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ID Date Author Typeup Category Subject
  10390   Thu Aug 14 18:31:45 2014 ericqUpdateLSCLSC Modeling Update

 Based on the game plan, I have created a slew of updated pretty plots about our signals and loops. 

First: With measured arm losses, when do we start to see REFL DC dip? At what arm buildup powers? 

I updated my MIST model with the arm losses I've measured (Y:130ppm, X:530ppm), and some measured transmissions from the wiki, vs. the design parameters, as I used to have. Here is the DC sweep plot which is now hanging up in the control room. 


In this plot, I also calculated what MIST thinks the full arm power buildup will be as compared to our single arm locking, and I get something of order 200, rather than the 600 we've tossed around in discussions. Nothing else is very different in this plot from the old version; though the REFLDC dip is a little bit wider. 

Now, here are some radiation-pressure inclusive sensing transfer functions, for the anti-spring case (which in Rob's day was easier to lock for unknown reasons):



Next: Include new AO path TFs into CM model Look at possibilities for engaging AO path 

With these TFs, and the recently measured+fit new AO TF, here are the open loop gains of the slow, digital, SqrtInv-sensed MCL CARM and fast, analog, REFLDC-sensed AO CARM loops for the region of offsets we've achieved and a little lower. The slow digital loop includes the 1k LP that we have used in the past, in addition to the normal CARM filters. I still need to figure out the right sequence of ( offset reduction / crossover frequency motion / overall gain adjustment ) that gets the coupled cavity resonance solidly within the loop bandwidth. 


  10393   Thu Aug 14 20:52:36 2014 ranaUpdateWikiViolin Mode table added to Wiki

Mech Resonance Wiki

I've updated the wiki by trawling the elog for violin entries. Please keep it up to date so that we can make violin notches.


  10394   Thu Aug 14 22:16:02 2014 JenneUpdateSUSViolin Mode filters for ETMs

The instigator of this was that we were seeing ring-ups of ETMs during our ALS locks this evening.  We measured the ETMY violin resonance to be 624.10 Hz, and Rana found an elog saying that the ETMX was around 631 Hz, so we made a 2 notch filter and added it to FM4 of the LSC-SUS filter banks for both ETMs. 

For the ETMY resonance, we measured the frequency in the DARM spectrum, and when we looked at the FINE_PHASE_OUT channels, the resonance was only in the Yarm sensor.  So, we conclude that it is coming from ETMY.

Also in the realm of filter modules, the FM3 boost for CARM, DARM, XARM and YARM was changed from zero crossing to ramp with a 1sec ramp time.

  10395   Thu Aug 14 22:31:12 2014 JenneUpdateLSCTRY gets mystery offset

I don't know why, but TRY has somehow gotten a 0.3 count offset in the last hour. 

Rana and I are witnesses for each other that neither of us has gone into the IFO room in the last several hours (and we're the only ones here).  For some reason though, the TRY PD now has a 0.3 count offset.  We have been doing some ALS locks, but we have not run the offset script in the last several hours.  Closing the green shutter doesn't change things, and we still see the offset when the MC loses lock, so it's not to do with the end or the PSL laser.  We haven't been in there, so there hasn't been a change in the room lights. 


  10398   Fri Aug 15 01:31:44 2014 JenneUpdateASCPOP QPD to ETMs model wiring complete

I have added a few things to the ASS model, and the ASC sub-block, so that we can send POP QPD information down to the ETMs for CARM angular control after we've reduced the CARM offset and gotten some carrier buildup.  I did not remove our ability to actuate on PRM, so that we can still play with it in PRMIsb cases.

The input matrix has been expanded so that it can send signals to new CARM_YAW and CARM_PIT filter banks.  The corresponding filter banks have been created.  The output matrix was also expanded to take in the 2 new servo outputs, and so it can send signals to both ETMs, pitch and yaw.  I did not include any triggering logic for this new CARM situation, since I assume we'll just turn it on and off with our scripts.  (We haven't really been using the triggering capability of the PRM ASC either lately, although it's all still there).  I added the inputs and outputs of the CARM servos to the list of acquired channels.

The ASC sub-block:


I also modified the top level of the ASS model.  This was just a simple addition of summing nodes for the ETMs, similar to what was already in place for the PRM, so that we can send both the ASS dither alignment signals and the ASC servo control signals to the optics.

The ASS top level:


I also quickly modified the ASC screen to expose all of the new options:


The ASS model was compiled, and restarted.  As usual, this temporarily removes the biases on the input pointing tip tilts, but the pointing seems to have come back without any trouble.

  10399   Fri Aug 15 02:05:55 2014 JenneUpdateLSCALS in-loop spectra

 Not sure why, but Rana and I didn't see the super high Xarm noise with ALS that we reported last night (elog 10382).  

The in-loop ALS noise seems fine.  The out of loop measurement while the ALS is locked is a little tricky, since ALS hold the arms within the POX/POY linear ranges.  

Here is the in-loop noise:


  10400   Fri Aug 15 13:29:31 2014 JenneUpdateGeneralGame plan: 15 Aug


The game plan graffle file is now in the 40m dropbox, so anyone can edit it.  Please just make sure to keep the date in the top right corner accurate.

  10401   Fri Aug 15 14:09:21 2014 JenneUpdateLSCTRY mystery offset gone

Again unknown, but about 6 hours ago (so ~8am) the offset disappeared. 

Here's a 1-day trend:


  10402   Fri Aug 15 14:35:57 2014 ericqUpdateLSCTRY mystery offset gone

One question answered, but another raised. The offset came from LSC-TRY switching to the ETMY-QPD signal from ETMY-TRY (Hi gain pd). 



  10403   Fri Aug 15 17:24:44 2014 HarryUpdateGeneralFiber Temp.

 Earlier today Q and I somewhat resurrected my old PER measurement setup so I could run the temperature characterization experiment.

Unfortunately, when I tried to use the fiber illuminator, no light came from the other end, causing me to fail my primary goal for the summer of "don't break anything." The fiber has been re-spooled and labeled appropriately. Also sorry.

In addition to this, Q and I scavenged parts from the telescopes on the PSL and Y End tables, which were either not functional, or needed to have their mode matching adjusted, since we're using the non-PM fibers for FOL, which have a different numerical aperture, and thus slightly different output modes.

Specifically, this is involved removing the rotational mounts, and appropriate beam dumping.

My "calorimeter" still remains intact, in case anyone wants to make this measurement in the future, as this is my last day in the lab.

It's also effective at keeping drinks cold, if you'd rather use it for that.

  10404   Fri Aug 15 20:26:37 2014 ericqUpdateGeneralGame plan


Q already did the tweak up of the PSL SHG crystal alignment.  HE SHOULD ELOG ABOUT THIS.  What was the final power of green that you got?  Do we have any record of a previous measurement to compare to?

As Jenne mentioned, I did this. 

Specifically, I first tweaked the mirror pointing the IR into the SGH in pitch and yaw to maximize the green power, and then adjusted the little set screws on the side of the SHG to maximize further. Power after the harmonic separator was of order 150uW. On the Y Green BBPD, I got ~48uW, instead of the 40uW Rana, Jenne, and myself saw the other night. 


now that I look through old ELOGs, I find some posts by Kiwamu saying the power should be around 650uWand that he was able to get 640uW out. So: I should do this again, systematically, more carefully, etc., etc. (Linked ELOG also states that optimum SHG temperature is alignment dependent...)



  10405   Fri Aug 15 20:38:17 2014 ericqUpdateGeneralELOG dump

 A few things that I have neglected to ELOG yet:

  • scripts/offsets/LSCoffsets is a new script that uses ezcaservo to set FM offsets of our LSC PDs. It still warns about large changes, and lets you revert. It reads the FM gain to pick the right gain for the ezcaservo call. 

  • MC refl DC was all over the place today, and has recently been "fuzzier" on the wall StripTool than I like. I touched the MC2 pointing a little bit, and the WFS seemed to find a sweet spot where the refl got steady back at around and under 0.5. I then ran "offload WFS" to try and stay there. 

  • Incidentally, the PMC transmission drifted up to 0.81 at some point today. This is weird, since not too long ago, we were not able to reach this level even with careful alignment. This coincided with the MC power being back up to ~17k, and arms locking at around 0.95. 

  • Last week I quickly tried cranking up the x-end green modulation frequency to ~1.3MHz (corresponding to a notch in the PZT AM response), and using a 550k lowpass on the mixer output, instead of a 70k, to try to buy more phase and increase the UGF. It didn't work. I didn't have a way to tune the mixer phase angle, and the mixer output was super noisy, but there were instants where I could convince myself that a mode was briefly locked to the arm... I'm going to do the Right Thing and characterize the loop properly, to figure out how to get at least 10kHz of control bandwidth out of these things. 

  10407   Mon Aug 18 18:33:57 2014 ericqUpdateGreen LockingYarm Green PDH

So far today, I've been working with the Y-end green PDH locking. Using a SR560 to roll off the AG4395A output to take a loop measurement at the servo output, I measured the following OLG, and inferred the CLG from it. The SR560 really helped it getting good coherence without introducing a big offset that changes the optical gain, thus distorting the loop shape, etc. etc. 


You would think this loop looks pretty good, 10k UGF, and 45 degrees of phase margin, gain peaking is sane, and pretty smooth slope. But, the thing still was flipping out of lock while I measured this. 

I suspect shenanigans at >100k. This is motivated by the fact that I've seen some big noise in the error signal around 150k. I don't have a good noise plot right now, because I'm trying to get a scheme going where I stitch together a bunch of 1 decade spectra from the 4395, but the noise floor isn't consistent across each patch (even though the attenuation stays the same, and I confirmed I'm in "noise" mode). I'm working on a loop measurement up there, too, but I haven't been able to get the right filter/amplitude settings yet. 

So, even though this plot is not totally correct (read: wrong and bad), I include it just for the sake of showing the big honking spike of noise at ~150K.  



  10408   Tue Aug 19 01:01:36 2014 Jenne, RanaUpdateGreen LockingYarm Green PDH

[ Rana, Jenne]

We remeasured the Yend PDH box.

When we first started, the green couldn't hold lock to the arm - it kept flickering between modes.  Changing the gain of the PDH box (from 7.5 to 6.0) helped.

We measured a calibration, from our injection point to our measurement point.

The concept was that we'd take the mixer output, and put that into an SR560, and put the swept sine injection into the other input port of the '560, and use A-B.  So, for this calibration, we left A unplugged, and just had the RF out of the 4395 going to input B of the '560.  The 600 Ohm output of the '560 went to the error point input on the PDH box (during normal operation the mixer output is connected directly to the error point input).  The SR560 was set to gain of 1, no filtering.  I don't recall if we were using high range or low noise, but we tried both and didn't really see a difference between them.

We had the 4395 take that calibration out, and then we measured the closed loop gain up to 1 MHz. (Same measurement setup as above, but we connected the mixer out to the input of the SR560 to close the loop, and made sure we were locked on a TEM00 green mode.) Rana used an ipython notebook to infer the open loop gain from our measurement.  Our conclusion is that we don't have nearly enough gain margin in our loop.  We found the PDH box gain knob at 7.5, and we turned it down to 6.0, but the loop is still pretty borderline. We used the high impedance active probe to measure the error point monitor, since we aren't sure that that point can drive a 50 Ohm load.


We also measured the error point spectra and the control point spectra.  Unfortunately, the saved data from the analyzer (no matter what is on the screen) comes out in spectrum, not spectral density.  So, we need to check our conversion, but right now to get from Watts power to Volts, we do sqrt(50 ohm * data).  We then need to get to spectral density, and right now we're just dividing by the square root of the bandwith that is reported in the .par file. This last step is the one we want to especially check, by perhaps putting some known amount of noise (from an SR785?) into the 4395, and checking that our calibration math returns the expected noise spectrum.

What still needs to be done is to calibrate this into Hz/rtHz.  To do this, we were thinking that we should look at the error point on a 'scope while the cavity is flashing.

Anyhow, here is the uncalibrated error point spectrum.  Purple is a measurement up to 30kHz, with 30Hz bandwidth.  Blue is a measurement up to 300kHz with 300Hz bandwidth.  The gain peaking schmutz above 10kHz sucks, and we'd like to get rid of it.  We also see the same peak at ~150kHz that Q saw earlier today.  We were using the high impedance probe here too.


 We have the data for the control point (all the data files are in /users/jenne/ALS/PDHloops/Yend_18Aug2014), but we haven't plotted it yet.

Things that need doing:

* (JCD) Think about this box's purpose in life.  What kind of gain do we need?  Do we need more / less than we're currently getting? NPRO freq noise is 1/f and is 10kHz/rtHz at 1Hz (this is from a plot of an iLIGO NPRO from Rana's thesis, but it's probably similar). Talk to Kiwamu; the noise budget in the paper seems to indicate that we had some kind of boost on or something.  Also, if we need much more gain than we already have, we'll definitely need a different box, maybe the PDH2 box that they have over in WBridge.

* (EQ, priority 1) Measure and calibrate error point noise down to lower freq for both arms.  What could we win by putting in a boost? If the residual noise is high, maybe the laser isn't good at following arm, so beatnote isn't good length info for the arm, and we can't succeed.

* (EQ, priority 2) Measure TF of PDH box, and a separate measurement of the Pomona box that is between the mixer and the error point - is that eating a bunch of phase?  It's already an LC circuit which is good, but do we really want a 120kHz lowpass when our modulation frequency is roughly 200kHz?  Ask ChrisW - he worked on one of these with Dmass.

* (EQ, priority 2ish) Measure TF of Xend PDH loop (unless you already have one, up to ~1MHz).

* (JCD) Make DCC tree leaf for PDH box #17.  Take photos of box.

  10409   Tue Aug 19 18:32:40 2014 ericqUpdateGreen LockingYarm Green PDH

Heading to dinner, going to come back for more green fun, but here's a quick update:

Xarm Peak-to-Peak of the PDH signal in the mixer output is about 70mV when GTRX was about 0.4. The sideband-generating function generator has an output of 2V (forgot to note rms or pp)

Yarm Peak-to-Peak of the PDH signal in the mixer output is about 640uV when GTRX was about 0.71. The sideband-generating function generator has an output of 0.091V (forgot to note rms or pp)

The Yarm signal thus correspondingly has a waaay noisier trace. I would've had scope plots to show here, but the scope freaked out about how large my USB drive capacity was and refused to talk to it >:|

This suggests to me that our modulation depth for the Yarm may be much too small, and may be part of our problems with it. 

  10410   Tue Aug 19 21:40:44 2014 AndresUpdateIMCNew Optical Setup for the IMC

IMC Calculation and Setup

I have been working in the calculation for improving the Gouy Phase separation between the WFSs. I tried different possible setup, but the three big constrains in choosing a good optical table setup are to have a Waist size that range from 1mm-2mm, the Gouy Phase  between the WFSs have to be greater than 75 degrees and there has to be a steering mirror before each WFS. I will be showing the best calculation because that calculation complies with Rana request of having both WFSs facing west and having the shortest beam path. I approximate the distances by measuring with a tape the distance where the current optics are located and by looking at the picture that I took I approximated the distance where the lenses will be placed. I'm using a la mode for calculating the gouy phase different. I attached a picture of the current optical table setup that we have. Using a la mode, I found that the current gouy phase that we have is 49.6750 degrees.

Now, for the new setup, a run a la mode and found a Gouy phase of 89.3728 degrees. I have to create a two independent beam path: one for the WFS1 and another one for WFS2. The reason for this is that a la mode place everything in one dimension so and since the WFS1 will have a divergence lens in order to increase the waist size, and since that lens should not be interacting with the waist size in the WFS2. We need two beam path for each WFS.  A la mode give us the following solution:

For the beam path of the WFS1

    label                z (m)           type             parameters        
    -----                  -----              ----             ----------        
    MC1                   0              flat mirror          none:           
    MC3                   0.1753     flat mirror          none:           
    MC2                   13.4587   curved mirror    ROC: 17.8700 (m)     
    Lens1                 29.3705   lens                  focalLength: 1.0201 (m)
    BS2                    29.9475   flat mirror          none:           
    First Mirror         30.0237   flat mirror          none:           
    Lens3                30.2000    lens                  focalLength: -0.100 (m)
    WFS1                30.4809    flat mirror         none: 

For the beam path of the WFS2

    label                   z (m)             type             parameters        
    -----                    -----                 ----             ----------        
    MC1                    0               flat mirror          none:           
    MC3                    0.1753      flat mirror          none:           
    MC2                    13.4587    curved mirror    ROC: 17.8700 (m)     
    Lens1                  29.3705    lens                   focalLength: 1.0201 (m)
    BS2                     29.9475    flat mirror          none:           
    Second Mirror    30.2650     flat mirror          none:           
    Lens2                 30.4809     lens                  focalLength: -0.075 (m)
    Third Mirror        30.5698     flat mirror          none:           
    WFS2                30.6968      flat mirror          none:  

I attached bellow how the new setup should look like in the second picture and also I include and attachment of the a la mode code.

 I used Mist to be able to see the read out that we get in the WFSs that take the Mode Cleaner Reflection and the QPD that take the transmitted from MC2. In the following, plots I'm misaligned the each mirrors: MC1, MC2 and MC3. The misalignment are in Yaw and Pitch. I'm dividing the WFSs reading by the total power reflect power, and I'm dividing the QPD for the MC2 transmission by the total transmitted power. In my Mist model, I have a laser of 1W and my EOM is modulated at 30MHz instead of 29.5MHz and the modulation depth was calculating by measuring the applied voltage using and Spectrum analyzer. I using Kiwamu measurement of modulation depth efficiency vs the applied voltage, https://dcc.ligo.org/DocDB/0010/G1000297/001/G1000297-v1.pdf,  I got a modulation depth of 0.6 mrad. I put this modulation depth and I got the following plots: The fourth and fifth attachment are for the current optical setup that we have. The sixth and seventh attachment is for the new optical setup. The eighth attachment is showing the mode cleaner cavity resonating. The last attachment contains the plots of WFS1 vs WFS2, MC2_QPD vs WFS1, MC2_QPD vs WFS3 for each mirror misaligned. The last two attachment are the MIST code for the calculation.

We have all the lenses that we need. I checked it last Friday and if everything is good we will be ready to do the new upgrade this coming Friday. For increasing the power, I check and we have different BS so we can just switch from the current setup the BS. Can you let me know if this setup look good or if I need to chance the setup? I would really love to do this upgrade before I leave.







  10411   Tue Aug 19 23:11:15 2014 JenneUpdateGreen LockingYarm Green PDH


 Here is a plot of last night's data with both the control and the error point on the same plot, in Volts.  Q is still working, so I don't have a calibration number yet to get these to Hz.

Note in the control spectrum that we have very significant 60Hz lines.  


EDIT:  I also added a new branch to the DCC Document Tree, and 2 leafs (one for each end).  Here's the ALS PDH servo branch: E1400350

  10412   Wed Aug 20 02:38:41 2014 JenneUpdateGreen LockingYarm Green PDH - requirement


* (JCD) Think about this box's purpose in life.  What kind of gain do we need?  Do we need more / less than we're currently getting? NPRO freq noise is 1/f and is 10kHz/rtHz at 1Hz (this is from a plot of an iLIGO NPRO from Rana's thesis, but it's probably similar). Talk to Kiwamu; the noise budget in the paper seems to indicate that we had some kind of boost on or something.  Also, if we need much more gain than we already have, we'll definitely need a different box, maybe the PDH2 box that they have over in WBridge.

It's not so impressive yet, but here's a plot that shows (a) Rana's guess for laser frequency noise, (b) The inferred in-loop version of that noise, (c) The CARM linewidth FWHM, translated to Hz.

For (b), I take the loop that Rana and I measured last night, and I assumed that it continued on forever as 1/f toward low frequency.  Then I do 1/(1+G) to get the closed loop version of the loop (which is a measurement with an artificial line tacked on the end), and multiply this with the laser freq noise, which is also totally artificial.

For (c), I do df/f = dL/L, with f = c/lambda_green, since the rest of the plot is meant to be in green frequency units.

This is my beginnings of trying to come up with a requirement for our green PDH boxes.  We weren't very clear in the MultiColor paper about the nitty-gritty details (obviously), but then Kiwamu didn't expand on those details in his thesis either.  He talks a lot more about the design considerations for the digital ALS loop, which isn't what I want today.  I will send him an email to see if he had any notes that didn't make it into his thesis.


  10413   Wed Aug 20 04:09:21 2014 ericqUpdateGreen LockingXarm Green PDH

I've made a whole bunch of measurements on the Xarm green situation.


  • GTRX was around 0.55 for all of the measurements tonight. 
  • Based on where I saw gain peaking in the CLG, it looked like UGF was 1-2kHz. I cranked the gain to 10kHz, ~20dB gain peaking followed, making it hard to measure. Currently sitting at 5kHz-ish. 
  • Measured CLG with AG4395A, calibrated for injection point response, inferred OLG. 
  • Took various PSDs, still need to calibrate into physically meaningful units. 

Reasonable amounts of time were spent bending the AG4395 to my will; i.e. figuring out the calibration things Jenne and Rana did, finding the right excitation amplitude and profile that would leave the light steadily locked, and finding the right GPIB incantation for getting spectra in PSD units instead of power units. I'm nearing completion of a newer version of AG4395 scripts that have proper units, and pseudo-log spectra (i.e. logarithmically spaced linear sweeps)

Transfer functions

Here is too many traces on one plot showing parts of the OLTF for the x green PDH. One notable omission is the PD response (note to self:check model and bandwidth). The servo oddly seems to have a notch around 100k. My calibration for the CLG injection may not have been perfect, instead of flattening out at 0dB, I had 2dB residual. I tried to correct for it after the fact, assuming that certain regions were truly flat at 0dB, but I want to revisit it to be thorough. I found some old measurements of the Innolight PZT PM response, which claims to be in rad/V, and have included that on the plot. 


In the end, the mixer and PZT response make it look like getting over 10kHz bandwidth may be tough. Even finding a good higher modulation frequency to be able to scoot the LP up would leave us with the sharp slope in the PZT phase loss, and could cause bad gain peaking. Maybe it's worth thinking about a faster way of modulating the green light?

Noise Spectra

Tomorrow morning, I'll calibrate all the noise spectra I have into real units. These include:

  • In loop error signal and control signal spectra
  • Mixer output spectrum when PD is dark, and when mixer input is terminated
  • Servo out spectrum when PD is dark, and when servo input is terminated

However, looking at the floors, it occurs to me that I may have left the attenuation on the input too high, in an effort to protect the input the PDH box, which rails all the time when not locked to a 00 mode, sometimes even with the input terminated or open. It's kind of a pain that the agilent makes it really hard to see the data when you're in V/rtHz mode, because I should've caught this while measuring :/

I used a scope to capture a pdh signal happening, which will let me transform the mixer output into cavity motion. The control signal goes to the innolight PZT with a ~1MHz/V factor. Here are the uncalibrated plots, for now. 




  10414   Wed Aug 20 15:31:27 2014 ericqUpdateCOCArm Loss Investigations Continue

 [ericq, Gabriele] 

Summary: After today's meeting, Gabriele and I looked into the arm loss situation, to see if we should really believe the losses that had been suggested by my previous measurements. We made some observations that we're not sure how to explain, and we're thinking about other ways to try and estimate the losses to corroborate previous findings. 

We first looked to see if the ASS had some effective offset, leaving the alignment not quite right. Once ASS'd, we twiddled each arm cavity mirror in pitch and yaw to see if we could achieve higher transmission. We could not, so this suggested that ASS works properly. 

We then looked at potential offsets in the Xarm loop. We found that an input offset of 25 counts increased the transmission, but only very slightly. With this offset adjusted, we confirmed the qualitative observation that locking/unlocking the xarm causes a much bigger change in ASDC than doing the same with the harm.

However, we noted that the ASDC data (which is the DC value of the AS55 RFPD) was quite noisy, hovering around 50 counts. Looking at the c1lsc model, we found that we were looking at direct ADC counts, so the signal conditioning was not so great. We went to the LSC rack and stole the SR560 that had been hooked up as a REFLDC offsetter, and used it to give ASDC a gain of 100, and a LP at 100Hz, since we only care about DC values. We then undid the gain in the input FM; and this calmed the trace down a fair bit. The effects due to each arm locking/unlocking was still consistent with previous observations. 

At this point, we looked at the arm transmission and ASDC signals simultaneously. Normally, when misaligning a cavity, one would expect the reflected power to rise and the transmission to fall.

However, we saw that when misalignment the Yarm in yaw in either direction, or the Xarm in one direction, both the IR transmission and ASDC would fall. This initially made us think of clipping effects. 

So, we checked out the AS beam situation on the AP table. On a card, the beam looks round as we could tell, and the beam spot on AS55 was nice and small. (We tweaked its steering a little bit in pitch to put it at the center of the "falling-off" points) The reflection and transmission falling effect remained. 

At this point, we're not really sure what could be causing this effect. After the reflected beams recombine at the BS, the output path is common, so it's strange that this odd effect would be the same for both arms. 

Lastly, we discussed other ways that we may be able to see if the Xarm really has ~500ppm loss. Since its transmission is ~1.4%, Gabriele estimated that we may be able to see a ~300Hz difference in the arm cavity pole frequency between the two arms, based on the modification of the cavity finesse due to loss. Since we don't currently have the AOM set up to inject intensity noise, we talked about using frequency noise injection to measure the arm cavity poles, though this would be coupled with the IMC pole, but this could hopefully be accounted for.

  10415   Wed Aug 20 16:10:43 2014 ericqUpdateGreen LockingXarm Green PDH

A MIST simulation tells me that the green pdh horn-to-horn displacement is about 1.2nm, or ~18kHz. I used this, along with the scope trace attached to the previous post, to calibrate the mixer output at 193419 Hz per V. (EDIT: I was a little too hasty here. What I'm really after is the slope of the zero crossing, which turns out to be almost exactly twice my earlier naïve estimate. See later post for correct spectra)

For the control signal, I assumed a flat Innolight PZT PM response of 1MHz/V. ( Under 10kHz, it is indeed flat, and this is the region where the control signal is above the servo output noise in yesterday's measurements)

Here are all of the same spectra from last night, with the above calibrations. 



Going off Jenne's earlier plot, it looks like the in-loop error signal RMS is ten times bigger than the CARM linewidth. 

  10416   Wed Aug 20 18:05:18 2014 JenneUpdateGreen LockingYarm Green PDH - requirement


 I calibrated the control signal from Volts to Hz using the rough PZT calibration of 5MHz/V for the Yend NPRO.  

For the error signal, Q said that the Yarm PDH peak-to-peak height was about a factor of 100 smaller than the Xarm, so I used a calibration of 1.9e7 Hz / V.

Then, from Q's Mist simulation including the high Xarm loss, and the plot that he posted in the control room, the CARM linewidth looks like it is about 2pm.  This is the number that I have included on today's plot.  Note though that yesterday I was using a linewidth of about 30pm, which I got from an Optical simulation about a year ago.  I do not know why these numbers come out an order of magnitude different!      The CARM linewidth is actually about 20 pm.  Both Q and I failed at reading log-x plots yesterday.  I have corrected this, and replotted.

Anyhow, here's the Yarm noise spectra calibrated plot:


I have emailed Kiwamu, but haven't heard back from him yet on what the original design considerations were, if he remembered us ever using a boost, etc.  What this looks like to me is that we need to do some serious work to get the noise down.  Maybe fixing the gain peaking and triggering the boost will get us most of the way there?

  10417   Wed Aug 20 21:09:16 2014 ericqUpdateGreen LockingXarm Green PDH

I remeasured all of the noise spectra again today, making sure the input attenuation was as low as it could safely be. I also got a snap of the y green PDH signal; it's fairly larger than I saw the other day, which is good. I used this to calibrate the error signal voltage spectra. 


Here are the noise traces for each arm. During these measurements GTRX was about .6, GTRY about 1.0 The Yarm noise doesn't look so good: the error signal is just barely above the mixer+lowpass output noise, and the RMS is plauged by 60Hz lines. (Is this related to what we see in IR TRY sometimes?)


Here are the arms error signals compared directly:


  10419   Thu Aug 21 15:07:48 2014 SteveUpdateVACRGA scan at day 197



  10422   Fri Aug 22 03:55:45 2014 JenneUpdateLSCXarm PDH fine, Yarm PDH/ALS needs work

[Rana, Jenne, EricQ]

We did several things tonight.  First, a list (so I can remember them all), and then some details.

(1) Jiggled ETMY SUS cables, removed kicks.

(2) Locked X and Y ALS, looked at POX, POY as out of loop sensors.

(3) Measured stuff (?) at the Yend.

(4) Reconnected REFL DC to SR560.

(5) Attempted CARM offset reduction.

Item 1:

When Rana and I started locking this evening, we saw (as Q has been witnessing for a while now) the ETMY kick a lot.  However, it seemed to be kicking even more than usual.  Since Q had been down at the end station recabling things, we wondered if a SUS-related cable got bumped.  Rana went down to the end and pushed all the cables into their receptacles.  One of the last sets that he pushed was the satellite box.  We didn't have walkie-talkie communication, but the DC offset of the ETMY oplevs changed just a minute or two before he returned to the control room.  So, we guess that it was the satellite box cables that were loose.  Unfortunately, there is no clear way to strain relieve them, which is why they can so often be troublesome.  Anyhow, the ETMY hasn't kicked since.

Item 2:

We locked the arms with ALS.  We saw that the POX signal was about 20% of the full pk-pk height of the PDH signal, so it's mostly within the linear range, but not entirely.  It is what it is, however, and we took measurements assuming that it's okay.  I calibrated POX by putting an excitation onto ETMX, and matching the height of the peak in POX and BEATX_FINE_PHASE_OUT_HZ.

Q and Rana had also [remembered / put in / something] a digital readback for the end green PDH error point.  Q went down to the end and gave me a number of 2600 Hz/V for the err mon port of the PDH board, which is what is connected to the ADC.  With that and 20/2^16 V/cts, I had a calibration of 0.8 Hz/ct. 

What we see in this plot is that the green end PDH is not the limiting noise for the POX out of loop measurement of the residual arm motion.  Also, in the multi-color metrology paper, Fig 7 (which is posted in the control room), we see at about a little over 1 Hz a ratio of about 4.5 between the residual motion and the AUX PDH error signal.  In today's plot, I see a ratio of about 20.  I infer from this that the green PDH for the Xarm is fine, and that we may want to re-look at the ALS digital loop, but we should leave the X PDH alone.

Here is the Xarm plot:


Q took the data for the Yarm plot, so hopefully he can give it to us in the morning.  What we did notice was that the noise was much worse for the Yarm.  This prompted Item 3, measuring the loop.

Item 3:

Q and Rana went down to the Yend and measured some things.  They came back, and said that they hadn't changed anything in analog while they were down there.  One thing that Q did note was that we have almost 90 degrees of phase margin (since it's a 1/f loop), and about 10 dB of gain margin, above the UGF.  So, we're in good shape for being able to try triggering the boost on the PDH box.  Q will give us more notes on this work, as well as plots, in the morning.

Item 4:

At some point, I remembered that Q and Gabriele had repurposed the SR560 that we had been using for the REFLDC input to the common mode board.  So, Q went and put it back, so that REFL DC goes into the SR560, and so does a DAC channel so that we can remotely set the offset.  The A-B output goes to the REFL11I whitening channel, since real REFL11I goes into the input of the CM board.  I think that today, the SR 560 was left at a gain of 1.

Item 5:

We decided to carry on and try to reduce the CARM offset some.  An annoyance is that the Yarm still has pretty significant low-frequency noise, but the idea is that if we can get over to the sqrtInvTrans signals, it will be fine.

So, we didn't get much farther than we had in the past, but it was nice to get there at all again.  I ran the carm_cm_up script (many times).  One of the times, all I wanted to do was see how much I could reduce the CARM offset.  CARM was on sqrtInvTrans, DARM was on ALS diff, and I was able to get the arm powers up to about 2.5.  I don't know why I lost lock.  The sqrtInv signals should be good until at least arm powers of 20 or so. 

I was able to see the REFL DC dip, but only a teensy tiny bit.  It went down by maybe 1 count.  Q suggested looking at how deep it could get while leaving CARM and DARM both on ALS, and setting both offsets to 0.  We were seeing arm flashes of about 50 counts, and REFL DC went from 0 to -800.  So, I wasn't seeing much of a REFL dip, but it was definitely there when I went to arm powers of 2ish.

We tried looking at different sqrtInv options for DARM, and haven't come to any real conclusion.  In the plot below, we are looking at a swept sine between DARM_IN1 (ALSdiff) and either MC_IN1 0.3*(sqrtInvX - sqrtInvY) or SRCL_IN1 (TRX - TRY / sqrt(TRX + TRY) ):



We have a few things to add to the to-do list:

* Put UGF servos for LSC loops in place.

* Implement UGF "servos" (per Koji's suggested method) for phase trackers.

* Write a lockloss script that is run by the ALS watch scripts - print a PDF of error and control signals for every lockloss, and save it somewhere.

* Fix up Ygreen modematching on the PSL table.  The X green spot is quite similar on the camera to the corresponding PSL green spot.  However the Y green spot is not at all the same as its PSL green spot. 


  10423   Fri Aug 22 13:51:00 2014 JenneUpdateGeneralUpdated game plan


  10427   Fri Aug 22 18:05:02 2014 AndresUpdateIMCUpgrade of the IMC WFSs for the reflection

 Upgrade of IMC Reflection Optical Setup

Nick and I upgrade the IMC. We move both WFSs and placed them facing west. When aligning the beam into the WFS, we make sure that the beam were hitting the center of the mirrors and then we placed the lenses in their corresponding position. We used the beam scanner to measure the waist and the waist in the second WFS was bigger than 1mm, and the second WFS was a little bit below than 1mm. We center the beam in the WFSs and in the PD. We did haven't measure whether we have a good Gouy Phase. Below I attached the picture of how the new setup look like.   


  10428   Mon Aug 25 09:56:21 2014 SteveUpdateIMC IMC WFSs upgrade


 The Napa earth quake magnitude 6 did not have any effect on the suspensions.

 The Goy phase upgrade was done nicely. The IOO pointing did not change. Credit owned to Nick and Andres.

  IFO is locked right on.

  10429   Mon Aug 25 15:49:44 2014 SteveUpdatePSLHEPA turned on

The PSL HEPA was off. It was turned on and it is running at 30VAC now.

  10431   Tue Aug 26 23:46:55 2014 ericqUpdateIMCWFS tuneup

 I decided to see what I could do with the new WFS setup. 

First, I adjusted the WFS digital demod angles. Once I ensured that the static MC alignment and DC alignment onto the WFS was good, I drove MC2 in pitch with the WFS output off. I then did the usual thing of making the Q peak at the excitation frequency go away. Here are the changes:

  • WFS1 Q1: 7 -> -24 (-31)
  • WFS1 Q2: 6.5 -> -9.5 (-16)
  • WFS1 Q3: -6.5 -> -26.5 (-20)
  • WFS1 Q4: 47 -> 30 (-17)
  • WFS2 Q1: -51 -> -39 (-12)
  • WFS2 Q2: -39 -> -21 (-18)
  • WFS2 Q3: -32 -> -20 (-12)
  • WFS2 Q4: -120 -> -108 (-12)

I then drove each MC mirror in pitch and yaw respectively, and measured the TF from excitation to the WFS signal (dB Magnitude, sign):


Mirror DoF WFS1 Pitch WFS1 Yaw WFS2 Pitch WFS2 Yaw
MC1 Pit -67.7,+ -81.9,+ -58.9,- -83.7,+
  Yaw -82.5,- -48.7,- -83.7,+ -112.3,-
MC2 Pit -50.4,- -77.1,- -54.2,- -67.9,+
  Yaw -82.1,- -52.9,+ -59.6,- -44.0,-
MC3 Pit -59.7,- -97.3,+ -62.0,+ -83.9,-
  Yaw -78.0,+ -52.9,+ -67.3,+ -51.4,+


I looked through some old ELOG's of Suresh's and used similar logic to scripts/MC/WFS/wfsmatrix2.m to generate a new output matrix. (This involves creating a null sensing vector that is orthogonal to the measured ones, and inverting that matrix) 


MC1 -1 0.044 0   MC1 -1 -0.294 0
MC2 0.19 1 1   MC2 -0.26 -0.045 -1
MC3 0.5 -0.681 0   MC3 -.9 1 0




MC1 0.835 -1 0   MC1 -1 -0.229 0
MC2 -0.948 -0.433 1   MC2 0.317 -1 -1
MC3 -1 0.865 0   MC3 0.743 0.628 0


I had to flip a gain or two to keep things stable, then measured the WFS error signal spectra to see if this made anything better. The WFS1 spectra look better, but WFS2 not so much. 


The loops would need a more thorough investigation, but for now, they're at least a little calmer. The MC is stabler than immediately after the upgrade, but there's still room for improvement. 


  10432   Wed Aug 27 09:12:47 2014 KojiUpdateIMCWFS tuneup

I'm sure that the 1~3Hz motion comes from the mirror motion, but not 100% sure what is causing
the broad stochastic noise. If this is the beam jitter, this penetrates to the IFO via the WFS servos.
Is there any way to characterize this noise in order to compare it with the actual (estimated) motion of the mirrors?

  10433   Wed Aug 27 18:03:47 2014 ericqUpdateGreen LockinguPDH Box Checkup

 Quick post of plots and data; I'll fill in more detail tonight. 

TL;DR: I pulled both green PDH boxes and made LISO models, compared TFs and noise levels. 

Pictures of X and Y boards, respectively



TF comparison to LISO. (Normalized to coincide at 1Hz)



Noise comparison to LISO



To Do:

  • Figure out why TFs were made differently. Check PM response curves of PZTs to see if they are fine, or need tweaking.
  • Make boosts useful. Both are currently integrators with corners under 10Hz, which is already pretty suppressed. 
  • I just noticed that the X board is missing C25, which should be a 1uF cap on the positive power pin of the primary TF stage opamp. This should be inserted. 

All data, EAGLE schematics, LISO source and plots in the attached zip. 


  10434   Thu Aug 28 01:41:03 2014 ericqUpdateLSCPhase Tracker UGF normalization

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.  

  10435   Thu Aug 28 08:31:16 2014 SteveUpdateGeneralone good day
  10436   Thu Aug 28 11:02:53 2014 SteveUpdateCalibration-RepairSR785 repair

SN 46,795 of 2003 is back.

  10437   Thu Aug 28 17:34:20 2014 ericqUpdateGreen LockinguPDH Box Checkup

I had noticed in the past, that the digital control signal monitor for the X end would saturate well before the ADC should saturate (C1:ALS-X_SLOW_SERVO_IN1, which is from the "output mon" BNC on the box). It turns out that there is some odd saturation happening inside the box itself.

In this scope trace, the servo input is being driven with a 0.02Vpp, 0.1Hz sine wave, gain knob at 1.0. This is bad. 


Evan and I poked around the board, and discover that for some reason currently unknown to us, the variable gain amplifier (AD8336) can't reach its negative rail, despite the +-12V arriving safely at its power supply pins. 

I also realized that the LF356 in the integrator stage in this box had been replaced with a LT1792 by Kiwamu in ELOG 4373. I've updated my schematic, and will upload both boxes' schematics to the DCC page Jenne created for them. (D1400293 and D1400294)

  10438   Fri Aug 29 17:28:07 2014 ranaUpdateGreen LockinguPDH Box Checkup


 I've been having trouble locking the X - green for the past few hours. Has there been some configuration change down there that anyone knows about?

I'm thinking that perhaps I need to replace the SHG crystal or perhaps remove the PZT alignment mirrors perhaps. Another possibility is that the NPRO down there is going bad. I'll start swapping the Y-end NPRO for the X-end one and see if that makes things better.

  10439   Fri Aug 29 23:53:35 2014 ericqUpdateGreen LockinguPDH Box Checkup

I had pulled out both X and Y servo boxes for inspection, put the Y box back, soldered in a missing op amp power capacitor on the X end box, and had not yet put back the X end box yet because of the saturation issue I was looking into. Otherwise nothing was changed at the ends; I didn't open the tables at all, or touch laser/SHG settings, just unplugged the servo boxes. 

  10440   Tue Sep 2 16:22:24 2014 JenneUpdateGeneralGame plan: 2 Sept

Slightly updated Game Plan.  Mostly, Q is continuing to check out the Xend PDH box saturation, and I am thinking on what our requirements are for ALS, and thus for the green PDH boxes.


  10441   Tue Sep 2 20:02:06 2014 ericqUpdateGreen LockinguPDH Box Checkup

I narrowed down the saturation point in the X green PDH box to the preamp inside the AD8336, but there is still no clear answer as to why it's happening. 

As per Jenne's request, I put the X end PDH box back for tonight's work. It locks, but we have an artificially low actuation range. With SR785, I confirmed a PDH UGF around 5k. Higher than that, and I couldn't reliably measure the UGF due to SR560 saturations. The analyzer is not currently in the loop. 

Both arms lock to green, but I haven't looked at beatnotes today. 

  10443   Wed Sep 3 00:17:22 2014 ranaUpdateGreen LockinguPDH Box Checkup

What monitor point is being plotted here? Or is it a scope probe output?

If this saturation is in the uPDH-X but not in the uPDH-Y, then just replace the VGA chip. Because these things have fixed attenuation inside, they often can't go the rails even when the chip is new.

In any case, we need to make a fix to get this box on the air in a fixed state before tomorrow evening.


I had noticed in the past, that the digital control signal monitor for the X end would saturate well before the ADC should saturate (C1:ALS-X_SLOW_SERVO_IN1, which is from the "output mon" BNC on the box). It turns out that there is some odd saturation happening inside the box itself.

In this scope trace, the servo input is being driven with a 0.02Vpp, 0.1Hz sine wave, gain knob at 1.0. This is bad. 


Evan and I poked around the board, and discover that for some reason currently unknown to us, the variable gain amplifier (AD8336) can't reach its negative rail, despite the +-12V arriving safely at its power supply pins. 

I also realized that the LF356 in the integrator stage in this box had been replaced with a LT1792 by Kiwamu in ELOG 4373. I've updated my schematic, and will upload both boxes' schematics to the DCC page Jenne created for them. (D1400293 and D1400294)


  10444   Wed Sep 3 04:17:21 2014 JenneUpdateLSCY green ALS (not PDH) needs investigation

Q put the X PDH box back, so that I could try locking, and remember which end is up after a week away.

I am unable to hold ALS comm/diff for any length of time. Only once today did I hold it through the FM3 boost turn-on.  So, I looked at the individual arms.

Xarm, even though it's the one that Q is seeing this saturation problem with, seems fine. 

Yarm however is having trouble holding lock for more than a few minutes at a time.  The green beam stays locked to the arm for ~infinity, so I'm not so worried about the PDH box right now.  If I look at the error and control points of the ALS digital servo, the Yarm is much more noisy above about 20 Hz.  Something that I might think of for this kind of mismatch at higher frequencies is poorly matched whitening / dewhitening, or none at all for the Yarm, however this doesn't look like that to me.  Based on the shape of the spectra, I don't think that we're running into ADC noise. For this plot, both arms are individually locked with ALS feeding back to the ETM, gain magnitude of 15 (Xarm gets a minus sign because of our temperature / beatnote moving direction convention), FMs 1,2,3,5,6 on.  Something that seems critical for getting the Yarm to have the FM3 boost without losing lock is having the SLOW temperature servos on for a little while so that the PZT output (as monitored on the temp servo screen) for the end lasers fluctuate around zero. Right now, both beatnotes are at about 62MHz, with an amplitude of about -31dBm.


I still need to do a somewhat more thorough investigation of what might be causing the Yarm locklosses.  Is the length-to-angle decoupling worse for ETMY than for ETMX?  Am I moving the arm length so far that the PZT can't follow within its actuation limits?  Does the Yend PDH box have a similar saturation to the Xend box, but somehow (a) worse, and (b) not as obvious so we didn't suspect it before? 

I need to put this plot into calibrated units, and also include the low frequency monitor that we have of the PDH error point (all of which are _DQ channels).

Things to do:

* Figure out Xend PDH box saturation issue.  Is Yend seeing same saturation in the variable gain amplifier?  We have 3 spares of these chips in the Plateau Tournant Bleu, if we need them. 

* Check Yarm ALS stability.  (NB:  The arms have been individually locked for the last 15 min or so while I've been writing, so maybe letting the slow servo settle is the key, and this is not something that needs work).

* Get CARM on DC Trans, DARM on AS55Q (after arm powers of about 1).  Can we see good REFL DC dip?  Should we try using just the transmission PD signal as the error signal for the CM board, if we aren't close enough to resonance to use REFL DC?

  10446   Wed Sep 3 18:42:43 2014 JenneUpdateLSCGreen PDH box boosts

From EricQ's simulations reported in elog 10390, we want to transition from ALS comm to DC transmission signals around 500 pm.  However, around 100 pm, the DC transmission signals have a sign flip, so we don't want to have the ALS swing that close to the CARM resonance.  So.  We want to be at about 500 pm, and not touch 100 pm.  So, we don't want our peak ALS motion to go beyond ~400 pm.  Which means that we need to have less than about 40 pm in-loop RMS, to avoid hitting 400 pm.  This is an ALS requirement, but since the analog PDH box is what forces the end laser to follow the arm cavity, and thus give us information about the arm length fluctuations, the PDH residual noise is part of our sensor noise for the full ALS.  So, we need to have the PDH in-loop RMS be less than 40 pm, integrated from a few kHz down to at least 30 mHz. Recall that above the ALS UGF (of about 200 Hz), the sensor noise will be suppressed by 1/f, so we should take that into account when we are looking at the PDH error signal, before we calculate the RMS motion.

Q also measured the in-loop error signal with the current Yend PDH box in elog 10430, and it looks like most of the RMS is coming from a few hundred Hz.  I designed a hack to the PDH board boost that has a zero at about 2kHz, and a gain of 30 at DC, so that we will win by squishing all that RMS.  Also, it shouldn't be too aggressive, so we should be able to leave it on all the time, and still acquire lock of the green laser to the arm, without having to do triggering.

The board schematic is at DCC D1400294.  The boost is also called the "integrator stage", although it will no longer be a simple integrator.

EDIT, JCD:  This cartoon is not correct for the non-boosted state, doesn't include effect of R16.


  10447   Wed Sep 3 20:38:40 2014 ericqUpdateGreen LockinguPDH Box Checkup

The traces were from the front panel output BNCs, but the VGA preamp exhibited this asymmetric saturation at its output.  

In any case, I tried to replace the Xend box's AD8336 with a new one, and in doing so, did some irreparable damage to the traces on the board  I was not able to get a new AD8336 into the board. There are some ATF ELOGs where Zach found the AD8336 noise to be bad at low frequencies (link), and its form factor is totally unsuitable for any design that may involve hand modification, since it doesn't even have legs, just tiny little pads. I suggest we never use it for anything in the future. 

Instead, I've hacked on a little daughter board with an OP27 as an inverting op-amp with the gain resistor on the front panel as its feedback resistor, which can swing from 0 to x20 gain (the old gain setting was around 15dB=~x6). I've checked out the TF and output noise, and they look ok. The board can output both rails as well. 

I don't really like this as a long term solution, but I didn't want to leave things in a totally broken state when I left for dinner. 

  10448   Thu Sep 4 00:56:44 2014 JenneUpdateLSCGreen PDH box boosts

Okay, went back to the drawing board with Rana and Koji on PDH box stuff.

Currently (at least for the Yend), in the boost OFF state, we have an overall gain of about 50.  This is crazy big.  Also, the zero in the "transfer function stage" is around 1kHz, however our green cavity pole is (calculated) to be around 20 kHz.  Since these are supposed to cancel but they're not, we have a wide weird flat region in our loop TF.

So.  I calculated the changes to the TF stage that I'll need so that I have an increase of about 20 in DC gain, kept the pole at the same ~20Hz, but moved the zero way out to 18kHz.  I also calculated the changes needed for the integrator stage to make it effective at much higher frequency than it was designed for.  Now the pole is at 75 Hz, and the zero will be at 1.6kHz, and the high frequency gain will stay pretty close to the same with and without the boost.

Planned new TF stage:


Planned boost stage (with and without boost activated):


New boost stage only, so you can see the phase:


The schematic, modified to show my planned changes (which I will put in the DCC after I make the changes):


  10449   Thu Sep 4 01:28:32 2014 ericqUpdateLSCRecycling cavity lengths

 Going off some discussion we had at lunch today, here is my current knowledge of the state of cavity lengths. 

Acknowledging that Koji changed the sideband modulation frequency recently, the ideal cavity lengths are (to the nearest mm):

  • Lprc = c / ( 4 * fmod) = 6.773 m
  • Lsrc = c / ( 5 * fmod) = 5.418 m

We when last hand measured distances, after moving PR2, we found:

  • Lprc  = 6.752 m = 2.1 cm short
  • Lsrc  = 5.474 m = 5.6 cm long. 

However, when I looked at the sideband splitting interferometrically, I found:

  • Lprc = 6.759m = 1.4 cm short

This is only 5mm from the hand measured value, so we can believe that the SRC length is between 5 and 6 cm too long. I'm building a MIST model to try and see what this may entail. 

  10450   Thu Sep 4 03:12:55 2014 ericqUpdateLSCGreen PDH box boosts

Jenne made her board modifications, and the measured TF agreed with the design. Alas, the green would not lock to the arm in this state. 

I think that the reason is that the new TF does not have nearly as much low frequency gain as the old one, for a given UGF. Thus, for example, the 1Hz noise due to the pendulum resonance, has 30dB less loop gain suppressing it. 



NEED MORE gain.jpg


  10451   Thu Sep 4 10:10:23 2014 KojiUpdateLSCRecycling cavity lengths

Com'on. This is just a 60ppm change of the mod frequency from the nominal. How can it change the recycling cav length by more than a cm?


This describes how the desirable recycling cavity lengths are affected by the phase of the sidebands at non-resonant reflection of the arms.

If we believe these numbers, L_PRC = 6.7538 [m] and L_SRC = 5.39915 [m].

Compare them with the measured numbers

  • Lprc = 6.752 m
  • Lsrc  = 5.474 m

You should definitely run MIST to see what is the optimal length of the RCs, and what is the effect of the given length deviations.

  10452   Thu Sep 4 16:45:10 2014 JenneUpdateLSCGreen PDH box boosts

As EricQ mentioned in last night's elog, the modifications were made to the Yend (SN 17) uPDH board.

R31 became 49.9 Ohms, R30 became 45.3kOhm, R24 became 1.02k, R16 became 1k, a new flying resistor is tombstoned up against R24 and connected by purple wire to C6 and it is 20k.  C28 is 183nF and C6 is 100nF.  These numbers were used in Q's simulation last night.




  10453   Thu Sep 4 18:16:20 2014 ericqUpdateLSCRecycling cavity lengths

Koji correctly points out that I naïvely overlooked various factors. With a similar analysis to the wiki page, I get:

  • Ideal arm length of 37.795 m
  • Ideal PRC length of 6.753 m
  • Ideal SRC length of 5.399 m

This means that:

  • The PRC, measured at 6.759m, is 6mm long. 
  • The SRC, measured at 5.474m, is 7.5 cm long

Next step is to see how this may affect our ability to sense, and thereby control, the SRC when the arms are going. 

MIST simulations and plots are in the attached zip. 

ELOG V3.1.3-