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.
I repeated the test of driving C1:SUS-<Optic>_<coil>_EXC individually and measuring the transfer function to C1:SUS-<Optic>_OPLEV_PERROR for Optic in (ITMX, ITMY, ETMX, ETMY, BS), coil in (LLCOIL, LRCOIL, ULCOIL, URCOIL).
There seems to be a few dB imbalance in the coils in both ETMs, as well as ITMX. ITMY and the BS seem to have pretty much identical TFs for all the coils - I will cross-check using OPLEV_YERROR, but is there any reason why we shouldn't adjust the gains in the coil output (not output matrix) filter banks to correct for this observed imbalance? The Oplev calibrations for the various optics are unknown, so it may not be fair to compare the TFs between optics (I guess the same applies to comparing TF magnitudes from coil to OPLEV_PERROR and OPLEV_YERROR, perhaps we should fix the OL calibrations before fiddling with coil gains...)
The anomalous behaviour of ITMY_UL (10dB greater than the others) was traced down to a rogue x3 gain in the filter module . This has been removed, and now Y arm ASS works fine (with the original dither servo settings). X arm dither still doesn't converge - I double checked the digital filters and all seems in order, will investigate the analog part of the drive electronics now.
I investigated the analog electronics in the coil driver chain by using awggui to drive a given channel with Uniform noise between DC and 8kHz, with an overall gain of 1000 cts. This test was done for both ITMs and the BS. The Whitening/De-Whitening was off during the test. I measured the spectra in
Attachment #1 - There is good agreement between all 3 measurements. To convert the DTT spectrum to Vrms/rtHz, I multiplied the Y-axis by 10V / ( 2*sqrt(2) * 2^15 cts). Between DC and ~1kHz, the measured spectrum everywhere is flat, as expected given the test conditions. The AI filter response is also seen.
Attachment #2 - Zoomed in view of Attachment #1 (without the AI filter part).
*The DTT plots have been coarse-grained to keep the PDF file size managable. X (Y) axes are shared for all the plots in columns (rows).
Similar verification remains to be done for the ETMs, after which the test has to be repeated with the Whitening/DeWhitening engaged. But it's encouraging that things make sense so far (except perhaps the coil balancing can be better as suggested by the previous elog).
I've left both arms locked. The Y-arm dither alignment is working well again, but for the X arm, the loops that actuate on the BS are still weird. Nothing obvious in the tests so far though.
GV 6pm 8 Jun 2017: I realized the X arm transmission was being monitored by the high-gain PD and not the QPD (which is how we usually run the ASS). The ASC mini screen suggested the transmitted beam was reasonably well centered on the X end QPD, and so I switched to this after which the X end dither alignment too converged. Possibly the beam was falling off the other PD, which is why the BS loops, which control the beam spot position on the ETM, were acting weirdly.
will investigate the analog part of the drive electronics now.
Not related to this work:
I noticed the X-arm LSC servo was often hitting its limit - so I reduced the gain from 0.03 to 0.02. This reduced the control signal RMS, and re-acquiring lock at this lower gain wasn't a problem either. See attachment #3 (will be rotated later) for control signal spectra at this revised setting.
Liyuon will set up a ~5 mW He/Ne laser for waist measurement for LIGO oplev telescope.
This will be between the beam tube and the CES wall. He will do his tests in the morning.
I've attached the results from my measurements of the noise characteristics of the Y-end auxiliary PDH system.
The following spectra were measured, in the range DC-1MHz:
In order to have good spectral resolution, the frequency range was divided into 5 subsections: DC-200Hz, 200Hz-3.4kHz, 3.4kHz-16.2kHz, 10kHz-100kHz, 100kHz-1MHz. The first three are measured using the SR785, while the last two ranges are measured with the Agilent network analyzer. The spectrum of the mixer output with its input terminated was quite close to the analyzer noise floor - hence, this was measured with an S560 preamplifier set to a gain of 100, and subsequently dividing the ASD by 100. To convert the Y-axis from V/rtHz to Hz/rtHz, I used two conversion factors: for the analyzer noise floor, PD dark noise, mixer noise and in-loop error signal, I made an Optickle simulation of a simple FP cavity (all parameters taken from the wiki optics page, except that I put in Yutaro's measured values for the arm loss and a modulation depth of 0.21 which I estimated as detailed here), and played around with the demodulation phase until I got an error signal that had the same qualitative shape as what I observed on an oscilloscope with the arms freely swinging (feedback to the laser PZT disabled). The number I finally used is 45.648 kHz/V (the main horns were 800mV peak-to-peak on an oscilloscope trace, results of the Optickle FP cavity simulation shown in Attachment #2 used to calibrate the X-axis). For the servo noise spectrum and in-loop control signal, I used the value of 2.43 MHz/V as determined here.
I'm not sure what to make of the strong peaks in the mixer noise spectrum between ~60Hz and 10kHz - some of the more prominent peaks are 60Hz harmonics, but there are several peaks in between as well (these have been confusing me for some time now, they were present even when I made the measurement in this frequency range using the Agilent network analyzer. My plan is to repeat these measurements for the Xend now.
I've re-measured the noise breakdown for the Y-end AUX PDH system. Spectra are attached. I've also measured the OLTF of the PDH loop, from which the UGF appears to be ~8.5kHz.
As Eric and Koji pointed out, the spectra uploaded here were clearly wrong as there were breaks in the spectra between decades of frequency. I redid the measurements, this time being extra careful about impedance mismatch effects. All measurements were made from the monitor points on the PDH box, which according to the schematic found here, have an output impedance of 49.9 ohms. So for all measurements made using the SR785 which has an input impedance of 1Mohm, or those which had an SR560 in the measurement chain (also high input impedance), I terminated the input with a 50ohm terminator so as to be able to directly match up spectra measured using the two different analyzers. I'm also using my more recent measurement of the actuator gain of the AUX laser to convert the control signal from V/rtHz to Hz/rtHz in the plotted spectra.
As a further check, I locked the IR to the Y-arm by actuating on MC2, and took the spectrum of the Y-arm mirror motion using the C1CAL model. We expect this to match up well with the in-loop control signal at low frequencies. However, though the shapes seem consistent in Attachment #2 (light orange and brown curves), I seem to be off by a factor of 5- not sure why. In converting the Y-arm mirror motion spectrum from m/rtHz to Hz/rtHz, I multiplied the measured spectrum by , which I think is the correct conversion factor (FSR/(0.5*wavelength))?
We have measured the open-loop transfer function of the Y-end green PDH loop. From the measurement, the loop UGF is ~12kHz.
We have been trying to measure this transfer function for some time now, and playing around with various points of injecting the excitation and measuring the output. Koji helped arrive at one that actually worked, and the scheme used to make this measurement is shown in the sketch below. The SR785 signal analyzer was used to make the measurement, while an SR560 preamp was used to sum the output from the PDH box (PZT-OUT) and the excitation, with this sum being delivered to the auxiliary laser PZT via a pomona box that sums the servo output and the signal from the LO. The transfer function measurement made was a1/a2 w.r.t the sketch attached.
Set-up to measure Y-end Green PDH transfer function:
Measured Open Loop Transfer Function:
I improved the alignment of the green beam into the Y arm cavity.
Other changes made today:
While aligning the Y-end aux laser light into the fiber we noticed that the green power out of the doubling crystal was in microwatts. I checked to see what was the trouble and found that the oven was cold as the temperature controller had been disabled. I enabled it and scanned the temperature to maximise the green output. Yet the power is less than 10% of that at the X end (7mW).
To verify I checked the power of various beams on the Y-end table. They are listed below in the picture
The green beam power is proportional to the square of the IR incident power and this explains the drop in green power by a factor of (210/730)^2 thus making 7 mW --> 0.5 mW. However we may be able to double the power at the Y-arm oven if the uncoated lenses in the IR path are exchaned for coated ones.
The green beam injection into the Y-arm cavity also needs to be cleaned up as noted here. As seen in the picture below two of the mirrors which launch the beam into the arm cavity need to be fixed as well.
Before changing setup at Y-end table, I measured the status value of the former setup as follows. These values will be compared to those of upgraded setup.
(These numbers are shown in the attachment #2.)
The setup I designed is here. It can bring 100% mode-matching and good separation of degrees of TEM01, however I found a probrem. The picture of setup is attached #3. You can see the reflection angle at Y7 and Y8 is not appropriate. I will consider the schematic again.
The SHG crystal has the conversion efficiency of ~2%W (i.e. if you have 1W input @1064, you get 2% conversion efficiency ->20mW@532nm)
It is not possible to produce 0.58mW@532nm from 20.9mW@1064nm because this is already 2.8% efficiency.
I measured it with the wrong setting of a powermeter. The correct ones are here:
The calculated conversion efficiency of SHG crystal is 1.2%W.
What about just copying the Xend layout? I think it has good MM (per calculations), reasonable (in)sensitivity to component positions, good Gouy phase separation, and I think it is good to have the same layout at both ends. Since the green waist has the same size and location in the doubling crystal, it should be possible to adapt the X end solution to the Yend table pretty easily I think.
I designed a new layout. It has good mode-matching efficiency, reasonable sensitivity to component positions, good Gouy phase separation. I'm setting optics in the Y-end table. The layout will be optimized again after finishing (rough) installation. (The picture will be posted later)
After installation I measured these power again.
There is a little power loss. That may be because of adding one lens in the beam path. I think it is allowable margin.
We were poking around and tried to make a button for the Y-green shutter, just like the X-green already has. I don't know where the X-green shutter button goes to in model-land, so I can't figure out if there is already a channel set up for the Y end. Just switching the X for a Y didn't work. Someone (maybe me) should fix this in the next soon.
[Jenne, with ample supervision by Kiwamu and Suresh]
Y-green was aligned, and is now flashing. The ETMY trans camera was very helpful for this alignment. I didn't end up needing to use a foil aperture.
Kiwamu and Suresh had just closed up the IOO doors, so we don't know yet where it's hitting on the PSL table (if the beam is making it that far). Tomorrow we'll look at ITMY to see if the green beam is centered there, and if it's coming out to the PSL table.
The Yarm green laser really wanted to lock on a 01/10 mode, so Kiwamu suggested I go inside and realign the green beam to the arm. I did so, and now it's much happier locked on 00 (the Yarm is resonating both green and IR right now).
I've applied FIR adaptive filter to YARM control. Feedback signal of the closed loop was used as adaptive filter error signal and OAF OUT -> IN transfer function I assumed to be flat because of the loop high gain at low frequencies. At 100 Hz deviation was 5 dB so I've ignored it.
I've added a filter bank YARM_OAF to C1LSC model to account for downsampling from 16 kHz to 2 kHz and put low-pass filter inside.
I've used GUR 1&2 XYZ channels as witnesses. Bandpass filters 0.4-10 Hz we applied to each of them. Error signal was filters using the same bandpass filter and 16 Hz 40 dB Q=10 notch filter. As an AI filter I used 32 Hz butterworth 4 order low-pass filter. Consequently, AI, bandpass and notch filters were added to adaptive path of witness signals.
I've used an FIR filter with 4000 taps, downsampling = 16, delay = 1, tau = 0, mu = 0.01 - 0.1. Convergence time was ~3 mins.
This is interesting. I suppose you are acting on the ETMY.
Can you construct the compensation filter with actuation on the MC length?
Also can you see how the X arm is stabilized?
This may stabilize or even unstabilize the MC length, but we don't care as the MC locking is easy.
If we can help to reduce the arm motion with the MCL feedforward trained with an arm sometime before,
this means the lock acquisition will become easier. And this may still be compatible with the ALS.
Why did you notched out the 16Hz peak? It is the dominant component for the RMS and we want to eliminate it.
Precise arm alignment is more demanded. as the PRMI locking requires good and reliable alignment of the ITMs.
I previously added the output matrix to ASS.
Now the input and output matrix as well as the gains and filters have been updated.
The current concept is
Fast loop: align the arms by the arm mirrors with regard to the given beam.
Slow loop: move the incident beam position and angle to make the spot at the center of the mirrors
This is actually opposite to Den's implementation.
In order to realize the faster alignment of the arm, I increased the corner frequency of the lockins for the arm signals from 0.5Hz to 1Hz.
With the new configuration the arm alignment converges in 10sec and the input pointing does in ~15sec.
The actuation to the input pointing TTs are done together with the feedforward actuation to the arms.
This way we can avoid too much coupling from the input pointing servos to the arm alignment servos.
The corresponding script /opt/rtcds/caltech/c1/scripts/ASS/YARM/DITHER_Arm_ON.py was also modified.
I modified my Simulink model of the YARM to match the new filter modules Rana installed on YARM. I also scaled the open loop transfer function of the model to fit the measured open loop transfer function at the unity gain frequency, as shown in the figure below. From this I produced the length response function correctly scaled, also shown below. Then I applied the calibration factor to the YARM data measured in /users/Templates/Y-Arm_120815.xml. Both the uncalibrated and calibrated spectra are included below.
Today I spent time locking the YARM in order to calibrate the arm cavity. Here's what I did:
1. Misalign all optics other than the beam splitter, ITMY, ETMY and PZT2
2. Restore BS, ITMY, ETMY, and PZT2
3. Open Dataviewer and run /users/Templates/JenneLockingDataviewer/Yarm.xml from the Restore Settings. This opens the signals C1:LSC-POY11_I_ERR (the Pound-Drever-Hall error signal for this measurement) and C1:LSC-TRY_OUT (the light transmitted through ETMY) in the plot window.
4. Adjust ITMY and ETMY pitch and yaw using the video screens looking at AS and ETMYT as a first, rough guide. It can be helpful at first to increase the gain on the YARM servo filter module in the C1LSC control screen to about 0.3 and decrease it back down to 0.1 as the beam becomes better aligned. You know when to decrease this gain when fuzzy, small oscillations appear on the C1:LSC-TRY_OUT signal. If the mode cleaner is locked you should see a bright spot on the AS camera.
5. Tinker with pitch and yaw while looking at the AS screen until you see a reasonably good circular spot without other fringes extending from a bright center.
6. The overall goal is to maximize C1:LSC-TRY_OUT because the power transmitted through EMTY is proportional to the power within the cavity. A decent target value is 0.85 and today I was able to get it to just over 0.80 at best. At first there will probably be small spikes in C1:LSC-TRY_OUT. You want to adjust pitch and yaw until the deviation in the signal from zero is no longer just a spike, but a sustained, flat signal above zero. By this time there should be light showing up on the ETMYT camera as well.
7. Once that happens, continue to successively adjust ITMY and ETMY doing the pitch adjustments on both first, and then the yaw adjustments, or vice versa. You can also tweak the PZT2 pitch and yaw. Once you've got C1:LSC-TRY_OUT as large as possible, you've locked the cavity.
I saved the pitch and yaw settings I ended up with for ITMY, ETMY, BS and PZT2 in the IFO_ALIGN screen. Before the end of the day I think Jenne restored the rest of the previously misaligned optics because they were restored when I got back from dinner.
I also worked on calibrating the YARM. I opened up DTT using C1:LSC-POY11_I_ERR as the measurement channel and C1:SUS-ITMY_LSC_EXC as the excitation channel. I ran a logarithmic swept sine response measurement with 100 points and an amplitude of 25. The mode cleaner kept losing its lock all day, and if this happened while making this measurement I tried to pause the sweep as quickly as possible. I analyzed the the transfer function and the coherence function that the sweep produced, and thought that some of the odd behavior was due to losing the lock and getting back to a slightly different locked state when resuming the measurement. The measured transfer function and coherence plots are attached below. Both the transfer function and the coherence look good above roughly 30 Hz, but do not look correct at low frequencies. There's also a roll-off in the measured transfer function around 200 Hz, while in the model the magnitude of the transfer function drops only after the corner frequency of the cavity, around several kHz. I have attached a plot of the roughly analogous transfer function from the DARM control loop model (the gains are very large due to the large arm cavity gain and the ADC conversion factor of 2^16/(20 V) ). The measured and the modeled transfer functions are slightly different in that the model does not include the individual mirrors, while the excitation was imposed on ITMY for the measurement.
The next steps are to figure out what's happening in DTT with the transfer function and coherence at low frequencies, and to understand the differences between the model and the measurement.
Once you've got C1:LSC-TRY_OUT as large as possible, you've locked the cavity.
Both the transfer function and the coherence look good above roughly 30 Hz, but do not look correct at low frequencies. There's also a roll-off in the measured transfer function around 200 Hz, while in the model the magnitude of the transfer function drops only after the corner frequency of the cavity, around several kHz. I have attached a plot of the roughly analogous transfer function from the DARM control loop model (the gains are very large due to the large arm cavity gain and the ADC conversion factor of 2^16/(20 V) ). The measured and the modeled transfer functions are slightly different in that the model does not include the individual mirrors, while the excitation was imposed on ITMY for the measurement.
The cavity is actually "locked" as soon as the feedback loop is successfully closed. One easy-to-spot symptom of this is that, as you mentioned elsewhere in your post, TRY is a ~constant non-zero, rather than spikey (or just zero). Once you've maximized TRY, you've got the cavity locked, and the alignment optimized.
We didn't get to this part of "The Talk" about the birds, the bees, and the DTTs, but we'll probably need to look into increasing the amplitude of the excitation by a little bit at low frequency. DTT has this capability, if you know where to look for it.
It would be great to see the model and your measurement overlayed on the same plot - they're easier to compare that way. You can export the data from DTT to a text file pretty easily, then import it into Matlab and plot away. Can you check and maybe repost your measured plots? I think they might have gotten attached as text files rather than images. At least I can't open them.
Here's the same plots in pdf format now. I originally posted them as jpg because I couldn't open the resulting pdf from DTT on rosalba, but I could open the jpg. I'll look into overlaying the measured and modeled curves as well.
I forgot to post this last night, but I locked the YARM again yesterday and misaligned the other optics. I took measurements on ITMY and ETMY with DTT again as well. At the end of the day I aligned the rest of the optics before I left.
From time to time the 20 dB jump in the transfer function still occurs. The new AD8336 op amp did not change that issue. I am sure that the op amp was broken,
because the amplitude of the sine did not change when I turned the gain knob.
The above two curves were measured with different input amplitude of the sine from the spectrum analyzer. Nothing changed in between except that there was no
jump when Kiwamu was around. Very strange. Testing the electronic board led to no clue what is happening.
For now, I will just use the PDH box as it is, but one should keep this odd behaviour in mind.
I could not improve the locking. So, I checked the transfer function of the PDH box again. The transfer function looks okay if the gain knob is <=2.0.
If the gain knob is >2.0 the 20dB step appears in the transfer function (see elog page 5713). This step is shifted to higher frequencies if the gain is
increased. The PZT drive out was not saturated at any time. Yesterday, I checked the electronic circuit with a gain of 2.0. Thus, I couldn't find the broken
gain amplifier (AD8336). The amplifier is ordered in will arrive on Monday.
With limited proof of working for a few times (but robustly), I'm happy to report that ASS on YARM and XARM is working now.
The issue is that PR3 is not placed in correct position in the chamber. It is offset enough that to send a beam through center of ITMY to ETMY, it has to reflect off the edge of PR3 leading to some clipping. Hence our usual ASS takes us to this point and results in loss of transmission due to clipping.
Solution: We can not solve this issue without moving PR3 inside the chamber. But meanwhile, we can find new spot positions on ITMY and ETMY, off the center in YAW direction only, which would allow us to mode match properly without clipping. This would mean that there will be YAW suspension noise to Length coupling in this cavity, but thankfully, YAW degree of freedom stays relatively calm in comparison to PIT or POS for our suspensions. Similarly, we need to allow for an offset in ETMX beam spot position in YAW. We do not control beam spot position on ITMX due to lack of enough actuators to control all 8 DOFs involved in mode matching input beam with a cavity. So instead I found the right offset for ITMX transmission error signal in YAW that works well.
I found these offsets (found empirically) to be:
These offsets have been saved in the burt snap file used for running ASS.
I'll reiterate here procedure to run ASS.
I took data from 1123495750 to 1123498750 GPS time (Aug 13 at 3AM, 50 mins of data) for C1:LSC-YARM_OUT_DQ, and all T240 and GUR1 channels.
Here is the PSD of the YARM_OUT, showing the data that I will use to train the FIR filter:
Coherence plots for YARM and all channels of T240 and GUR1 sesimometers are shown below. This will help determine what regions to preweight the best before computing FIR filter. They also show how GUR1 is back to work compared to those of elog:11457.
error signal = signal measured behind the low-pass filter
feedback signal = output of the gain servo, going to the PZT
First of all both signals in V/sqrt(Hz) just in case I mess up the next calibration step.
The 60 Hz line (and its multiple) are a new feature. They show up as soon as the feedback loop is closed. So far, I couldn't find their origin.
For the next calibration step:
I measured the power spectrum of channel C1:GCY_SLOW_SERVO1_IN1, which is the PZT driving voltage.
I converted the output to a PSD. Next, I converted counts/sqrt(Hz) to volts/sqrt(Hz) by multiplying with 40 V / 2^16 counts.
Finally, I multiplied it with 5MHz/V for the PZT to end up with Hz/sqrt(Hz).
This corresponds to a cavity length fluctuation of
with lambda = 532nm and a YARM cavity length of 37.757m (elog # 5626).
All in one plot
Today, I could lock the YARM laser for 2h to the YARM cavity. After to hours the output of the servo is saturated. I need to work on thermal feedback to the laser.
It is a nice TEM00 mode and the green light enters PSL table.
Measured with pin-ball machine spectrum analyzer (I forgot the real name, but it is the one that makes sounds like a pin-ball machine), source power10mVp, Lb1005 gain 2.05.
Input offset of LB1005 is zero
On Thursday, Oct 27, lock for 3 min
On Friday, Oct 28, lock up to 18 min, improvements done by
On Monday,Oct 31, careful adjustment of summing box (rear of of LB1005), lock up to 2h, limited by saturated feedback signal --> work on slow control
Some more plots
I scripted a series of YARM DC reflectivity measurements last night alternating between locked state and unlocked state (with ETMY misaligned) for measuring the after-vent armloss. The general procedure is based on elog 11810, but I'll also give a brief summary here.
I did this back in June (but strangely never posted what I found, shame on me). What I found back then was a YARM loss of 237 ppm +/- 41 ppm and an XARM loss of 501 ppm +/- 105 ppm
Last night's data indicates a YARM loss of 143 ppm +/- 24 ppm after cleaning with first contact.
THIS IS STILL ASSUMING THAT THE MODE-MATCHING HASN'T CHANGED. We had however moved ETMY closer to ITMY during the vent by 19mm. Gautam and I had some trouble setting up the ALS to confirm the mode-matching, but we're in the process of recovering the XARM IR beat.
Rana pointed out that another way to mitigate seismic motion at in the mode cleaner would be to look at the YAW and PITCH output channels of the WFS sensors that control the angular alignment of the mode cleaner.
I downloaded 45 mins of data from the following two channels:
And did some quick offline Wiener filtering with no preweighting, the results are shown in the PSD's below,
I'm quite surprised at the Wiener subtraction obtained for the YAW signal, it required no preweighting and there is about an order of magnitude improvement in our region of interest, 1-3 Hz. The PIT channel didn't do so bad either.
In preparation for the armloss map I checked the calibration of the Y-Arm ITM and ETM OpLevs with the method originally described in https://nodus.ligo.caltech.edu:8081/40m/1247. I was getting a little confused about the math though, so I attached a document at the end of this post in which I work it out for myself and posteriority. Stepping through an introduced offset in the control filter for the corresponding degree of freedom, I recorded the change in transmitted power and the reading of the OpLev channel with the current calibration. One thing I noticed is that the calibration for ITM PIT is inverted with respect to the others. This can of course be compensated at any point in any readout/feedback chain, but it might be nice to establish some sort of convention where positive feedback to the mirror will increase the OpLev reading.
The calibration factors I get are within ~10% of the currently stored values. The table (still incomplete, need to relate to the current values) summarizes the results:
The individual graphs:
We looked at beam spots on ITMY and ETMY. We switched to smaller apertures on the other side of the rulers. For ITMY beam spot was 1mm below and 1mm south (right if you look in the direction ITMY -> ETMY) from the aperture center, for ETMY - 4 mm up and 3mm north from the aperture center. We made a correction for this using PZT 1 and 2. Now beam spots are in the middle of the apertures on ITMY and ETMY.
We tried to look at reflected beam from ETMY but it was hard to see the dependence between ETMY DC offset and reflected beam. We'll continue tomorrow.
Spot centering on Y arm - DONE!
1. I went back to the IFO alignment slider positions from Friday. The Y arm was flashing in HOM because the earthquake this morning tripped all suspensions and the slider values were not real. X arm did not have any flashes.
2. Y arm aligned using TT1 and TT2. Spot centering measured using Jenne's A2L_Yarm script.
Pitch 6.48 4.39
Yaw -7.42 -3.135
3. I started centering in pitch. I used the same in-vac alignment method (down on TT1 and up on TT2 in pitch) and measured spot positions.
4. When the spot positions were centered in pitch, I started with yaw alignment.
5. I had to use TT1 to center on ITMY and move TT2 and ITMY to center on ETMY.
6. Spot positions after centering:
Pitch -1.22 -1.277
Yaw 0.42 -0.731
7. I wanted to go back and tweak the pitch cenetering; but framebuilder failed and dataviewer kept loosing connection to fb
AS seems clipped. Although it could be because of the misaligned BS.
IPANG was centered on the QPD, but it is so clipped, that I'm not sure we can trust it. Max sum right now is -4, rather than the usual -8 or -9.
Once fb is fixed, we should align the X-arm which will be followed by green alignment.
Over the last few weeks, it has been observed that there is some strong seismic activity that starts at around 9PM everyday and goes on for a couple of hours. It seems unlikely that it is our geologist neighbour (Jenne met with the grad student who works on the noisy experiment).
The high frequency noise, which has been a dominant noise above 30 Hz in the Y arm ALS (#6133), decreased by a factor of 5.
This reduction was done by increasing the modulation depth at the Y end PDH locking. Now the noise floor at 100 Hz went to 0.2 pm/sqrtHz.
However the noise source is not yet identified and hence it needs a further investigation.
(Increasing the modulation depth)
As this entry, Yarm ALS is not stable enough to lock PRMI + 2 arms. We tried to figure out what is the reason.
What we did
Check connection and alignment
1. Check the Green REFL PD.
Reflection is hitting the center of PD.
2. Check all the BNC connections
All connection are fine.
3. Check which power supply the PDH box is connected to.
PDH box is connected to 1Y4 AC power supply.
Check the control signal and error signal
4. Connected the PZT OUTMON to PC
Before the PZT output was not connected to the monitor channel. We connected that.
5. Saw the time series of the error signal and control signal (PZT output)
When the Yarm lost end PDH lock, we found that control signal kicked the PZT of end green laser. And also we saw the saturation of control signal. We are not sure where this saturation comes from.
With these check, we couldn't find any problem in connection or alignment. But the PDH control signal looks somehow strange. We tried to compare the Yarm signals with that of the Xarm, but we could not conclude anything meaningful.
We don't understand right now but we will continue to check that. We will add more details to the discussion once we have looked into the PDH box signals using oscilloscope.
Jamie and I were doing some locking, and we found that the Yarm green wasn't locking. It would flash, but not really stay locked for more than a few seconds, and sometimes the green light would totally disappear. If the end shutter is open, you can always see some green light on the arm transmission cameras. So if the shutter is open but there is nothing on the camera, that means something is wrong.
I went down to the end, and indeed, sometimes the green light completely disappears from the end table. At those times, the LED on the front of the laser goes off, then it comes back on, and the green light is back. This also corresponds to the POWER display on the lcd on the laser driver going to ~0 (usually it reads ~680mW, but then it goes to ~40mW). The laser stays off for 1-2 seconds, then comes back and stays on for 1-2 minutes, before turning off for a few seconds again.
Koji suggested turning the laser off for an hour or so to see if letting it cool down helps (I just turned it off ~10min ago), otherwise we may have to ship it somewhere for repairs :(
I turned the Yend laser back on....it hasn't turned itself off yet, but I'm watching it. As long as we leave the shutter open, we can watch the C1:ALS-Y_REFL_DC value to see if there's light on the diode.
This is happening again to the Yend laser. It's been fine for the afternoon, and I've been playing with the temperature. First I have been making big sweeps, to figure out what offset values do to the actual temperature, and more recently was starting to do a finer sweep. Using the 'max hold' function on the 8591, I have seen the beat appear during my big sweeps. Currently, the laser temperature measurement is at the Yend, and the RF analyzer is here in the control room, so I don't know what temp it was at when the peaks appeared.
Anyhow, while trying to reaquire lock of the TEM00 mode after changing the temperature, I find that it is very difficult (the green seems misaligned in pitch), and every minute or so the light disappears, and I can no longer see the straight-through beam on the camera. I went down to the end, and the same symptoms of LED on the laser head turning off, power out display goes to ~40mW, are happening. I have turned off the laser as was the solution last time, in hopes that that will fix things.
Manasa has done some work to get the Xgreen aligned, so I'll switch to trying to find that beatnote for now.