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  13117   Fri Jul 14 17:47:03 2017 gautamUpdateGeneralDisks from LLO have arrived

[jamie, gautam]

Today morning, the disks from LLO arrived. Jamie and I have been trying to get things back up and running, but have not had much success today. Here is a summary of what we tried.

Keith Thorne sent us two disks: one has the daqd code and the second is the boot disk for the FE machines. Since Jamie managed to successfully compile the daqd code on FB1 yesterday, we decided to try the following: mount the boot disk KT sent us (using a SATA/USB adapter) on /mnt on FB1, get the FEs booted up, and restart the RT models. 


I just want to mention that the situation is actually much more dire than we originally thought.  The diskless NFS root filesystem for all the front-ends was on that fb disk.  If we can't recover it we'll have to rebuilt the front end OS as well.

As of right now none of the front ends are accessible, since obviously their root filesystem has disappeared.

While on FB1, Jamie realized he actually had a copy of the /diskless/root directory, which is the NFS filesystem for the FEs, on FB1. So we decided to try and boot some of the FEs with this (instead of starting from scratch with the disks KT sent us). The way things were set up, the FEs were querying the FB machine as the DHCP server. But today, we followed the instructions here to get the FEs to get their IP address from chiara instead. We also added the line 

/diskless/root *(sync,rw,no_root_squash,no_all_squash,no_subtree_check)

to /etc/exports followed by exportfs -ra on FB1. At which point the FE machine we were testing (c1lsc) was able to boot up. 

However, it looks like the NFS filesystem isn't being mounted correctly, for reasons unknown. We commented out some of the rtcds related lines in /etc/rc.local because they were causing a whole bunch of errors at boot (the lines that were touched have been tagged with today's date).

So in summary, the status as of now is:

  1. Front-end machines are able to boot
  2. There seems to be some problem during the boot process, leading to the NFS file system not being correctly mounted. The closest related thing I could find from an elog search is this entry, but I think we are facing a different probelm.
  3. We wanted to see if we could start the realtime models (but without daqd for now), but we weren't even able to get that far today.

We will resume recovery efforts on Monday.

  13124   Wed Jul 19 00:59:47 2017 gautamUpdateGeneralFINESSE model of DRMI (no arms)


I've been working on improving the 40m FINESSE model I set up sometime last year (where the goal was to model various RC folding mirror scenarios). Specifically, I wanted to get the locking feature of FINESSE working, and also simulate the DRMI (no arms) configuration, which is what I have been working on locking the real IFO to. This elog is a summary of what I have from the last few days of working on this.

Model details:

  • No IMC included for now.
  • Core optics R and T from the 40m wiki page.
  • Cavity lengths are the "ideal" ones - see the attached ipynb for the values used.
  • RF modulation depths from here. But for now, the relative phase between f1 and f2 at the EOM is set to 0.
  • I've not included flipped folding mirrors - instead, I put a loss of 0.5% on PR3 and SR3 in the model to account for the AR surface of these optics being inside the RCs. 
  • I've made the AR surfaces of all optics FINESSE "beamsplitters" - there was some discussion on the FINESSE mattermost channel about how not doing this can lead to slightly inaccurate results, so I've tried to be more careful in this respect.
  • I'm using "maxtem 1" in my FINESSE file, which means TEM_mn modes up to (m+n=1) are taken into account - setting this to 0 makes it a plane wave model. This parameter can significantly increase the computational time. 

Model validation:

  • As a first check, I made the PRM and SRM transparent, and used the in-built routines in FINESSE to mode-match the input beam to the arm cavities.
  • I then scanned one arm cavity about a resonance, and compared the transmisison profile to the analytical FP cavity expression - agreement was good.
  • Next, I wanted to get a sensing matrix for the DRMI (no arms) configuration (see attached ipynb notebook).
    • First, I make the ETMs in the model transparent
    • I started with the phases for the BS, PRM and SRM set to their "naive" values of 0, 0 and 90 (for the standard DRMI configuration)
    • I then scanned these optics around, used various PDs to look at the points where appropriate circulating fields reached their maximum values, and updated the phase of the optic with these values.
    • Next, I set the demod phase of various RFPDs such that the PDH error signal is entirely in one quadrature. I use the RFPDs in pairs, with demod phases separated by 90 degrees. I arbitrarily set the demod phase of the Q phase PD as 90 + phase of I phase PD. I also tried to mimic the RFPD-IFO DoF pairing that we use for the actual IFO - so for example, PRCL is controlled by REFL11_I.
    • Confident that I was close enough to the ideal operating point, I then fed the error signals from these RFPDs to the "lock" routine in FINESSE. The manual recommends setting the locking loop gain to 1/optical gain, which is what I did.
    • The tunings for the BS and RMs in the attached kat file are the result of this tuning.
    • For the actual sensing matrix, I moved each of PRM, BS and SRM +/-5 degrees (~15nm) around each resonance. I then computed the numerical derivative around the zero crossing of each RFPD signal, and then plotted all of this in some RADAR plots - see Attachment #1.

Explanation of Attachments and Discussion:

  • Attachment #1 - Computed sensing matrix from this model. Compare to an actual measurement, for example here - the relative angle between the sensing matrix elements dont exactly line up with what is measured. EQ suggested today that I should look into tuning the relative phase between the RF frequencies at the EOM. Nevertheless, I tried comparing the magnitudes of the MICH sensing element in AS55 Q - the model tells me that it should be ~7.8*10^5 W/m. In this elog, I measured it to be 2.37*10^5 W/m. On the AS table, there is a 50-50 BS splitting the light between the AS55 and AS110 photodiodes which is not accounted for in the model. Factoring this in, along with the fact that there are 6 in-vaccuum steering mirrors (assume 98% reflectivity for these), 3 in air steering mirrors, and the window, the sensing matrix element from the model starts to be in the same ballpark as the measurement, at ~3*10^5 W/m. So the model isn't giving completely crazy results.
  • Attachment #2 - Example of the signals at various RFPDs in response to sweeping the PRM around its resonance. To be compared with actual IFO data. Teal lines are the "I" phase, and orange lines are "Q" phase.
  • Attachment #3 - FINESSE kat file and the IPython notebook I used to make these plots. 
  • Next steps
    • More validation against measurements from the actual IFO.
    • Try and resolve differences between modeled and measured sensing matrices.
    • Get locking working with full IFO - there was a discussion on the mattermost thread about sequential/parallel locking some time ago, I need to dig that up to see what is the right way to get this going. Probably the DRMI operating point will also change, because of the complex reflectivities of the arm cavities seen by the RF sidebands (this effect is not present in the current configuration where I've made the ETMs transparent).

GV Edit: EQ pointed out that my method of taking the slope of the error signal to compute the sensing element isn't the most robust - it relies on choosing points to compute the slope that are close enough to the zero crossing and also well within the linear region of the error signal. Instead, FINESSE allows this computation to be done as we do in the real IFO - apply an excitation at a given frequency to an optic and look at the twice-demodulated output of the relevant RFPD (e.g. for PRCL sensing element in the 1f DRMI configuration, drive PRM and demodulate REFL11 at 11MHz and the drive frequenct). Attachment #4 is the sensing matrix recomputed in this way - in this case, it produces almost identical results as the slope method, but I think the double-demod technique is better in that you don't have to worry about selecting points for computing the slope etc. 


Attachment 1: DRMI_sensingMat.pdf
Attachment 2: DRMI_errSigs.pdf
Attachment 3: 40m_DRMI_FINESSE.zip
Attachment 4: DRMI_sensingMat_19Jul.pdf
  13148   Fri Jul 28 16:47:16 2017 gautamUpdateGeneralPSL StripTool flatlined

About 3.5 hours ago, all the PSL wall StripTool traces "flatlined", as happens when we had the EPICS freezes in the past - except that all these traces were flat for more than 3 hours. I checked that the c1psl slow machine responded to ping, and I could also telnet into it. I tried opening the StripTool on pianosa and all the traces were responsive. So I simply re-started the PSL StripTool on zita. All traces look responsive now.

  13150   Sat Jul 29 14:05:19 2017 gautamUpdateGeneralPSL StripTool flatlined

The PMC was unlocked when I came in ~10mins ago. The wall StripTool traces suggest it has been this way for > 8hours. I was unable to get the PMC to re-lock by using the PMC MEDM screen. The c1psl slow machine responded to ping, and I could also telnet into it. But despite burt-restoring c1psl, I could not get the PMC to lock. So I re-started c1psl by keying the crate, and then burt-restored the EPICS values again. This seems to have done the trick. Both the PMC and IMC are now locked.

Unrelated to this work: It looks like some/all of the FE models were re-started. The x3 gain on the coil outputs of the 2 ITMs and BS, which I had manually engaged when I re-aligned the IFO on Monday, were off, and in general, the IMC and IFO alignment seem much worse now than it was yesterday. I will do the re-alignment later as I'm not planning to use the IFO today.

  13151   Sat Jul 29 16:24:55 2017 jamieUpdateGeneralPSL StripTool flatlined
Unrelated to this work: It looks like some/all of the FE models were re-started. The x3 gain on the coil outputs of the 2 ITMs and BS, which I had manually engaged when I re-aligned the IFO on Monday, were off, and in general, the IMC and IFO alignment seem much worse now than it was yesterday. I will do the re-alignment later as I'm not planning to use the IFO today.

This was me.  I restarted the front ends when I was getting the MX streams working yesterday.  I'll try to me more conscientious about logging front end restarts.

  13167   Fri Aug 4 18:25:15 2017 gautamUpdateGeneralBilinear noise coupling

[Nikhil, gautam]

We repeated the test that EricQ detailed here today. We have downloaded ~10min of data (between GPS times 11885925523 - 11885926117), and Nikhil will analyze it.

Attachment 1: bilinearTest.pdf
  13169   Mon Aug 7 16:00:41 2017 ranaUpdateGeneralBilinear noise coupling

These are not the angular test parameters that we're looking for:

 recall that what we want is the low frequency beam spot variations and the feedback to be limited to a small high frequency band.

e.g. only inject noise at 40-50 Hz, not loud enough to find at 2x the injected frequency.

It should NOT be the case that the high frequency injected noise be dominating the RMS.

The coupling should be ~1e-3; some combination of beam spot mis-centering and beam spot motion.

  13170   Mon Aug 7 22:50:57 2017 KojiUpdateGeneralNew wifi router for the GC network installed

I have replaced the old 11n wifi router (CISCO / Linksys) for the GC network with a new one with 11ac technology.

The new one is a 3band wifi router. Thus it has one 2.4GHz (11n) SSID and two 5GHz (11ac) SSIDs. All these have been set to be hidden. Just come to the 40m and find the necessary info for the connection.

Note that the user id / password for the admin tool have been changed from the default values.

  13181   Thu Aug 10 09:10:55 2017 steveUpdateGeneraldataviewer is recovering

It can look back 7 days trends now. There is still no vacuum channels. I can bring back the channels through the restore directory, but there are no data.

Attachment 1: 7dm.png
  13208   Tue Aug 15 08:22:16 2017 SteveUpdateGeneralprojector light bulb replaced

BL-FS300C-PH-LE was replaced after 2,904 hrs  It did not explode this time.  The 4 monts life period is actually pritty good because this is a $73.  cheap bulb. The best-high priced warranty is 5 months.


PS: future option_bulbless laser projector


This would give you everything you are requesting, plus a lamp-less design and 5yr warranty.  Ground shipping would be free anywhere in the lower 48, and we would not charge sales tax on orders billing/shipping outside of AZ.  If you have any questions or if you would like to order... just let me know!
  13221   Wed Aug 16 20:01:03 2017 gautamUpdateGeneralSUS model ASC input weirdness

I'm not sure if this has something to do with the model restarts / new RCG, but while I was re-enabling the MC watchdogs, I noticed the RMS sensor voltage channels on ITMX hovering around ~100mV, even though local damping was on (in which configuration I would expect <1mV if everything is working normally).  I was confused by this behaviour, and after staring at the ITMX suspension screen for a while, I noticed that the input to the "ASCP" and "ASCY" servos were "-nan", and the outputs were 10^20 cts frown (see Attachment #1).

Digging a little deeper, I found that the same problem existed on ITMY, ETMX, ETMY, PRM (but not BS or SRM) - reasons unknown for now.

I have to check where this signal is coming from, but for now I just turned the "ASC Input" switch off. More investigation to be done, but in the meantime, ASS dither alignment may not be possible.

After consulting with Jamie, I have just disabled all outputs to the suspensions other than local damping loop outputs. I need to figure out how to get this configuration into the safe.snap file such that until we are sure of what is going on, the models start up in this safer configuration.

gedit 28 Oct 0026: Seems like this problem is seen at the sites as well. I wonder if the problem is related.

Attachment 1: ITMX_ASC.png
  13228   Fri Aug 18 21:58:35 2017 gautamUpdateGeneralSUS model ASC input weirdness

I spent some time today trying to debug this issue.

Jamie and I had opened up the c1sus frontend to try and replace the RFM card before we realized that the problem was in the RCG code generator. During this process, we had disconnected all of the back-panel cabling to this machine (2 ethernet cables, dolphin cable, and RFM cables/fibers). I thought I may have accidentally returned the cables to the wrong positions - but all the status indicator lights indicate that everything is working as it should, and I also confirmed that the cabling is as it is in the pictures of the rack on the wiki page.

Looking at the SimuLink model diagram (see Attachment #1 for example), it looks like (at least some of) these channels are actually on the dolphin network, and not the RFM network (with which we were experiencing problems). This suggests that the problem is something deeper. Although I did see nans in some of the ETMX ASC channels as well, for which the channels are piped over the RFM network. Even more puzzling is that the ASC MEDM screen (Attachment #3) and the SimuLink diagram (Attachment #2) suggest that there is an output matrix in between the input signals and the output angular control signals to the suspensions. As Attachment #4 shows, the rows corresponding to ITMX PIT and YAW are zero (I confirmed using z read <matrixElement>). Attachment #3 shows that the output of all the servo banks except CARM_YAW is zero, but CARM_YAW has no matrix element going to the ITMs (also confirmed with z read <servoOutputChannel>). So 0 x 0 should be 0, but for some reason the model doesn't give this output?

GV Edit: As EricQ just pointed out to me, nan x 0 is still nan, which probably explains the whole issue. Poking a little further, it seems like this is an SDF issue - the SDF table isn't able to catch differences for this hold output channel.

As I was writing this elog, I noticed that, as mentioned above, the CARM_YAW output was "nan". When I restart the model (thankfully this didn't crash c1lsc!), it seems to default to this state. Opening up the filter module, I saw that the "hold output" was enabled.

Toggling that switch made the nans in all the SUS ASC channels disappear. Mysterious indecision.

All the points above stand - CARM_YAW output shouldn't have been going anywhere as per the output matrix, but it seems to have been responsible? Seems like a bug in any case if a model restarts with a field as "nan".

Anyways the problem seems to have been resolved so I'm going to try locking and dither aligning the arms now.

Rolf mentioned that a simple update could fix several of the CDS issues we are facing (e.g. inability to open up testpoints), but he didn't seem to have any insight into this particular issue. Jamie will try and recompile all the models and then we have to see if that fixes the remaining problems.


I have to check where this signal is coming from, but for now I just turned the "ASC Input" switch off. More investigation to be done, but in the meantime, ASS dither alignment may not be possible.

After consulting with Jamie, I have just disabled all outputs to the suspensions other than local damping loop outputs. I need to figure out how to get this configuration into the safe.snap file such that until we are sure of what is going on, the models start up in this safer configuration.


Attachment 1: ITMXP.png
Attachment 2: ASC_model_outmatrix.png
Attachment 3: ASC_medm.png
Attachment 4: ASC_outMat.png
  13235   Mon Aug 21 20:11:25 2017 johannesSummaryGeneralLoss measurements plan

There are three methods we (will soon) have available to evaluate the round-trip dissipative losses in the arms that do not suffer from the ITM loss dominance:

  • DC reflection method:
    • Compare reflected light levels from [ITM only] vs [arm cavity on resonance]
  • Basler CCDs:
    • Infer large (or small) angle scatter loss with calibrated CCDs
  • Reflection ringdowns:
    • Need AS port light injection, principle is similar to DC method but better (?)


The DC method comparing reflectivities has been used in the past and is relatively easy to do. After the recent vacuum troubles the first step should be to re-perform these as CDS permits (needs some ASS functionality and of course the MC to behave). It wouldn't hurt to know the parameters this depends on, aka mode overlap and modulation depths with better certainty. Maybe the SURF scripts for mode-spectroscopy can be applied?


With the new CCD cameras calibrated, pre-vent we can determine the magnitude of the large-angle scatter loss (assuming isotropic scatter) of ETMX and possibly ETMY. Can we look past ETMX/ETMY from the viewports? Then we can probably also look at the small angle scatter of ITMX and ITMY. If not, once we open one of the chambers there's the option of installing mirrors as close as possible to the main beam path. The easiest is probably to look at ITMX, since there is plenty of space in the XEND chamber, and the camera is already installed.


This requires a lot of up-front work. We decided to use the spare 200mW NPRO. It will be placed on the PSL table and injected into an optical fiber, which terminates on the AS table. The again free space beam there needs to be sort-of mode-matched into the SRC ("sort-of" because mode-spectroscopy). We want to be able to phaselock this secondary beam to the PSL with at least a couple kHz bandwidth and also completely extinguish the beam on time-scales of a few microseconds. We will likely need to purchase a few components that we can salvage from other labs, I'm still going through the inventory and will know more soon (more detailed post to follow). We need to settle for the polarization we want to send in from the back.


Tentative Schedule (aggressive)

Week Aug 21 - Aug 27:

  • Update mode-overlap estimates
  • Obtain current DC refl estimates
  • Spatial profile of auxiliary NPRO
  • Fiber setup concept; purchasing
  • CCD software prep work

Week Aug 28 - Sep 3:

  • Re-evaluate modulation indices if necessary
  • Optical beat AS Port Auxiliary Laser (ASAL) - PSL
  • PLL setup
  • CCD large angle prep work

Week Sep 4 - Sep 10:

  • PLL CDS integration
  • Amplitude-modulation preparation
  • CCD large angles

Week Sep 11 - Sep 17:

  • Fiber-injection
  • AS table preliminary mode-matching
  • Faraday setup
  • CCD small angle prep work

Week Sep 18 - Sep 24:

  • ASAL amplitude switching
  • CCD small angles

Week Sep 25 - Oct 1:

  • AS port ringdowns


  13236   Mon Aug 21 21:26:41 2017 gautamSummaryGeneralLoss measurements plan

In case you want to use it, I had profiled the Lightwave NPRO sometime back, and we were even using it as the AUX X laser for a short period of time. 

As for using the AS laser for mode spectroscopy: don't we want to match the beam into the cavity as best as possible, and then use some technique to disturb the input mode (like the dental tooth scraper technique from Chris Mueller's thesis)? 

Johannes and I did an arm scan of the X arm today (arm controlled with ALS, monitoring IR transmission) - only 2 IR FSRs were scanned, but there should be sufficient information in there to extract the modulation depth and mode matching - can we use Kaustubh's/Naomi's code?. The Y arm ALS needs to be touched up so I don't have a Y arm scan yet. Note that to get a good arm scan measurement, the High Gain Thorlabs PD should be used as the transmission PD.


Week Aug 21 - Aug 27:

  • Update mode-overlap estimates
  • Obtain current DC refl estimates
  • Spatial profile of auxiliary NPRO
  • Fiber setup concept; purchasing
  • CCD software prep work


  13241   Tue Aug 22 16:56:54 2017 johannesSummaryGeneralAS laser existing components inventory

I surveyed the lab today to see what we may need to buy for the AS laser setup.

We have:

NPRO 200 mW + Driver

Faraday Isolator from cabinet

ISOMET Model 1201E: This is a free space AOM I found in the modulator cabinet. It needs to be driven at 40MHz (to be confirmed) with ~6W of electrical power. For a 500 micron beam it can allegedly achieve rise times of '93' [units not specified, could this be nanoseconds?]. I did not find a dedicated driver for it, however there was a 5W minicircuits amplifier ZHL-5W-1 in the RF cabinet and a switch ZSDR-230, which has a typical switch time of 2 microseconds, however I'm not sure how this translates to rise/fall times of the deflected power. It seems we have everything to set this up, so we'll by the end of the week if we can use a combination of these things or if we need to buy additional driver electronics.

New Focus model 4004 broadband phase modulator which is labeled as dusty, and in fact quite dirty when looking through. We should attempt to clean this thing and maybe we can use it here or at the ends.

Probably all the optics we need for the PSL table setup.


We need:

Beat PD: How about one of these: EOT ET-3000A? I didn't find a broadband PD for the beat with the PSL

Fiber Stuff: coupler & polarization maintaining fiber 20m & collimator. There are a couple options here, which we can discuss in the meeting.

Faraday Isolator: If we want to inject P-polarization. If S is okay we can use a polarizing plate beamsplitter instead.

Possibly some large lenses for mode-matching to IFO (TBD)



  13253   Fri Aug 25 11:11:26 2017 gautamUpdateGeneralMC1 kicked again

Looks like MC1 got another big kick just under 4 hours ago. None of the other optics show any evidence of a glitch so it seems unlikely that this was some sort of global event. It's been well behaved for ~2weeks now. IMC was unlocked. I manually re-aligned MC1, at which point the autolocker was able to lock the IMC.

Looking at this plot, it seems that LR and UL coils seem to have the largest kicks. UR barely saw it. Not sure what (if anything) to make of this - apparently the optic moved by ~20urad with the UR magnet approximately the pivot.

Attachment 1: MC1_glitch.png
  13275   Wed Aug 30 15:00:06 2017 gautamUpdateGeneralEdgeswitch fiber swap

A couple of minutes ago, Larry W swapped the fibers to our 40m Edgeswitch (BROCADE FWS 648G) to a faster connection. This is the switch to which our gateway machine, NODUS, is connected. The actual swap itself happened at the core router in Bridge, and took only a few seconds. After the switch, I double checked that I was able to ssh into nodus from my laptop, and Larry informed me that everything is working as expected on his end.

Larry also tells us that the other edgeswitch at the 40m (Foundry Networks), to which most of our GC network machines are connected, is a 100MBPS switch, and so we should re-route the connections from this switch to the BROCADE switch at our convenience to take advantage of the faster connection.

  13283   Thu Aug 31 21:40:24 2017 gautamUpdateGeneralMC1 kicked again

There was a pretty large glitch in MC1 about an hour ago. The misalignment was so large that the autolocker wasn't able to lock the IMC. I manually re-aligned MC1 using the bias sliders, and now IMC locks fine. Attached is a 90 second plot of 2K data from the OSEMs showing the glitch. Judging from the wall StripTool, the IMC was well behaved for ~4 hours before this glitch - there is no evidence of any sort of misalignment building up, judging from the WFS control signals.

Attachment 1: MC1_glitch.png
  13305   Mon Sep 11 09:47:53 2017 SteveUpdateGeneralWIMA caps refilled

Instock WIMA caps refilled to a minimum 50 pieces each.

Attachment 1: WIMA.png
  13308   Mon Sep 11 15:58:02 2017 SteveUpdateGeneralNPRO for repair

This NPRO has a tripping power output******


" Hi Eric,

I checked with the Engineer as Vincent is travelling.

“The lasers have serial number below 2000 which we cannot repair them, we only can repair NPRO laser has serial number 2000 or later.”


Betty-Ann Watt

Customer Service Professional
Global Customer Service/Communication & Commercial Optical Products "




Attachment 1: NPRO_tripping.jpg
  13393   Wed Oct 18 19:17:42 2017 gautamUpdateGeneralPRC angular feedforward

Last night, I collected ~30mins of data for the vertex seismometer channels and the POP QPD PIT/YAW signals with the PRMI locked on carrier (angular FF OFF). The ITM Oplev loops weren't DC coupled, as they are in the full IFO locking sequence, but I feel like the angular FF filters can be improved - there are frequent sharp dives in the AS110 signal level which are correlated with large amplitude motion of the POP spot on the control room CCD monitor.

Repeating the frequency domain multicoherence analysis using BS_X and BS_Y seismometer channels as witnesses suggest that we can win significantly (see Attachment #1).

I've never really implemented feedforward filters - I was planning on using ericq's latest entry on this subject as a guide. From what I gather, the procedure is as follows:

  1. Pre-filter the target (POP QPD PIT or YAW) and witness (BS_X, BS_Y) channels
    • Downsample the 2k target data and 256Hz witness data to 32 Hz (how to choose this?)
    • Detrend (linear?)
    • Apply elliptic low pass filter (previously, a 3rd order Elliptic Low pass with 3dB ripple, 40dB stopband attenuation, corner at 5Hz was used).
  2. Filter the target signal (i.e. POP QPD PIT/YAW) by the inverse actuator TF.
    • This "actuator TF" is a measurement of how actuating on the angular DoFs of the PRM affects the POP QPD spot.
    • So by pre-filtering the target signal through the inverse actuator TF, we get a measure of how much the PRM angular motion is.
    • The reason we want to do this is to give the FIR filter that produces optic motion (output) given ground motion sensed by the seismometer (input) fewer poles/zeros to fit (?).
    • The actual actuator TF has to be measured using DTT, and fit - is there anything critical about this fitting? Seems like this should be just a simple pendulum transfer function so a pair of complex poles should be sufficient?
  3. The actual Wiener filter is calculated by the function miso_firlev.m. There are many versions of this floating around from what I can gather.
    • This function requires 3 input parameters.
      • Order of filter to be fit
      • Witness channels (can be multiple)
      • Target channel (has to be single, hence the "miso" in the function name).
    • Today, at the meeting, we talked about weighting the cost function that the optimal Wiener filter calculator minimizes.
    • The canonical wiener filter minimizes the mean squared error between the output of the filter and the desired signal profile (which for this particular problem is the angular motion of the PRM, calculated by dividing the target signal by the actuator TF, knowing which we can cancel it out).
    • But as seen in Attachment #1, the main reduction in RMS comes below f=5Hz.
    • So can we weight the cost function more heavily at lower frequencies? From what I can find in previous calculations, it looks like this weighting happens in the pre-filtering stage, which is not the same thing as including the frequency dependent weighting in the calculation of the Weiner filter? The PSD and acf are F.T. pairs per the Wiener-Khinchin theorem so intuitively I would think that weighting in the frequency domain corresponds to weighting on the lags at which the acf is calculated, but I need to think about this.
    • What kind of low-pass filter do we use to prevent noise injection at higher frequencies? Does the optimal filter calculation automatically roll-off the filter response at high frequencies?
  4. As I write this, seems like there is another level of optimization of "meta-parameters" possible in this whole process - e.g. what is the optimal order of filter to fit? what is the optimal pre-filtering of training data? Not sure how much we can gain from this though.

Some notes from Rana from some years ago: https://nodus.ligo.caltech.edu:8081/40m/11519

If anyone has pointers / other considerations I should take into account, please post here.

Attachment 1: pop_feedforward_potential.pdf
  13418   Wed Nov 8 14:28:35 2017 gautamUpdateGeneralMC1 glitches return

There hasn't been a big glitch that misaligns MC1 by so much that the autolocker can't lock for at least 3 months, seems like there was one ~an hour ago.

I disabled autolocker and feedback to the PSL, manually aligned MC1 till the MC_REFL spot looked right on the CCD to me, and then re-engaged the autolocker, all seems to have gone smoothly.


Attachment 1: MC1_glitchy.png
Attachment 2: 6AFDA67D-79B1-469C-A58A-9EC5F8F01D32.jpeg
  13460   Fri Dec 1 17:09:29 2017 Udit KhandelwalSummaryGeneral 

Current objectives and statuses:

  • CAD Model of 40m lab (facility, chambers, invacuum components etc)
    • Status: On hold since I'm unable to acquire general dimensions of 
  13472   Wed Dec 13 17:46:08 2017 Udit KhandelwalSummaryGeneralSummary of Current Tasks

40m Lab CAD

1. 40m_bldg.dwg has 2D drawing of the 40m building

  • After importing file as a 2D sketch into solidworks, make sure to retrace all the lines before performing any 3D extrusion stuff.
  • Made walls 3m high

2. 40m_VE.dwg has the Vaccuum Envelope.

  • Divided the file into individual sketches for the tubes, test mass, and beam splitter chambers (so they can be individually modified later if required).

3. 40melev.dwg has the relative positioning between (1) and (2).

  • Using this file to position objects inside building cad.

4. All files can be found in Dropbox folder [40m SOS Modeling], which should be renamed to [40m CAD].

5. Next step would be to add the optical table, mirrors.

Tip-Tilt Suspension

1. Current objective: (refer to D070172) - Increase the length of the side arms (so it matches the dimensions of D960001), while keeping the test mass subassembly at the same height.
2. Future objective: Resonant frequency FEM of the frame (sans the test mass), and then change height to get the desired frequency.

Past Work

  • Completed solidworks model of SOS (D960001). I understand this is not the focus right now so this is for reference that the model is ready to be used.


  • I will be in India from 16th December until 6th January so this is my final visit for this year. I have enough material to work from home, and will correspond with Koji over email regarding Lab CAD and tip-tilt suspension.
  13475   Thu Dec 14 08:59:17 2017 SteveUpdateGeneralwe are here
Attachment 1: 8_days.png
  13489   Wed Dec 20 00:43:58 2017 KevinSummaryGeneralDAC noise contribution to squeezing noise budget

Gautam and I looked into the DAC noise contribution to the noise budget for homodyne detection at the 40m. DAC noise is currently the most likely limiting source of technical noise.

Several of us have previously looked into the optimal SRC detuning and homodyne angle to observe pondermotive squeezing at the 40m. The first attachment summarizes these investigations and shows the amount of squeezing below vacuum obtainable as a function of homodyne angle for an optimal SRC detuning including fundamental classical sources of noise (seismic, CTN, and suspension thermal). These calculations are done with an Optickle model. According to the calculations, it's possible to see 6 dBvac of squeezing around 100 Hz.

The second attachment shows the amount of squeezing obtainable including DAC noise as a function of current noise in the DAC electronics. These calculations are done at the optimal -0.45 deg SRC detuning and 97 deg homodyne angle. Estimates of this noise are computed as is done in elog 13146 and include de-whitening. It is not possible to observe squeezing with the current 400 Ω series resistor which corresponds to 30 pA/rtHz current noise at 100 Hz. We can get to 0 dBvac for current noise of around 10 pA/rtHz (1.2 kΩ series resistor) and can see 3 dBvac of squeezing with current noise of about 5 pA/rtHz at 100 Hz (2.5 kΩ series resistor). At this point it will be difficult to control the optics however.

So it seems reasonable to reduce the DAC noise to sufficient levels to observe squeezing, but we will need to think about the controls problem more.

Attachment 1: 40m_squeezing.pdf
Attachment 2: 40mDAC_squeezing.pdf
  13490   Thu Dec 21 19:25:48 2017 KevinSummaryGeneralDAC noise contribution to squeezing noise budget

Gautam and I redid our calculations, and the updated plot of squeezing as a function of DAC current noise per coil is shown in the attachment. The current noise is calculated as the maximum of the filtered DAC noise and the Johnson noise of the series resistor. The total noise is for four optics with four coils each.

The numbers are worse than we quoted before: according to these calculations we can get to 0 dBvac for current noise per coil of about 2.4 pA/rtHz at 100 Hz.


Gautam and I looked into the DAC noise contribution to the noise budget for homodyne detection at the 40m. DAC noise is currently the most likely limiting source of technical noise.

Several of us have previously looked into the optimal SRC detuning and homodyne angle to observe pondermotive squeezing at the 40m. The first attachment summarizes these investigations and shows the amount of squeezing below vacuum obtainable as a function of homodyne angle for an optimal SRC detuning including fundamental classical sources of noise (seismic, CTN, and suspension thermal). These calculations are done with an Optickle model. According to the calculations, it's possible to see 6 dBvac of squeezing around 100 Hz.

The second attachment shows the amount of squeezing obtainable including DAC noise as a function of current noise in the DAC electronics. These calculations are done at the optimal -0.45 deg SRC detuning and 97 deg homodyne angle. Estimates of this noise are computed as is done in elog 13146 and include de-whitening. It is not possible to observe squeezing with the current 400 Ω series resistor which corresponds to 30 pA/rtHz current noise at 100 Hz. We can get to 0 dBvac for current noise of around 10 pA/rtHz (1.2 kΩ series resistor) and can see 3 dBvac of squeezing with current noise of about 5 pA/rtHz at 100 Hz (2.5 kΩ series resistor). At this point it will be difficult to control the optics however.

So it seems reasonable to reduce the DAC noise to sufficient levels to observe squeezing, but we will need to think about the controls problem more.


Attachment 1: 40mDAC_squeezing.pdf
  13491   Fri Dec 22 09:40:19 2017 ranaSummaryGeneralDAC noise contribution to squeezing noise budget
  1. Should not count the ITMs. On those we can use big resistors / filters to cut out the noise.
  2. For the initial LIGO, we used 7 K resistors and the mass was 10 kg. But...the output driver went +/- 150 V.

So we had a max F/m = (20 mA * 0.064 N/A)/(10 kg) = 0.0001. For the 40m, to get the same thing, we would need 40x less current (~0.5 mA). At the moment we have (12 V / 400 Ohm) = 30 mA.

We need to get a spectrum and times series of the required coil current for acquiring and holding the DRMI, and also the single arm. Then we can see where to make noise reductions to allow this drastic force reduction. 

Coil Driver Upgrade wiki here.

  13492   Tue Dec 26 17:24:24 2017 SteveUpdateGeneralpower outage

There was a power outage.

The IFO pressure is 12.8 mTorr-it and it is not pumped. V1 is still closed. TP1 is not running. The Rga is not powered.

The PSL output shutter is still closed. 2W Innolight turned on and manual beam block placed in its beampath.

3 AC units turned on at room temp 84F

Attachment 1: powerOutage.png
  13493   Thu Dec 28 17:22:02 2017 gautamUpdateGeneralpower outage - CDS recovery
  1. I had to manually reboot c1lsc, c1sus and c1ioo.
  2. I edited the line in /etc/rt.sh (specifically, on FB /diskless/root.jessie/etc/rt.sh) that lists models running on a given frontend, to exclude c1dnn and c1oaf, as these are the models that have been giving us most trouble on startup. After this, I was able to bring back all models on these three machines using rtcds restart --all. The original line in this file has just been commented out, and can be restored whenever we wish to do so.
  3. mx_stream processes are showing failed status on all the frontends. As a result, the daqd processes are still not working. Usual debugging methods didn't work.
  4. Restored all sus dampings.
  5. Slow computers all seem to be responsive, so no action was required there.
  6. Burtrestored c1psl to solve the "sticky slider" problem, relocked PMC. I didn't do anything further on the PSL table w.r.t. the manual beam block Steve has placed there till the vacuum situation returns to normal.

@Steve: I noticed that we are down to our final bottle of N2, not sure if it will last till 2 Jan which is presumably when the next delivery will come in. Since V1 is closed and the PSL beam is blocked, perhaps this doesn't matter.

from Steve: there are spare full N2 bottles at the south end outside and inside. I replaced the N2 on Sunday night. So the system should be Ok as is.

I also hard-rebooted megatron and optimus as these were unresponsive to ping.

*Seems like the mx_stream errors were due to the mx process not being started on FB. I could fix this by running sudo systemctl start mx on FB. After which I ran sudo systemctl restart daqd_*. But the DC errors persist - not sure how to fix this. Elogging suggests that "0x4000" errors are connected to timing problems on FB, but restarting the ntp service on FB (which is the suggested fix in said elogs) didn't fix it. Also unsure if mx process is supposed to automatically start on FB at startup.

Attachment 1: 28.png
  13499   Wed Jan 3 15:13:55 2018 SteveUpdateGeneralprojector light bulb replaced

Bulb  is replaced.


I noticed this behaviour since ~Dec 20th, before the power failure. The bulb itself seems to work fine, but the projector turns itself off after <1 minute after being manually turned on by the power button. AFAIK, there was no changes made to the projector/Zita. Perhaps this is some kind of in-built mechanism that is signalling that the bulb is at the end of its lifetime? It has been ~4.5 months (3240 hours) since the last bulb replacement (according to the little sticker on the back which says the last bulb replacement was on 15 Aug 2017


  13503   Thu Jan 4 14:39:50 2018 gautamUpdateGeneralpower outage - timing error

As mentioned in my previous elog, the CDS overview screen "DC" indicators are all RED (everything else is green). Opening up the displays for individual CPUs, the error message shown is "0x4000", which is indicative of some sort of timing error. Indeed, it seems to me that on the FB machine, the gpstime command shows a gps time that is ~1 second ahead of the times on other FE machines.

Running gpstime on other FE machines throws up an error, saying that it cannot connect to the network to update leap second data. Not sure what this is about...

I double checked the GPS timing module, we had some issues with this in the recent past. But judging by its front panel display, everything seems to be in order...

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/gpstime", line 9, in <module>
    load_entry_point('gpstime==0.2', 'console_scripts', 'gpstime')()
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 356, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 2476, in load_entry_point
    return ep.load()
  File "/usr/lib/python3/dist-packages/pkg_resources.py", line 2190, in load
  File "/usr/lib/python3/dist-packages/gpstime/__init__.py", line 41, in <module>
    LEAPDATA = ietf_leap_seconds.load_leapdata(notify=True)
  File "/usr/lib/python3/dist-packages/ietf_leap_seconds.py", line 158, in load_leapdata
  File "/usr/lib/python3/dist-packages/ietf_leap_seconds.py", line 115, in fetch_leapfile
    r = requests.get(LEAPFILE_IETF)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 60, in get
    return request('get', url, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 49, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 457, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 569, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 407, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', OSError(101, 'Network is unreachable'))



  13506   Fri Jan 5 21:54:28 2018 ranaUpdateGeneralpower outage - timing error

Rolf came here in the morning, but not sure what he did or if Jamie remotely did something. But the screen is green.

Attachment 1: huh.png
  13507   Fri Jan 5 22:19:53 2018 gautamUpdateGeneralpower outage - timing error

Just putting the relevant line from email from Rolf which at least identifies the problem here:

Looks like FB time is actually off by 1 year, as your timing system does not get year info.

There still seems to be something funky with the X arm transmission PDs - I can't seem to get the triggering to switch between the QPD and the Thorlabs PD, and the QPD signal seems to be wildly fluctuating by several orders of magnitude from 0.01-100. The c1iscex FE was pulled out, and it seemed to me like someone was doing some cable re-arrangement at the X end.

I will look into this tomorrow. 


Rolf came here in the morning, but not sure what he did or if Jamie remotely did something. But the screen is green.


  13510   Sat Jan 6 18:27:37 2018 gautamUpdateGeneralpower outage - IFO recovery

Mostly back to nominal operating conditions now.

  1. EX TransMon QPD is not giving any sensible output. Seems like only one quadrant is problematic, see Attachment #1. I blame team EX_Acromag for bumping some cabling somewhere. In any case, I've disabled output of the QPD, and forced the LSC servo to always use the Thorlabs "High Gain" PD for now. Dither alignment servo for X arm does not work so well with this configuration - to be investigated.
  2. BS Seismometer (Trillium) is still not giving any sensible output.
    • I looked under the can, the little spirit level on the seismometer is well centered.
    • I jiggled all the cabling to rule out any obvious loose connections - found none at the seismometer, or at the interface unit (labelled D1002694 on the front panel) in 1X5/1X6.
    • All 3 axes are giving outputs with DC values of a few hundred - I guess there could've been some big earthquake in early December which screwed the internal alignment of the sensing mass in the seismometer. I don't know how to fix this.
    • Attachment #2 = spectra for the 3 channels. Can't say they look very seismicy frown. I've assumed the units are in um/sec.
    • This is mainly bothering me in the short term because I can't use the angular feedforward on PRC alignment, which is usually quite helpful in DRMI locking.
    • But I think the PRM Oplev loop is actually poorly tuned, in which case perhaps the feedforward won't really be necessary once I touch that up.

What I did today (may have missed some minor stuff but I think this is all of it):

  1. At EX:
    • Toggled power to Thorlabs trans monitoring PD, checked that it was actually powered, squished some cables in the e- rack.
    • Removed PDA55 in the green path (put there for EX laser AM/PM measurement). So green beam can now enter the X arm cavity.
    • Re-connected ALS cabling.
    • Turned on HV supply for EX Green PZT steering mirrors (this has to be done every time there is a power failure).
  2. At ITMY table:
    • Removed temporary HeNe RIN/ Oplev sensing noise measurement setup. HeNe + 1" vis-coated steering mirror moved to SP table.
    • Turned on ITMY/SRM Oplev HeNe.
    • Undid changes on ITMY Oplev QPD and returned it to its original position.
    • Centered ITMY reflected beam on this QPD.
  3. At vertex area
    • Looked under Trillium seismometer can - I've left the clamps undone for now while we debug this problem.
    • Power-cycled Trillium interface box.
    • Touched up PMC alignment.
  4. Control room
    • Recover IFO alignment using combination of IR and Green beams.
    • Single arm locking recovered, dither alignment servos run to maximize arm transmission. Single arm locks holding for hours, that's good.
    • The X arm dither alignment isn't working so well, the transmission never quite hits 1 and it undergoes some low frequency (T~30secs) oscillations once the transmission reaches its peak value.
    • Had to do the usual ipcrm thing to get dataviewer to run on pianosa.

Next order of business:

  1. Recover ALS:
    • aim is to replace the vertex area ALS signals derived from 532nm with their 1064nm counterparts.
    • Need to touch up end PDH servos, alignment/MM into arms, and into Fibers at ends etc.
    • Control the arms (with RMs misaligned) in the CARM/DARM basis using the revised ALS setup.
    • Make a noise budget - specifically, we are interested in how much actuation range is required to maintain DARM control in this config.
  2. Recover DRMI locking
    • Continue NBing.
    • Do a statistical study of actuation range required for acquiring and maintaining DRMI locking.
Attachment 1: EX_QPD_Quad1_Faulty.pdf
Attachment 2: Trillium_faulty.pdf
  13541   Fri Jan 12 18:08:55 2018 gautamUpdateGeneralpip installed on nodus

After much googling, I figured out how to install pip on SL7:

sudo easy_install pip

Next, I installed git:

sudo yum install git A

Turns out, actually, pip can be installed via yum using

sudo yum install python-pip
  13544   Fri Jan 12 20:35:34 2018 Udit KhandelwalSummaryGeneral2018/01/12 Summary
  1. 40m Lab CAD
    1. Worked further on positioning vacuum tubes and chambers in the building.
    2. Next step would be to find some drawings for optical table positions and vibration isolation stack. Need help with this! 
  2. Tip Tilt Suspension (D070172)
    1. Increased the length of side arms. The overall height of D070172 assembly matches that of D960001.
    2. The files are present in dropbox in [40mShare] > [40m_cad_models] > [TT - Tip Tilt Suspension]
  13555   Wed Jan 17 23:36:12 2018 johannesConfigurationGeneralAS port laser injection

Status of the AS-port auxiliary laser injection

  • Auxiliary laser with AOM setup exists, first order diffracted beam is coupled into fiber that leads to the AS table.
  • There is a post-PMC picked-off beam available that is currently just dumped (see picture). I want to use it for a beat note with the auxiliary laser pre-AOM so we can phaselock the lasers and then fast-switch the phaselocked light on and off.
  • I was going to use the ET3010 PD for the beat note unless someone else has plans for it.
  • I obtained a fixed triple-aspheric-lens collimator which is supposed to have a very small M^2 value for the collimation on the AS table. I still have the PSL-lab beam profiler and will measure its output mode.
  • Second attached picture shows the space on the AS table that we have for mode-matching into the IFO. Need to figure out the desired mode and how to merge the beams best.
Attachment 1: PSLbeat.svg.png
Attachment 2: ASpath.svg.png
  13560   Fri Jan 19 15:22:19 2018 Udit KhandelwalSummaryGeneral40m CAD update 2018/01/19

40m CAD Project

  1. All parts will be now named according to the numbering system in this excel sheet: LIGO 40m Parts List in dropbox folder [40mShare] > [40m_cad_models] > [40m Lab CAD]
  2. I've placed optical tables in the chambers at 34.82" from the bottom for now. This was chosen by aligning the centre of test mass of SOS assembly (D960001) with that of vacuum tube (Steve however pointed out last week they might not necessarily be concentric).

  13561   Fri Jan 19 20:59:07 2018 Udit KhandelwalUpdateGeneralSolidworks Rendering

Rendered the SOS assembly (D960001) with correct materials and all and it looks very nice. Will extend this to the building cad later.

  13566   Mon Jan 22 12:48:48 2018 KojiSummaryGeneralBeat setup for aLIGO EOM test

I'm planning to construct a beat setup between the PSL and AUX beams. I am going to make it in the area shown in a blue square in the attached photo. This does not disturb Johannes' and PSL setups. The beams are obtained from the PBS reflection of the PSL and the dumped beam of the aux path (0th or 1st order beam of the AOM).

Attachment 1: IMG_3048.JPG
  13567   Mon Jan 22 20:54:58 2018 KojiSummaryGeneralAUX-PSL beat setup

The beat setup has been made on the PSL table. The BS and the PD were setup. The beat was found at 29.42degC and 50.58degC for the PSL and AUX crystal temperatures, respectively.
We are ready for the EOM test. I have instruments stacked around the PSL table. Please leave them as they are for a while. If you need to move them, please contact with me. Thanks.

A picked-off PSL after the main modulator was used as the PSL beam. This was already introduced close to the setup thanks to the previous 3f cancellation test ELOG 11029. The AUX beam was obtained from the transmission of 90% mirror. Both paths have S polarization. The beams are combined with a S-pol 50% BS. The combined beam is detected by a new focus 1GHz PD.

The PSL crystal temp (actual) was 50.58degC. The AUX crystal temp was swept upward and the string beat was found at 50.58degC. After a bit of alignment, the beat strength was -18dBm (at 700V/A RF transimpedance of NF1611) .

Attachment 1: IMG_3051.JPG
  13601   Fri Feb 2 21:12:46 2018 Udit KhandelwalSummaryGeneralSummary - 2018/02/02

Tip-Tilt Suspension CAD:
Discussed with Koji about motivation to simplify the design of this assembly, which has many unnecessary over-constraints. I have started to cad alternate parts with the aim of removing these over-constraints.

40m Lab CAD:
Acquired a stack of original engineering drawings of the vacuum chambers from Steve which I will take home, get scanned, and then use as reference for the cad i'm working on.

Started paperwork at west bridge office to get paid as an "occasional employee". Hopefully I receive old money.

  13602   Fri Feb 2 22:47:00 2018 KojiSummaryGeneralAP1053: Packaging & Performance

I've packaged an AP1053 in a Thorlabs box. The gain and the input noise level were measured. It has the gain of ~10 and the input noise of ~0.6nV/rtHz@50MHz~200MHz.


AP1053 was soldered on Thorlabs' PCB EEAPB1 (forgot to take a picture). The corresponding chassis is Thorlabs' EEA17. There is a 0.1uF high-K ceramic cap between DC and GND pins. The power is supplied via a DC feedthru capacitor (Newark / Power Line Filter / 90F2268 / 5500pF) found in the WB EE shop. The power cable has a connector to make the long side of the wires detachable. Because I did not want to leave the RF signal path just mechanically touched, the SMA connectors were soldered to the PCB. As the housing has no access hole, I had to make it at one of the sides.

The gain of the unit was measured using the setup shown in the upper figure of Attachment 2. When the unit was energized, it drew the current of about 0.1A. The measued gain was compensated by the pick off ratio of the coupler (20dB). The gain was measured with the input power of -20, -10, 0, 10, and 15dBm. The measurement  result is shown in Attachment 3. The small signal gain was actually 10dB and showed slight degradation above 100MHz. At the input of 10dB some compression of the gain is already visible. It looks consistent with the specification of +26.0dBm output for 1dB compression above 50MHz and +24.0dBm output below 50MHz.

The noise level was characterized with the setup shown in the bottom figure of Attachment 3. The noise figure of the amplifier is supposed to be 1.5dB above 200MHz and 3.5dB below 200MHz. This is quite low and the output noise of AP1053 can not be measured directly by the analyzer. So, another LN amplifier (ZFL-500HLN) was stacked. The total gain of the system was measured in the same way as above. The measured noise level was ~0.7nV/rtHz between 50MHz and 200MHz. Considering the measurement noise level of the system, it is consistent with the input referred noise of 0.6nV/rtHz. I could not confirm the advertized noise figure of 1.5dB above 200MHz. The noise goes up below 50MHz. But still 2nV/rtHz at 3MHz. I'd say this is a very good performance.

Attachment 1: AP1053.JPG
Attachment 2: AP1053_measurement.pdf
Attachment 3: AP1053_gain.pdf
Attachment 4: AP1053_noise.pdf
  13638   Fri Feb 16 21:03:17 2018 Udit KhandelwalSummaryGeneralSummary 2018/02/16

40m Lab Cad:
Updated the dimensions of and fleshed out the chambers in greater detail, by referring to the engineering drawings that Steve gave to me. I have scanned and uploaded most of these drawings to Dropbox in [40mShare]>[40m_cad_models]>[Vacuum Chamber Drawing Scans]. The excel file "LIGO 40m Parts List" in the [40m Lab CAD] folder also lists the Steve drawings I referenced for dimensions of each part.

Next steps:
1. Finish details of all chambers.
2. Start placing representative blocks on the optical table.

  13639   Fri Feb 16 22:15:30 2018 gautamUpdateGeneralc1mcs model restarted

c1mcs had died for some reason. Looking at dmesg, I see:

[769312.996875] c1mcsepics[1140]: segfault at 7f5000000012 ip 00007f50ea8ded8f sp 00007f50e9f53a10 error 4 in libc-2.19.so[7f50ea865000+1a1000]

None of the other EPICS processes died. Not sure what to make of this. I was at the PSL table working, and had closed the PSL shutter to avoid MC autolocker trying to keep the MC locked while I was mucking about, but this shouldn't have had any effect on an EPICS process?

Anyway, I just logged into c1sus, stopped and restarted the model. IMC locks fine now.

  13640   Fri Feb 16 22:19:07 2018 gautamUpdateGeneralFibel ALS input polarization tuning

After discussing with Koji, I decided to try and align the input beam polarization at the PSL fiber coupler to one of the special axes of the PM fiber. The motivation is to try and narrow down the source of the large RF beatnote amplitude drift I noticed and reported last night.

The setup for doing so is shown in Attachment #1 - essentially, I setup one of the newly purchased couplers in a mount, set up a PBS, and placed two photodiodes at the S and P ports of the PBS. The idea is to rotate the input coupler in its mount, thereby maximizing the PER (monitored on two Thorlabs PDA520s - I didn't check the gain balance of them). 

I spent ~30mins doing some preliminary trials just now, and, I was able to achieve a PER of ~1/20. But I think much better numbers were reported in this SURF project (although I'm not entirely sure I understand that measurement). I will spend a little more time tweaking the alignment. The procedure is tricky as at some point, simply rotating the mount reduces the mode-matching efficiency into the fiber so much that it is not possible to get a meaningful PER measurement from the photodiodes. I'm adjouring for now, more to follow...

Attachment 1: PER_setup.JPG
  13641   Mon Feb 19 14:27:25 2018 gautamUpdateGeneralFibel ALS input polarization tuning


Current configuration of PSL free-space to fiber coupling is:

  • 3.25 mW / 4.55mW (~71%) coupling efficiency, both numbers measured with Ophir power meter, Filter OFF
  • \mathrm{PER} \doteq \frac{\mathrm{P_{fast}}}{\mathrm{P_{slow}}} (I choose to define it in this way as opposed to the reciprocal) of 75 (~19dB). The uncertainty in this number is large (see discussion), but I am confident that we have >10dB, which while isn't as good as can be, is sufficient for the main motivation behind this work.


I had noticed that the RF beat amplitude was fluctuating by up to 20dBm as viewed on the control room analyzer. As detailed in my earlier elog, I suspected this to be because of random polarization drift between the PSL and EX fields incident on the Fiber coupled PDs. Since I am confident the problem is optical (as opposed to something funny in the electronics), we'd like to be able to isolate which of the many fiber segments is dominating the contribution to this random polarization drift.

Some useful references:

  1. General writeup about how PM fibers work and PER. Gives maximum achievable PERs for a given misalignment of incident beam relative to one of the two birefringent axes.
  2. Another similar writeup. This one put me onto the usefulness of the alignment keys on the fibers.
  3. Thorlabs PM980 specs - this tells us about the orientation of the two axes for the kind of fibers we use.

Procedure and details:

  • The principle of operation behind polarization maintaining (PM) fibers is that intentional birefringence is introduced along two perpendicular axes in the fiber.
  • As a result of which light propagates with different phase velocity along these axes.
  • For an arbitrary incident field with E-field components along both axes, it is almost impossible to predict the output polarization as we do not know the length of propagation along each axis to sufficient precision (it is also uncontrolled w.r.t. environmental fluctuations). So even if you launch linear polarization into the fiber, it is most likely that the output polarization state will be elliptical.
  • But if we align the incident, linear polarization along one of the two axes, then we can accurately predict the polarizaiton at the output, to the extent that the fiber doesn't couple power in the two axes during propagation. I cant find a spec for the isolation between axes for the fiber we use, but the specs I could find for other fiber manufacturers suggest that this number is >30dB, so I think the assumption is a fair one.
  • A useful piece of information is that the alignment key on the fibers gives us information about the orientation of the birefringent axes inside the fiber. For the Thorlabs fibers, it seems that the alignment key lines up with the stress-inducing rods inside the fiber (i.e. the slow axis). I confirmed this by looking at the fiber with the fiber scope.
  • The PSL pickoff beam I am using for this setup is from the transmission of the PBS after the Faraday. So this field should have relatively pure P-polarization.
  • The way I have set up the fiber on the PSL table, the fast axis of the fiber corresponds to P-polarization (i.e. E field oscillates parallel to the plane of the optical table). Actually, it was this alignment that I tweaked in this work.
  • Using the information about the alignment key defining polarization axes on the fiber, I also set up the output fiber coupler such that the fast axis lined up as near parallel to the plane of the optical table as possible. In this way, the beam incident on the PBS at the output of my setup should be pure P-polarizaiton if I setup my input alignment into the fiber well.
  • I tweaked the rotation of the Fiber mount at the input coupler to maximize the ratio of P_p / P_s, as measured by the pair of PDs at the output. 
  • As #1 of my listed references details, you need to align the incident linear polarization to one of the two birefringent axes to closer than 6 degrees to achieve a PER of >20dB. While this sounds like a pretty relaxed requirement, in practise, it is about as good as we can hope to achieve with the mounts we have, as there is no feature that allows us to lock the rotational degree of freedom once we have optimized the alignment. Any kind of makeshift arrangement like taping the rotating part to the mount is also flaky, as during the taping, we may ruin the alignment.
  • Attachment #1 shows the result of my alignment optimization - the ratio P_p / P_s is about 75.
  • The uncertainty on the above number is large. Possible sources of error:
    • Output coupler is not really aligned such that fast axis corresponds to P-polarizaiton for the output PBS.
    • The two photodiodes' gain balance was not checked.
    • The polarization content of the input beam was not checked.
    • The PBS at the output could be slightly misaligned relative to the S/P polarization directions defined by the tabletop.
    • The PBS extinction ratio was not checked.
  • But anyways, this is a definite improvement on the situation before. And despite the large uncertainty, I am confident that P_p / P_s is better than 10dB.
  • Moreover, Steve and I installed protective tubing on the lengths of fiber that were unprotected on the PSL table, this should help in reducing stress induced polarization drifts in the fiber, at least in these sections of fiber.
  • So I think the next step is to monitor the stability of the RF beatnote amplitude after these improvements. At some point, we need to repeat this procedure for the EX and EY fibers as well.
  • If the large drifts are still seen, the only thing we can exclude as a result of this work is the section of fiber from the PSL light coupler to the beat mouth.
Attachment 1: IMG_6900.JPG
  13642   Tue Feb 20 13:59:30 2018 KojiUpdateGeneralModulation depth measurement for an aLIGO EOM

Last night I worked at the PSL table for the modulation depth measurement for an aLIGO EOM. Let me know if the IFO behavior is unusual.

What I did was:

  • Cranked up the HEPA speed to 100
  • Placed an aLIGO EOM in the AUX beat path (south side of the PSL laser). (It is still on the PSL table as of Feb 20, 2018)
  • Closed the PSL shutter
  • Turned off the main Marconi forr 11MHz. The freq and output power of this marconi have not been touched.
  • Turned off the freq generation unit
  • Worked on my measurement with the spectrum and network analyzers + aux marconi.
  • Turned down the HEPA speed to 30
  • Turned on the freq generation unit
  • Turned on the main Marconi
  • Opened the PSL shutter => IMC locked
  13650   Thu Feb 22 16:11:14 2018 KojiUpdateGeneralaLIGO EOM crystal replacement

aLIGO EOM crystal replacement

  • The entire operation has been performed at the south flow bench @40m.
  • We knew that the original crystal in the aLIGO EOM we are testing has some problem. This was replaced with a spare RTP crystal.
  • Once the housing was removed, it was obvious that the crystal has a crack (Attachment 1).
    It seemed that it was produced by either a mechanical stress or a thermally induced stress (e.g. soldering).
  • I wanted to make sure the new crystal is properly aligned interms of the crystal axis.
    The original crystal has the pencil marking at the top saying "Z" "12". The new (spare) crystal has "Z" and "11".
    So the new crystal was aligned in the same way as the original one. (Attachment 2)
  • I took an opportunity to measure the distribution of the electrode lengths (Attachment 3). The lengths are 14, 5, and 14mm, respectively.
Attachment 1: IMG_3421.JPG
Attachment 2: IMG_3426.JPG
Attachment 3: IMG_3427.JPG
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