About Noise Budget
How to improve it ?
I was working around the PSL table and Y endtable today.
I modified the Y arm optical layout that couples the 1064nm light leaking from the SHG crystal into the fiber for frequency offset locking.
The ND filter that was used to attenuate the power coupled into the fiber has been replaced with a beam sampler (Thor labs BSF-10C). The reflected power after this optic is ~1.3mW and the trasmitted power has been dumped to a razor blade beam dump (~210mW).
Since we have a spare fiber running from the Y end to the PSL table, I installed an FC/APC fiber connector on the PSL table to connect them and monitored the output power at the Y end itself. After setting up, I have ~620uW of Y arm light on the PSL table (~48% coupling).
During the course of the alignment, I lowered the power of the Y end NPRO and disengaged the ETMY oplev. These were reset after I closed the end table.
Attached is the out of loop noise measurement of the Y arm ALS error signal before (ref plots) and after.
Prior to the works on the Y end setup I propose to perform the temperature scan business like Koji and Suresh did before (see this entry).
This business will allow us to easily find a beatnote at 532nm after the installation on the Y end.
I guess the right persons for this work are Bryan and Suresh.
Bryan will have a safety guidance from Steve in this after noon. So after that they can start working on it.
/* - - - coarse plan - - - */
* remove Alberto's laser from the AS table
* setup Alberto's laser on the PSL table
* put some stuff such as lenses, mirrors and etc. (Use the IR beam picked off after the doubling crystal for the main laser source)
* mode matching
Which laser are we going to use, Alberto's laser or MOPA laser ?
I found the beat note for the Y arm. Nothing was changed with respect to yesterday night, but the beat is back!
As I didn't have the green laser PZT feedback for the laser temp control, I went to the yend to check out what's the situation.
I found horrible and disgusting "remnants".
WHAT ARE THESE BSs AT THE Y END?
- The table enclosure was left open
- A (hacky) DB25 cable with clips was blocking the corridor and I was about to trip with the cable.
- This DB25 cable went to the table without going through the air tight feedthrough that is designed for this purpose.
- An SR560 (presumably for the openloop TF measurement) was left inserted in the loop with entangled cables connected to the servo box.
- Of course the laser PZT out mon was left unplugged.
Even after cleaning these cables (a bit), the end setups (including the X end too) are too amature.
Everything is so hacky. We should not allow ourselves to construct this level of setup everytime
we work on any system. This just adds more and more mysteries and eventually we can't handle
Beam colors: 1064 nm red, 514 nm green and 633 nm yellow.
There should be room for lens in front of the pd at red3 and a mirror for alignment in the new layout.
This picture may help you how to improve the new ETMX 4' x 3' optical layout.
[Tomotada / Kiwamu]
The open loop transfer function of the Y end PDH loop was remeasured : the UGF was found to be at 17 kHz.
The phase margin at the UGF was about 27 deg.
While the measurement we noticed that the modulation onto the laser PZT was too big
and it was creating a big AM on the reflected light with an amplitude of a few mV.
So we put a 20 dB attenuator to decrease the modulations and the reflected light became much quitter.
Also the servo shape formed by Newfocus LB1005 looks too simple : we should have a more sophisticated servo filter (i.e. PDH box!!).
As promised, I will get on this this week.
The doubling oven is now ready to go for the Y arm. The PPKTP crystal is mounted in the oven:
Note - the crystal isn't as badly misaligned as it looks in this photo. It's just an odd perspective shot. I then closed it up and checked to make sure the IR beam on the Y bench passes through the crystal. It does. Just need to tweak the waist size/position a bit and then we can actually double some frequencies!
The Y arm green PDH servo is working fine with a sufficient amount of suppression.
And the servo configuration looks like this :
(the Error signal)
I took a spectrum of the error signal when the laser was locked to the Y arm and found that it meets the requirement.
Another way to make a 1:100 pole:zero boost is to use resistors and capacitors in a Pomona box
mixer -> LB box -> Pomona box -> PZT
Pomona Box = R1 = 7.2 kOhm, C2 = 22 uF, R2 = 72 Ohms (sr560 = $2400, pomona ~ $50)
For the RMS calculation, it would be good to notch out the harmonics. They don't matter since our ALS feedback will have notches at those frequencies.
I wouldn't bother...
I measured the PZT actuator gain for the Lightwave NPRO at the Y-end to be 3.6 +/- 0.3 MHz/V. This is somewhat lower than the value of 5 MHz/V reported here, but I think is consistent with that measurement.
In order to calibrate the Y-axis of my Aux PDH loop noise budget plots, I wanted a measurement of the end laser actuator gain. I proceeded to measure this as follows:
The attached plot shows the measured data. The X-axis is shown after the conversion mentioned in the last bullet point. The error bars are the standard deviations of the averaging at each DC offset.
After the discussions at the Wednesday meeting, I redid this measurement using a sinusoidal excitation summed at the error-point of the PDH servo as opposed to a DC offset. From the data I collected, I measured the actuator gain to be 2.43 +/- 0.04 MHz/V. This is almost half the value we expect, I'm not sure if I'm missing something obvious.
I redid this measurement and have now determined the actuator gain to be 4.61 +/- 0.10 MHz/V. This is now pretty consistent with the expected value of ~5MHz/V as reported here.
I made the following changes to the old methodology:
I also took spectra of the phase tracker output and error signal to make sure I was choosing my excitation frequencies in regions where there were no peaks already present (Attachment #1).
The scatter of measured actuator gains at various excitation frequencies is shown in Attachment #2.
Indeed it is strange. I took a quick look at it.
In order to recover the same condition (e.g. the same amount of the reflected DC light and the same temperature readout),
it needed to have +8.9V in the slow input from the DAC through EPICS.
Obviously applying an offset in the slow input to maintain the same condition is not good.
It needs another solution to maintain the sweet frequency where the frequency of the PSL and the Y end laser is close in a range of 200 MHz.
Plugging in the thermal feedback BNC cable to the laser reduced the DC voltage of the green PDH photo diode from 3.12 V to 1.5V off resonance.
Currently, the y end plant is yep.mdl. In order to compile it properly (for the moment at least) requires running the normal makefile, then commenting out the line in the makefile which does the parsing of the mdl, and rerunning after modifying the /cds/advLigo/src/fe/yep/yep.c file.
The modifications to the yep.c file are to change the six lines that look like:
"plant_mux = plant_gndx" into lines that look like "plant_mux = plant_delayx". You also have to add initialization of the plant_delayx type variables to zero in the if(feInt) section, near where plant_gndx is set to zero.
This is necessary to get the position feedback within the plant model to work properly.
#NOTE by Koji
#NOTE by Koji
This entry means that Makefile was modified not to parse the mdl file.
This affects making any of the models on megatron.
#NOTE by Koji
This entry means that Makefile was modified not to parse the mdl file.
This affects making any of the models on megatron.
To prevent this confusion in the future, at Koji's suggestion I've created a Makefile.no_parse_mdl in /home/controls/cds/advLIGO on megatron. The normal makefile is the original one (with correct parsing now). So the correct procedure is:
1) "make yep"
2) Modify yep.c code
3) "make -f Makefile.no_parse_mdl yep"
Attachments #1 is the current setup of AUX Y Green locking and it has to be improved because:
About the above two:
One of the example for improvement is just adding a new lens (f=10cm) soon after the doubling crystal. That will make mode matching better (100%) and also make separation better (85 deg) (Attachments #4 and #5). I'm checking whether we have the lens and there is space to set it. And I will measure current power of transmitted main laser in order to confirm the improvement of alignment.
About the last:
I am considering what component is needed.
[ Yuki, Gautam ]
The setup I designed before has abrupt gouy phase shift between two steering mirrors which makes alignment much sensitive. So I designed a new one (Attached #1, #2 and #3). It improves the slope of gouy phase and the difference between steering mirrors is about 100 deg. To install this, we need new lenses: f=100mm, f=200mm, f=-250mm which have 532nm coating. If this setup is OK, I will order them.
There may be a problem: One lens should be put soon after dichroic mirror, but there is little room for fix it. (Attached #4, It will be put where the pedestal is.) Tomorrow we will check this problem again.
And another problem; one steering mirror on the corner of the box is not easy to access. (Attached #5) I have to design a new seup with considering this problem.
[ Yuki, Steve ]
With Steve's help, we checked a new lens can be set soon after dichroic mirror.
We want to remotely control steeing PZT mirrors so its driver is needed. We already have a PZT driver board (D980323-C) and the output voltage is expected to be verified to be in the range 0-100 V DC for input voltages in the range -10 to 10 V DC.
Then I checked to make sure ir perform as we expected. The input signal was supplied using voltage calibrator and the output was monitored using a multimeter.
But it didn't perform well. Some tuning of voltage bias seemed to be needed. I will calculate its transfer function by simulation and check the performance again tommorow. And I found one solder was off so it needs fixing.
diagram --> elog 8932
Plan of Action:
Interim Procedure Report:
The current setup of AUX Y-arm Green locking has to be improved because:
What to do
Final Procedure Report for Green Locking in YARM:
To do for Green Locking in YARM:
The auto-alignment servo should be completed. This servo requires many parameters to be optimized: demodulation frequency, demodulation phase, servo gain (for each M1/2 PIT/YAW), and matrix elements which can remove PIT-YAW coupling.
The MON outputs of the Y end QPD whitening board were hot earlier today while pulling it out of the crate. After swapping the 4 pin lemo connector with an isolated panel mount bnc connector, I stuck the board back into the crate and this immediately kicked the ETMY suspension. Jenne and I went to the Y end to look at what was going on. We removed the board from the crate after smelling something burning. The MON output ports of the whitening board were super hot this time. There is no sign of any components melting on the board (comparing the board with its pictures that were taken earlier) and a tester board stuck into the crate lights up just fine.
So the back panel is still ok. We need to troubleshoot or replace the whitening board.
Edit, JCD: The attached photos are from right after I replaced the "Rgain" resistor, elog 9823. What they show is that it looks like some of the melting / burning may have already been happening before I pulled the board, and I just never noticed :( In particular, look at the resistors on the main board above the blue "G" sticker. There isn't a difference that I can tell between this photo from last week, and today's situation.
maybe the tantalum caps on the daughter board power supply lines are blown? If so, replace with 35V+ ceramic.
The main problem was a panel fixing bolt that caused the short circuits between power supply layers.
This burned the PCB and secondarily caused permanent short circuit between +15V/-15V/+5V layers.
- The resistances between +15V, +5V, and -15V were low. The resistance between +15V and -15V is 13 Ohm.
The one between +5V and -15V is 7Ohm. And the one between +15 and +5 is 19Ohm. So the situation is
+15V o-(13 Ohm)-+-(9 Ohm)-o +5V
Even after removing all of the active components from the board, they remained the same.
- The tantalum caps were removed from the board and it was confirmed that they are not the cause of the issue.
- The panel was removed from the module for the component migration to a spare board (to be described in the other entry).
I found that the screw hole and the screw have burnt marks. The screw need an insulation tube to avoid short circuit.
The other screw was also bare. The spare board has the screws with the insulation tubes.
Q put the X PDH box back, so that I could try locking, and remember which end is up after a week away.
I am unable to hold ALS comm/diff for any length of time. Only once today did I hold it through the FM3 boost turn-on. So, I looked at the individual arms.
Xarm, even though it's the one that Q is seeing this saturation problem with, seems fine.
Yarm however is having trouble holding lock for more than a few minutes at a time. The green beam stays locked to the arm for ~infinity, so I'm not so worried about the PDH box right now. If I look at the error and control points of the ALS digital servo, the Yarm is much more noisy above about 20 Hz. Something that I might think of for this kind of mismatch at higher frequencies is poorly matched whitening / dewhitening, or none at all for the Yarm, however this doesn't look like that to me. Based on the shape of the spectra, I don't think that we're running into ADC noise. For this plot, both arms are individually locked with ALS feeding back to the ETM, gain magnitude of 15 (Xarm gets a minus sign because of our temperature / beatnote moving direction convention), FMs 1,2,3,5,6 on. Something that seems critical for getting the Yarm to have the FM3 boost without losing lock is having the SLOW temperature servos on for a little while so that the PZT output (as monitored on the temp servo screen) for the end lasers fluctuate around zero. Right now, both beatnotes are at about 62MHz, with an amplitude of about -31dBm.
I still need to do a somewhat more thorough investigation of what might be causing the Yarm locklosses. Is the length-to-angle decoupling worse for ETMY than for ETMX? Am I moving the arm length so far that the PZT can't follow within its actuation limits? Does the Yend PDH box have a similar saturation to the Xend box, but somehow (a) worse, and (b) not as obvious so we didn't suspect it before?
I need to put this plot into calibrated units, and also include the low frequency monitor that we have of the PDH error point (all of which are _DQ channels).
Things to do:
* Figure out Xend PDH box saturation issue. Is Yend seeing same saturation in the variable gain amplifier? We have 3 spares of these chips in the Plateau Tournant Bleu, if we need them.
* Check Yarm ALS stability. (NB: The arms have been individually locked for the last 15 min or so while I've been writing, so maybe letting the slow servo settle is the key, and this is not something that needs work).
* Get CARM on DC Trans, DARM on AS55Q (after arm powers of about 1). Can we see good REFL DC dip? Should we try using just the transmission PD signal as the error signal for the CM board, if we aren't close enough to resonance to use REFL DC?
I found the big big Y green beat. Details will be posted later.
I found the big green beat note for the Y arm. The alignment of the green optics on the PSL table was crappy.
What I did:
1. By adjusting PSL laser temperature, I found tiny beat note when
PSL laser temperature on display: 31.35 deg C (PSL HEPA 100%)
C1:PSL-FSS_SLOWDC = 1.75
PSL laser temperature on display: 33.21 deg C (PSL HEPA 100%)
C1:PSL-FSS_SLOWDC = -6.82
Y end laser temperature settings are fixed as follows during the measurement.
Y end laser "T+": 34.049 deg C
Y end laser "ADJ": 0
Y end laser measured temperature: 34.13 deg C (*)
C1:GCY-SLOW_SERVO2_OFFSET = 29845
Bryan's formula (swapped one; see elog #6746), suggests the paring
(Yend laser temp, PSL laser temp) = (34.13 deg C, 31.09 deg C).
2. Checked that beat PD is working by swapping the beat PDs for Y arm and X arm.
3. Checked that the mode-matching of the two beams, one from Y arm and the other from PSL, is OK by moving mode-matching lens and measuring the beam spot size at near/far field are the same.
4. When checking the beam spot size at far field(~ 1 m from the BS), I noticed the relative beam tilt by ~ 1 mrad. We aligned them few days ago, but I think the green beam from the Y arm has shifted. Of course we align IR to the Y arm first, but we difinitely need dither servo or A2L for the arm, too.
5. As soon as aligning the PSL green optics near the BS, I found a large beat note. The measured amplitude was ~ -26 dBm, without any amplifiers after the PD.
Currently the measured green beam power onto the beat PD from Y end is 75 uW and from PSL is 92 uW. So the calculated beat amplitude will be ~ -10 dBm (see calculation in elog #6746). So there is about 84% loss. Anyway, I will go on to the mode scan.
I tried to find Y arm green beat in order to do the mode scan.
I found a beat peak(see attached picture), but the amplitude seems too small.
It is may be because the alignment/mode matching of the green beams at the PSL table is so bad. Or, the peak I found might be a beat from junk light.
What I did:
1. Aligned Y arm to the IR beam from MC.
2. Re-aligned Y end green beam to the Y arm using steering mirrors on the Y end table.
3. Re-aligned PSL green optics.
# C1:GCV-GREEN_TRY is temporary connected to the DC output of the Y green beat PD.
4. Temperature of the PSL laser was 31.48 deg C, so I set "T+" of the Y end laser to 34.47 deg C, according to Bryan's formula (elog #4439);
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
5. Scanned Y end laser temperature by C1:GCY-SLOW_SERVO2_OFFSET. Starting value was 29725 and I scanned from 27515 to 31805, by 10 or 100. Laser frequency changes ~ 6 MHz / 10 counts, so it means that I scanned ~ 2.5 GHz. During the scan, I toggled C1:AUX-GREEN_Y_Shutter to make sure the green beam resonates in TEM00 mode.
# I made a revolutionary python script for toggling channels(/opt/rtcds/caltech/c1/scripts/general/toggler.py). I made it executable.
6. Found a tiny beat note when C1:GCY-SLOW_SERVO2_OFFSET = 29815. I confirmed it is a beat signal by blocking each PSL and Y arm green beam into the beat PD. I left C1:GCY-SLOW_SERVO2_OFFSET = 29815.
7. I found that Bryan's formula;
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
Y_arm_Temp_set = 0.95152*T_PSL + 3.8672
Y_arm_Temp_meas = 0.87326*T_PSL + 6.9825
according to his graph(elog #4439). So, I set "T+" of the Y end laser to 33.82 deg C.
8. This time, I scanned PSL laser temperature by C1:PSL-FSS_SLOWDC. I found a tiny beat note when C1:PSL-FSS_SLOWDC = 1.0995. C1:PSL-FSS_SLOWDC has 10 V range, so I scanned ~ 10 GHz, assuming the laser frequency changes 1 GHz/K and the temperature changes 1 K/V.
9. Re-aligned PSL green optics so that the beam hits optics at their center, and checked that the poralization of the two green beams are the same.
10. Checked that amplifier ZFL-100LN+ on the beat PD is working correctly. The power was supplied correctly (+15 V) and measured gain was ~ 25 dBm.
11. Exchanged BNC cable which connects the beat PD to the spectrum analyzer. Previous one we used was too long and it had -15 dB loss(measured). I exchanged to shorter one which has -2 dB loss.
Beat note amplitude estimation:
The amplitude of the beat note observed in the spectrum analyzer was ~ -54 dBm. According to the estimation below, it seems too small.
The measured power of the two green beams are
P_Y = 4 uW
P_PSL = 90 uW
So, the power of the beat signal should be
P_beat ~ 2 sqrt(P_Y * P_PSL) = 37 uW
Responsivity and transimpedance of the beat PD (Broadband PD, LIGO-T0900582) are 0.3 A/W and 2 kOhm. So, the power of the electrical signal is
W = (P_beat * 0.3 A/W * 2 kOhm / sqrt(2))^2 / 50 Ohm = 5 uW
5 uW is -23 dBm. We have +25 dB amplifier after the PD and the loss of the BNC cable is -2 dB. So, if the two beams interfere perfectly, the peak height of the beat signal should be ~ 0 dBm. The measured value -54 dBm seems too small. According to elog #5860, measured value by Kiwamu and Katrin was -36 dBm.
PSL laser temperature: 31.48 deg C (PSL HEPA 100%)
Y end laser "T+": 33.821 deg C
Y end laser "ADJ": 0
C1:GCY-SLOW_SERVO2_OFFSET = 29815 (was 29725)
Koji soldered new 50" long cable for the Y station.
Please check the spectra. If something is wrong, please swap the cables between X and Y in order to see if the cable is still the issue. I believe the cable was nicely made as I carefully checked the connection twice or more during and after the soldering work.
Atm1, New short-50" long cable was installed at ETMY end ( Y-station ) between Guralp-B ( MIT ) and granite base.
Interface box input 2 was left connected to cable 1 and input 1 to cable 2. This plot shows no change.
Atm2, Than I swapped the two long cables at the interface box
Now the signal seems to be ok <2 Hz,
>2 Hz some problem exist.
50" short cable
I will look for more bad soldering tomorrow. How many cables did she make?
The optics on the Y-end table which required to be moved have been repositioned. Please see the attached pic for details.
The green beam is not yet aligned to the cavity. That is my next task.
The Y-arm can now be locked with green light using the universal PDH servo. Modulation frequency is now 277kHz - chosen because it seems to produce smaller offsets due to AM effects
To lock, turn on the servo, align the system to give nice circular-looking TEM_00 resonances, and wait for a good one. It'll lock on a decent mode for a few seconds and then you can turn on the local boost and watch it lock for minutes and minutes and minutes.
The suspensions are bouncing around a bit on the Y-arm and the spot is quite low on the ETMY and a little low on ITMY, but from this point it can be tweaked and optimised.
0) Did a bunch of alignment to get beams roughly centered on ETMY and ITMY and maximize power. Adjusted the aperture and focus on ETMY camera to get nice image. Camera needs to be screwed in tightly and cables given some real strain relief, Steve.
1) snapshots not working on many MEDM screens. Who's on top of this?
2) save/restore not working for PZT2 sliders
3) changed power and filter triggers on yarm to match xarm
4) yarm filters copied from xarm (need to handtune RGs)
5) DTT wasn't working on rossa. Used the Date/Time GUI to reset the system time to match fb and then it stopped giving 'Test Timed Out'. Jamie check rossa ntpd.
6) Removed the high 3.2 Hz RG filter. We don't have any sharp features like that in the spectrum.
---then added it back. The 3.2 Hz comes and goes depending on what Yoichi is doing over in the MC area. Leaving it in by default, but lowering the Q from 2 to 1.5.
7) Attached is the noise spectra, coherence, and loop gain model for this yarm condition. For the plant model, I assume a pendulum (f=1 Hz, Q = 9) and a cavity pole of 1600 Hz. Gain is scaled to set the UGF at 165 Hz (as guessed by looking at the servo gain peaking frequency). This cheezy model doesn't include any of the delays from DAC, AA, or AI. Eric and Sasha should have something more useful for us by Friday.
8) Change the DQ channels: need XARM and YARM IN1 at 16k. e.g. XARM_ERR, etc.
9) To get the DTT plots to make thumbnails in the elog, I print a .ps file and then use 'epstopdf' to make the PDF.
I modified the Y-Arm PDH box (S/N 17) to have the same TF as the one of the temporary setup described in Kiwamu's earlier entry. Note that the TF below was taken with the gain knob set to 0, so that the proper DC gain is achieved with a setting of ~4. This is desirable because it gives us wiggle room.
The changes were:
Below is the TF along with the LISO model. They are different at low frequencies because the box must have been railing internally (though the phase shows that the result is as expected), and there is a feature around 60 kHz that probably arises from some op amp instability. I will see if adding a small cap somewhere does the trick, and then take a new TF with a lower source voltage.
I'll try to lock the arm with the box tomorrow.
I installed the newly modified PDH box #17 and locked the Y-Arm.
I wasn't able to bring the REFL level down to the 30% that Kiwamu claimed to get, despite readjusting the alignment---I got ~40-45%. I attained a UGF of ~8 kHz, lower than the 20 kHz that Kiwamu said he got with the temporary setup, probably because the PDH box just isn't as fast. Despite that, it looks like the error suppression is actually better than before...
Here is an error spectrum:
I have to admit that this calibration is worthy of suspicion and should be done more rigorously. I simply used the measured UGF frequency and known servo TF and PZT actuator gain to estimate the optical response. I am pretty confident that it's accurate to within a factor of 3 or so.
Using the ALS green beat and armlength feedback I mapped an IR resonance of the Y-Arm by stepping through a ramp of offset values.
First I optimized the IR alignment with the dither scripts while LSC kept the arm on resonance, and then transitioned the length control to ALS. The beat frequency I obtained between the Y-arm green and the PSL was about 25 MHz. Then I applied a controlled ramp signal (stepping through small offset increments applied to LSC-ALSY_OFFSET, while logging the readback from channels LSC-TRY_OUT16 and ALS-Y_FC_SERVO_INMON with an averaging time of 1s.
The plots show the acquired data with fits to and , respectively.
The fits, weighted with inverse rms uncertainty of the data points as reported by the cds system, returned HWHM = 0.6663 ± 0.0013 [offset units] and m = -0.007666 ± 0.000023 [MHz/offset unit], which gives a combined FWHM = 10,215 ± 36 Hz. The error is based purely on the fit and does not reflect uncertainties in the calibration of the phase tracker.
This yields a finesse of 388.4 ± 1.4, corresponding to a total loss (including transmissivities) of 16178 ± 58 ppm. These uncertainties include the reported accuracies of FSR and phase tracker calibration from elog 9804 and elog 11761.
The resulting loss is a little lower than that of elog 11712, which was done before the phase tracker re-calibration. Need to check for consistency.
With the exception of a 2" mirror mount, I've confirmed that we have everything for the Y-end green production and mode-matching.
We need to calculate a mode-matching solution for the Lightwave laser so that it gives the correct beam size in the doubling crystal.
Additionally, Rana has suggested that we change the pedestals from the normal 1" diameter pedestal+fork combo to the 3/4" diameter posts and wider bases that are used on the PSL table (as shown in the attached image).
There was a 2" mirror mount among the spares on the PSL table. It has a window LW-3-2050 UV mounted in it. I
have moved it to the Y-end table. We seem to have run out of 2" mirror mounts ...
Our goal is to realize PRMI+one arm again. However we found that the noise level of the Y-arm is worse than before (entry).
Today we went through into the servo gains of the ALS related loops.
- What we did
Step 1 to 6 is for Yarm
Alignment of the cavity and the green:
1. Locked arms using IR PDH, aligned the green beam to increase the transmission. Now the value of ALS-TRY_OUTPUT is more than 0.8.
Checking and adjustment of the end green PDH gain:
2. Measured the OLTF of green PDH loop.
3. The gain of the PDH box was 8.2. We found that the UGF was too high and the phase mergin was too low (20deg)
Therefore, the gain was reduced to the gain to 6.8. Now, the UGF and phase margin are 17.7 kHz, 41.96 degree, respectively.
Phase tracker loop:
4. Measured the OLTF of the phase tracker loop. The UGF was 2 kHz, and phase margin was 45 degree.
We found that these were already the nominal and optimized numbers.
For a reference: the filter bank C1:ALS-BEATX_FINE_PHASE has the gain of 110.
5. Disable the IR PDH lock, and stabilized Yarm by ALS. We measured the OLTF of the ALS loop (attachment 1).
The UGF and phase margin were turned out to be 125 Hz and 41 degree. respectively. This looks pretty optimal.
The ALS servo gain (the gain of the C1:ALS-YARM module) was 15.0.
6. We measured the in-loop noise of the ALS loop (C1:BEATY_FINE_PHASE_OUT_HZ) (attachment 2).
The comparison of the in-loop performance is discussed below.
After these adjustment, we found that the ALS in-loop noise of Yarm decreased in high frequency band.
(see this entry for the comparison. Sorry for my laziness! I don't have the overlaid plot)
If we believe this is true, lowering the end PDH gain improved the noise level between 100Hz to 1kHz.
This sounds weird as we decreased the PDH gain, rather than increased. We should confirm this effect by increasing the gain.
Now the in-loop RMS is started to be dominated by the peaks at 3, 16, and 24 Hz.
We should compare the current in-loop spectrum with the previous spectrum when the ALS was working fine.
By the way:
We suffered from frequent disruptions of the ALS servo during our investigation.
As we speculated that this was caused by the malfunction of the green PDH loop, we left the arm still and observed
how the green PDH lock is robust. Our discovery was that the green PDH loop had frequent interruptions (every 5~10min).
From this observation, we strongly feel that we need to look into the entire end PDH loop.
We found that we need to look into the entire end PDH loop to figure out what causes the worse noise level of the Y-arm than before.(entry)
Today, I measured in-loop noise of the end PDH loop and the ALS loop with different end PDH servo gain of Y-arm to make sure the PDH servo gain change the noise level of the ALS control loop.
- What I did
Measuring the OLTF of the end PDH loop:
1. Measured the OLTF of the PDH loop with the end PDH servo gain 6 and 7.
The UGF and phase margine: 16 kHz and 53 degree(gain 7)
7.8 kHz and 86 degree(gain 6)
I couldn't measure the OLTF with higher servo gain than 7 because the loop was not stable enough. I guess that is because of the noise of the SR560, which I used for node of the excitation signal.
Calibration of the end PDH error signal
2. Locked the cavity using IR and turn on the notch filter at 580 Hz of the C1:LSC-XARM. Excited the ETMY using awg with sinusoidal signal at 580 Hz. Set the end PDH servo gain to 6 and measured error signal of the end PDH. The calibration factor of the end PDH error signal H is calculated by
H = abs(G + 1) / A * Verr / Vin
where G is the OLTF of the end PDH, A is the actuator response of the ETMY, Vin is the amplitude of the excitation signal and Verr is the error signal at 580 Hz. This H convert the error signal to the fluctuation of the cavity length, so it has the unit of V/m. We can change that unit to V/Hz by multiplying f/L, where f is the laser frequency of IR and L is the length of the arm. In this case the H convert the error signal to the fluctuation of the resonant frequency of the cavity.
The actual number was
H = 1.4e7 [V/m] (2.0e-6 [V/Hz])
In-loop noise of the end PDH loop
3. Measured the error signal of the PDH loop with the end PDH servo gain of 6.0, 7.0, 8.0 and 9.0. I calibrated these signals with above H, so these unit is Hz/rHz. I attached the result of these in-loop noise. When the end PDH servo gain is 9.0, the end PDH loop looks unstable. And 8.0 looks to be the optimal gain in terms of the in-loop noise of end PDH loop.
ALS in-loop noise:
4. Stabilized the Y-arm with ALS control loop with different end PDH servo gain, and measured in-loop noise of the ALS control loop. I attached these results and discussed about this results below.
Now we can say that too high PDH servo gain makes ALS loop very noisy. Compare to when the PDH servo gain is 7 or 8, the ALS in-loop noise is roughly 4 times higher when the PDH servo gain is 9.0, which means the PDH loop is not stable. However between 100 Hz and the end PDH in-loop noise has no big difference between when the servo gain is 6 and 9. If this high frequency noise comes from the end PDH control and this effect is linear, these noises should be same level. Also the PDH servo gain of 7.0 looks optimal gain in terms of the in-loop noise of ALS control loop, although the 8.0 has smallest end PDH in-loop noise. Actually PDH in-loop noise are smaller than ALS in-loop noise.
I'm wondering what causes the 60 Hz peak in black curve. When the gain become higher, the peak at 60 Hz looks to become larger. The UGF of the ALS loop is above 100Hz, so it's not because of that. I feel there is some hint for understanding this result in this peak.
From this observation, I could make sure that the end PDH servo gain change the ALS in-loop noise, but that effect doesn't look so simple.
By the way
We should take care about 60 Hz comb peaks. You can see huge peaks in PDH in-loop noise and also in ALS in-loop noise.
The Y-arm ASS was tuned to be in a workable state. Basically, I followed Koji's recipe.
The SNR of the dither lines in the TRY and YARM control signals were checked - Attachment #1. The dither frequencies are marked with vertical dashed lines (can't figure out how to add 4 cursors in DTT so there's two in each row for a total of 4). A couple of days ago, when I was doing some preliminary checks, I found that the oscillator at 24.91 Hz caused a broadband increase in the TRY noise between DC and ~100 Hz. But today I saw no evidence of such behaviour. So I decided against changing the frequency.
The linearity of the demodulated error signals around the quadratic maxima of the TRY level was checked. I did not, however, investigate in detail the frequency-dependent offset Koji has reported in his elog.
After this work, the TRY level is at 0.95. This is commensurate with the MC trans level being lower by ~7% relative to July 2018. Furthermore, the ASS servo is able to return to TRY~0.95 with a time-constant of ~5 seconds in response to misalignment of the cavity optics. After I investigate the X-arm ASS, I will reset the normalization for TRX and TRY.
Update 645pm: In the spirit of general IFO recovery, I re-centered the ITM and ETM oplev spots, and also the IR beam on the IPPOS QPD to mark the new input pointing alignment (the spot is slightly lower on the AS camera than what I remember). I then tweaked the XARM transmission to maximize it, and re-set the TransMon normalization. I edited the normalization script to comment out the normalizing of the TransMon QPD gains as the QPDs are in some kind of indeterminate state now. Attachment #2 shows the current status, you can also see the normalization being reset. LSC mode disabled for overnight.
Once the XARM ASS is also checked out, I propose moving back to locking the DRMI / PRFPMI configs.
We measured the the openloop transfer function of the PDH green lock of the y-arm.The measurement setup was same as yesterday's measurement.elog 9047
In this measurement, the servo gain was 7 and the source amplitude for the excitation was 1 mV. As you can see in below figure, the measured UGF was 15 kHz and the phase margin was 45 degree.
attatchment1 - OLTF with servo gain of 7
The east end AC unit is arching over and running rough at CES. Called for mechanic.......
Both belts were replaced and the unit is running happily.
Rana suggested taking a look at the Y-arm test mass actuator TFs (measured by driving the coils one at a time, with only local damping loops on, using the Oplev to measure the response to a given drive). Attached are the results from this measurement (I used the Oplev pitch error signal for all 8 measurements). Although the magnitude response for all coils have the expected 1/f^2 shape, there seems to be some significant (~10dB) asymmetry in both the ETM and ITM coils. The phase-response is also not well understood. If we are just measuring the TF of a pendulum with 1 Hz resonant frequency, then at and above 10Hz, I would expect the phase to be either 0 or 180 deg. Looks like there is a notch at 60 Hz somewhere, but it is unclear to me where the ~90 degree phase at ~100Hz is coming from.
For the ITM, the UL OSEM was replaced during the 2016 summer vent - the coil that is in there is now of the short OSEM variety, perhaps it has a different number of turns or something. I don't recall any coil balancing being done after this OSEM swap. For the ETM, it is unclear to me how long this situation has been like this.
Yesterday night, I tried to measure the ASS output matrix by stepping the ITM, ETM and TTs in PIT and YAW, and looking at the response in the various ASS error signals. During this test, I found the ETM and ITM pitch and yaw error signals to be highly coupled (the input matrix was diagonal). As Rana suggested, I think the whole coil driver signal chain from DAC output to coil driver board output has to be checked before attempting to fix ASS. Results from this investigation to follow.
Note: The OSEM calibration hasn't been done in a while (though the HeNes have been swapped out), but as Attachment #2 shows, if we believe the shadow sensor calibration, then the relative calibrations of the ITM and ETM Oplevs agree. So we can directly compare the TFs for the ITM and ETM.