see attachment).

First, we calibrated the Agilent 4395 Network Analyzer (NA) to account for differences in cable features between the Ref PD and Test PD connections. This was done using the 'Cal' softkey on the NA. 

Then we performed transimpedance measurements for the test PD and reference PD relative to the RF output of the NA and relative to each other (see 2nd attachment. Note that the NA's RF output is split and sent to both the IR Laser and the NA's Ref input).

Next, we made DC measurements of the outputs of the photodetectors to estimate the photocurrent distribution of the transimpedance setup (like the 2nd attachment, but with the outputs of the PDs going to a multimeter). By photocurrent distribution, we mean how the beamsplitter and respective quantum efficiencies/generalized impedance/etc. of the PDs influence how much current flows through each PD at with a DC input.

Finally, we measured the output noise as a function of photocurrent (like the 2nd attachment, but with a lightbulb instead of the IR Laser). Input voltages for the lightbulb ranged from 0mV to 6V. Data was downloaded from the NA using netgpibdata from the scripts directory. Analysis is currently in progress; graphs to come soon.

 [Eric, Alex]

We successfully used our system to modulate the input to a single photodetector. The RF Out of the network analyzer went to the Mod In of our laser, which was operating at 98 mA. The laser's output was sent to our 1x16 optical splitter. This provided input signals for both our reference detector and AS55. Our reference detector's output was sent to the network analyzer's R input, while the AS55's output was sent to the network analyzer's A input. 

We still need to work out the specifics of how the modulation works. Specifically, we want to look at the amplitude of the network analyzer's output. Additionally, we may have been saturating our reference detector, causing noise problems.

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.