We were poking around and tried to make a button for the Y-green shutter, just like the X-green already has. I don't know where the X-green shutter button goes to in model-land, so I can't figure out if there is already a channel set up for the Y end. Just switching the X for a Y didn't work. Someone (maybe me) should fix this in the next soon.
[Jenne, with ample supervision by Kiwamu and Suresh]
Y-green was aligned, and is now flashing. The ETMY trans camera was very helpful for this alignment. I didn't end up needing to use a foil aperture.
Kiwamu and Suresh had just closed up the IOO doors, so we don't know yet where it's hitting on the PSL table (if the beam is making it that far). Tomorrow we'll look at ITMY to see if the green beam is centered there, and if it's coming out to the PSL table.
The Yarm green laser really wanted to lock on a 01/10 mode, so Kiwamu suggested I go inside and realign the green beam to the arm. I did so, and now it's much happier locked on 00 (the Yarm is resonating both green and IR right now).
I've applied FIR adaptive filter to YARM control. Feedback signal of the closed loop was used as adaptive filter error signal and OAF OUT -> IN transfer function I assumed to be flat because of the loop high gain at low frequencies. At 100 Hz deviation was 5 dB so I've ignored it.
I've added a filter bank YARM_OAF to C1LSC model to account for downsampling from 16 kHz to 2 kHz and put low-pass filter inside.
I've used GUR 1&2 XYZ channels as witnesses. Bandpass filters 0.4-10 Hz we applied to each of them. Error signal was filters using the same bandpass filter and 16 Hz 40 dB Q=10 notch filter. As an AI filter I used 32 Hz butterworth 4 order low-pass filter. Consequently, AI, bandpass and notch filters were added to adaptive path of witness signals.
I've used an FIR filter with 4000 taps, downsampling = 16, delay = 1, tau = 0, mu = 0.01 - 0.1. Convergence time was ~3 mins.
This is interesting. I suppose you are acting on the ETMY.
Can you construct the compensation filter with actuation on the MC length?
Also can you see how the X arm is stabilized?
This may stabilize or even unstabilize the MC length, but we don't care as the MC locking is easy.
If we can help to reduce the arm motion with the MCL feedforward trained with an arm sometime before,
this means the lock acquisition will become easier. And this may still be compatible with the ALS.
Why did you notched out the 16Hz peak? It is the dominant component for the RMS and we want to eliminate it.
Precise arm alignment is more demanded. as the PRMI locking requires good and reliable alignment of the ITMs.
I previously added the output matrix to ASS.
Now the input and output matrix as well as the gains and filters have been updated.
The current concept is
Fast loop: align the arms by the arm mirrors with regard to the given beam.
Slow loop: move the incident beam position and angle to make the spot at the center of the mirrors
This is actually opposite to Den's implementation.
In order to realize the faster alignment of the arm, I increased the corner frequency of the lockins for the arm signals from 0.5Hz to 1Hz.
With the new configuration the arm alignment converges in 10sec and the input pointing does in ~15sec.
The actuation to the input pointing TTs are done together with the feedforward actuation to the arms.
This way we can avoid too much coupling from the input pointing servos to the arm alignment servos.
The corresponding script /opt/rtcds/caltech/c1/scripts/ASS/YARM/DITHER_Arm_ON.py was also modified.
I modified my Simulink model of the YARM to match the new filter modules Rana installed on YARM. I also scaled the open loop transfer function of the model to fit the measured open loop transfer function at the unity gain frequency, as shown in the figure below. From this I produced the length response function correctly scaled, also shown below. Then I applied the calibration factor to the YARM data measured in /users/Templates/Y-Arm_120815.xml. Both the uncalibrated and calibrated spectra are included below.
Today I spent time locking the YARM in order to calibrate the arm cavity. Here's what I did:
1. Misalign all optics other than the beam splitter, ITMY, ETMY and PZT2
2. Restore BS, ITMY, ETMY, and PZT2
3. Open Dataviewer and run /users/Templates/JenneLockingDataviewer/Yarm.xml from the Restore Settings. This opens the signals C1:LSC-POY11_I_ERR (the Pound-Drever-Hall error signal for this measurement) and C1:LSC-TRY_OUT (the light transmitted through ETMY) in the plot window.
4. Adjust ITMY and ETMY pitch and yaw using the video screens looking at AS and ETMYT as a first, rough guide. It can be helpful at first to increase the gain on the YARM servo filter module in the C1LSC control screen to about 0.3 and decrease it back down to 0.1 as the beam becomes better aligned. You know when to decrease this gain when fuzzy, small oscillations appear on the C1:LSC-TRY_OUT signal. If the mode cleaner is locked you should see a bright spot on the AS camera.
5. Tinker with pitch and yaw while looking at the AS screen until you see a reasonably good circular spot without other fringes extending from a bright center.
6. The overall goal is to maximize C1:LSC-TRY_OUT because the power transmitted through EMTY is proportional to the power within the cavity. A decent target value is 0.85 and today I was able to get it to just over 0.80 at best. At first there will probably be small spikes in C1:LSC-TRY_OUT. You want to adjust pitch and yaw until the deviation in the signal from zero is no longer just a spike, but a sustained, flat signal above zero. By this time there should be light showing up on the ETMYT camera as well.
7. Once that happens, continue to successively adjust ITMY and ETMY doing the pitch adjustments on both first, and then the yaw adjustments, or vice versa. You can also tweak the PZT2 pitch and yaw. Once you've got C1:LSC-TRY_OUT as large as possible, you've locked the cavity.
I saved the pitch and yaw settings I ended up with for ITMY, ETMY, BS and PZT2 in the IFO_ALIGN screen. Before the end of the day I think Jenne restored the rest of the previously misaligned optics because they were restored when I got back from dinner.
I also worked on calibrating the YARM. I opened up DTT using C1:LSC-POY11_I_ERR as the measurement channel and C1:SUS-ITMY_LSC_EXC as the excitation channel. I ran a logarithmic swept sine response measurement with 100 points and an amplitude of 25. The mode cleaner kept losing its lock all day, and if this happened while making this measurement I tried to pause the sweep as quickly as possible. I analyzed the the transfer function and the coherence function that the sweep produced, and thought that some of the odd behavior was due to losing the lock and getting back to a slightly different locked state when resuming the measurement. The measured transfer function and coherence plots are attached below. Both the transfer function and the coherence look good above roughly 30 Hz, but do not look correct at low frequencies. There's also a roll-off in the measured transfer function around 200 Hz, while in the model the magnitude of the transfer function drops only after the corner frequency of the cavity, around several kHz. I have attached a plot of the roughly analogous transfer function from the DARM control loop model (the gains are very large due to the large arm cavity gain and the ADC conversion factor of 2^16/(20 V) ). The measured and the modeled transfer functions are slightly different in that the model does not include the individual mirrors, while the excitation was imposed on ITMY for the measurement.
The next steps are to figure out what's happening in DTT with the transfer function and coherence at low frequencies, and to understand the differences between the model and the measurement.
Once you've got C1:LSC-TRY_OUT as large as possible, you've locked the cavity.
Both the transfer function and the coherence look good above roughly 30 Hz, but do not look correct at low frequencies. There's also a roll-off in the measured transfer function around 200 Hz, while in the model the magnitude of the transfer function drops only after the corner frequency of the cavity, around several kHz. I have attached a plot of the roughly analogous transfer function from the DARM control loop model (the gains are very large due to the large arm cavity gain and the ADC conversion factor of 2^16/(20 V) ). The measured and the modeled transfer functions are slightly different in that the model does not include the individual mirrors, while the excitation was imposed on ITMY for the measurement.
The cavity is actually "locked" as soon as the feedback loop is successfully closed. One easy-to-spot symptom of this is that, as you mentioned elsewhere in your post, TRY is a ~constant non-zero, rather than spikey (or just zero). Once you've maximized TRY, you've got the cavity locked, and the alignment optimized.
We didn't get to this part of "The Talk" about the birds, the bees, and the DTTs, but we'll probably need to look into increasing the amplitude of the excitation by a little bit at low frequency. DTT has this capability, if you know where to look for it.
It would be great to see the model and your measurement overlayed on the same plot - they're easier to compare that way. You can export the data from DTT to a text file pretty easily, then import it into Matlab and plot away. Can you check and maybe repost your measured plots? I think they might have gotten attached as text files rather than images. At least I can't open them.
Here's the same plots in pdf format now. I originally posted them as jpg because I couldn't open the resulting pdf from DTT on rosalba, but I could open the jpg. I'll look into overlaying the measured and modeled curves as well.
I forgot to post this last night, but I locked the YARM again yesterday and misaligned the other optics. I took measurements on ITMY and ETMY with DTT again as well. At the end of the day I aligned the rest of the optics before I left.
From time to time the 20 dB jump in the transfer function still occurs. The new AD8336 op amp did not change that issue. I am sure that the op amp was broken,
because the amplitude of the sine did not change when I turned the gain knob.
The above two curves were measured with different input amplitude of the sine from the spectrum analyzer. Nothing changed in between except that there was no
jump when Kiwamu was around. Very strange. Testing the electronic board led to no clue what is happening.
For now, I will just use the PDH box as it is, but one should keep this odd behaviour in mind.
I could not improve the locking. So, I checked the transfer function of the PDH box again. The transfer function looks okay if the gain knob is <=2.0.
If the gain knob is >2.0 the 20dB step appears in the transfer function (see elog page 5713). This step is shifted to higher frequencies if the gain is
increased. The PZT drive out was not saturated at any time. Yesterday, I checked the electronic circuit with a gain of 2.0. Thus, I couldn't find the broken
gain amplifier (AD8336). The amplifier is ordered in will arrive on Monday.
With limited proof of working for a few times (but robustly), I'm happy to report that ASS on YARM and XARM is working now.
The issue is that PR3 is not placed in correct position in the chamber. It is offset enough that to send a beam through center of ITMY to ETMY, it has to reflect off the edge of PR3 leading to some clipping. Hence our usual ASS takes us to this point and results in loss of transmission due to clipping.
Solution: We can not solve this issue without moving PR3 inside the chamber. But meanwhile, we can find new spot positions on ITMY and ETMY, off the center in YAW direction only, which would allow us to mode match properly without clipping. This would mean that there will be YAW suspension noise to Length coupling in this cavity, but thankfully, YAW degree of freedom stays relatively calm in comparison to PIT or POS for our suspensions. Similarly, we need to allow for an offset in ETMX beam spot position in YAW. We do not control beam spot position on ITMX due to lack of enough actuators to control all 8 DOFs involved in mode matching input beam with a cavity. So instead I found the right offset for ITMX transmission error signal in YAW that works well.
I found these offsets (found empirically) to be:
These offsets have been saved in the burt snap file used for running ASS.
I'll reiterate here procedure to run ASS.
I took data from 1123495750 to 1123498750 GPS time (Aug 13 at 3AM, 50 mins of data) for C1:LSC-YARM_OUT_DQ, and all T240 and GUR1 channels.
Here is the PSD of the YARM_OUT, showing the data that I will use to train the FIR filter:
Coherence plots for YARM and all channels of T240 and GUR1 sesimometers are shown below. This will help determine what regions to preweight the best before computing FIR filter. They also show how GUR1 is back to work compared to those of elog:11457.
error signal = signal measured behind the low-pass filter
feedback signal = output of the gain servo, going to the PZT
First of all both signals in V/sqrt(Hz) just in case I mess up the next calibration step.
The 60 Hz line (and its multiple) are a new feature. They show up as soon as the feedback loop is closed. So far, I couldn't find their origin.
For the next calibration step:
I measured the power spectrum of channel C1:GCY_SLOW_SERVO1_IN1, which is the PZT driving voltage.
I converted the output to a PSD. Next, I converted counts/sqrt(Hz) to volts/sqrt(Hz) by multiplying with 40 V / 2^16 counts.
Finally, I multiplied it with 5MHz/V for the PZT to end up with Hz/sqrt(Hz).
This corresponds to a cavity length fluctuation of
with lambda = 532nm and a YARM cavity length of 37.757m (elog # 5626).
All in one plot
Today, I could lock the YARM laser for 2h to the YARM cavity. After to hours the output of the servo is saturated. I need to work on thermal feedback to the laser.
It is a nice TEM00 mode and the green light enters PSL table.
Measured with pin-ball machine spectrum analyzer (I forgot the real name, but it is the one that makes sounds like a pin-ball machine), source power10mVp, Lb1005 gain 2.05.
Input offset of LB1005 is zero
On Thursday, Oct 27, lock for 3 min
On Friday, Oct 28, lock up to 18 min, improvements done by
On Monday,Oct 31, careful adjustment of summing box (rear of of LB1005), lock up to 2h, limited by saturated feedback signal --> work on slow control
Some more plots
I scripted a series of YARM DC reflectivity measurements last night alternating between locked state and unlocked state (with ETMY misaligned) for measuring the after-vent armloss. The general procedure is based on elog 11810, but I'll also give a brief summary here.
I did this back in June (but strangely never posted what I found, shame on me). What I found back then was a YARM loss of 237 ppm +/- 41 ppm and an XARM loss of 501 ppm +/- 105 ppm
Last night's data indicates a YARM loss of 143 ppm +/- 24 ppm after cleaning with first contact.
THIS IS STILL ASSUMING THAT THE MODE-MATCHING HASN'T CHANGED. We had however moved ETMY closer to ITMY during the vent by 19mm. Gautam and I had some trouble setting up the ALS to confirm the mode-matching, but we're in the process of recovering the XARM IR beat.
Rana pointed out that another way to mitigate seismic motion at in the mode cleaner would be to look at the YAW and PITCH output channels of the WFS sensors that control the angular alignment of the mode cleaner.
I downloaded 45 mins of data from the following two channels:
And did some quick offline Wiener filtering with no preweighting, the results are shown in the PSD's below,
I'm quite surprised at the Wiener subtraction obtained for the YAW signal, it required no preweighting and there is about an order of magnitude improvement in our region of interest, 1-3 Hz. The PIT channel didn't do so bad either.
In preparation for the armloss map I checked the calibration of the Y-Arm ITM and ETM OpLevs with the method originally described in https://nodus.ligo.caltech.edu:8081/40m/1247. I was getting a little confused about the math though, so I attached a document at the end of this post in which I work it out for myself and posteriority. Stepping through an introduced offset in the control filter for the corresponding degree of freedom, I recorded the change in transmitted power and the reading of the OpLev channel with the current calibration. One thing I noticed is that the calibration for ITM PIT is inverted with respect to the others. This can of course be compensated at any point in any readout/feedback chain, but it might be nice to establish some sort of convention where positive feedback to the mirror will increase the OpLev reading.
The calibration factors I get are within ~10% of the currently stored values. The table (still incomplete, need to relate to the current values) summarizes the results:
The individual graphs:
We looked at beam spots on ITMY and ETMY. We switched to smaller apertures on the other side of the rulers. For ITMY beam spot was 1mm below and 1mm south (right if you look in the direction ITMY -> ETMY) from the aperture center, for ETMY - 4 mm up and 3mm north from the aperture center. We made a correction for this using PZT 1 and 2. Now beam spots are in the middle of the apertures on ITMY and ETMY.
We tried to look at reflected beam from ETMY but it was hard to see the dependence between ETMY DC offset and reflected beam. We'll continue tomorrow.
Spot centering on Y arm - DONE!
1. I went back to the IFO alignment slider positions from Friday. The Y arm was flashing in HOM because the earthquake this morning tripped all suspensions and the slider values were not real. X arm did not have any flashes.
2. Y arm aligned using TT1 and TT2. Spot centering measured using Jenne's A2L_Yarm script.
Pitch 6.48 4.39
Yaw -7.42 -3.135
3. I started centering in pitch. I used the same in-vac alignment method (down on TT1 and up on TT2 in pitch) and measured spot positions.
4. When the spot positions were centered in pitch, I started with yaw alignment.
5. I had to use TT1 to center on ITMY and move TT2 and ITMY to center on ETMY.
6. Spot positions after centering:
Pitch -1.22 -1.277
Yaw 0.42 -0.731
7. I wanted to go back and tweak the pitch cenetering; but framebuilder failed and dataviewer kept loosing connection to fb
AS seems clipped. Although it could be because of the misaligned BS.
IPANG was centered on the QPD, but it is so clipped, that I'm not sure we can trust it. Max sum right now is -4, rather than the usual -8 or -9.
Once fb is fixed, we should align the X-arm which will be followed by green alignment.
Over the last few weeks, it has been observed that there is some strong seismic activity that starts at around 9PM everyday and goes on for a couple of hours. It seems unlikely that it is our geologist neighbour (Jenne met with the grad student who works on the noisy experiment).
The high frequency noise, which has been a dominant noise above 30 Hz in the Y arm ALS (#6133), decreased by a factor of 5.
This reduction was done by increasing the modulation depth at the Y end PDH locking. Now the noise floor at 100 Hz went to 0.2 pm/sqrtHz.
However the noise source is not yet identified and hence it needs a further investigation.
(Increasing the modulation depth)
As this entry, Yarm ALS is not stable enough to lock PRMI + 2 arms. We tried to figure out what is the reason.
What we did
Check connection and alignment
1. Check the Green REFL PD.
Reflection is hitting the center of PD.
2. Check all the BNC connections
All connection are fine.
3. Check which power supply the PDH box is connected to.
PDH box is connected to 1Y4 AC power supply.
Check the control signal and error signal
4. Connected the PZT OUTMON to PC
Before the PZT output was not connected to the monitor channel. We connected that.
5. Saw the time series of the error signal and control signal (PZT output)
When the Yarm lost end PDH lock, we found that control signal kicked the PZT of end green laser. And also we saw the saturation of control signal. We are not sure where this saturation comes from.
With these check, we couldn't find any problem in connection or alignment. But the PDH control signal looks somehow strange. We tried to compare the Yarm signals with that of the Xarm, but we could not conclude anything meaningful.
We don't understand right now but we will continue to check that. We will add more details to the discussion once we have looked into the PDH box signals using oscilloscope.
Jamie and I were doing some locking, and we found that the Yarm green wasn't locking. It would flash, but not really stay locked for more than a few seconds, and sometimes the green light would totally disappear. If the end shutter is open, you can always see some green light on the arm transmission cameras. So if the shutter is open but there is nothing on the camera, that means something is wrong.
I went down to the end, and indeed, sometimes the green light completely disappears from the end table. At those times, the LED on the front of the laser goes off, then it comes back on, and the green light is back. This also corresponds to the POWER display on the lcd on the laser driver going to ~0 (usually it reads ~680mW, but then it goes to ~40mW). The laser stays off for 1-2 seconds, then comes back and stays on for 1-2 minutes, before turning off for a few seconds again.
Koji suggested turning the laser off for an hour or so to see if letting it cool down helps (I just turned it off ~10min ago), otherwise we may have to ship it somewhere for repairs :(
I turned the Yend laser back on....it hasn't turned itself off yet, but I'm watching it. As long as we leave the shutter open, we can watch the C1:ALS-Y_REFL_DC value to see if there's light on the diode.
This is happening again to the Yend laser. It's been fine for the afternoon, and I've been playing with the temperature. First I have been making big sweeps, to figure out what offset values do to the actual temperature, and more recently was starting to do a finer sweep. Using the 'max hold' function on the 8591, I have seen the beat appear during my big sweeps. Currently, the laser temperature measurement is at the Yend, and the RF analyzer is here in the control room, so I don't know what temp it was at when the peaks appeared.
Anyhow, while trying to reaquire lock of the TEM00 mode after changing the temperature, I find that it is very difficult (the green seems misaligned in pitch), and every minute or so the light disappears, and I can no longer see the straight-through beam on the camera. I went down to the end, and the same symptoms of LED on the laser head turning off, power out display goes to ~40mW, are happening. I have turned off the laser as was the solution last time, in hopes that that will fix things.
Manasa has done some work to get the Xgreen aligned, so I'll switch to trying to find that beatnote for now.
I turned the laser back on around 1am. This is still happening, although right now it is turning off more often than before, maybe every 15 seconds or so. I am going to turn off the laser for the night.
The measured laser temperature is about 45C (I have a 25,000 count offset in the Y ALS Slow control right now....higher offset, lower temp), although the measured laser temp drops to ~43.5C when the power goes down.
I took a look at the laser. It is probably the LD TEC (DTEC) failure.
As the temperature of the LD (DTMP) gradually deviated from 25degCish,
the DTEC voltage also went up from 2Vish to 2.1, 2.2...
When DTEC reaches 3V, it stopped lasing. This cools the diode a bit, and
it start lasing but repeat the above process.
I am not sure which of the head and controller has the issue.
The situation did not improve much by reducing the pumping current (ADJ: -15).
BTW, Turning on/off the noise eater did not change the situation.
I think the head/controller set should be sent out to JDSU and find how they will say.
We tried to re-tune Yarm ASS today. It cannot be fully closed as of now. I think we need to play with signs.
- We want to make sure Yarm ASS work with current ITMY coil matrix (40m/16899).
- ASS makes the beam positions on test masses to be the same every day.
What we did:
- Adjusted A2L paths of C1:ASS-YARM_OUT_MTRX based on cavity geometry. For the paths to maximize the transmission using TT1 and TT2, we just assumed they are correctly calculated by someone in the past.
- Adjusted OSC_CLKGAINs so that ITMY and ETMY will be shaken in the same amplitude in terms of radians. The ratio of the excitation was determined to take into account for the oscillator frequency difference between DOFs.
- Checked the time constant of A2L paths by turning on A2L paths only, and checked that of max-transmission paths by turining on them only.
- Adjusted DEMOD_SIG_GAINs so that their time constants will be roughly the same, with C1:ASS-YARM_SEN_MTRX fully identity matrix and all servo GAINs to be +1.
- Re-tuned DEMOD_PHASEs to minimize Q signal. C1:ASS-YARM_ITM_PIT_L_DEMOD_PHASE and C1:ASS-YARM_ITM_YAW_T_DEMOD_PHASE were re-tuned within +/- 5 deg.
- These changes are recorded in /opt/rtcds/caltech/c1/Git/40m/scripts/ASS/ASS_DITHER_ON.snap now.
- A2L loops seems to be working, but max-transmission paths seems to diverge at some point. I think we need to play with the signs/gains of max-transmission paths for C1:ASS-YARM_OUT_MTRX.
- Attached is the current configuration we achieved so far.
We are still in the progress of re-commissioning Yarm ASS.
Today, we tried to adjust output matrix by measuring the sensing matrix at DC.
Turning on yaw loops kind of works, but pitch does not. It seems like there is too much coupling in pitch to yaw.
We might need to adjust the coil output matrix of ITMY and ETMY to go further, and/or try measuring the sensing matrix including pitch - yaw coupling.
What we did:
- Confirmed that turning on TT1 and TT2 loops (max-transmission loops) work fine. When we intentionally misalign TT1/2, the ASS loops correct it. So, we moved on to measure the sensing matrix of A2L paths, instead of using theoretical matrix caluclated from cavity geometry we used last week (40m/16909).
- Instead of +/-1's, we put +/-2's in the ITMY coil output matrix to balance the actuation between ETMY and ITMY to take into account that ITMY is now using only two coils for actuating pitch and yaw (40m/16899).
- Measured the change in C1:ASS-YARM_(E|I)TM_(PIT|YAW)_L_DEMOD_I_OUT16 error signals when offset was added to C1:SUS-(E|I)TMY_ASC(PIT|YAW)_OFFSET. We assumed pitch-yaw coupling is small enough here. Below was the result.
ETM PIT error ITM PIT error
ETM PIT OFFSET of +100cnts: -3.0cnts -2.99cnts
ITM PIT OFFSET of +100cnts: -11.94cnts -5.38cnts
ETM PIT error ITM PIT error
ETM PIT OFFSET of +100cnts: -3.0cnts -2.99cnts
ITM PIT OFFSET of +100cnts
ETM PIT error ITM PIT error
ETM YAW OFFSET of +100cnts: -3.42cnts -16.93cnts
ITM YAW OFFSET of +10 cnts: +1.41cnts +0.543cnts
- Inverted the matrix to get A2L part of C1:ASS-YARM_OUT_MTRX. Attachment #1 is the current configuration so far.
- With this, we could close all yaw loops when pitch loops were not on. But vise versa didn't work.
- Anyway, we aligned the IFO by centering the beams on test masses by our eyes and centered all the oplevs (Attachment #2).
- Do coil balancing to reduce pitch-yaw coupling
- Measure sensing matrix also for pitch-yaw coupling
- Xarm ASS is also not working now. We need to do similar steps also for Xarm
ETM PIT error ITM PIT error
ETM YAW OFFSET of +100cnts:
ITM YAW OFFSET of +10 cnts
I finally got YARM AS to work today. It is hard to describe what worked, I did a lot of monkey business and some dirty offset measurements to create the ASS output matrix that gave results. Note that I still had to leave out ITMY PIT L error signal, but transmission was maximizing without it. The beam does not center fully on ITMY in Pit direction right now, but we'll mvoe on from this problem for now. Future people are welcome to try to make it work for this last remaining error signal as well.
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.
[ 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.
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.
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
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.
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?
I finished installation of optics in the Y-end and recovered green locking. Current ALS-TRY_OUTPUT is about 0.25, which is lower than before. So I still continue the alignment of the beam. The simulation code was attached. (Sorry. The optic shown as QWP2 is NOT QWP. It's HWP.)
[ Yuki, Gautam, Steve ]
To align the green beam in Y-end these hardware were installed:
I made sure that DAC CH9~16 and cable to AI-board worked correctly.
When we applied +100V to PZT driver and connected DAC, AI-board and PZT drive, the output voltage of the driver was not correct. I'll check it tomorrow.
[EricQ, Jenne, brains of other people]
Checked at ETMY that the pointing hadn't changed a whole lot. Jamie and Koji pointed out that we were half falling off of the IPANG QPD. Adjusted PZT2 sliders so that we were again centered on IPANG's QPD. Before we close up, we'll want to put the sliders back to their nominal positions, and use the knobs to hit IPANG, but this is equivalent and fine for now. The tip tilts don't seem to have moved much overnight, since the beam drift on both IPANG and ETMY was fixed simultaneously with PZT2 (recall, IPANG picked off before tip tilts exist in the beam path). This left us hitting the center of ETMY. We moved ETMY sliders to make the reflected beam hit the center of ITMY (same spot position as transmitted beam from BS). Then moved ITMY to get prompt reflection to hit same spot on ETMY as original primary beam from BS. Checked at ITMY that we didn't need to move ETMY anymore. (Actually, I forget how many iterations we did, but in the end, all of the reflections that we can find are co-located on the test masses.)
Align BS so we're hitting the center of ETMX
Tap / readjust ITMX OSEM which is at 0.3 to get it back to the center of its range
Align ITMX to lock MICH
Check no clipping on POX / POY optics, no clipping around BS
Check PRM, SRM alignment (what exactly do we want to do here? Try to lock PRMI / SRMI?)
Get IPPOS out of vac
Fix clipped ITMY / SRM oplev
Install 'black' glass beam dumps - forward-going POP beam, 2 places in BS chamber (check old elog from Jenne/Yuta for the places).
Get green spots co-located with IR spots on ETMs, ITMs, check path of leakage through the arms, make sure both greens get out to PSL table
I fitted intra-cavity power dependance on mirror misalignment plot with parabola to get the g-factor.
Y arm (tangential) g = 0.44 +0.01 -0.01 (measured value before was 0.3765 +/- 0.003 elog #6938)
PRC (sagittal) g = 0.97 +0.01 -0.04 (expected value is 0.939 elog #8068)
PRC (tangential) g = 0.96 +0.02 -0.05 (expected value is 0.966 elog #8068)
Error bars are just statistical errors from the fitting. Estimated systematic error is ~0.04 (or more).
Here, I assumed PR2/PR3 to be flat to make the calculation simple. I assumed PRC to be curved PRM - flat ITM cavity, and Y arm to be curved ETMY - flat ITMY cavity.
Intra-cavity power decrease can be written as
dP/P = (dx/w0)**2 + (dt/a0)**2
where dx and dt are translation and tilt of the beam axis introduced by mirror misalignment. w0 is waist size and a0 is divergence angle (= lamb/(pi*w0)).
When considering a flat-curved cavity with cavity length L, dx and dt can be expressed as;
(dx) 1 ( L*g L ) (a2)
( ) = --- ( )*( )
(dt) 1-g ( -(1-g) 0 ) (a1)
using misalignments of mirrors(a1,a2). Here, mirror1 is curved, and mirror2 is flat. See Kakeru document /users/OLD/kakeru/oplev_calibration/oplev.pdf for derivation.
So, power decrease by flat mirror misalignment can be expressed as
dP/P = pi*L/lamb * g/(1-g)/sqrt(g*(1-g)) * a2**2
For curved mirror is
dP/P = pi*L/lamb * 1/(1-g)/sqrt(g*(1-g)) * a1**2
We can derive g-factor by measuring dP dependance on a1/a2.
My script lives in /opt/rtcds/caltech/c1/scripts/dither/gfactormeasurement/plotgfactor.py.
It least fitts data with parabola (scipy.optimize.leastsq) and gets g-factor value from bisection (scipy.optimize.bisect).
Below are the plots of fitted curves.
[oplev calibration] We noticed QPD rotation when calibration oplevs (elog #8232). ~5 deg of rotation makes 10% of systematic error to the oplev calibration and this introduces ~0.04 of error to g-factor values. This
[oplev linear range] Oplev linear range is ~100 urad, so this is OK.
[assumption of flat PR2/PR3] Result here doesn't tell you g-factor of PRM itself, but some "effective g-factor" of PRM/PR2/PR3 combination. We can compare with FINESSE result.
[intra-cavity power drift] If there's significant intra-cavity power drift during the measurement, if effects parabola fitting. We can make this affect small by sweeping the mirror alignment in both direction and take average.
By the way:
I kept getting PRC g-factor of something like 0.999999 because I had power normalization mistake in my calculation. My script worked for Yarm because TRY is already normalized.
Also, I was multiplying the oplev calibration factor wrong last night (see elog #8230).
- Compare with FINESSE result.
- Is this g-factor enough? Is this presicion enough? Calculate from mirror angluar motion.
- More stable lock of PRMI.
- Try dithering method to measure g-factor to check consistency and also to study systematic effect.
dP/P = pi*L/lamb * g/(1-g)/sqrt(g*(1-g)) * a2**2
dP/P = pi*L/lamb * 1/(1-g)/sqrt(g*(1-g)) * a1**2
The ~16Hz bounce mode of some optic is showing up in the Yarm error signal.
MC is kind of 'windy' looking, so maybe it's from that? (Yuta's guess).
We need to make sure that the SUS damping and oplev paths both have notches at the correct bounce mode, not the old, old MOS frequency. If that doesn't work, may need to put a resgain in Yarm path.
Made the Bounce notch in the BounceRoll filter (ITMY OLPIT, ITMY OLYAW) wider, so it actually spans the peak we see in the error spectra. When we next lock the arm later today, I'll retake this spectra to see if the ETMY oplev fix (Koji, Yuta) and this notch fix both helped.
We extracted the fiber that Suresh and Sonali laid over the summer, for the IR beat for the Ygreen laser, and Frank took it back to Bridge to be used in the new fiber distributed reference laser setup.
Because I'm too lazy to write a cohenrent elog right now, here's my notes that I wrote while working tonight:
Elog notes, 27July2011
Aligned Xarm, just to check on it. Had to flip sign of TRX in DCPD filter bank (to gain of -1) to make the signal positive.
Restored Yarm, see some slight flashing, but no lock yet.
Adjusted phase rotation of AS55 from 56.5deg to 60deg, just by-eye trying to maximize AS55I, my arm error signal. AS55I goes from ~ -40 to +60 counts
Tried fitzing with Yarm gain, flipping sign, incr gain. No real change in signals, or flashing.
Incr. ETMY oplev gains to -0.4 from -0.2
Engaged ELP35's on Pit and Yaw, to be more similar to other optics. However, right now all of the optics have different things in their filter banks. Why??
Arm is flashing pretty reliably now, but still not locking. The trigger threshold is always satisfied, so that's not it.
... it was not possible to get the cavity locked in green. So we decided to do the first measurement with infrared locked only.
When we sat down to align the Yarm to the green, the green light was happy to flash in the cavity, but wouldn't lock, even after Jan had tweaked the mirrors such that we were flashing the TEM00 mode. When we went down to the end to investigate, the Universal PDH box was saturating both negative and positive. Turning down the gain knob all the way to zero didn't change anything, so I put it back to 52.5. Curiously, when we unplugged the Servo OUT monitor cable (which was presumably going to the rack to be acquired), the saturation happened much less frequently. I think (but I need to look at the PDH box schematic) that that's just a monitor, so I don't know why that had to do with anything, but it was repeatable - plug cable in, almost constant saturation....unplug cable, almost no saturation.
Also, even with the cable unplugged, the light wouldn't flash in the cavity. When I blocked the beam going to the green REFL PD (used for the PDH signal), the light would flash.
Moral of the story - I'm confused. I'm going to look up the PDH box schematic before going back down there to investigate.
[Annalisa, Jenne, Nic]
After having troubles with the Xarm earlier (maybe Manasa can write/say something about this? Something about perhaps seeing the phase tracker jump, and cause it to lose lock?), we moved on to the Y arm.
Annalisa locked the Yarm green, and closed the ALS loop. I believe that earlier today, she tuned the gain such that we don't start getting gain peaking at a few hundred Hz. We would like to get a script going, so that it's not so labor intensive to reclose the ALS loop after an MC lockloss....but that's a daytime task.
We then found the IR resonance, using only the Yarm ALS system. After Manasa's work yesterday, the Yarm was very stable while locked with the ALS. We took a power spectrum of POY11_I_ERR, which I have calibrated using the number in elog 6834 of 1.4e12 cts/m, or 7.14e-13 m/ct. See the figure below.
After that, we changed the offsetter2 offset such that the arm was off resonance, but not so far off that we crossed any significant resonances (in particular, we wanted to not go as far as the 55MHz resonance).
Then, I tried to lock the PRMI for a while, but the alignment wasn't very good. We knew that the Yarm was well aligned, since our IR resonance was > 0.98, but it had been a while since we had aligned the X arm. I tweaked the ITMX position to make the Michelson dark, and then tried acquiring PRMI lock. At first, I tried with REFL165 I and Q, but with the non-ideal alignment and the offset in the 165 diode (LSC offsets was not run this evening), I wasn't catching any locks. I then switched to AS55Q and REFL33I, but wasn't able to catch lock there either.
The MC lost lock, which made us lose the ALS loop, but the ALS had been locked for more than 30 minutes, at least. I tried locking the PRMI with the current alignment (after having misaligned ETMY), but was only able to get lock stretches of 1 second at maximum.
We are calling it a great success for the night, since we have confirmed that, at least for the Yarm, Manasa's beatbox work has improved things. Also, we have a pretty solid plan for trying the PRMI+arm tomorrow, even though it didn't work out tonight.
We knew that the Yarm was well aligned, since our IR resonance was > 0.98, but it had been a while since we had aligned the X arm.
The X arm was locked with TRX>0.98 earlier last night while I was measuring the out of loop noise of the phase tracker.