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.
PRMI is locked on sideband, starting ~4 minutes ago, to collect ASC/seismic data for feedforward.
First up for me this evening was getting the PRMI locked.
I used the IFO configure screen to lock the X and Y arms, then aligned them using the ASS scripts. Then used the IFO config screen to restore the Michelson, and did some fine tune tweaking of the BS alignment by looking at the AS camera. Then, I restored the PRMI from the IFO config screen, tweaked the PRM a little bit in yaw, and was able to get a lock using REFL 165 I&Q for ~25 minutes before I got bored and unlocked things. I used the ASS for the PRM to align the PRM, then turned off the ASS. POP110 and POP22 both drifted down, but by a small amount, and at the end (when I turned the ASS back on for PRM), they picked back up to about their original levels.
(Note to self: to get it to print both plots, chose custom paper size, make it 14.5 by 11. Don't ask why, just do it, because it works. Also, in PNG device properties, increase the compression to 9.)
After I played with the PRMI, I started looking at the ALS system.
I had both arms locked on IR using the regular LSC system (so POX and POY for the error signals). Then I opened up the green shutters, and got both arms locked on green (so the green lasers were just following the arms...no digital ALS business). I went out to the PSL table and tweaked up the alignment of the green beams (didn't need much at all, just an itsy bitsy bit in yaw, mostly). I saw a very strong peak for the Yarm vs. PSL (around -19dBm), and there was a harmonic of that beat. Opening and closing the Xarm green shutter had no effect on these peaks, so there wasn't any kind of X-Y cross beat sneaking around that I could see. That's really as far as I got - I think (but haven't checked) that Manasa may have removed the power splitter / combiner, so that the RF analyzer is only looking at the Y beat PD (she mentioned earlier today that she was going to give that a try to narrow things down).
After that, Rana and I went back to the PRMI for some noise stuff, and worked on the PMC. See those separate elogs for info on those activites.
I am locking some things, and have the PRM aligned, and it will stay locked for short periods of time, but as Kiwamu warned me, when the PRM alignment is better, the lock is more "crazy" and unstable. This should go on our list of mysteries.
One of the things that I looked at tonight was whether or not I could hold the PRMI on REFL165 at CARM offset of 0, and it turns out that I can. Hooray. The next step was having a look to see if it is actually less noisy than the REFL33 lock.
I calibrated REFL33 and REFL165 to meters (I have the data to do the same for 11 and 55, but haven't done so yet). This way, we can directly compare the signals from each PD.
I scanned between +3 and -3 CARM digital offset (which we think is about 1nm/count while held on ALS), with a ramp time of 10 seconds. I did this several times while the PRMI was locked on both REFL33 and REFL165. Here are the gps times for 8 examples where the PRMI did not lose lock during the sweep:
Here are screen shots from the first REFL33 sweep, and the first REFL165 sweep. DTT can't print 3 plots together, so I'll have to make this nicer later. The top plot is the error signals, calibrated to meters. The middle plot is the control signals, that need to be calibrated to Newtons. The bottom plot is the arm powers, so you can see roughly where we were in the sweep.
We'd like to see a MIST simulation, or perhaps e2e, to see what the predicted disturbance is for each of the error signals during the CARM resonance. We want to make sure that the loops are engaged for all of the degrees of freedom for the simulation.
Recipes for tonight:
REFL165 sometimes has a tough time catching lock by itself, but if you add either REFL33 or REFL55 error signals to the REFL165 signals, it'll catch, and then you can just remove the extra error signals. Also, it doesn't stay locked very robustly unless you include the PRCL FM1 boost.
Here are a bunch of PDFs of time series from last night's CARM sweeps. The y-axes are all calibrated (except for the TRX/TRY, which are just normalized to single arm power, as usual) to real units - meters for the error signals, and Newtons for the control signals. The y-axes for each plot are the same on all PDFs (ex, the control signal plot in the lower left has the same range for all cases) so that it is easy to compare directly.
The most striking thing is that while the PRMI is held on REFL33, the MICH control signal saturates as we go through arm resonance. If the PRMI is held on REFL165, there is no such problem. I think we're going to have a lot more luck keeping the PRMI on REFL 165.
Plots while held on REFL 33:
Plots while held on REFL 165:
I was able to get the PRMI locked on REFL33 I&Q, but it wasn't overly stable, since there is so little separation between the MICH and PRCL signals in that PD.
We have already adjusted the phase to maximize PRCL in the I-phase. Since MICH is ~45 degrees separated from PRCL, there is some projection of MICH in the I-phase, and some in the Q-phase.
To remove this MICH component, I locked the PRMI on REFL55, and drove MICH. I looked at REFL33I at the CARM filter bank input (as just a dummy location to get a signal into DTT). I then added REFL33Q to the CARM row of the input matrix, to try to get the MICH line minimized. I then used these values for PRCL, and used just REFL33Q for MICH, and re-locked the PRMI. The PRMI was much more stable and happy.
The input matrix values that I used were:
MICH: REFL33Q = -0.487, Servo Gain = -20.0
PRCL: REFL33I = 1.556, REFL33Q = 1.8, Servo Gain = -0.020
Some locking notes:
The PRMI is very sensitive to alignment, and the PRM tends to drift away from optimal alignment on a ~1 hour timescale. When the PRM was not well aligned, it looked like MICH had a locking offset (manifested as non-equally sized blobs at AS). The MICH offset seemed to go away when we realigned the PRM.
The PRMI was locked with the carrier field resonant in the PRC 🙌. The lock is pretty stable (I only let it stay locked for ~10mins and then deliberately unlocked to see if I could readily re-lock, but it has stayed locked for the last ~20mins while I typed this up). See Attachment #1 for the DC power monitor StripTool for a short section of lock.
Next (for LSC activities):
I'm leaving the LSC mode off for tonight, but with the PRMI optics aligned and ETMs misaligned.
Finally, we managed to lock PRMI on sidebands:
- The new REFL165 PD was installed on the AP table
- The REFL165I/Q signals are now showing sensible and robust PRCL/MICH signals
- PRMIsb was locked only with these REFL165 signals
- Installation of the REFL165 PD
We prepared the REFL165 PD for the 4" optical height. The actual issue was the power supply for the PD.
We soldered wires between the PD and the RF PD interface break-out board. Then the PD interface
cable for the old REFL165 (iLIGO style) was connected.
At the REFL port, most of the light is rejected by the first beam splitter (R=90%?). We attenuated the beam by a factor of 10
using an ND filter. The new PD showed the DC output of ~10V. This corresponds to the photocurrent of 5mA.
(cf. the shot-noise intercept current is ~1mA)
The output of the REFL165 PD was checked with the RF spectrum analyzer. It was a bit surprising but we had a forest of
RF signals betwen 11MHz and 178MHz. We tried to use a high-pass filter with fc=100MHz (SPH-100) but still the rejection
was not enough. We ended up with using SPH-150 (fc=150MHz).
- Whitening / Demodulation phase
Then we connected the RF output to the SMA cable to the LSC rack. We immediately saw the nice signals from REFL165I/Q
channels, namely sensible structure of pendulum resonances (1/3/16Hz peaks) and floor level.
The whitening level was changed from 21dB to 45dB (max). The DC offsets in the I/Q channels (of the order of 2000~4000)
were removed by the ./LSC/LSCoffset script.
Firstly we locked the PRMI with the usual signals (REFL33I and AS55Q).
The demodulation phase was roughtly tuned (1deg precision) such that the Q phase signal is minimized,
assuming most of the signal is coming from PRCL. Our choise is 74deg.
In this configuration, PRCL shows same quality of signal as our prefered PRCL (i.e. REFL33I) in the amplitude and the sign.
We switched to the REFL165 signal by handing off at the input matrix. The input matrix element for REFL165_I was gradually
increasded up to 0.8 while the element for REFL33I was gradually reduced to 0. We did the same for REFL165_Q with the element of 0.2.
Now we tried locking with REFL165I/Q from the beginning. Once the alignment is adjusted, the lock was immediately obtained
only with REFL165I/Q. Today we did not adjusted the ASC stuff (OPLEVs and PRM ASC) so the lock was not long (<1min). Particularly
ITMX poiting kept drifting and it made the lock difficult. We should check the oplev setup carefully.
- LSC summary
Signal source: REFL165I (74deg) / Whitening gain 45dB
Normalization sqrt(POP110I x 0.1) / Trigger POP110I 100up 3down
Servo: input matrix 0.80 -> PRCL Servo FM3/4/5 Always ON G=+2.50
Actuator: output matrix 1.00 -> PRM
Signal source: REFL165Q (74deg) / Whitening gain 45dB
Normalization sqrt(POP110I x 10.0) / Trigger POP110I 100up 3down
Servo: input matrix 0.20 -> MICH Servo FM4/5 Always On G=-40
Actuator output matrix -1.00 -> ITMX / +1.00 -> ITMY
- Refine the PRM asc servo (AC coupled)
- Align oplevs
- ITMX oplev is drifting quickly (~1min time scale)
Since Q has found that REFL165 will be better for holding the PRMI while we reduce the CARM offset, I had a look at locking PRMI sideband locking with both 3f PDs.
I checked the REFL165 demod phase, and changed it from -142.5 deg to -138.5 deg. to minimize the Q signal while driving PRM length.
I found that keeping the MICH and PRCL loop gains the same, and using matrix elements +0.1 for both I and Q for REFL165, rather than +1 for both I and Q for REFL 33.
MICH gain is +0.8, PRCL gain is -0.02. FMs 4,5 on for both, FM 2 triggered for MICH, FMs 2,3,6 triggered for PRCL.
I then locked the PRMI on sideband with REFL 33 and then REFL 165, and measured the other one as an out of loop sensor of the motion. I find that REFL33 and 165 are both comparable, and so we shouldn't have any trouble using REFL165 for locking.
[Rana, Gabriele, Jenne]
We have now locked the PRMI using REFL55 I&Q for more than one minute!!!!!
This isn't really the most useful plot as is, but it was created using:
/opt/rtcds/caltech/c1/scripts/general/getdata C1:LSC-POP22_I_ERR_DQ C1:LSC-REFL55_I_ERR_DQ C1:LSC-REFL55_Q_ERR_DQ C1:LSC-MICH_IN1_DQ C1:LSC-MICH_OUT_DQ C1:LSC-PRCL_IN1_DQ C1:LSC-PRCL_OUT_DQ -d 80 -s 1049013520 -c
This is just one of several long lock stretches. If I can get the TRIG_MON channels to be saved, we can automatically (versus my by-hand search) find lock stretches and make this kind of plot. Although we want them saved in some raw format so we can zoom in on selected axes, I think. This might require some python-fu from Jamie, or learning of python-fu for Jenne.
The secret sauce:
* The big key was changing REFL55's phase. It was -4 when we looked at the I&Q signals, and minimized the PRCL information in the Q-phase. We were able to get short lock stretches with this. During these stretches, Rana changed the REFL55 phase until the lock sounded (audibly) quieter. The final phase we settled on was +26. As we changed the phase, the lock stretches got longer and longer.
* We also tweaked up the POP22 phase. It was close from our previous efforts of looking at non-locked time series, but we perfected it by minimizing the signal in the Q-phase during lock stretches. We also found that it drifted (according to this method) by ~5 degrees over ~half an hour (I don't remember the exact time between our phase tunings).
* POP22's low pass filters (both options, ELP10 and ELP50) must be OFF for any lock to be acquired. Turning on either filter prevents locking.
* Normalization helped a lot. Without normalization we weren't really able to catch any locks, certainly not of any significant length. (0.004, using POP22I, for both MICH and PRCL).
** Normalization: use POP22I for both MICH and PRCL, value = 0.004
** Input matrix: MICH with REFL55Q, value = 0.01; PRCL with REFL55I, value = 0.01 (we used the small number in the matrix so our servo gains weren't too tiny).
** POP22 lowpass filters OFF
** Analog whitening OFF for REFL55, POP22.
** Analog gain for REFL55 I&Q = 27 dB
** Analog gain for POP22 I&Q = 15 dB
** Output matrix: MICH with -1 to ITMX, +1 to ITMY. PRCL with +1 to PRM.
** Servo gains: PRCL = 0.75; MICH anywhere between -3 and -20. Best in the -8 to -15 range.
** Vio2 filters in ITMX, ITMY, PRM (all actuated-on mirrors) were OFF. (Still need to lower the Q on these so they don't ring).
** PRCL and MICH triggering on POP22I. The trigger-off was always 20, but the trigger-on changed throughout the night from ~170 to ~50. I think 130 was a trigger value for at least some of the long-time locks.
** Low frequency seismic was small (i.e. no anomalous 0.1 Hz - 1 Hz noise) during successful lock times. (Not to say it must be low, but it was low when we were able to lock for long stretches).
Things we had looked at and thought about throughout the evening:
* Oplev calibration. See elog 8391 and 8393. Optimized BS and PRM to reduce yaw angular motion.
* Actuators all functioning as expected. We checked transfer functions of MICH_OUT/MICH_IN1 for locking with different optics, to ensure that at high frequency the response was 1/f^2. Also, we locked MICH with (a) both ITMs, (b) BS, (c) ITMX and (d) ITMY. We locked the PR-ITMY half-cav with (a) PRM and (b) ITMY. We locked the PR-ITMX half-cav with (a) PRM and (b) ITMX. Thus, we conclude that all of the PRMI-related optics are functioning as expected.
* Realigned REFL55 beam onto PD. It was clipping a bit, so the DC power wasn't steady (when ITMs were misaligned, PRM aligned). After alignment, the DC power as seen on a 'scope was much smoother.
* Turning off the limiters for the MICH and PRCL control signals allowed us to hear a high-pitched whine. From looking at the time series, it's predominantly in MICH_OUT. Rana speculates that perhaps the normalization is causing the UGF to wander temporarily to an unstable place. For a time there was a high-Q peak between 500 and 600Hz, but reducing the gain (of MICH?) eliminated that. Then we heard several times, irrespective of gain setting, the ~400Hz broad peak (I say broad because I was able to see it on DTT looking at the error and control signals, and it spanned +/-100Hz).
Things to investigate:
* Is there a good reason that we should switch to triggering on POP110, rather than the current POP22? From Gabriele, Jamie and my Finesee/Mist modelling last week, without the arms, the 11MHz and 55MHz resonate at different PRC lengths. If this difference is very small, then we are fine, but if the difference is large, it could be causing trouble - we're trying to catch the lock at the linear part of the 55MHz signal, but if that does not coincide with the linear part of the 11MHz signal, we're doing the wrong thing.
* For the POP normalization, should we be using the amplitude or the power ( POP22 or sqrt(POP22) )? Why? Look at this with a modelling sweep and/or analytically.
* Look at different noise sources, potentially sensing noise, coil actuator noise,..... We should check these out, and make sure we're not limited by anything obvious.
* Make a "restore" medm screen, rather than restore script. IFO Configure restore script can pull in values from the screen (EPICS values). One screen per configuration.
* Get TRIG_MON signals saved, write script to search for triggered lock times (between given gps times), then plot interesting signals for just before lock, during lock, and until just after a lockloss.
Koji is working on PRMI locking, and while he was doing that I glanced at the oplevs' spectra for the ITMs and PRM.
I found that when the PRMI was locked (for only 1 second or so max lock time) on the 55MHz sideband, and the error signals show a big peak around 400Hz (definitely audible in the control room), the only OpLev that I see a similar peak in is ITMX pitch.
In the plot below, I have grabbed a time when the PRMI was flashing as the black reference traces, and then a time when the PRMI was locked as the active traces. You can see that there is a similar peak in both REFL55I and ITMX_OL_PIT when the cavity is locked.
I tried to reproduce the locking situation described in this entry tonight.
The momentary lock was regularly seen but there was no stable lock.
I wonder why the actuators are always saturated. The feedback signals have the dominant component at ~400Hz.
It would also be nice if the servos have some immunity to gain fluctuation.
I didn't check how the situation of the AP table is. I'll look into some details tomorrow.
- Disabled MCL path in mcdown/mcupscript.
Nominal gain in mcdown/mcup was -50 and -100 respectively.
- Confirmed the stable lock was just because of the quiet seismic of the Friday night.
- Improvement of the PRM ASC servo
RG3.2 (3.2Hz Q=2 Height 30dB)
RG3.2 (3.2Hz Q=10 Height 30dB) + zero[f, 1, .5] pole[f, 2, 3] zero[f, 4.5, .5] pole[f, 3.5, 3]
Filter shape comparison is found in the second plot attached.
The resulting spectra (freerun vs controlled) is found in the first plot.
Nominal PRM ASC gain is +70
- Openloop TF measurement
OLTF PRCL 250Hz 30deg / MICH 200Hz 45deg
- REFL55/REFL33 phase adjustment (in lock)
REFL55 phase fine tune (95.25deg) (x1,x0.3)
REFL33 phase (-13.0deg) (x1, x2)
We wanted to try the PRC length measurement,but we ended up spending all the afternoon to lock the PRMI on sidebands. Here are some results
- Locked PRMI with REFL165 I/Q
- Aligned the POP beam on the QPD. We found that the vertical motion of the beam appeared in the yaw signal, and horizontal motion in the pitch signal.
This was fixed by swapping the cables to the ADC. Later it turned out that this was caused by the calibration setup for the QPD.
We requested Jenne to fix the QPD on the table with the current orientation.
- Re-implemented the AC-coupled ASC servo. The filters were just copied from the previous PRM ASC servo (in the SUS ASC filter).
The same filter was installed to the pitch and yaw filter modules for now. The gains were adjusted to have some stable lock stretches.
The power spectra of C1:ASC-PRCL_YAW_IN1 and C1:ASC-PRCL_PIT_IN1 were attached.
The reference curves are the ones with the servo on. The other two are the free-running stability of the QPD output.
- Modified the up and down scripts for the PRM ASC for the new setup.
It first turns on the inputs of the filters and then turn on FM2/3.
It assumes that the outputs are engaged all time.
Here are some details about the PRMI locking done last night.
REFL11 has been installed on the AP table. The RF signal from the RFPD is sent by a heliax cable which has been called ASDD133.
Before the beam goes into the RFPD a HWP and PBS are installed such that we can adjust the amount of light entering to the photo diode.
One thing I didn't like was that I had to introduce a big amount of the light into the PD to get a reasonably big RF signal.
I was trying to look for an RF signal by looking at a spectrum analyzer, then I realized that the RF signal at 11 MHz was quite tiny when the DC_MON was less than 1.5 V.
After I increased the amount of the light up to 1.9 V in DC_MON, which sounds already too much, I then got able to see the 11 MHz signal on the analyzer.
Note that I decreased the amount of the light down to 0.5 V after I finished locking the PRMI.
We should make sure what is going on with the 11 MHz modulation.
First I started locking the MIchelson with AS55. The demodulation phase was already somewhat optimized to the I-signal port.
So I decided not to touch the demodulation phase matrix because it may take some times.
After I eliminated electrical offsets in the digital side, I was easily able to lock the Michelson. The control sign was plus.
Then I started playing with the PRC control too. The demodulation phase in REFL11 looked nearly 45 deg although I didn't carefully measure it.
I made a 45 deg rotational matrix to maximize the I-port signal and tried to lock the PRC. Then immediately I was able to lock PRC as well as MICH.
GAIN_MICH = 100
GAIN_PRC = 100
Also GAIN_PRC = -100 gave a carrier resonant lock.
The control filters are the same in MICH and PRC. I used my favorite filters as usual.
FM1 = 1000 : 10
FM6 = 0.1 : 1
FM7 = 1 : 50
Somehow I frequently failed to engage the boost filters (i.e. FM6 and FM7) it looks offsets in the control path kicks either BS or PRM.
The PRMI has been successfully locked
Details will be posted in the morning.
Also changing the sign of the PRC control gave me the lock of the carrier resonant condition.
The screenshot above is the time series of the error signals when I was locking the PRMI in the sideband resonant condition (i.e. carrier is non-resonant).
Note that I used REFL11 for the PRC control and AS55 for the MICH control as planed.
Details will be posted in the morning.
Today we will try to lock the PRMI.
I was trying to measure the sensing matrix in the PRMI configuration, but basically gave up.
It is mainly because the lock of PRMI wasn't so stable and it didn't stay locked for more than a minute.
It looked like an angular motion fluctuated a lot around 1- 3 Hz. The beam spot on the AS camera moved a lot during the lock.
I have to figure out who is the bad suspension and why.
All the suspensions are bad until you fix them. But, ... there is a script which can be used to diagnose them today: