I found the beat note for X arm. I did not change anything this morning (to the best of my knowledge). Hooking up the spectrum analyzer, I could find the beatnote signal at the PD RF output, after the amplifier and also at the MON port of the beatbox. I still don't know what changed from the last night set of trials
I found the beat note for the Y arm. Nothing was changed with respect to yesterday night, but the beat is back!
Craig, Gautam and Steve,
Single mode fiber 50m long is layed out into cable tray that is attached to the beam tube of the Y arm.
It goes from ETMY to PSL enclosure. It is protected at both ends with " clear- nylon slit corrugated loom tubing " 1.5" ID
The fiber is not protected between 1Y1 and 1Y4
Installed 0.5" ID 10 ft long protective tubing at the PSL end of the ETMY fiber this morning. Here I had to cable tie a bunch of cables at the east side of the PSL enclosure.
They were hanging off the table blocking space were the sliding doors move.
At the ETMX end of the X-arm fiber received the same protective tubing.
I ran a series of diagnostics on the X arm ALS to look at how the beatbox behaves after the makeover.
Diagnostic tests run:
1. X arm ALS in-loop spectrum
2. X arm ALS out-of loop spectrum
3. X ALS scan of the X arm cavity
The noise suppression looks better after the makeover at the lower frequencies. To suppress the noise at high frequencies, we would have to add more whitening filters.
Yesterday I did a cavity scan with IR while holding the Yarm with green.
ALS servo tuning:
The gain of the loop is set such that BEATY_FINE_Q_ERR x GAIN = 120k. This is a kind of "empirical low" in order to have the UGF around 1kHz.
Start with FM5 [1000:1] enabled, determine the sign of the gain increasing it in small steps and making sure that the mirror doesn't get a kick. Then gradually raise it while looking at the BEATY_PHASE_OUT power spectrum.
Enable FM7 [RG16.5], FM6 [RG3.2], FM3 [1:5], FM2[0:1], FM10 [40:7].
Plot 1 shows the power spectrum of BEATY_PHASE_OUT (calibrated in Hz).
Offset setting and cavity scan
The C1ALS_OFFSETTER2 was used to set an offset for ALS scan.
Many scans have been done to find the optimal offset conditions, I only attached one (Plot 2).
I also misaligned the END mirror in pitch to enhance the HOMs peaks, but it turned out that it was not enough, because I didn't see a very big difference between the "aligned" and the "slightly misaligned" measurements (Plot 3).
Increase the cavity misalignment both in pitch and in yaw and repeat the measurement.
[Annalisa, Koji, Manasa]
In order to improve the ALS stability we went ahead to check if we are limited by the sensor noise of ALS.
What we did:
RF signals similar to the beatnote were given at the RF inputs of the beatbox.
The frequency of the RF signal was set such that I_OUT was zero (zero-crossing point of the beatbox).
We measured the noise spectrum of the phase tracker output.
Plot 1: X ALS noise spectrum
Plot 2: Y ALS noise spectrum
The X arm ALS noise is not limited by the sensor noise...which means we shoudl come up with clever ideas to hunt for other noise sources.
But this does not seem to be the case for the Y arm ALS. The Y arm part of the beatbox is noisy for frequencies < 100Hz.
After looking into the details and comparing the X and Y arm parts of beatbox, it looks that amplitude of the beat signal seem to affect the Y arm ALS noise significantly and changes the noise spectrum.
Investigate the effect/limitations of amplitude of the beatnote on the X arm and Y arm beatbox.
I started doing a scan of the Y arm cavity with IR with ALS enabled.
The servo tuning procedure is basically the same as described in elog 8831.
This time I had a stronger beat note(-14 dBm instead of -24 dBm of the last measurement) thanks to a better alignment.
Plot1 shows the Power spectrum of the BEATY_PHASE_OUT. The RMS is smaller by a factor of 2 (400Hz), corresponding to a residual motion of about 25 pm.
Offset setting avity scan
In order to give an offset linearly growing in time, I used the ezcastep script instead of giving the offset in OFFSETTER2. If the ramp time is long enough, it is not necessary to enable the 30mHz filter.
To span 2 FSR, I started from an offset of 450 and I gave a maximum value of 1600 with a delay of 0.2s between two consecutive steps.
I did a first scan with the cavity well aligned, basically to know the position of the 00 peaks and choose the best offset range (Plot2)
Then I misaligned the TT2, first in PITCH and yhen in YAW, in order to enhance the HOMs. (Plot3 and Plot4)
More investigation and measurements needed.
Yesterday evening Nic and me were in the lab. The Mode Cleaner was unlocked, but after many attempt we could fix it and we did many scans of the Y arm cavity.
Today I was not able to keep the MC locked. Koji helped me remotely, and eventually the MC locked back, but after half an hour of measurements I had to stop.
I made some more scan of the Y arm though. I also tried to do the same for the X arm, but the MC unlocked before the measurement was finished. I'll try to come back in the night.
We did the same mod of the beatbox for the Y arm too. See
The new whitening filters improved the out-of-loop ALS stability of the Y arm down to 300Hz (20pm_rms in displacement).
- After modifying the whitening filters, the out-of-loop stability of the arms were tested with the IR PDH signals.
- The X arm showed non-stationarity and it made the ALS servo frequenctly fell out of lock.
- For now we decided to use the Y arm for the PRMI+one arm trial.
- The performance of the ALS was tested with several measurements. (attachment 1)
Cyan: Stability of the beatnote frequency with the MC and the arm freely running. The RMS of the day was ~6MHz.
Blue: Sensing limit of the beat box was tested by giving a signal from Marconi. The same amplitude as the X arm beat was given as the test signal.
This yielded the DC output of ~1200 counts.
Green: Out-of-loop estimation of the beatbox performance. This beat note stability was measured by controlling the arm with the IR PDH signal.
Assuming the PDH signal has better SNR than the beat signal, this gives us the out-of-loop estimation of the stability below 150Hz, which is the
unity gain frequency of the ALS loop.
Above 150Hz the loop does not force this noise to the suspension. Just the noise is injected via a residual control gain (<1).
Black: In-loop evaluation of the ALS loop. This becomes the left over noise for the true stability of the arm (for the IR beam).
Red: The arm was brought to the IR resonance using the ALS offset. The out-of-loop stability was evaluated by the IR PDH signal.
This indeed agreed with the evaluation with the other out-of-loop evaluation above (Green) below 150Hz.
Attachment 2 shows the time series data to show how the arm is brought to the resonance.
1 count of the offset corresponds to ~20kHz. So the arm started from 200kHz away from the resonance
and brought to the middle of the resonance.
(Manasa downloaded the 2k sampled data so that we can use this for presentations.)
Path to data (retreived using getdata)
Yesterday and today I was in the lab doing many cavity scan.
First I did many measurement with the cavity aligned in order to get the position of the 00 modes, then I misaligned the beam in many different ways to enhance the higher order modes.
In particular, I first misaligned the mode cleaner to make the beam clipping into the Faraday. To do this, I set to 0 the WFS gain, but I left the autolocker still enabled. In this way, the autolocker couldn't bring the mirrors back to the aligned position.
Then I misaligned also the TT2 to get even more HOMs.
Eventually, Rana came and we misaligned TT1 to clip the beam, and using TT2 we aligned back the beam to the arm.
To increase the SNR, we changed the gain of the TRY PD, setting it to 20dB (which corresponds to a factor 100 in digital scale)
I attached one scan that I did with Rana on Sunday night. I could not upload a better resolution image because the file size was too big, but here's the path to find all of the scans:
There are many folders, one per each day I measured. In each folder there are measurements relative to aligned cavity, Pitch and Yaw misalignment.
The PDA520 used for TRY was set to 0 dB analog gain. This corresponds to ~500 counts out of 32768. The change to 20 dB actually increases the gain by 100. This makes the single arm lock saturate at ~25000 counts (obviously in analog before the ADC). The right setting for our usual running is probably 10 dB.
For the IMC WFS, we had disabled the turn on in the autolocker to use the IMC to steer the beam in the FI, but that was a flop (not enough range, not enough lever arm). In the end, I think we didn't get any clipping.
I did a calibration measurement for the Y part of the BeatBox using a Marconi. This is in order to get a more accurate calibration for the arm cavity scan measurement.
The calibration factor I found is:
C1:ALS-BEATX_FINE_PHASE_OUT 50.801 +/- 0.009 deg/MHz
During my cavity scan measurement, I had recorded the beat frequency and amplitude from the Spectrum Analyzer at each zero crossing.
I connected the Marconi to the RF in of the Y part of the BeatBox, and I set the Marconi carrier frequency at one of this zero-crossing frequency that I had recorded, while I set the amplitude in way to have on the spectrum analyzer the same beat amplitude that I read during the measurements or, equivalently, in order to have C1:ALS_BEATY_FINE_Q of the order of 1200 (which is the same value I had during my measurements).
I started with
Then I monitored the C1:ALS_BEATY_FINE_I on the oscilloscope and I adjusted the carrier frequency so that I had zero signal on the oscilloscope. Eventually the frequency corresponding to the zero crossing was 79.989 MHz.
I resetted the phase (clear history in the BEATY_FINE_PHASE panel) and I started changing the frequency by steps of 0.2 MHz, and I spanned about 70 MHz (from 32 to 102 MHz).
The calibration coefficient I found is not so different from the one that Yuta measured (elog 8199).
Here are the fit parameters:
y = a + bx
a = -4239.7 +/- 0.6 deg
b = 50.801 +/- 0.009 deg/MHz
I aligned both the X and Y end green to the arms.
[Koji, Nic, Manasa]
Update from last night.
Koji and I realigned the green optics on the PSL to start working on the ALS.
We set on a beat note search. We couldn't find the beat note between any of the arm green transmission and the PSL green. All we could see was the beat between the X arm and the Y arm green leakage.
Since we had the beatnote between the 2 green transmission beams, we decided to scan the PSl temperature. We scanned the SLOW actuator adjust of PSL; but couldn't locate any beat note. The search will continue again today.
Beat notes were recovered for both the arms.
I locked the arms to IR using PDH and measured the ALS out of loop noise at the phase tracker output.
The Y arm has the same 300Hz/rtHz rms. The X arm rms noise measures nearly the same as the Y arm in the 5-500Hz region (X arm has improved nearly 10 times after the last whitening filter stage change old elog ).
The noise in the ALSX error signals could be related to the bad alignment and conditions at the X end.
This is an elog about the activity on Friday night.
- The X arm green beam was aligned with assist of the ASX system.
- M1 PZT alignment was swept while M2 PZT was under the control of ASX.
- Everytime M1 was touched, M2 was restored by manual alignment so that the REFL beam hits the center of the REFL PD.
This way we could recover the lock of TEM00. Once TEM00 is recovered, ASX took care of the alignment of M2
- The error signal used by the cavity dither did not give us a good indication where the optimal alignment is.
- Thus the best alignment of M1 had to be manually scanned. The resulting maximum green transmission was ~0.88
- Once the beam was aligned, the out-of-loop stability of the Xarm was measured.
There has been no indication of the improvement compared to Manasa's measurement taken before our beam alignment.
ASX scripts for PZT dither have been fixed appropriately. Script resides in scripts/ASX.
You can run the scripts from the ASX medm screen now.
Shutter moved, no more clipping.
Pick-off mirror 2" replaced by 1" one. Laseroptik HR 532nm, incident angle 30-45 degrees, AR 532 nm
Green REFL PD moved to 4" close to pick-off mirror. Pd being close to pick-off does not separate multiple reflections on it. I'll replace Laseroptic mirror with Al one. It is not easy to find.
Hole cut into side wall for doubler oven cable to exit.
- An Aluminum mirror instead of 2" unknown mirror for the pick-off for the rejected beam from the green faraday isolator (Steve)
=> Replaced. To be reviewed
- Faraday mount replacement. Check what we have for the replacement. (Steve)
- The green REFL PD should be closer to the pick-off mirror. (Steve)
=> Moved. To be reviewed
- A beam dump should be placed for the green REFL PD
- Move the green shutter to the place where the spot is small (Steve)
=> Moved. To be reviewed.
- The pole of the PZT mounting should be replaced with a reasonable one. (Steve with Manasa's supervision)
- Tidying up doubling oven cable. Make a hole on the wall. (Steve)
=> Done. To be reviewed.
- Tidying up the PZT cabling (Steve)
- The optics are dirty. To be drag wiped. (Manasa, Masayuki)
Beam trap for Pd refl is in place. Cabeling is ti·died up.
Laseroptic 1" mirror is replaced by Al 1" mirror. Problem remains the same. This diffraction patter has to be coming from the Faraday.
Atm1, good separation when Pd is far
Atm2, bad separation when Pd is close
Today we measured the openloop transfer function of the PDH green lock of the x-arm.
Edit //manasa// The excitation was given from SR785 source. SR560 was used as the summing node at the PDH servo box output where the loop was broken to measure the OLTF. The SR785 was used to measure the frequency response (CH2/CH1; CH1 A SR560 output and CH2 A PDH servo output) in sweptsine mode.
We measured with two different servo gain. We started with the servo gain of 3 and at that gain the UGF was 1.5 kHz and the phase margin was 50 degree. After that we increase the servo gain to 5.5 and at that gain the UGF was 6.2 kHz and the phase margin was 55 degree. In all the measurement we use the source amplitude of 1.0 mV for all frequencies (from 100 Hz to 100 kHz). We could not increase the gain and also the source amplitude any more because the green was kicked out of lock.
Next work list
1. In the earlier measurements we found the UGF of the PDH green lock of the x-arm as 10 kHz and the phase margin as 45 degree, so we will investigate what has changed from these measurements.elog 4490
2. We will measure the power spectrum of the error signal and the feedback signal.
3. We will calibrate the above signals to compare with ALS out of loop noise.
netgpib was taking forever to transfer data. So the measurements are just photos of the display.
attachment1 - servo gain 3
attachment2 - servo gain 5.5
Today we measured OLTF of PDH green lock of x-arm again. In the previous measurement the excitation signal was injected at the PDH servo box output(elog 9044), but in this measurement we changed the injection point to the RFPD mixer output (just before the servo input).
We measured the OLTF with the servo gain of 6.5 and source amplitude of 5 mV for all frequency band. The measured UGF was 11 kHz and the phase margin was 48 degree.
Next that measurement, we tried to measure the power spectrum density of the error signal and feedback signal. But the alignment was not so good, so we aligned the green light injection point. Tomorrow we will continue the alignment and will measure the PSD.
attatchment1 - OLTF of PDH green lock with servo gain of 6.5
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 Y arm green transmission has been measuring in counts all along. I modified the gain in the ALS-TRY filter module to normalise the transmission.
Transmission has been normalised with GTRY = 1 corresponding to 600 counts.
Meh. 600 counts is too weak. You should fix the electronics so that the maximized green laser transmission gives more like ~10000 counts.
Beam trap for Pd refl is in place. Cabeling is ti·died up.
The extra high post 3.375" for PZT is ready. We also have 2 more 2" green Laseroptik mirrors. I'm ready to swap them in.
The 75 mm focal length lens was placed in front of the green REFL PD yesterday.
We locked the XARM and YARM with using ALS control loop and we succeeded to lock stably both arms. The performance of the ALS was tested with a measurement of the calibrated error signal. (attachment 1)
- red and blue : the in-loop noise of ALS of each arm.
- green and purple:Stability of the beat-note frequency with the MC and the arm freely running.
In the high frequency region, YARM has larger noise than XARM, and these noises were not there in previous measurements by Koji and Manasa (elog8865). You can see that in both of in-loop noise and free running noise. These noises may be caused by the Green PDH servo or hte phase tracker servo or any other electrical staff. We will start noise budget of these servo.
At higher frequency than UGF of ASL control loop, the loop does not suppress the noises at all, but the inloop and free running noise are not equivalent. I have no idea about that so far.
What was the beat freq for each arm?
The HF noise level depends on the frequency of the beat note.
As the BBPD has the freq dependent noise level. (See this entry)
What was the beat freq for each arm?
The HF noise level depends on the frequency of the beat note.
As the BBPD has the freq dependent noise level. (See this entry)
I'm not sure about the actual number of the beat frequency, but the beat frequency was almost same in both arms. And I took this measurement sometimes with slightly different beat frequency but the noise level didn't change so much.
Then we can estimate the noises.
Measurement with ARMs
ITMX: 5.0843e-9 /f^2 [m/count]
ITMY: 4.9677e-9 / f^2 [m/count]
In high frequency region there is the difference between xarm and yarm. These difference are already there in error signal. I'm not sure where these noise comes from. We will make measurement with Green PDH from tomorrow, so we can also check with those measurement.
In other region the two noises are very close and also very similar to the plot of the seismic motion in the control room (attached on the front of TV screen).
Measurement with FPMI
i)By locking the FPMI with AS55Q and arms using POX,POY we measured the OLTF on AS55Q, the response from BS actuation to error signal on AS55Q for H_mich. The fitted, measured OLTF and the residual function is in attachment1. I fitted two parameters and they are time-delay and the gain. The time delay is -275 usec. The time delay in three different control are almost same. The response from BS to AS55Q is in attachment 2.
With these two measuremets, I calclated the H_mich in FPMI. This H_mich should be different from simple MI because the cavity refrectivity is different from the front mirror. Acrually it changed and the value was
Hmich = 4.4026e7
ii) I excited the ETMX and ETMY and measure the response from actuation to the error signal of MICH on AS55Q. The response is in attachment 3 and 4. from these result I calculated the H_L-l by using the formula as I mentioned. The value was
H_Lx-l = 175.7650 (XARM)
H_Ly-l = 169.8451 (YARM)
iii) I measured the error signal of MICH and XARM and YARM and with measured H_L-l, I estimated the FPMI noise caused by ARM locking. You can see in the higher frequency region than 10 Hz is dominated by noise caused by ARM control in-loop noises. 150 Hz and 220Hz are the UGF of each arms, so the two peaks are caused by arm control. You can see the small difference between FPMI noise and noise from arms. There are two possibilities, one is that these measurement is not same time measurement so they should have small difference. and other possibility is the error of the caliculation. But I think it doesn't look so bad estimation.
We will do same measurement with lock the arms the ALS system on tomorrow. Then we will check the PDH servo or other noise source and investigate the ALS system
While trying to lock the arms using ALS we found that the locks were not very stable and the in-loop noise was higher than seen before.
I looked into things and checked the out-of loop noise for ALS and found that the Y arm ALS noise (rms) was higher than the X arm.
To troubleshoot, I measured the OLTF of the phase tracking loop. While X arm was healthy, things weren't looking good for the Y arm. Sadly, the Y phase tracking loop gain was set too high with a phase margin of -2 degrees. We brought down the gain from 300 to 150 and set the phase margin close to ~55 degrees.
X arm Phase tracker loop:
UGF = 1.8 K Hz
Phase margin = 50 degrees
Y arm Phase tracker loop:
UGF = 1.6 KHz
Phase margin = 55 degrees
I. ALS servo loops
After fixing things with the phase tracking loop, we checked if things were good with the ALS servo loops.
We measured the OLTF of the X and Y arm ALS servo loops. In both cases the phase margin was ~20 degrees. There was no room to set enough phase margin. So we looked at the servo filters. We tried to modify the filters so that we could bring enough phase margin, but could not get at it. So we put back the old filters as they were.
attachment1: OLTF of the ALS XARM and YARM control loops
attachment2: Current phase budget. FM4 and FM10 are the boost filters.
II. ALS in-loop noise
Also, I found that the overall noise of the ALS servo has gone up by about two orders of magnitude (in Hz/rtHz) over the whole range of frequencies for both the arms from the last time the measurements were made. I suspect this could be from some change in the calibration factor. Did anybody touch things around that could have caused this? Or can somebody recollect any changes that I made in the past which might have affected the calibration? Anyways, I will do the calibration again.
We wanted to lock both the arms using ALS and get IR to resonate while arms are held using ALS. The X arm was locked using ALS and offsetter2 was used to scan the arm and find IR resonance. The Y arm was locked using ALS. But as the Y arm was brought closer to IR resonance, the X arm ALS loses lock. (attachment 1)
We believe that this comes from the X and Y transmission not being well separated at the PSL table. The PBS is not sufficient to decouple them (A strong beatnote ~35dB between the X and the Y arm green lasers can be seen on the spectrum analyzer).
Decouple the X and Y arm transmitted beams at the PSL table. I am trying to find a wedged mirror/window that can separate the 2 beams at the PSL table before the beat PD (sadly the laseroptik HR532nm optics have no wedge)
Flowchart for ALS autolocker. The error signal thresholds will be decided by trial and error.
I think we can use the IMC autolocker to start with getting this started. Once Jamie fixes the NDSSERVER environment variable bug, we should be able to use his more slick automation code to make it auto lock.
[revised at 10/1 pm 5:00]
As we mentioned in previous entry (elog#9171), the phase margin of ALS control was at most 20 degree. We modified the filter of C1ALS_XARM and C1ALS_YARM. The OLTF is in attachment1. Now the phase margins of both arms are more than 35 degree. I modified the FM5 filters of both servo.
FM5 filter is the filter for the phase compensation. It had the one pole at 1000 Hz and one zero at 1Hz. As you can see in attachment2, it start to lose the phase at 50 Hz. But the UGF of our ALS control loop is higher than 100 Hz, so I changed the pole from 1 kHz to 3 kHz in order to get more phase margin at UGF. The new servo have 10dB larger gain than previous filter at higer than 1kHz, but the control loop do nothing in that region, so it's no problem.
We have phase lag between 2 arms. I used same filters for both arms, so I'm wondering where these phase lag came from.
As I was trying to solve the 2 arm ALS problem, I found the Y arm ALS not so stable AGAIN :( . I measured the in-loop noise of the X arm as ~400Hz/rtHz (60 picometers).
I went ahead and checked the out of loop noise of the ALS and found there is some high frequency noise creeping in above 20Hz for the Y arm ALS (blue curve). I checked the UGFs and phase margins of the phase tracker loops and found they were good (UGF above 1.4KHz and phase margins between 40 and 60 degrees).
So the suspect now is the PDH servo loop of both the arms which has to be checked.
Attached is the out-of loop noise plots of X and Y arm ALS.
We made a new flowchart of ALS autolocker. We added the additional step to find the beat note frequency. We have to find a way to read the PSL temperature. By reading the PSL temperature we can decide the sweep range for the end green laser temperature with the curve which measured in previous measurement (in this entry)
We have three thresholds of error signal. One is the threshold for checking the arms are stabilized or not. It should be some hundreds count. Another threshold is to check that the suspensions are not kicked. This should be some thousands counts (in flow chart, it is 2K counts). The other is to check the optimal servo gain. If the servo gain is too high, the UGF is also too high and we will not have enough gain margin. The error signal start to oscillate at the UGF. We will check this oscillation and find the optimal gain. In flow chart this threshold is 1K counts.
We found the PDH servo gain for Y arm green was set at 2 (too low). The gain was set to 8.6 (based on earlier OLTF measurement elog 8817).
The ALS out-of loop noise was remeasured. We also measured the out-of loop noise of each arm while the other arm had no green (shutter closed). There doesn't seem to be any difference in the noise (between green and orange for Y arm and red and pink for the X arm) except that the noise in the X arm was slightly low for the same conditions (blue and red) when measurement was repeated.
TRANSLATION by Jenne: We first locked both X and Y for IR using the LSC, and X and Y for green using the analog PDH servos. We measured the _PHASE_OUT_Hz calibrated error signals for both X and Y in this configuration - this gives us the out of loop noise for the ALS system, the Green and Blue traces in the plot. We then closed the X end shutter, and measured the Y arm's error signal (to check to see if there is any noise contribution from the suspected X-Y cross beatnote). Then, we closed the Y end shutter, relocked the Xarm on green's 00 mode, and measured the X arm's error signal. We weren't sure why the Pink curve was smaller than the Blue curve below a few Hz, so we repeated the original measurement with both arms dichroic. We then got the Red curve. So, we should ignore the blue curve (although I still wonder why the noise changed in such a short time period - I don't think we did anything other than unlock and relock the cavity), and just see that the Green and Gold curves look similar to one another, and the Red and Pink curves look similar to one another. This tells us that at least the out of loop noise is not affected by any X-Y cross beatnote.
We succeeded in stabilizing both the arms using ALS and get IR to resonate at the sametime.
At each step we measured the _PHASE_OUT_Hz calibrated error signals for Y in this configuration so as to get the in-loop noise of ALS control of YARM
1. we stabilized YARM off IR resonance by using ALS, misaligned ETMX, closed XARM green shutter. That means no IR flashing and no green in XARM.
2. we aligned the ETMX with XARM green shutter closed.
3. we opened the green shutter and locked the green laser with PDH to the XARM.
4. we stabilized the XARM using ALS and off resonance for IR.
5.We brought the XARM to IR resonance with YARM stabilized off IR resonance.
6. we brought the YARM to IR resonance
Beat frequencies when both the arms were stabilized and had IR resonating :
X arm beat frequency = 73.2 MHz; Y arm beat frequency = 26.6 MHz.
1.the ALS in-loop noise in X and Y arms with IR off resonance and resonating.
2.the ALS in-loop noise in Y arm in each step from 1 to 6.(will follow soon)
The Y arm ALS in-loop noise doesn't seem to be different in any of the configurations in step 1 to 6. This seems to mean that the ALS of the two arms are decoupled.
Actually we are not sure what changed from the last few days (when we were seeing some sort of coupling between the ALS of X and Y arm) except for YARM green PDH servo gain changed (see this entry),
I checked the BK precision 1856D manual. I found that although this frequency counter can measure upto 3.5GHz, it has 2 separate input channels to measure two range of frequencies.
One input to measure between 0.1Hz to 100 MHz and the other to measure between 80MHz to 3.5GHz. Our beat frequency desirable range is <100MHz for stable ALS. Also, the beat PD response falls off beyond ~150MHz . Should we be happy with this frequency counter and use it in the 0.1Hz-100MHz range or look for one with a better measuring range?
P.S. Right now we are using the spectrum analyzer in the control room set to frequency range from 10MHz - 140 MHz for beat note search.
We locked MICH with 2 arms stabilized by ALS control.
We measured the power spectrum of the LSC-MICH_IN1 at each step so as to know the in-loop noise of MICH. And also we measured the OLTF of MICH loop and the error signal with BS excited at 580 Hz and MICH notch filter at same frequency enabled to obtain the MICH calibration factor.
1. We locked MICH using the AS55Q error signal and fedback to BS actuator. (Red curve)
2. We locked MICH and locked both the arms using POX11 and POY11 error signals and fedback to ETMs actuators.(Blue curve)
3. We stabilized both the arms using ALS. We use the ALS error signals and fedback to ETMs actuators. And then we locked MICH.(Magenta curve)
The green and brown curve are the ALS in-loop noise, which is the _PHASE_OUT_Hz calibrated error signals. So for these two curves the unit of vertical axis is Hz/rHz. The other curves are the MICH in-loop noises and these are not calibrated. So for these curves the unit of vertical axis is counts/rHz.
The UGF of MICH loop is 10 Hz with phase margin of 45 degrees (measured today). The FPMI noise with ALS stabilized arms is much larger than the FPMI with IR PDH locked arms above 30 Hz. That is because the ALS arm stability is not as good as the stability of PDH locked arms. We have to analyze and verify the calibrated numbers for FPMI + ALS with model.
I wrote the down script for ALS. This script is (script)/ALS/ALSdown.py When this script is running, it watches the feedback signal of the ALS control loop so as to shut down the servo immediately when the suspension is kicked.
When the value of C1:ALS-X(Y)ARM_OUT becomes larger than the threshold (right now it is 4500 counts), it changes the servo gain to 0, turns off all filters except for FM5 (the filter for phase compensation), resets the history of the phase tracker of each arm and prints the time on window when the suspension kicked
I put the switch on the C1ALS screen, and if you push this switch the window will open (like when you turn on the c1ass script) and the script start to run. For stopping this script, you have to close that window or press Ctrl + C on that window. This is little bit inconvenient, but we will make autolocker script for ALS and this downscript will be included that script soon. So I think it is enough to protect the suspensions right now.
Step by step procedure for stabilizing arms using ALS servo:
The procedure is the same for both the arms.
0. Check that the ALS arm servos are turned OFF and not sending any signals to the ETM suspensions.
1. Find the beat note by varying the laser temperature (moving the slider for SLOW_SERVO2_OFFSET).
Tip: It is easier to have the arms locked using IR PDH while searching for the beat note. Also check the stability of the MC. Unstable MC will cause the PSL temperature to drift and thereby affect the beat frequency.
2. Once you have the beat note, check if the beat amplitude is ~ -15 to -20 dBm. If the amplitude is small, then the alignment needs to be fixed (either the green input pointing at the end tables or the PSL green alignment). This is important because the UGF of the phase tracking loop (should be above 1KHz) changes with the amplitude of the beat note.
Also the beat frequency should be < 100 MHz; preferably below 80 MHz.
3. Disable IR PDH locking if you had used it while searching for the beat note.
4. Press CLEAR HISTORY button for the phase tracker servo. Check if the phase tracking loop is stable (phase tracker servo output counts should not be ramping up). If the phase tracker is not stable, check the servo gain and phase margin of the loop.
5. Turn OFF all filters in the ALS arm servo filter module except for FM5 (phase compensation filter). With ALS arm servo gain set to zero, enable the arm servo and allow ALS control signals to be sent to the ETM suspensions.
5. Open dtt and look at the power spectrum of the ALS error signal (C1:ALS-BEAT?_FINE_PHASE_OUT_HZ).
6. Set ALS arm servo gain +/- 0.1 to check the sign of the servo gain. Wrong sign of gain will make the loop unstable (beat note moving all over the frequency range on the spectrum analyzer). If this happens, set the gain to zero immediately and clear history of phase tracker servo. If you have set the correct sign for gain, the servo will stabilize the beat note frequency right away.
7. Once you know the correct sign of the servo gain, increase the gain in steps while simultaneously looking at the power spectrum of error signal on dtt (it is convenient to set dtt measurements to low bandwidth and exponential measurement settings). Increase the gain until you can see a slight bump close to the UGF of the ALS servo (>100Hz).
There have been times when this servo gain was in a few hundreds; but right now it varies from +/- 10-20 for both the arms. So we are stepping up gain in steps of +/- 2.
8. Enable filters (FM2, FM3, FM6, FM7, FM8). Wait to see the rms noise of the error signal go down (a few seconds).
9. Enable boost filter (FM10). There also exists a weaker boost filter (FM4) which we don't use any more.
1. Beat frequency changes affect both the servo gain and sign of gain. So if you lose stability of ALS servo at any point, you should go through all the steps again.
2. At any point if the ALS arm servo becomes unstable (which can happen if the MC loses lock or if the beat frequency was too high ), change the servo gain to zero immediately. Turn OFF all the filters except for FM5 (if they were enabled) and reset phase tracker servo (CLEAR HISTORY button in the phase tracker filter module). Masayuki has written the down script that does all this. The script will detect arm servo loop instability by continuously looking at the feedback signal. Details about the script can be found here.
Here is a cheat sheet that can give you an idea of the SLOW SERVO2 offset range to scan in order to find the beat note:
PSL temperature X offset Y offset
31.58 5278 -10890
31.52 5140 (not recorded)
31.45 4810 (not recorded)
31.33 4640 -10440
31.28 4500 -10340
I modified the ALS down script. When the value of C1:ALS-X(Y)ARM_OUT becomes larger than the threshold, it turn off the output ON/OFF switch immediately. That is because the ALS servo has ramp time. When script changes the gain to 0, it takes some seconds. That is not good for suspensions.
After changing servo gain to 0 and turning off the filters, the script waits ramp time and turn on the servo output switch.