Wow! What's happened?
As the video showed good quality of resonances, I stopped by at the 40m on the way back home.
I looked at the error signals and found that they indicate high finesse and clear resonance of the sidebands.
The lock was immediate once the gain is set to be -0.004 (previous 0.05ish). This implies the optical gain is ~10 times larger than the previous configration.
The alignment was not easy as POPDC was saturated at ~27000. I leave this as a daytime job.
As I misaligned the PRM, I could see that the lock hopped into the next higher order. i.e .from TEM00 to TEM01, from TEM01 to TEM02, etc
This means that the modes are closely located each other, but sufficiently separated to sustain each mode.
I definitely certify that cavity scans will give us meaningful information about the cavity.
I replaced the BS1 between the POPDC PD and the camera with a 98 reflector, and moved the 50 up before the BS to dump half the light. Still saturating POPDC, but hopefully the ratio between POPDC and the camera should be better. We just need to dump more of the power before we get there. I'll come back to this after C&D if no one else has already gotten to it.
I don't know why I didn't pay more attention last night, but things look way WAY better. The beams are much cleaner and the power level is much much higher.
After Jamie did all the work this morning on the POP table, I was able to get the cavity to lock. It's not very stable until I engage the boost filters in the PRCL loop. After locking, I tuned up the alignment a bit more. Now we're taking mode scan data. Look for results hopefully shortly after Journal Club!
[Jamie, Koji, Jenne]
We are looking at the mode scan data, and have some preliminary results! We have data from when the cavity was aligned, when it was slightly misaligned in pitch, and slightly misaligned in yaw.
Inverting the equation for transverse mode spacing, we infer (for pitch misalignment) a cavity g-factor of 0.99, and from there (assuming the G&H mirror is flat and so has a g-factor of 1), we infer a PRM radius of curvature of 168 meters which is ~50% longer than we expected.
More results to come over the weekend from Jamie.
During the scanning we were riddled by the fact the PDH error and the transmission peaks do not happen simultaneously.
After a little investigation, it was found that "LP100^2" filter is left on in the POPDC filter.
Moreover, it was also found that the whitening filter switches for the POPDC does not switch the analog counterpart.
These were the culprit why we never saw accidental hitting of the max transmission by the peaks when the cavity was not locked.
I know that the most of the whitening filter in the RF paths were checked before (by Keiko?), but the similar failure still exists in the POX path.
We should check for the whitening filters in the DC path as well and fix everything at once. I can offer assistance on the fixing part.
Very exciting result, if true. I suppose we should try to reconfirm this result by doing another phase map of PRM03.
Is it possible that PR2 is not flat? How would we test to see if the tip-tilt frame screw gives it a curvature? Perhaps we can check with COMSOL.
Kiwamu and Koji
The PRM/SRM were balanced with the standoffs. We glued them to the mirror.
This was the last gluing so far until we get new PRM/ETMs.
[Masayuki, Jenne, Rana]
We have, for the past hour and a few minutes, had PRMI + 2 arms locked. Yup, that's right, we did it! (We never gave control of the arms to the IR LSC system, so it's kind of cheating, but it was still cool.)
A little after midnight, we felt that the Yarm was behaving well enough that we could give PRMI + 2 arms a try. So we did. Probably around 1am-ish, or maybe a little bit before, we had the system locked.
How did we do it?
* Locked arms in IR to help find green beatnotes.
* Misalign ETMs, lock and align PRMI.
* Misalign PRM.
* Restore ETMs, find arm resonances, then step away (I did +3 counts, which is 29 kHz).
* Restore PRM, lock PRMI.
* Brought Xarm back close to resonance using ALS (-3 counts). It seems like this may not actually have gotten us back to perfect resonance, but that actually made bringing in the other arm easier.
* Brought Yarm back close to resonance using ALS (-3 counts).
* Turned on Sensing Matrix notches and oscillators (10,000 counts for MICH, actuating on BS and PRM at 562.01 Hz, 200 counts for PRCL actuating on PRM at 564.01 Hz).
* Stepped arms back and forth to see how things responded.
During this process, particularly during the various arm steps, the PRMI lost lock many times. However, the ALS system never lost lock for either arm, for an hour and a half or so. Good work, ALS team!! The PRMI would reaquire lock (sometimes we'd have to undo whatever arm step we just took, to get farther away from resonance) without any intervention. It seemed that as we came closer to full arm resonance, we were never able to hold PRMI locked. This is what is instigating some of our investigations for tomorrow.
Also, Rana reported to me that he turned the c1tst model back off, and opened the door(s?) to the ETMY rack to allow more air flow sometime before midnight, which seems to have reduced the rate of the CPU going over 61 microseconds, as well as reduced the number of times the ETMY suspension glitches. We definitely need to make some changes so that we're not so close to the edge. This may have been one of the big things that allowed our success tonight.
The transmission PDs at the ends of the arms are saturating around 50 counts (they have gains of 2e-3 so that they are roughly normalized to 1 being the max power in a single arm). We need to commission the end transmission QPDs.
All of the signals looked a little ratty, and we heard lots of noise - Rana suggests that we recommission our CARM servo.
ALS beat info: [Xarm 40.9 MHz, -11.4 dB], [Yarm 50.5 MHz, -17.7 dB]
Things to look at tomorrow:
Data! I should be able to extract sensing matrix information, even though my sensing matrix software isn't totally ready yet. I know what the oscillators were doing, and I can look at the PD error signals. We also save the Offsetter numbers, so I can kind of tell what the PRMI+arms situation was.
Can we tell by looking at the end laser PZT feedback signals whether we're making our arms longer or shorter? So that we can tell if we're putting on DARM or CARM offsets.
Spectrum and time series of REFL 165 (our PRMI LSC locking PD) to see if we're saturating while we bring the arms into resonance. Basically, does anything bad happen, particularly since the PD is not a resonant PD, so there are some 1f signals floating around in addition to the 3f signals. We want to put in a directional coupler after the PD, before the demod board, and send that signal to a spectrum analyzer and a 'scope. Hopefully we can use the power of the internet to not need to sit in the IFO room saving data as we move the arms around. Do we need to put bandpass filters on the PD signal before it goes to the demod board?
Optickle model of 1f vs. 3f signals in the different ports, as the CARM offset is reduced.
Violin notches for the arms - should be put into ALS and LSC models. It looks like the modes are around 631 Hz, but we should check.
Hardware for end low gain transmission QPDs.
Software (schmidt triggering) for end transmission QPDs.
Modifying / preparing a matrix in the ALS system so that we can give CARM and DARM offsets conveniently.
Nice work. Congratulation
Just in case people were confused, although the PRMI + 2 ALS arms were controlled, we weren't able to bring them in to resonance. They were in some unknown off-resonant state.
We can try to calculate the expected recycling gain (ignoring losses in the PRM) following section F.2.1 of my Manifesto:
T_PRM = 5.6%, R_ARMS ~ 98%, G_PRC ~38.
So the full TRX/TRY powers should be G_PRC/T_PRM = 690.
In our stable configuration, we were sitting at TRX/Y powers of ~5-10. Once in awhile we could get a state where the power was saturating the detectors at ~50 and possibly would have gone up to 100, but it was all oscillation at that point. (we've got to find and notch the ETM violin mode frequencies in the ALS feedback servos.
As we move in towards resonance, we have to now consider all of complications of handing off to various error signals and CARM optical spring compensation and RF saturation that have been discussed in Rob's thesis and Lisa's lock acquisition modeling.
> all of complications of handing off
- ALS error signals transfered to the LSC input matrix.
- Handing off from the ALS to the 1/sqrt(TRX)+offset signal
- Handing off to the RF signal
- And, of course, CM servo.
PRCL Open Loop Transfer Function. PRMI locked on REFL 165 I&Q, Xarm held on IR resonance using ALS, ETMY misaligned:
MICH Open Loop Transfer Function. PRMI locked on REFL 165 I&Q, Xarm held on IR resonance using ALS, ETMY misaligned:
Time series data during our PRMI + 2 arm attempt:
its time to get the CM servo hardware turned back on. We're going to want to switch it on when we're about ~1/50th of the way up the CARM fringe.
A good way to re-commission it is to lock it to the single arm, using a Pomona box filter to move the arm pole down to the coupled cavity pole frequency.
Koji reminded me that we should also save the data from the PRMI+Xarm, just in case we want to look at it later.
Here is the time series, in which you can see us finding the Xarm IR resonance, moving the arm off resonance, locking PRMI, and bringing the arm back into resonance. At the very end, the arm is still held on resonance, but I had disabled the LSC locking, so we see very large flashes at TRX (of order 40, rather than 1).
The data is in the same folder as the 2arm data: /users/jenne/PRCL/PRMI_Xarm_ALS_16Oct2013/
The text files have been differentiated, so that the 2arm data has "_2arms" at the end of the filename, while the Xarm data had "_Xarm" appended to the filename. Since we left the cavities locked for many minutes (during which transfer functions were taken), the data set for the PRMI+Xarm is very long.
We talked about how it should be automated.
We'll gradually offload the switching works on scripts.
Here is the list of automations that we need to work on for less hectic PRMI+ALS trials.
1. Enable/Disable ASC when PRMI is locked/unlocked.
2. Smooth transfer from REFL33/AS55 to REFL165 when PRMI is locked.
3. Change actuation from the ITMs to BS and PRM after PRMI lock.
4. Enable ALS.
5. IR resonance scan using ALS.
X arm stabilized using ALS while PRMI stayed locked
[Rana, Lisa, Jenne, Manasa]
Time series : ALS enabled at t = 0 and disabled at t = 95s
What we did:
1. Jenne will elog about ASC (POP QPD) updates.
2. Found the beat note between Xarm green and PSL green.
3. Stabilized arm fluctuation by enabling ALS servo.
4. Scanned the arm for carrier resonance by ramping on the offset and set the offset such that we had IR resonating (TRX fluctuated between 0.1 and 0.8 counts).
5. Disabled the ALS servo and locked PRMI using AS55 for MICH and REFL33 for PRCL.
6. Enabled ALS.
Enabling ALS to detune the arm out of resonance kept PRMI locked (currently for a span of few tens of seconds). However we could not see PRMI locked as stably compared to when the arms are misaligned. Everytime the offset was set IR to resonate, the PRMI was kicked out of lock.
Also there is some leakage at the arm transmission when PRMI was locked. The leakage was visible at ETMX transmission as flashes in different higher order modes indicating the not-so sufficient ALS stability. The leakage sets an offset at TRX measuring 0.01-0.05 counts.
To do list:
The ALS_OFFSETTER1 has to be calibrated in FSR. We were giving random offsets to do the offset scan.
Installed a filter before ETMXT camera to remove the refl green. (Note to myself: The filter needs to go on a better mount/adapter).
[Koji, Manasa, Annalisa]
I made several trials to scan the arm on the IR TEM00 resonance while the PRMI was held with REFL165I&Q.
It was so hectic to keep multiple systems running correctly. We talked about how it should be automated.
We'll gradually offload the switching works on scripts.
In a good alignment condition, when I swept on the resonance, everytime the PRMI lost the lock. It reacquired
once the arm passed the resonance.
Lately I got difficulty to acquire lock of the PRMI while the arm is waiting at its off resonance.
If I change the ALS offset I got a stable lock in a certain offset, and did not get in another offset
so there could be something systematic. (The arm was in between the carrier resonance and the next sideband (55MHz) resonance).
- Run LSCoffset script.
- Misalign PRM. Lock and align the arms with ASS.
- Go into the tables. Align the oplevs for ETMX/Y, ITMX/Y, and BS. (Very important for alignment stability)
- Align PRMI and lock PRMI. Unlock once.
- Go into the BS/PRM table. Align the oplev for PRM.
- Misalign PRM by -0.2
- Find the beat note at around 50MHz by changing the Yarm SLOW control. Today the PSL SLOW was ~0.24, and the Yarm SLOW was -10981.
- Reset Phase Tracker History (Important)
- Engage Yarm ALS with FM5. Tested the sign of the servo by giving 0.01 or -0.01. In my case, the negative number worked fine.
Gradually increase the gain up to -10. Turn on FM2/3/6/7/10.
- Use Filter module "C1ALS-OFFSETTER2" to give the ALS sweep. I used FM1 (30mHz LPF). Change the offset while looking at the IR TRY and POY11 error signal.
- Once the resonance is found, shift the beat note by giving +10 or -10 offset.
- While the arm is kept off resonance, align PRM.
- Lock PRMI with REFL33I and AS55Q. Turn on PRM ASC.
- Once the stable lock is obtained, switch the input signals to REFL165I&Q. I used REF33I x1.0->REFL165I x0.8 and AS55Q x1.0 -> REFL165Q x0.5
[PRMI + one arm]
- Revert the ALS offset by 10 to bring the arm on the resonance the see what happens.
While Jenne was plotting, I locked and aligned the MICH with AS55_Q. Then I aligned the PRM and locked PRMI using REFL55_I/Q with triggering on POP22, but no power normalization.
I used this to set the phase for REFL11 and REFL55 (driving PRM at 111.3 Hz and minimizing the Q response using the DTT Sine Response tool). I flipped the sign on REFL11 by
The REFL11 gain is ~50x larger than REFL55; this is with the 15 dB whitening gain on REFL55 and none for REFL11. What's going on here? The attached PDF shows the two time series with the free swinging PRMI and both phases set to ~ +/- 2 deg. The REFL55 signals have been scaled up by 50x.
So then we went in and looked at the RF signals at the demod boards. To do this we disconnected the RFPD test cables and hooked the RF Mon outputs into the 50 Ohm inputs on a scope. The following PNG images show the scope traces. The REFL11 (yellow) traces are too big!! See how small the REFL55 (green) are. REFL11 is saturating - need to fix.
Now that the REFL55 signal chain is capable of providing balanced, orthogonal readout of the two quadratures, I was able to recover the 1f SB resonant lock pretty easily. Ran sensing lines for ~5mins, still looks weird. But I didn't try to optimize anything / do other checks (e.g. actuate MICH using ITMs instead of BS) tonight, and I'm craving the Blueberry pie Rana left me. Will continue to do more systematic tests in the next days.
This afternoon, I kept the PRM locked for ~1hour and then measured transfer functions from the PRM angular actuators to the POP QPD spot motion for pitch and yaw between ~1pm and 4pm. After this work, the PRM was misaligned again. I will now work on the feedforward filter design.
Since it seems like the entire electronics chain has no obvious failure, I decided to compensate for the apparent increased optical gain by turning the flat whitening gain down from +18dB to 0dB. Then, after some fiddling around with alignment, settings etc, I was able to lock the PRMI once again, with the ETMs misaligned, using REFL55_I to sense PRCL, and REFL55_Q to sense MICH. Some sensing matrices attached. Some notes:
So there is clearly something funky with the nominal MICH actuation scheme (MICH suspension, PRM suspension or both), which we should get to the bottom of before trying any low noise locking. I think using the ITMs as the MICH actuator in the full lock will not be a good low nosie strategy, as we would then be "polluting" all our suspended optics with our control loops, which seems highly suboptimal for technical noise sources like coil driver noise etc.
After making sure the beams were hitting the 3f photodiodes on the "AP" table, I was able to lock the PRMI with the sidebands resonant inside the RC using 3f error signals. This would be the config we run in when trying to lock some more complicated configuration, such as the PRFPMI (i.e. start with the arms controlled by ALS, held off resonance). Tonight, I will try this (even though obviously I am not ready for the CARM transition step). The 3f lock is pretty robust, I was able to stay locked for minutes at a time and re-acquisition was also pretty quick. See Attachment #1. Not sure how significant it is, but I set the offsets to the 3f paths by averaging the REFL33_I and REFL33_Q signals when the PRMI was locked with the 1f error signals.
As usual, there's a lot of angular motion of the POP spot on the CCD monitor, but the lock seems to be able to ride it out.
Lock-settings (I modified the .snap file accordingly):
REFL33_I --> PRCL, loop gain = -0.019, Trigger on POP22, ON @ 20cts, OFF@0.5cts.
REFL33_Q --> MICH, loop gain = +1.4, Trigger on POP22, ON @ 20cts, OFF@0.5 cts.
I tried implementing a basic PRMI ASC using the POP QPD as a sensor. The POP22 buildup RMS is reduced by a factor of a few. This is just a first attempt, I think the loop shape can be made much better, but the stability of the lock is already pretty impressive. For some past work, see here.
I made a change to the c1ass model to normalize the PIT and YAW POP QPD outputs by the SUM channel. A saturation block is used to prevent divide-by-zero errors, I set the saturation limits to [1,1e5], since the SUM channel is being recorded as counts right now. Model change is shown in the attached screenshots. I compiled and installed the model. Ran the reboot script to reboot all the vertex FEs to avoid the issue of crashing c1lsc.
Attachment #1 - comparison of the POP QPD PIT and YAW output signal spectra with and without them being normalized by the SUM channel. I guess the shape is different between 30-100 Hz because we have subtracted out the correlated singal due to RIN?
This did not have the effect I desired - I was hoping that by normalizing the signals, I wouldn't need to change the gain of the ASC servo as the buildup in the PRC changed, but I found that the settings that worked well for PRMI locked with the carrier resonant (no arm cavities, see Attachment #2, buildup RIN reduced by a factor of ~4) did not work for the PRMI locked with the sideband resonant. Moreover, Koji raised the point that there will be some point in the transition from arms off resonance to on resonance where the dominant field in the PRC will change from being the circulating PRC carrier to the leaking arm carrier. So the response of the actuator (PRM) to correct for the misalignment may change sign.
In conclusion, we decided that the best approach to improve the angular stability of the PRC will be to revive the PRC angualr feedforward, which in turn requires the characterization and repair of the apparently faulty vertex seismometer.
I have resaved the PRMI locking settings in the IFO Config screen. Nothing has changed, except that I have put a 1e-4 into the PRCL matrix elements for REFL11I, REFL33I and REFL55I. So, PRMI still locks on REFL165 I&Q, but the other 3 REFL diodes' whitening gets triggered when the cavity is locked. I think this will help the LSC sensing matrix measurements, which I'm going to test out now.
It looks like PRMI LSC is making PRM motion (and BS motion) at ~3Hz worse.
I concluded this from measuring feedback signal of suspension servo and LSC servo.
1. BS and PRM moves alot at ~3 Hz.
2. LSC senses fake signal at ~3Hz from beam spot motion on PD
3. LSC feedback this motion to position of PRM
4. Suspension damping servo try to cancel this because ~3 Hz motion is not actual length signal
x: Orignal longitudinal motion of PRM
n_L: Sensing noise in LSC (including ITM motion, fake ~3Hz motion)
n_S: Sensing noise in suspension damping (OSEM sesor noise, fake ~3Hz motion)
G_L: Openloop transfer function of PRCL LSC
G_S: Openloop transfer function of suspension damping (PRM SUSPOS)
H: LSC sensor transferfunction (PDH signal on REFL_33_I)
F_S: Filter for suspension damping
A: Actuator transfer function (PRM OSEM coils)
Since G_L >> G_S and G_L >> 1 for below 100Hz (see elogs #6950 and #6967), feedback signal of LSC and suspensiton damping can be written as
f_L = x - A*F_S*n_S - (1+G_S)/H*n_L
f_S = 1/G_L*x - A*F_S*n_S - G_S/H*n_L
So, basically, LSC supresses PRM motion but puts n_L to PRM. Suspension servo try to surpress n_L, which was not there when LSC is off.
1. Below left is uncalibrated spectra of
Red: suspension damping feedback to PRM/BS when PRMI is locked
Blue: LSC feeed back to PRM/BS when PRMI is locked
Pink: suspension damping feedback to PRM/BS when PRMI is not locked
As you can see, PRM suspension damping feed back increases at ~ 1.5-3 Hz because of LSC. This is the same for BS at ~1 Hz and ~3 Hz.
2. Above right is same spectra for ITMX/ITMY. There's no change in suspension damping feedback. This means, radiation pressure coupling or something is not related in this issue. LSC servo is not engaged for ITMs.
3. Below is oplev spectra for PRM/BS
Red: Oplev pitch error signal of PRM/BS when PRMI is locked
Blue: Oplev yaw error signal of PRM/BS to PRM/BS when PRMI is locked
Pink: Oplev pitch error signal of PRM/BS when PRMI is not locked
Cyan: Oplev yaw error signal of PRM/BS to PRM/BS when PRMI is not locked
You can see the increase in pitch/yaw motion at ~ 1.5-3 Hz for PRM, and ~1Hz/~3Hz for BS. They are consistent with measurement of feedback spectra.
By the way:
I adjusted oplev servo gains for ITMX. They were crazy this evening. They now have UGF ~ 2.5 Hz.
C1:SUS-ITMX_OLPIT_GAIN = 1.0 (was 2.6)
C1:SUS-ITMX_OLYAW_GAIN = -0.5 (was -1.6)
- Can we notch ~3 Hz feedback so that LSC doesn't feedback this motion?
- Why ~3 Hz motion is high for BS/PRM? Too much load on BS chamber stack?
- Can we reduce ~3 Hz motion?
- If BS chamber stack is bad, PR3 might have ~3 Hz motion, too. Does this make PRMI beam spot motion crazy?
- How about PR2?
As stephanie did a few years ago, the idea should be to match the damping between the DRMI optics so as to minimize the differential motion. No notching is necessary. Read her SURF report about the IMC.
To taste the strangeness of the current 40m PRC, I locked the PRMI with the guide of Koji.
We first aligned MICH by mostly tweaking ITMX, assuming that ITMY is in a good place as the Y-arm locks. MICH lock was stable.
Then we restored the IFO to the PRM_SBres mode. With a bit of alignment work on PRM and gain tweaking, the PRMI locked.
Also the PRMI was not so stable. Especially, when the alignment fluctuates, the optical gain changes and the loop becomes temporarily unstable. We took POP_DC as the guide for the gain change and normalized the PRCL error signal with it. To do this smoothly, we first changed the input matrix to route the PRCL error signal, which is REFL33_I, so that the signal also goes to the MC filter bank. Then with dtt, we monitored the spectra of the PRCL_IN1 and MC_IN1. We tweaked the value of the element in the normalization matrix for the MC path until the two spectra look the same (at this moment, the normalizing factor for the PRCL path was still zero). During this process, we noticed that the MC path signal (normalized by POP_DC) is noisier at above 500Hz. This was because the POP_DC has a large noise at high frequencies. So we put a low pass filter (100Hz 2nd order Butterworth) to the POP_DC filter bank to reduce the noise. Then the two spectra looked almost the same. The correct normalization factor found in this way was 0.03. So we put this number in the normalization matrix for PRCL. It did not break the PRMI lock.
After the normalization is turned on, the PRMI lock became somewhat more stable. However, the POP_DC was still fluctuating a lot, especially when the alignment is good. So I made a boost filter: 5Hz pole Q=2, 15Hz zero Q=1.5. I also made this filter automatically triggered when the PRMI is locked. This made the PRMI lock acquisition quicker. However, still the POP_DC fluctuation is large. It seems that the alignment of PRC is really fluctuating a lot.
The current UGF of PRMI is about 150Hz with the phase margin over 50deg.
Yesterday we discussed a bit about working on the PRMI sensing matrix.
In particular we will start with the "issue" of non-orthogonality in the MICH actuated by BS + PRM. Yesterday afternoon we played a little with the oscillators and ran sensing lines in MICH and PRCL (gains of 50 and 5 respectively) in the times spanning [1312671582 -> 1312672300], [1312673242 -> 1312677350] for PRMI carrier and [1312673832 -> 1312674104] for PRMI sideband. Today we realize that we could have enabled the notchSensMat filter, which is a notch filter exactly at the oscillator's frequency, in FM10 and run a lower gain to get a similar SNR. We anyways want to investigate this in more depth, so here is our tentative plan of action which implies redoing these measurements:
Task: investigate orthogonality (or lack thereof) in the MICH when actuated by BS & PRM.
1) Run sensing MICH and PRCL oscillators with PRMI Carrier locked (remember to turn NotchSensMat filter on).
2) Analyze data and establish the reference sensing matrix.
3) Write a script that performs steps 2 and 3 in a robust and safe way.
4) Scan the C1:LSC-LOCKIN_OUTMTRX, MICH to BS and PRM elements around their nominal values.
5) Scan the MICH and PRCL RFPD rotation angles around their nominal values.
We also talked about the possibility that the sensing matrix is strongly frequnecy dependant such that measuring it at 311Hz doesn't give us accurate estimation of it. Is it worthwhile to try and measure it at lower frequencies using an appropriate notch filter?
Wed Aug 11 15:28:32 2021 Updated plan after group meeting
- The problem may be in the actuators since the orthogonality seems fine when actuating on the ITMX/ITMY, so we should instead focus on measuring the actuator transfer functions using OpLevs for example (same high freq. excitation so no OSEM will work > 10 Hz).
I measured the OLTF of both the PRM Oplev loops. Nothing odd sticks out as odd to me in this measurement - there seems to be ~40 degrees of phase margin and >10 dB gain margin for both loops, see Attachment #1. I didn't measure down to the second UGF at ~0.2 Hz (the Oplev loops are AC coupled), so there could be something funky going on there. The problem still persists - if I misalign and realign the PRM using the ifoalign scripts, the automatic engagement of Oplev loops causes the loop to oscillate. Could be that the script doesn't wait for long enough for the alignment transient to die out.
Update 1230pm: Indeed, this was due to the integrator transient. It dies away after a couple of seconds.
The PRMI Oplev servo needs some tuning, it is currently susceptible to oscillations in Pitch.
Something is really excellent with the alignment today, or something has changed with the POP path / electronics. While usually we see ~120 counts on POP22_I and ~175 counts on POP110_I (cf elog 9193), today I have ~175 counts on POP22_I and ~265 counts on POP110_I.
My hypothesis from the measurements below, to explain PRMI beam spot motion is;
Stack motion at 3.3 Hz largely couples to BS and PRM angular motion.
LSC for PRMI try to compensate this 3.3 Hz motion because they appear in the error signal.
But since it's not length, failing and even adding more angular motion.
1. Uncalibrated spectra of POPDC and ASDC when PRMI is locked. This tells you that beam motion seen at POP is 3.3 Hz.
2. Uncalibrated spectra of feedback signal to BS and PRM. This tells you that LSC is actuating BS and PRM mainly at 3.3 Hz. I think this is because beam spot on PD moves at 3.3 Hz and so faking the error signal.
3. Below left is uncalibrated spectra of BS, ITMX, ITMY, PRM (and ETMY) angular motion measured using oplevs. I centered oplevs on these optics (except ETMY, which was mis-aligned during PRMI lock). It looks like BS and PRM motion at 3.3 Hz is larger than other optics. Also, there's some coherence between POPDC and BS/PRM motion. We see some coherence with ITMs and even with ETMY, which is completely independent from PRMI. I think this is because 3.3 Hz motion is originated from the ground (stack) motion.
4. Above right is the same spectra, but when PRMI is not locked. It looks like there's no big change compared with PRMI locked. When locked, there's some excess for BS and PRM at ~1-3 Hz. I think this is from LSC feedback, which in principle, doesn't affect any angular motion.
- Why BS and PRM has large 3.3 Hz peak compared with other optics?
- Is 3.3 Hz peak effecting MI lock or arm lock?
- How can we monitor PR2/3 angular motion?
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?
Since we have never tried to lock PRMI on carrier after the folding mirrors were flipped, I tried to lock PRCL on carrier.
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.
EDIT by JCD:
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.
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.
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.
We aligned the IFO in the PRMI state and let it swing freely.
PRMI glitch certainly comes from power recylcing gain fluctuation.
I confirmed this by
- Reading the value of POPDC at the time when there's glitch in error signals
-> There was some threshold for POPDC to make a glitch
- Look closer to the glitch
-> It was oscillation in ~400Hz, where we have phase flip in PRCL/MICH servo
Next is to find why we have power recycling gain fluctuation. I want to see the correlation between alignment fluctuation of optics and POPDC.
Below is the plot of
Red PRCL error signal (C1:LSC-REFL33_I_ERR)
Green MICH erorr signal (C1:LSC-AS55_Q_ERR)
Blue PRC intra-cavity power (C1:LSC-POPDC_OUT)
when PRMI is carrier locked.
Time when there is a glitch in error signal is marked. Value of POPDC at that time is also marked. It looks like there's some threshold (dotted blue line).
It sometimes doesn't show glitch even if POPDC is above the "threshold". It is maybe because of alignment fluctuation. Intra-cavity power gets high, but power at PDs get low, or vice versa.
Right plot is closer look. Glitch is a sudden oscillation at ~400 Hz. It is the frequency where we have phase flip in PRCL/MICH openloop transfer function now(see elog #6950).
I modified filiters for LSC_MICH and LSC_PRCL.
Although modes we can see at POP and AS look still bad, error signals are less glitchy than I see before (elog #6886).
Measured power recylcing gain for PRMI was 1.6 (??)
Openloop transfer function for LSC_MICH:
UGF ~130Hz, phase margin ~30 deg
550 usec delay
APOLOGIES: I forgot "pi" in previous delay calculation. (I put notes on elogs #6940 and #6941)
Openloop transfer function for LSC_PRCL:
UGF ~130Hz, phase margin ~30 deg
550 usec delay
A bump cam be seen in ~200 Hz. Coupling of DOFs?
Beam shape and motion:
Below left is the Sensoray capture of AS/REFL/POP when PRMI is carrier locked.
Beam spot motion looks less bouncy than before, but it still shows motion mostly at ~3.3Hz. This might be from PRM motion. Above right is uncalibrated spectra of POPDC and REFLDC. You can see 3.3 Hz peak. This peak has some coherence with PRM motion measured by oplevs. I centered BS/PRM oplev to do this measurement.
Power recycling gain:
- Definition and designed value
Power recylcing gain is
G = (PRC intracavity power) / (incident power)
When MI is perfectly symmetric, this can be written as
G = (t_PRM/1-r_PRM*r_ITM)**2
where t_i, r_i is amplitude transmissivity, reflectivity. Inserting the designed values;
t_PRM = sqrt(0.0575)
r_ITM = sqrt(1-0.014)
designed power recycling gain for PRMI is
G = 44
POP power when PRM is misaligned and MI is locked at dark fringe is
P_mis = P_in * T_PRM * (1-T_PR3) * (1-T_ITM) * T_PR3
POP power when PRMI is locked is
P_PR = P_intra * T_PR3
G = P_intra / P_in = (P_PR / P_mis) * T_PRM * (1-T_PR3) * (1-T_ITM) ~ (P_PR / P_mis) * 0.06
I measured power of POP using C1:LSC-POPDC_OUT. It was 268 when PRM is misalined and MI is locked at dark fringe. Also, it was ~850 when PRMI is carrier locked. When closing PSL shutter, it was ~246. So,
G_PR = (850-246)/(268-246) * 0.06 = 1.6
It looks too small.
The phase margins looks still too small.
Do You need such high gain at LF? This is not a high finesse cavity so can we sacrifice
some DC gain while gaining more phase around UGFs?
Otherwise, the gain fluctuation should be nicely compensated (i.e. fancy normalization).
I modified filiters for LSC_MICH and LSC_PRCL again to cope with power recycling gain fluctuation.
After some more alignment, power recycling gain increased (but still ~3.7). It fluctuates more than a factor of 2, and I began to see glitches again. So I needed more gain margin, as Koji pointed out.
I played around with filters, but I couldn't remove all the glitches. Gain margin now look OK in principle.
It looks like PRM motion is related. Since PRM doesn't have oplev now, I will see PRM oplev tomorrow.
New openloop transfer function:
UGF ~100 Hz, phase margin ~ 50 deg
no phase flip in less than factor of ~5 gain change
550 usec delay
UGF ~100 Hz, phase margin ~ 70 deg (phase bump at UGF)
no phase flip in less than factor of ~5 gain change
550 usec delay
Power recylcing gain:
It is now ~3.7. It fluctuates pretty much. See time series data below when I locked PRMI. MICH and PRCL locks at the same time.
G = (1600-244)/(266-244)*0.06 = 3.7
From Finesse simulation (and also analytic calcs), the expected PRCL optical gain is ~1 MW/m (there is a large uncertainty, let's say a factor of 5, because of unknown losses e.g. PRC, Faraday, steering mirrors, splitting fractions on the AP table between the REFL photodiodes). From the same simulation, the MICH optical gain in the Q-phase signal is expected to be a factor of ~10 smaller. I measured the REFL55 RF transimpedance to be ~400 ohms in June last year, which was already a little lower than the previous number I found on the wiki (Koji's?) of 615 ohms. So we expect, across the ~3nm PRCL linewidth, a PDH horn-to-horn voltage of ~1 V (equivalently, the optical gain in units of V/m for PRCL is ~0.3 GV/m).
In the measurement, the MICH gain is indeed ~x10 smaller than the PRCL gain. However, the measured optical gain (~0.1GV/m, but this is after the x10 gain of the daughter board) is ~10 times smaller than what is expected (after accounting for the various splitting fractions on the AS table between REFL photodiodes). We've established that the modulation depth isn't to blame I think. I will check (i) REFL55 transimpedance, (ii) cable loss between AP table and 1Y2 and (iii) is the beam well centered on the REFL55 photodiode.
Basically, with the 400ohm transimpedance gain, we should be running with a whitening gain of 0dB before digitization as we expect a signal of O(1V). We are currently running at +18dB.
Then I put the RF signal directly into the scope and saw that the 55 MHz signal is ~30 mVpp into 50 Ohms. I waited a few minutes with triggering to make sure I was getting the largest flashes. Why is the optical/RF signal so puny? This is ~100x smaller than I think we want...its OK to saturate the RF stuff a little during lock acquisition as long as the loop can suppress it so that the RMS is < 3 dBm in the steady state.
I did all these checks today.
I will check (i) REFL55 transimpedance, (ii) cable loss between AP table and 1Y2 and (iii) is the beam well centered on the REFL55 photodiode.
So it would seem that there is nothing wrong with the sensing electronics. I also think we can rule out any funkiness with the modulation depths since they have been confirmed with multiple different measurements.
One thing I checked was the splitting ratios on the AP table. Jenne's diagram is still accurate (assuming the components are labelled correctly). Let's assume 0.8 W makes it through the IMC to the PRM - then, I would expect, according to the linked diagram, 0.8 W * 0.8 * (1-5.637e-2) * 0.8 * 0.1 * 0.5 * 0.9 ~ 22 mW to make it onto the REFL55 PD. With the PRM aligned and the beam centered on the PD (using DC monitor but I also looked through an IR viewer, looked pretty well centered), I measured 500 mV DC level. Assuming 50 ohm DC transimpedance, that's 500 / 50 / 0.8 ~ 12.5 mW of power on this photodiode, which while is consistent with what's annotated on Jenne's diagram, is ~50% off from expectation. Is the uncertainty in the Faraday transmission and IMC transmission enough to account for this large deviation?
If we want more optical gain, we'd have to put more light on this PD. I suppose we could have ~10x the power since that's what is on IMC REFL when the MC is unlocked? If we want x100 increase in optical gain, we'd also have to increase the transimpedance by 10. I'll double check the simulation but I"m inclined to believe that the sensing electronics are not to blame.
Unconnected to this work but I feel like I'm flying blind without the wall StripTool traces so I restored them on zita (ran /opt/rtcds/caltech/c1/scripts/general/startStrip.sh).
We locked the PRMI, this time really on the sidebands, using the two REFL55 signals.
Here are the parameters: triggering on POP22_I in at 140, out at 20. No normalization. MICH gain -0.15, PRCL gain 0.1
It seems that the lock is not very stable. It seems likely to come from some large angular motion of one of the mirrors. We'll need to calibrate the optical lever signals to understand which one is moving too much.
> The two REFL55 signals
Wow! It's a good news.
I think this is our first ever lock of PRMI with the REFL I/Q signals.
We kept having difficulty to obtain MICH from the REFL beam.
Next time could you make calibration of REFL55 MICH and AS55 MICH and compare the ratio with any simulation?
We began the evening, after alignment of all optics was good (arms flashing, PRC flashing, assumed SRM last saved alignment was okay), centering all oplevs and aligning beam onto AS55, REFL11, REFL55 and REFL33 and POPDC.
After a quick check to make sure that the input pointing was still okay for Yarm (TRY was 0.88 when we began PRMI work, which we called okay), we aligned and locked the Michelson with AS55Q. We were able to use a gain as large (abs val) as -15 before the loop started oscillating. (ETMs, PRM, SRM all misaligned during this). We measured the UGF of the MICH loop to be 170Hz, with phase margin of 40 degrees.
We then restored the PRM, and tweaked the pointing until the PRC beam at AS overlapped the MICH beam.
We then started playing with locking. We were not very successful in using REFL 11 or REFL 55 (I for both, although we also tried 11Q just for kicks). We then switched to using REFL33I, and had success!! We are reliably able to lock to the "sideband", and not so reliably lock to the carrier (by flipping the sign of the PRCL loop gain). I say "sideband" with quotes, since we aren't sure that it is the sideband. We are, however, confident that it is locking, and it's certainly not locked to the carrier. Videos are at the bottom of the entry.
A list of some values:
Other notes: We changed AS55's demod phase back to 24.5, from it's atmosphere half-cavity value. The change from the original value was recorded in elog 8030.
We changed REFL11's demod phase back to 150, which is the value that it was when we had PRMI locked on ~July 10th, 2012. (We looked up the burt snapshot to check).
FI back, upper right is POP, lower left is REFL, lower right is AS. It seems as though we may need to redo coil balancing now that we're at vacuum / with the current OSEM values.
One of the biggest issues we had was that any Q signals (i.e. the quadrature where PRCL is absent.) of REFL11/33/55/(165) haven't been consistent each other.
i.e. We never had reliable lock of MICH with REFL_any_Q, regardless of the resonant condition. This is definitely one of the things to be tried in order to prepare for the full lock.
We don't trust any demodulation phases of any PDs any more as the previous PRC mode (or say, absence of the stable mode) was unreasonable to determine any of the demodulation phases.
I remember that the POP DC saturates at 27000. You may need to reduce the gain switch again.
The AS OSA and/or POP BBPD would be useful for the sideband PR gain estimation.