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ID Date Author Type Categorydown Subject
  9407   Tue Nov 19 01:11:19 2013 JenneUpdateLSCGreen status

I am able to lock the Yarm ALS, but not at the full gain that I should be.  I attribute this to my mediocre alignment of the path on the PSL table.  EDIT: Manasa pointed out that I forgot to set the PSL FSS slow adjust to ~zero, so the PSL temperature was off, so there wasn't really any hope for me last night.

However, I decided that I should write down the ALS locking procedure, as shown to me by Masayuki on 29Oct2013, that is written in one of the Control Room notebooks.  So, here it is.  I will write channel names and DTT template names for the Y arm, but the procedure is the same for both arms.

  1. Lock and align arms using IR.
  2. Lock green beams to arms.
  3. Align green beams to arms.
  4. Check beatnote alignment on PSL table.
  5. Find beatnote by changing end laser temperature (C1:ALS-Y_SLOW_SERVO2_OFFSET) in steps of ~30, watch spectrum analyser for peak.  Easier if arms are locked in IR, but disable LSC system before moving to step 6.  Beatnote should be less than ~50 MHz, and should have a peak height of about -20dBm or more.  When doing 2 arms, be careful that beatnotes of the different arms do not overlap in frequency.  Manasa reminds me that you must also remember to set the PSL FSS SLOW actuator adjust to near zero, to get the PSL back near its nominal temperature.
  6. Check UGF of phase tracker loop.  (DTT template in /users/Templates/ALS/YALS_PT_OLTF.xml) Want UGF to be ~2kHz.  Change C1:ALS0BEATY_FINE_PHASE_GAIN as necessary.
  7. Start the watch script from the ALS screen to watch for lockloss.
  8. Look at the PHASE_OUT spectrum (DTT template in /users/Templates/ALS/ALS.xml).
  9. Clear history of Phase Tracker Loop (clear hist button on C1:ALS-BEATY-FINE_PHASE screen).  Very important to do this before step 10, every time you get to step 10  (i.e. if you lose lock and are starting over)!
  10. Check sign of loop gain by using + or - 0.1 for the gain (C1:ALS-YARM_GAIN).  Beatnote should immediately stop moving if you have the sign right.  Otherwise, it'll zip around (if it does, repeat step 9, step 10).
  11. Turn gain of ALS up to ~15.  Watch the PHASE_OUT spectrum, look for the servo bump.  When you see it, back off the gain a little.  Gain of ~15 is usually about the right ballpark.
  12. Turn on FM 2, 3, 6, 7, 8 of C1:ALS-YARM.  (FM5 should already have been on).
  13. Wait for PHASE_OUT spectrum to come down.  Turn on FM10 of C1:ALS-YARM.
  14. Check UGF of ALS loop (DTT template in /users/Templates/ALS/YALS_OLTF.xml).  Want UGF to be about 150 or 170 Hz (at the peak of the phase bubble).  Adjust C1:ALS-YARM_GAIN as necessary.
  15. ALS is locked!  Use something like the "Scan Arm" script from the ALS screen to find IR resonance, or do whatever measurement you want.  Dataviewer template /users/Templates/Dataviewer_Templates/ALSdtv.xml may be useful.
  9409   Tue Nov 19 11:56:10 2013 manasa, jenne, ericQUpdateLSCPRM- OSEM side ccd camera is in place



Can't we somehow hook up this camera to the MUX with the movie mode?
I think both the MUX and the sensoray are compatible with the color video signal.
Only the old CRT is B/W.

 Watek ccd with Tamron lens is hooked up to MUX

This set up close to the viewport glass! Please be careful!

 Video captures when power recycling cavity is locked (videos 1 & 2) and flashing (video 3). Arms stayed misaligned.

1. CH1 and CH2 are loooking at PRM front and back faces. CH3 and CH4 are looking at POP and REFL

 2. CH1 and CH2 are loooking at PRM front and back faces. CH3 and CH4 are looking at the ITMs

3. CH1 and CH2 are loooking at PRM front and back faces. CH3 and CH4 are looking at POP and REFL

  9410   Tue Nov 19 14:47:44 2013 JenneUpdateLSCPRM oplev measured and modeled TF


I forgot how we could turn on the PRM oplev servo and the PRM ASC servo at the same time without conflict.
It seems that this new oplev servo covers 0.04 to 8Hz. It's pretty broadband. Do we inject the ASC signal to the oplev error?

 Right now all 3 servos that control PRM angle (OSEM damping, Oplev, and ASC) run in parallel, and they're all AC coupled. 

  9411   Tue Nov 19 14:47:59 2013 manasaUpdateLSCGreen status


After I aligned the IR interferometer (no ASS - we still need to figure out what's going on with that), I am trying to find the green beatnotes for each arm.

First, I locked the green lasers to each arm.

I then went out to the PSL table and aligned the Green Yarm path by overlapping the near-field and far-field of the yarm transmission and the PSL green pickoff.  I then turned on the power for the Beat PDs, since it was off (I confirmed that the outputs were plugged into the beatbox, so they are seeing 50 ohms).  I assume that the beat PDs were off since Manasa pulled the Beatbox last week, but there is no elog reference!!  Anyhow, after seeing a real signal, I maximized the DC power on the beat PD for the Yarm.  I then maximized the light on the DC transmission PD for the Yarm.

I looked at the Xarm, and the near-field alignment looks okay, but I haven't checked the far-field.

I started looking for the beatnotes from the control room:

I am changing the SLOW_SERVO2_OFFSETs by 30 counts, and then unlocking and relocking the arms, and checking to see if I see a peak on the RF spectrum analyser. 

The Y offset started at -10320, and I found a beatnote at -11230 (beatnote is about 26MHz).  The X offset started at 4500.  Going larger seemed to get me to a less bright TEM00 mode, so I switched and have been searching by going down in offset, but haven't yet found the beatnote.  I suspect that I actually need to align the X path on the PSL table.  The Y beatnote is very small, about -30dBm, so I also need to tweak the alignment by maximizing the peak value.

 I found the beatnotes for both the X and Y arm ALS this morning. The beat amplitudes measured -5dBm and -18dBm respectively and occurred at SLOW SERVO2 OFFSET 4550 and -10340. I had to only tweak the Y green PSL alignment to increase the beat amplitude.

I locked both the arms using ALS and they were stably locked until MC unlocked for a moment (nearly 16 minutes).

The only thing missing in the list of things you looked into is the status of the PSL slow actuator adjust. Check if this is near zero.

  9413   Tue Nov 19 17:47:17 2013 JenneUpdateLSCPRMI+2arms attempt

So far this afternoon, I have redone the IFO alignment, locked both arms with ALS, moved both arms off resonance, locked PRMI, and started bringing one arm back to resonance. 

The alignment was really not good, which I knew yesterday, but the ASS wasn't working yesterday.  I hand-did the alignment, and tried locking, which was easier with the slightly better alignment.

I locked both arms with ALS, found the resonances, and then moved them off resonance using Masayuki's scripts.

I then restored the PRM alignment, and locked the PRMI. 

I started bringing the Yarm back, but I kept losing lock when I got to about 0.1 transmission.

After losing lock several times, I switched over to looking at the ASS. I have figured out the problem, and fixed it.  The ASS for the arms now works again.

Looking at the StripTool plots of the lockin outputs for each arm, it was clear that the "L" traces were their usual size, but the "T" traces, which are demodulated versions of the transmission DC PDs, were tiny.  I investigated in the model, and the answer is obvious:  both the LSC and the ASS get the transmission information directly from the end sus computers.  Since we recently moved the normalization gain for the transmission diodes into the SUS models from the LSC model, this means that the ASS was seeing a differently sized signal than it had in the past. 

To fix this, I put a gain into the T_DEMOD_SIG filter banks for all 8 lockins that use info from the transmission DC PDs.  I used 1/g , where g is the gain that is in the C1:SUS-ETM#_TR#_GAIN channels.  For TRX, that number is -0.003, and for TRY that number is 0.002 .  So, in the .snap file that is used when turning on the ASS, I have given the Xarm lockins a gain of -333, and the Yarm lockins a gain of 500.  I chose this place, because the only thing that has happened to the signal until this point is a bandpass, so the rest of the servo gains can remain the same. 

I tested the ASS, and it works just like it used to.  I let it run, and align all of the optics, then I misaligned by a small amount each of the ETMs, saw that the lockin output values changed, and then were servoed back to zero.  So, it seems all good. 

  9414   Tue Nov 19 21:28:05 2013 manasaUpdateLSCPRMI carrier locking

Since we have never tried to lock PRMI on carrier after the folding mirrors were flipped, I tried to lock PRCL on carrier.

I thought this might give us some idea about the PRC stability for resonance or some clue as to what happens to the PRM suspensions and PRMI stability when we have carrier resonating in the cavity.

I changed the sign of the PRCL gain and also tried increasing the gain. But this did not work and I was not able to carrier lock PRMI. May be I am missing to change some parameter that is very trivial?

  9415   Tue Nov 19 21:59:35 2013 manasaUpdateLSCPRMI carrier locking


Since we have never tried to lock PRMI on carrier after the folding mirrors were flipped, I tried to lock PRCL on carrier.

I thought this might give us some idea about the PRC stability for resonance or some clue as to what happens to the PRM suspensions and PRMI stability when we have carrier resonating in the cavity.

I changed the sign of the PRCL gain and also tried increasing the gain. But this did not work and I was not able to carrier lock PRMI. May be I am missing to change some parameter that is very trivial?

PRMI could not be locked on carrier using 3f. The configuration from the last time when PRMI was carrier locked (elog) were used and PRMI locked on carrier with these settings.

== PRMI carrier ==
  MICH: AS55_Q_ERR, AS55_PHASE_R = -12 deg,  MICH_GAIN = -0.2, feedback to ITMX(-1),ITMY(+1)
  PRCL: REFL55_I_ERR, REFL55_PHASE_R = 70 deg, PRCL_GAIN = 1.0, feedback to PRM

Below is the video capture showing the PRM front and back face when carrier flashes with few second locks.


The demod phase numbers that Manasa is quoting above were correct back in March, when the elog she's quoting from was written.  They are not true now, since we've adjusted things in the last 8 months.  Also, I'm using a gain of -1.5 for MICH, and +1.5 for PRCL.  MICH has no FMs triggered, PRCL has FM 2,3,6 triggered.  Since we won't be using this configuration for full locking, but just for some tests, I'm currently using AS55 Q for MICH, and REFL 55I for PRCL, and using the ITMs to actuate on MICH for today.

  9416   Wed Nov 20 01:10:38 2013 JenneUpdateLSCPRMI carrier locking

I have increased the gain of the MICH loop to -100, and set FMs 2,3,7 to be triggered.  I have also increased the PRCL gain to 2.  The PRCL ASC pitch and yaw gains used to be -0.004, but I have increased them both to -0.01.

Now, I'm seeing power fluctuations in POPDC of ~200 pk-pk, at an average value of 2650.  That's a RIN of 7.5% .  If I turn off all OSEM damping for the PRM (after the cavities are already locked), I get POP DC fluctuations of 100 pk-pk at the same average value, so a RIN of 4%. 

Back on October 30th (elog 9338), we had an average POPDC of 400, with fluctuations of 200 pk-pk, so a RIN of 50%. 

So, I am pleased that, with the carrier locking, I have lower power fluctuations.  And, since there is more overall power in the PRC right now than we had 3 weeks ago, I'm hopeful that a PRMI+arms test will have lower power fluctuation.

Also, a note, when my MICH gain was still low, I had lots of power fluctuation at the AS port, which was coherent with my POPDC power fluctuations (which makes sense).  At that time, my overall RIN was higher than it is now (although I neglected to write down the numbers), but more significantly, I saw occasional 'kicks', where the ASDC and POPDC powers would ring for 1 or 2 seconds, with power fluctuations of order 40%.  I have not seen any of those kicks since increasing the MICH gain.

  9417   Wed Nov 20 16:05:52 2013 JenneUpdateLSCPRMI carrier locking, OpLev Check

[Jenne, EricQ]

We locked the PRMI on carrier again today, after lunch.  Following a suggestion from the 40m meeting, we wanted to compare the PRMI carrier fluctuations with the new vs. old OpLev servo for the PRM. 

To do change between the servo shapes, I put in an elliptic lowpass at 35Hz, since I overwrote that with the 55Hz lowpass the other day.  The only other change between shapes is turning on and off my boost / emphasis filter. 

So, the scenarios were:

(1) New OpLev servo

(2) Old OpLev servo (no boost, but 3.2Hz res gain and bounce roll notches on), with 55Hz lowpass

(3) Old OpLev servo with 35Hz lowpass

For scenario (1), like last night, there were small power fluctuations.  For scenario (2), most of the time there were small power fluctuations, but occasionally there would be a kick somewhere, and the power would dip down by ~50%, and the fluctuations would continue like a ringdown for a few seconds, and then we'd be back to small fluctuations until the next kick.  For scenario (3), even with trying different LSC servo gains, we could not get the PRMI to lock on carrier for more than a few tenths of a second.  During that time, the power fluctuations were very large. 

So, the old oplev servo was kind of okay, but the lowpass at 35 Hz was bad, bad, bad.  It seems that the new OpLev servo is doing good things for us.

  9418   Wed Nov 20 17:05:15 2013 JenneUpdateLSCXend QPD and Whitening board replaced

[EricQ, Jenne]

We have put the Xend QPD back in place, and centered it.  The whitening board was replaced by me a few days ago.

We also went down to the Yend and centered the Yend QPD.

I used the offset.py script that Masayuki wrote to zero the offsets of the individual quadrants when the PSL shutter was closed, and then I averaged the output of the SUM filter banks, and made the gains 1/AvgSum, so that both the Thorlabs PD and the QPD are normalized to 1 at single-arm resonance, for each arm.

I don't know what the gain is of the QPD head off the top of my head, relative to the Thorlabs PD, but eventually we want them to be the same, so that 1=1 and 700=700 on each PD.

  9430   Wed Nov 27 18:31:26 2013 KojiSummaryLSCAdittion of the ALS error signals to the LSC input matrix

The Phase tracker outputs (= ALS X/Y error signals) are now conveyed to the LSC model.

Their entry points at the LSC model are C1:LSC-ALSX_IN1 and C1:LSC-ALSY_IN1.
They are connected to the signal matrix (28th and 29th signals) via signal conditioning filters (C1:LSC-ALSX and C1:LSC-ALSY).

The main LSC screen has not been updated. The conventional ALS servos are still remains as they were.

This renovation required the recompilation of c1als, c1rfm, and c1lsc. Two PCIe-RFM bridge paths were added resulting in
increase of the c1rfm timing budget from 38 to 44.

  9439   Wed Dec 4 14:16:42 2013 JenneUpdateLSCPRFPMI flashes on transmission QPDs

2 weeks ago I took some data, and remembered today at the 40m meeting that I hadn't posted it.  Bad grad student.

All I'm trying to show here is that we see flashes in the arms that are larger than the ~50 units that we see saturate the Thorlabs transmission PDs. For arm power values below ~50, the QPD sum and Thorlabs PDs give approximately the same values.  So, 1 unit on the Thorlabs PDs is equivalent to 1 unit on the QPD sum, and 50 units on the Thorlabs diode is equivalent to 50 units on the QPD sum.

The situation was arms held on resonance with ALS, and the PRMI was flashing.


Arm powers of ~140 imply a power recycling gain of ~7.

  9440   Wed Dec 4 15:43:13 2013 JenneSummaryLSCPut LSC DAQ channels back

Last week, Koji cleaned up the LSC model to make it much more readable, while he was working on piping the ALS signals to the LSC model.  However, somehow the DAQ Channels block got deleted before the model was committed to the svn.  Since there were 2 months between svn checkins for c1lsc.mdl, it's possible that someone had the model open just to look at, and the block got deleted, and that's the version that Koji started with. 

Anyhow, thankfully we have the svn, so Koji and I found that the DAQ Channels block was (as expected) in the previously checked-in version of the LSC model.  I put a copy of the old model onto my desktop, opened it up, copied the DAQ Channels block, and then pasted it into the new cleaned-up version of the model.  (Jamie - is there a way to conveniently download a previous version through the web interface?)

I have checked it in, compiled and restarted the lsc model.  The _DQ channels are back now.

  9449   Fri Dec 6 21:38:27 2013 KojiUpdateLSCCDS related activities for LSC

I worked on the CDS related stuffs for LSC yesterday and today.

1. Slow machines:

I checked the database files for c1iscaux and c1iscaux2 (slow machines). They are mainly
used for the control of LSC whitening filters. The channel names were totally random as we
reconfigured the RF PDs while the channel names had been unchanged.

- Now the database was modified so that the PD name and the channels are related.
- saverestore.req and autoBurt.req were also changed accordingly.

- PD interface channels are completely random. Don't use them.
- I found the whitening of DCPDs are not effective.

- We need to clean up /cvs/cds/caltech/target directory. The autoBurt requests in the old targets
are making unnecessary burt files.

2. LSC screens

- The channel names on the LSC OVERVIEW screen was modified. (Attachment 1)
- A new LSC Whitening screen was made. (Attachment 2)

3. LSC screen generator

To touch the main LSC screen is very tough. The screen was split in to several sub screens
and combined with a command.


This command combines the multiple adl files into a single file with x&y offsets.
This way, you can work with the each section of the screen.
Also, moving the blocks are just easy.

4. LSC Code Bug?

During the screen making, I found that a couple of the whitening switches are not
working properly.
e.g. When AS165 (either I or Q) FM1 is activated throught the whitening trigger,
the MSB bit (bit15) of the binary I/O (C1:LSC-BIO_0_0) does not .

SImilarly ASDC FM1 does not toggle bit15 of C1:LSC-BIO_0_1.

The other channels seems OK.

At first, I thought this is a bug of "Bit2Word" block. But an individual test of the block showed that
the block is not guilty. So why is only Bit15 malfunctioning???


Attachment 1: LSC1.png
Attachment 2: LSC2.png
  9456   Thu Dec 12 00:47:45 2013 DenUpdateLSClocking activity

Jenne, Den

Today we worked on PRM angular servos and Y-arm ALS stabilization.

In the current PRMI angular control configuration two servos simultaneously drive PRM - oplev and POP ASC. We considered 2 ways to redesign this topology:

  • once lock is acquired, turn on POP ASC servo that corrects oplev error signal
  • turn off PRM oplev and turn on POP ASC  servo

The first option requires model rewiring so we started from the second one. We had to redesign POP ASC pitch and yaw servos for this because PRM TF has changed. Attached is servo OLTF.

This method worked out well and once PRMI is locked we turned off oplev servo with ramp of 0.5 sec and enable ASC POP servo with ramp of 1 sec.

Once PRMI was locked and ASC running we have turned off PRM angular local damping that presumably prevents us from bringing arms into resonance due to IR coupling to shadow sensors.

PRMI was stable using only ASC POP servo and we moved on to ALS. We found Y-arm beatnote and enabled control to ETMY.

Cavity was stabilized but not robust - we were loosing IR in a minute because green relocked to 01 mode with transmission equal to more than half of 00 mode. This is probably due to angle to length coupling of ETMY.

We were also loosing IMC during cavity stabilization. We made MCL servo and will tune it tomorrow looking at the arm spectrum as an OOL sensor.

Attachment 1: POP_ASC.pdf
  9462   Fri Dec 13 02:19:57 2013 JenneUpdateLSClocking activity

[Jenne, Den]

Tonight we worked on tweaking up the PRCL new ASC, and then PRMI+1 arm locking.  We were unable to get the Xarm to stay locked on a TEM00 mode for very long, and after an hour or two of using the PZTs to try to align the beam to the cavity, we gave up and just used Yarm green.

NB: We haven't done anything to MCL, although it is not in use.  Den is still going to get around to elogging what servo shaping he changed on that last night.

I wrote a script that will handle the transitions between the new PRCL ASC and the PRM oplev and local damping.  The script is accessible from the PRC ASC screen, and will detect when the PRMI is locked or not.  When it is locked, it will turn down the PRM oplev gains and turn on the ASC, and then it will turn off the local shadow sensor damping for PRM pitch and yaw.  When the PRMI unlocks, the script will turn off the ASC and restore oplev and local shadow sensor damping. 

We saw that the bounce mode of the PRM was getting rung up with our new ASC, so we included a band stop in the ASC, and also turned on the triggering for the PRCL LSC FM6, which has the resonant gain for the bounce mode (as well as roll, and the stack mode).  This made the PRMI spot very stable. 

We then moved on to green arm locking.  The Yarm is behaving perfectly nicely (as nice as it has been lately - it's alignment and mode matching could also use some work), but Xarm was giving us a bit of trouble.  As always (since the PZTs were installed?), the mode matching isn't excellent for the green to the arm, so it can be hard to catch a TEM00 mode.  Also, even if we did catch a good mode, it would often not stay locked for more than a few tens of seconds.  We tried several alignment tweakings, and several different end laser temperatures (within the confines of seeing the beatnote under 100MHz), and didn't have a lot of success.  It looks like Eric had the slow servo engaged for the Xend laser, so the temperature offset was something like +300,000, which seemed totally crazy.  I turned that off, and found the beatnote somewhere around output of -10,300.  So, I haven't gone to the end to look at the temperature, but it's going to be different than when Eric was taking measurements this afternoon.  It seems like the main problem with the Xarm is poor mode matching - the maximized input pointing for TEM00, when you unlock and relock the cavity, is just as likely to give you a TEM_9_0 mode, as TEM00.

So, we gave up on the Xarm for the evening, and tried PRMI+1arm, with the new PRCL ASC.  This was successful!   The Yarm beatnote was around laser slow servo output of +4450.  Beatnote at 46.0MHz, -26dBm.  We found the IR resonance, moved off, locked the PRMI, transitioned to the new ASC, and brought the Yarm back to IR resonance.  What we see is that the power fluctuations in the PRC are much smaller than they were back around Halloween (elog 9338), however the arm power fluctuations still seem very, very large.  This is certainly partly due to the fact that we haven't done a thorough Yarm alignment since before messing with the greens, so we will have drifted somewhat.  Also, the ALS beatnote sensor isn't perfect, so won't be perfect at holding us near resonance. 

Den is thinking about whether we can use the arm transmission QPD signals to feed back to the ETM ASC servos, to try to reduce the RIN in the arms.  I feel like we should also see if this amount of power fluctuation can be explained by our ALS noise, because maybe we'll be fine once we transition to IR and turn off the ALS system.   Attached is a plot showing that the arm's RIN is coherent with the spot motion seen by the transmission QPD, so we need to check the alignment of the cavity, as well as consider using the trans QPD in an ASC feedback loop.

Here is a plot of the PRC sideband power, as well as the Yarm transmission.  The GPS time for this plot is approximately 1070963372.


Attachment 2: try_dc.pdf
  9463   Fri Dec 13 02:30:22 2013 KojiUpdateLSClocking activity

According to the measurement by Eric, the X-arm green PDH UGF is too low. We still have some room to increase the gain.

The out of loop stability of the ALS for each arm should be measured everyday.
Otherwise we can't tell whether the arm is prepared for advanced locking activities or not.

We expect to see the arm stablity of ~50pm_rms for the Y arm and ~150pm_rms for the X arm.

  9465   Fri Dec 13 13:28:07 2013 DenUpdateLSCarm calibration template

I have calibrated ETMX and ETMY actuators and added a template armSpectra.xml into /users/Templates directory.

Template shows control and error signals of both arms. Procedure is standard: calibrate control to meters and match error based on UGF measurement. XARM UGF: 200 Hz, YARM UGF 210 Hz.

Noise level at high frequencies (>100 Hz) for YARM is 3*10-15 and is factor of 3 better then for XARM. Servo gains are in the same ratio. I think there is less light on POX than on POY RF PD because I checked phase rotation and analog gain. I assume transimpedances are the same.

Attachment 1: armsCal.pdf
  9471   Sat Dec 14 02:51:47 2013 DenUpdateLSClocking activity

I had a look on x,y arms stabilization using ALS. Input green beam was misaligned and I was loosing 00 every few minutes. I vent on the floor and realigned green beams.

YARM alignemt was smooth - transmission increased from 0.4 to 0.85 with PSL shutter off.

XARM was tough. Steering mirrors did not have any derivatives when transmission power was 0.5. I walked the beam with piezos but got only 0.55. It seems that the input beam is mismatched to the cavity. When the transmission was 1 last time? Does anyone have a model of the xend table to compute mode matching?

Input green alignent was improved and I could keep arms stabilized for periods of ~30min - 1 hour. Still not forever.

I noticed that ALS_XARM and ALS_YARM servos have limiters of 6000 and control signal had high frequency components that were not rolled off as shown on the plot "ETMY_DRIVE". I have added a low pass filter that reduced RMS by factor of 5 and took 7 degrees of phase at UGF=150 Hz. Now margin is 33 degrees.

Then I excited ETMY longitudinally at 100 Hz and measured first and second harmonics of the YARM RIN. I got total DC offset of 0.3 nm. This means significant length coupling to RIN. First of all, "scan arm" script does not tune the offset very precise. I guess it looks at DC power, checks when cavity passes through symmetrical points of the resonance and takes the average. It is also useful to look at POX/POY and confirm that average is 0. Plot "ALS_RIN" shows comparison of YARM power fluctuations when it is locked using IR and stabilized using ALS. By manually correcting the offset I could reduce length coupling into RIN, coherence was ~0.1.

Cavity RMS motion also couples length to RIN. Plot "ALS_IR" shows YARM error signal. I also looked at POY signal (LSC-YARM_IN1) as an OOL sensor. At low frequencies POY sees only IMC length fluctuations converted to frequency. I have engaged MCL path and ALS error and LSC error signals overlaped. Cavity RMS motion is measured to be 200 pm.

Attachment 1: ETMY_DRIVE.pdf
Attachment 2: ALS_RIN.pdf
Attachment 3: ALS_IR.pdf
  9472   Sat Dec 14 11:56:54 2013 ranaUpdateLSCcommon mode servo



 It seems to me that current design of the common mode servo is already fine. Attached plots show common mode open and closed loop transfer function.

 These seem like pretty terrible loop shapes. Can you give us a plot with the breakdown of several of the TFs and some .m file?

We should be able to estimate the noise coming out of the MC using the single arm and then make a guess for the CM loop gain requirement. There's no reason to keep the old Boost shapes; those were used in the old MC configuration which had a RefCav. In addition to minimizing the EOM range, we should also minimize the AO signal as Koji has pointed out. In practice, I've seen that using ~300 Hz of offset makes no harm with 4 kHz MC pole.

  9474   Sat Dec 14 14:21:46 2013 DenUpdateLSCcommon mode servo



 These seem like pretty terrible loop shapes. Can you give us a plot with the breakdown of several of the TFs and some .m file?

Attached is matlab code that I used

G = zpk(-2*pi*8964, 2*pi*[-10; -10; -10; -1000; -274000], db2mag(242.5)) * ...
    tf([1 0.8*1.55e+05 3.1806e+10], 1);

CARM = G/(1+G);

% Common mode boosts
BOOST = zpk(-2*pi*4000, -2*pi*40, 1);
BOOST1 = zpk(-2*pi*20000, -2*pi*1000, 1);
BOOST2 = zpk(-2*pi*20000, -2*pi*1000, 1);
BOOST3 = zpk(-2*pi*4500, -2*pi*300, 1);

% Coupled cavity pole
CCPole = zpk([], -2*pi*100, 2*pi*100);

% Servo gain
Gain = db2mag(43);

% CARM OL with boosts
H = CARM * CCPole * BOOST * Gain;
H1 = H * BOOST1;
H2 = H1 * BOOST2;
H3 = H2 * BOOST3;

% Plot
% bode(H, H1, H2, H3, 2*pi*logspace(3, 5, 10000));
% bode(1/(1+H), 1/(1+H1), 1/(1+H2), 1/(1+H3), 2*pi*logspace(3, 5, 10000));

  9475   Sun Dec 15 03:13:15 2013 DenUpdateLSCattempt to reduce carm offset

X,Y arms were stabilized using ALS and moved 5 nm from the resonance, PRMI was locked on sideband using REFL165 I&Q. POP angular servo was running; PRMI RIN was good (~2-3%)

During slow offset reduction I was sweeping MICH, PRCL and POP servos for instabilities due to possible optical gain variations, loops were fine.

I could reduce offset down to ~200 pm and then lost lock due to 60 Hz oscillations as shown on the attached plot "arm_offset"

Arms were stabilized with RMS comparable to the offset and power in arms was fluctuating from 3 to 45.

60 Hz line most probably comes from MICH. RMS is dominated by the power lines and is ~ 1 nm as seen on the plot "PRMI_CAL". I think this is too much but we need to do simulations.

Attachment 1: ARM_OFFSET.pdf
Attachment 2: PRMI_CAL.pdf
  9477   Sun Dec 15 21:01:19 2013 KojiUpdateLSCCM servo module installed

Now the module is inserted at the 2nd crate from the top of 1Y2 (LSC analog rack). It is next to the DCPD whitening module.

I found the backplane cable for the Common Mode servo module.
I traced a cable form XY220 at the right most module on the crate where iscaux2 is living.
This cable was connected to the upper backplane connector.

Switching of the module is tested. All the switches and gain control are doing their job.

It was found that the offset and slow readback are not responding.
I checked the schematic of the CM servo module (D040180).
It seems that there is another cable for the offset and read back voltages.

  9478   Mon Dec 16 02:20:49 2013 DenUpdateLSCMICH rms is improved

When PRMI is locked on REFL 165 I&Q signals MICH rms is dominated by the 60 Hz line and harmonics. It comes from demodulation board.

To increase SNR ZFL-100 LN amplifier (+23.5dB) was installed in LSC analog rack. MICH 60 Hz and harmonics are improved as shown on the plot "mich_err"

I have also added a few resg at low frequencies. MICH rms is not 3*10-10. In Optickle I simulated power dependence in PRC and ARMs on MICH motion. Plot is attached.

 I think we need to stabilize MICH even more, down to ~3*10-11 . We can think about increasing RF amplifier gain, modulation index and power on BB PD.

CARM offset reduction was a little better today due to improved MICH RMS. Power in arms increases up to 15 and than starts to oscillate up to 70 and then PRMI looses lock.

Tomorrow we need to discuss where to put RF amplifier. Current design has several drawbacks:

  • DC power for the amplifier is wired from a custom (not rack based) +15V power supply that was already inside the lsc rack and used for other ZFL-100LN
  • BNC cables are used because I could not find any long SMA cables
  • we would like gain of ~40 dB instead of 23.5 dB
Attachment 1: MICH_ERR.pdf
Attachment 2: DC_power.pdf
Attachment 3: ARM_OFFSET.pdf
  9479   Mon Dec 16 20:08:43 2013 KojiUpdateLSCCM servo module installed

I found another backplane cable for the CM servo module. It is plugged to the module now.

I can see that C1:LSC-CM_SLOW_MON is responding to C1:LSC-CM_REFL_OFFSET.
But C1:LSC-CM_SUM_MON and C1:LSC-CM_FAST_MON are not replated to the given offset.
I probably need to check the cross connects.

  9480   Tue Dec 17 02:10:29 2013 DenUpdateLSClocking activity

Koji, Den

Some results and conclusions from tonight:

PRC macroscopic length is detuned. We measured REFL phases in carrier and sideband configurations - they are different by ~45 degrees for both 11 and 55 MHz sidebands. Additional measurement with phase locked lasers is required.

We got stable lock of PRMI+2arms with CARM offset of ~200 pm. We think this is the point when we should transition to 1/sqrt(TR) signals. We plan to rewire LSC model and also test CM servo with 1 arm during the day.

POP ASC OL shape changes when we reduce CARM offset probably due to normalization by sum inside the PD. Servo gets almost useless when PRMI power fluctuates by a factor of few.

SMA cables were made and installed for the REFL165 RF amplifier in lsc rack.

  9481   Tue Dec 17 14:06:59 2013 ranaUpdateLSCNew Broadband PD for POP 22/110

 I looked at the BBPD design so that we could make a POP22/110. It looks like it will be easy (I hope).

 The first attachment shows the schematic with the RF notch modified to handle 55 MHz. As long as the capacitor in this notch can be kept to below 20 pF, it doesn't degrade the noise so much,

The second attachment shows the TF and input referred noise. We ought to be able to get 20 pA/rHz at the input to the first RF amplifier.

The LISO files are in the svn under liso/examples/aLIGO_BBPD/,

Later, if we have to notch more than just 55 MHz, we can add a notch between the 2 RF amplifiers as Koji has done for the REFL165.

Attachment 1: POP22110sch.pdf
Attachment 2: POP22110.pdf
POP22110.pdf POP22110.pdf
  9482   Tue Dec 17 20:59:23 2013 JenneUpdateLSC1/sqrt(TR) signals added to frames

I noticed that we have not been saving the 1/sqrt(TRX) and 1/sqrt(TRY) data, so I modified the c1lsc model and added them to the DAQ channels block.  I restarted the c1lsc model, and the _DQ channels are now archived.

  9483   Tue Dec 17 21:28:36 2013 JenneUpdateLSCCM servo slow output digitized

Den just plugged an output from the common mode board into an LSC whitening board (the spare channel that used to be called "PD_XXX_I" in the LSC model).  I have modified the LSC model to reflect the new name of the new signal ("CM_SLOW"), and have added this channel to the LSC input matrix.  Koji is, I believe, adding this channel to the LSC screen in the auxiliary error signals section.  I am also adding the _OUT of the filter bank to the DAQ channels block.

  9484   Wed Dec 18 00:26:15 2013 JenneUpdateLSCXend QPD schematic investigation

I have looked at the photo of the Xend QPD from elog 9367, as well as the schematic for the board (D990272). 

Things that will need swapping out:

  • Thick film resistors in the signal path need to be changed to thin film.
  • MAX333 needs to be replaced with MAX333A.  The 333 has "ON Resistance" of 140-175 ohms, whereas the 333A has "ON Resistance" of 20-35 ohms.
  • AD797 needs to be replaced by OPA140.  The 797 is a low voltage noise op-amp, but for a diode we want low current noise.  AD797 has 2pA/rtHz at 1kHz, whereas the OP140 has 0.8fA/rtHz at 1kHz (see Zach's elog 8125 re: OPA140).

I have ordered from digikey via techmart 10 each of the MAX333A's and the OPA140's.  (4 per QPD times 2 QPDs plus 2 spares = 10).  Both of these new chips have the same footprint and pinout as the part that they are replacing, so it'll be a fairly easy task.

Next up, I need to make a LISO model for the circuit for one of the quadrants, to see what shape it'll turn out to be.  Part of this will include deciding what resistors and capacitors to put in the OPA140 gain stage. 

Right now, the AD797s say on the schematic that the gain options are different by a factor of 5, but the actual QPD has a different resistor than is on the schematic, and there is also a capacitor in parallel with each resistor, so I need to just pull those out, and pick some values that make sense to me. 

Rana and I have discussed ignoring the 2nd and 3rd gain switching options on each quadrant, as that is getting to be more fine control than we need. 

Other things on the board:

  • The 50 ohm resistors to ground for the "QPD_rtn" have all shorted.  Rana says this is good, so leave it as-is.
  • The positive input to the AD797's all have a 100 ohm resistor to ground, rather than just being connected to ground.  Why is this??

For now, I will probably just work on the QPD head, and not the whitening board.  For now, we can run with 1 stage of whitening, and if we need lower noise, we can revisit the whitening board (including replacing the thick film resistors with thin film).

When thinking about what gains I want on my gain stages, I want to have my full arm power (~700 TRX units) be ~20,000 counts from the ADC.  So, I want my single arm power (1 TRX unit) to be ~30 counts from the ADC.  This is not such a big number, so this may also require more thinking. 

  9485   Wed Dec 18 03:29:48 2013 DenUpdateLSCyarm locked on mc

As a CM slow path test I locked free swinging yarm by actuating on MC length with bandwidth of 200 Hz. Crossover with AO is not stable so far.

I used xarm as an ool frequency noise sensor. MC2 violin mode is at 645 Hz, I have added a notch filter to LSC-MC2 bank.

Attachment 1: MC_ARM.pdf
  9486   Wed Dec 18 11:32:34 2013 ranaUpdateLSCXend QPD schematic investigation


 Since we use the TransMon QPD for triggering the high/low gain switching we need to run with the whitening OFF during lock acquisition and the turn it on after we have the arms locked with ALS. This should be put into the up/down scripts.

  9487   Wed Dec 18 11:37:12 2013 JenneUpdateLSCPOP22 and POP110 demod phases

Somehow the POP22 and POP110 demod phases weren't correct anymore.  I guess Den saw this after he changed the setup for the REFL165 PD at the LSC rack, but didn't elog it. 

I went out to the LSC rack, and found that the power supply that is supplying the amplifiers for both POP22/110 and REFL165 was set to ~16V each channel.  I put it back to 15V for each channel.  I don't know what Den intended for the 165 amplifier (more volts is more gain), but the POP22/110 amplifier usually runs with 15V. 

I also reset the POP22 and POP110 demod phases.  Since I'm not able to lock PRMI on sideband this morning (why?!?!), I locked on the carrier, and moved the phases around until POP22 and POP110 were both maximally negative.  The phases are/were:

  OLD [deg] NEW [deg]
POP22 107.2 -165.0
POP110 95.0 150.0

This is a ~60 degree change for both PDs. 

I am not sure if Den ever checked the demod phase of REFL165 after he put in the new SMA cable (there's no mention of it in the elog!), so I'm going to check that to see if it helps get PRMI locking back.  I know that Den had also been using REFL11 for PRMI locking, but the parameters he used for that aren't in the log either.

  9490   Wed Dec 18 17:28:50 2013 GabrieleSummaryLSCEstimate of the PRC length error


Looking back at what I did in april (see log #8411) I realized that it is possible to get an estimate of how much the PRC length is wrong looking at the splitting of the sideband resonant peak as visible in the POP_110_I signal. With the help of Jenne the PRMI was aligned and left swinging. The first plot shows a typical example of the peak splitting of 55MHz sidebands. This is much larger than what was observed in April.

When the sidebands resonate inside the PRC they get a differential dephasing given by 

dPhi = 4*pi*f_mod/c * dL

where dL is the cavity length error with respect to the one that makes the sidebands perfectly resonant when the arms are not there. This is not exactly the error we are interested in, since we should take into account the shift from anti-resonance of the SBs in the arm cavities.

Nevertheless, I can measure the splitting of the peak in units of the peaks full width at half maximum (FWHM). I did this fitting few peaks with the sum of two Airy peaks. Here is an example of the result


The average splitting is 1.8 times the FWHM. Knowing the PRC finesse, one can determine the length error:

dL = c / (4 * f_mod * Finesse) * (dPhi / FWHM)

Assuming a finesse of 60, I get a length error of 4 cm.

To get another estimate, we kicked the PRM in order to get some almost linear sweeps of the PRC length. Here is one of the best results:


The distance between consecutive peaks is the free spectral range (FSR) of the PRC cavity. Again, I can measure the peak splitting in units of the FSR and determine the length error:

dL = c / (4 * f_mod) * (dPhi / FSR)

The result is again a length error of 4 cm.


An error of 4 cm seems pretty big. Therefore I set up a quick simulation with MIST to check if this makes sense. Indeed, if I simulate a PRMI with the 40m parameters and move the PRC length from the optimal one, I get the following result for POP_110_I, which is consistent with the measurement.



 Therefore, we can quite confidently assume that the PRC is off by 4 cm with respect to the position that would make the 55 MHz sideband resonant without arms. Unfortunately, it is not possible with this technique to infere the direction of the error.







Attachment 3: pop110_vs_dL.png
  9491   Wed Dec 18 18:45:39 2013 JenneUpdateLSCREFL 165 demod phases

I checked out the REFL165 demod phase, and it looks like it was okay.  it was -20.9 degrees.  I turned on my sensing matrix oscillators, and maximized the PRCL signal in the REFL165_I_ERR channel, and got a pretty good maximization at +155 degrees.  I used this to lock the PRMI on sidebands, with MICH gain of +0.3, and PRCL gain of +0.1 . 

Since this is working, I'm leaving the REFL 165 phase here, at +155 degrees, although this is almost exactly 180 degrees from what Den left it at, so I'm not sure why I was not able to lock with a demod phase of -20.9.  (I tried all 4 permutations of signs, with gain values of the same magnitude (0.3 for MICH, 0.1 for PRCL), and wasn't able to lock.  I'll try to figure this out tomorrow, but it was time for the meeting, then the IFO has been busy doing more important things the rest of the afternoon.

Plan for checking:  Lock with demod phase of 155, measure TF to one of the other REFL diodes (11, 33 or 55), lock on that other REFL diode.  Then, change the REFL 165 demod phase back to -20.9, and measure the transfer function again.  Hopefully the answer is just that I was doing something dumb, and it works easily.  This test/measurement should only take a few minutes, but it'll make me happier knowing that things still work as they should.

  9492   Thu Dec 19 03:29:34 2013 DenUpdateLSCCM servo test using yarm is complete

Koji, Den


  • lock yarm on IR, wire POY to CM input
  • transition arm to CM length path by actuating on IMC
  • increase AO gain for a stable crossover
  • engage CM boosts


  • arm can be kept on resonance and even acquired on MC2
  • stable length / AO crossover is achieved
  • high bandwidth loop can not be engaged because POY signal is too noisy and EOM is running out of range

We spent some time tuning CM slow servo such that fast path would be stable in the AO gain range -32db -> 29dB (UGF=20kHz) when all boosts are turned off and common gain is 25dB. Current filters that we use for locking are not good enough - AO can not be engaged due to oscillations around 1kHz. This is clearly seen from slow path closed loop transfer function. I will attach servo shapes tomorrow.

Attached plot "EOM" shows EOM rms voltage while changing AO gain from -10dB to 4dB. For UGF of 20kHz we need AO gain of 29dB.

It seems we can start using CM servo for CARM offset but the sensor should be at least factor of 30 better than POY. Add another factor of 10 if we would like to use BOOST 2 and BOOST 3.

Attachment 1: EOM.png
  9493   Thu Dec 19 12:51:57 2013 JenneSummaryLSCEstimate of the sign of the PRC length error

My hunch is that the PRC is SHORT by a few cm, not long. 

In my Optickle simulation, the sidebands are not perfectly co-resonating in the PRMI when the arms are not locked.  See Fig 1, which is the fields in the PRMI using the design PRC length.  If I add 5cm to the PRC length, I get Fig 2, where the peaks are about the same separation, but the upper and lower sidebands have swapped sides of the 0 mark.  However, if I remove 5cm from the PRC length, I get Fig 3, where the peaks are much farther apart than in Fig 1.  This case looks more similar to the data that Gabriele plotted in his elog entry, where the peaks are separated by at least a linewidth.  This is not at all conclusive, but it's a guess for which direction we need to move.  Obviously doing an actual measurement will be better. 

My tummy feelings also agree with this simulation:  When we flipped PR3 (the only optic change in the PRC since Gabriele and I measured the 55MHz peak separation in April), since the HR side of the optic is now at the back, we had to push the whole suspension cage forward to get the beam aligned to the Yarm.  Conversely, however, transmitting through the glass substrate adds to the optical path length.  So, my tummy feelings may be wrong.

Figure 1, PRC at design length, PRMI sweep:


Figure 2, PRC 5cm longer than design length:


Figure 3, PRC 5cm shorter than design length:



  9495   Thu Dec 19 18:33:25 2013 GabrieleSummaryLSCEstimate of the sign of the PRC length error

Maybe I'm getting confused, but I still believe there is no way to decide the direction from yesterday's measurement.

Let's say for example that the arm sideband detuning from antiresonance is equivalent to a PRC length change of +1cm away from the position of ideal resonance of the sidebands without arms. Then we can get a measured separation of the sidebands, without arms, corresponding to 5cm both if the PRC is off by +4cm or by -6cm...

  9499   Fri Dec 20 01:24:11 2013 DenUpdateLSChigh bandwidth loop achieved for yarm

Koji, Den

CM Servo with POY11 successfully engaged. UGF: ~15kHz.

Tonight we decided to repeat one arm locking using high-bandwidth CM servo. We low-passed AO signal to avoid saturations of the EOM. We tried different configurations that compromise between noise and loop phase margin and ended up with a pole at 30kHz. SR560 is used as a low-pass filter.

Another problem that we faced was big (~2.6V) electronic offset at the input of 40:4000 BOOST. Once engaged, cavity would be kicked out of lock. We calibrated this offset to be almost half linewidth of the cavity (~300pm). To avoid lock loss during engaging the boost we increased common mode gain to maximum (31 dB).

Measured OL is attached. UGF is 15kHz, phase margin is 60 degrees. We have also simulated evolution of loop shape during bringing AO path. Plot is attached.

The final procedure is

  • set common gain up to 31dB, AO gain to 8dB, MC IN2 gain 10dB, CM offset 0.7V
  • lock arm with CM slow path with bandwidth of 200 Hz
  • enable AO path, gradually increase slow and fast gains by 12 dB
  • enable boost
Attachment 1: CM_OL_meas.pdf
Attachment 2: cm_ol_sim.pdf
Attachment 3: CM_slow_fast_cross.pdf
  9500   Fri Dec 20 03:31:07 2013 KojiUpdateLSClock acquisition path for the CM servo

up/down scripts are to be made

(Offset Edit on Dec 20 10:38PM)

POY11QMon -> CM Servo In1 -> CM Servo -->Out1 -> ADC -> CM Slow FM -> LSC MC Servo FM -> ETMY(x1.0) -> DAC -> ETMY
                                       -->Servo Out -> SR560 (DC, 1st order 30kHz LPF) -> MC In2

Lock acquisition path 1

Initial condition:

CM Slow FM:

  • Gain 2.6

CM Servo setting:

  • In1 Gain 31dB, SW:OFF, Offset -1.880, Boost Off, Super Boost Off, AO +8dB

MC Servo setting:

  • In2 10dB, SW:OFF


  • Engage LSC
  • LSC MC servo gain +0.1, FM7/FM10 (Trigger FM2 with 3sec delay)
  • Turn on MC


  • Enable AO path (CM servo In1 SW:ON, MC servo In2 SW:ON)
  • LSC MC gain +0.1 -> +0.2
  • AO path gain 8dB->14dB
  • LSC MC gain +0.2 -> +0.35
  • AO path gain 14dB->18dB
  • CM servo offset -1.88 -2.7 -> 0.8 0.0 (gradually)
  • Enable CM servo Boost

Lock acquisition path 2

Initial condition:

CM Slow FM:

  • Gain 2.6

CM Servo setting:

  • In1 Gain 31dB, SW:OFF, Offset -1.880, Boost Off, Super Boost Off, AO +20dB

MC Servo setting:

  • In2 10dB, SW:OFF


  • Engage LSC
  • LSC Yarm G=+0.25 FM4/5 (Trigger FM3/6/7/8)


  • Enable MC servo In2 (SW:ON)
  • Set LSC MC gain +0.2 FM7/10
  • Enable LSC MC (On)
  • Enable CM servo In1 (SW:ON)
  • Disable LSC Yarm (OFF)
  • Change CM servo offset -1.88 -> +0.700 -2.70 -> 0.0
  • Enable CM servo Boost
  • Turn on LSC CM FM2 (optional)

Transition to ETMY LSC to MCL

  • After all of the transition: LSC output matrix ETMY (+1.00)
  • LSC output matrix MC2 (-1.00)
  • LSC output matrix ETMY (0.00)
  9501   Fri Dec 20 03:34:40 2013 KojiUpdateLSChigh bandwidth loop achieved for yarm

This too huge offset difference with/without "BOOST" switch should be checked.

  9507   Fri Dec 20 22:45:02 2013 KojiUpdateLSChigh bandwidth loop achieved for yarm

I checked the offset situation in the CM servo boost circuit. 

- The offset voltage on the CM servo screen is a raw DAC output. This number is diluted by the voltage divider before the amplifier.
  So, the actual offset of the boost circuit was much smaller. (~20mV)

- There is a offset trimmer on the board. This was adjusted so that the boost does not generate an output offset.

- So the default offset is 0V.

- When the arm was locked with (digital) POY11, the CM servo offset is necessary to be -2.7 (now).
  This means that analog POY11Q and digital POY11 has different offset for the best resonance transmission.
  That is believable if POY11I is contributing to the digital POY11 signal.

  9509   Sat Dec 21 01:54:04 2013 KojiConfigurationLSCLSC Whitening for the DCPDs/CM servo replaced

The previous LSC whitening filters for the DCPDs were in an unknown state (although the transfer functions were actually measured and fit a while ago)
They had no DC gain control and some of the channels had modifications.

To make the setup clear, the filter module was replaced with the spare module without any modification.

The channels are now respoding to the whitening gain switches. So far there is no screen for the new  whitening gains yet.

Also I found that POX11 DC cable has not been connected. Now it is connected.

  9523   Mon Jan 6 22:11:46 2014 JenneUpdateLSCPRCL sideband locking still not so happy


 The PRCL once again doesn't want to lock on sidebands for me.  I can lock on the carrier just fine (using the IFO Config settings, along with some hand-alignment of the PRM). 

However, I can't convince it to lock on sidebands.  Using the configs that I used on Dec 18th (elog 9491), I'm not getting it.  I've done the arm ASS alignment, and I've run LSCoffsets, both of which seemed to do their things appropriately. 

I'm going to attribute this today to not being in the groove yet, and I'll look at it again in the morning.

  9537   Wed Jan 8 13:01:48 2014 GabrieleSummaryLSCEffect of PRC length mismatch on error signals

I ran a simulation of a double cavity with a PRC length mismatched w.r.t. the modulation frequency. I summarized the results in the attached PDF. I think it would be important to have a cross check of the results.

In brief:

A mismatch between PRC length and modulation frequency do have an effect on error signals

Multiple zeros appear in REFL_3f/PRCL that can be removed by careful tuning of the demodulation phase (however, the shape of the signal makes difficult to understand which phase is good…)

No visible effect on REFL_1f/CARM

But a large PRCL signal appears in REFL_1f_I, which is used to control CARM. This is not good.

A mismatch of the order of 0.5 cm has a small effect.






Attachment 1: REFL_vs_PRClength.pdf
REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf REFL_vs_PRClength.pdf
  9539   Wed Jan 8 16:08:52 2014 ericqSummaryLSCEffect of PRC length mismatch on error signals


So, we want an relatively quick measurement of the PRC length error (with sign!) at the order of .5 centimeter or so. Rana suggested the "demodulation phase method," i.e. lock the simple Michelson, measure what demodulation phase brings the 1F signal entirely within the phase quadrature, then lock the PRMI and measure the demodulation phase again. This tells you something about the length of the PRC. 

Gabriele and I worked through a simulation using MIST to determine how to actually do this. We simulated the case of injecting a line at 1kHz in the laser frequency via the laser's PZT and looking at the transfer function of the 1kHz signal to the I and Q at the 1F AS demodulated signal when locked. (Michelson locked on the dark fringe, PRC locked on 11MHz sideband) With the I and Q in hand, we can measure some demodulation phase angle that would bring everything into I. 

When the PRC length is in the ideal location, the demodulation phases in the two cases are the just about the same. Sweeping the length of the PRC around the ideal length gives us a monotonic function in the difference in the demodulation phases:


So, with this simulation, we should be able to calibrate a measured difference in demod phase into the length error of the cavity! We will proceed and report...

  9541   Wed Jan 8 19:05:30 2014 GabrieleSummaryLSCEffect of PRC length mismatch on error signals

 [Gabriele, EricQ]

Actually it is difficult to see any laser frequency line in the dark fringe signal, since the Schnupp asymmetry is small. It is much better to use a differential MICH excitation which gives a better signal at the dark port.

We repeated the simulation explained before. We can use both the AS55 or the AS11 signals, bout the first one has a limited linear range and the expected 4cm value is very close to saturation.


  9544   Thu Jan 9 17:58:31 2014 ericqSummaryLSCEffect of PRC length mismatch on error signals

[ericq, Gabriele, Manasa]

 We wanted to perform the PRC length measurement today with an AS11 signal, but such a signal didn't exist. So, we have temporarily connected the AS110 PD signal (which is some Thorlabs PD, and not a resonant one) into the REFL11 demod board. 

We then proceeded with the goal of locking the PRC with REFL165. A few parameters that were changed along the way as we aligned and locked things:

  • the XARM gain was increased from 0.4 to 0.5 to help it acquire lock
  • the MICH gain was decreased from -10 to -5 since there was some gain peaking in its servo output
  • the REFL165 demodulation phase was changed from 155 to 122, to place a PRCL excitation entirely within I (we did this while locked on the carrier)

Sadly, in the end, we couldn't lock the PRC on a sideband in a stable manner. The alignment would drift faster than we could optimize the alignment and gains for the PRC. I.e. we would lock the PRC on the carrier, align PRM (and maybe touch ITMX) to maximize POPDC, switch to sideband locking, try to lock, and things would start looking misaligned. Switching back to carrier locking, the beam spots on REFL (for example) would have moved.

Manasa noted the MC_TRANS_Y has been substantially drifting along with small drift in MC_TRANS_P as well. So we need to fix the source of the mode cleaner beam drifting if we want to make this measurement. 

  9546   Fri Jan 10 15:31:07 2014 ranaSummaryLSCEffect of PRC length mismatch on error signals


 Its very doubtful that the MC yaw drift matters for the IFO. That's just a qualitative correlation; the numbers don't hang together.

  9548   Sun Jan 12 09:57:24 2014 GabrieleSummaryLSCEffect of PRC length mismatch on error signals



 Its very doubtful that the MC yaw drift matters for the IFO. That's just a qualitative correlation; the numbers don't hang together.

 Then there must be something else slowly drifting. It was very clear that the good alignment of the IFO was every time lost after few minutes...

ELOG V3.1.3-