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.
After Jenne and Masayuki told that they were not able to stabilize the ALS for either arms yesterday, I looked into things with the ALS servo.
I had trouble initially trying to even stabilize the loop for a few minutes. So I measured the OLTF of the phase tracker loop and the ALS X arm servo. I changed phase tracker gain to 125 and that rendered UGF of 2KHz and phase margin of 45 degrees for the phase tracker loop.
The ALS servo gain was set such that UGF was 125Hz and phase margin 38 degrees (attached is the transfer function measurement for the servo).
I could stabilize the arm to ~500 Hz/rtHz (rms), which is twice that of what we had while we did the (PRMI+1arm ALS).
But ALS was still not stable long enough with the higher rms to even allow a cavity scan to find IR resonance. I suspect the problem to now lie with the PDH loop. We should be looking to stabilize the PDH for green if we need a stable ALS.
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 have stabilized the ALS for Y arm and concluded that although the PDH servo could be stabilized, it drifts and loses stability over a span of few hours. (See masayuki's elog today)
We wanted to follow the same systematic procedure like in the previous elog to look at the condition of the X arm as well.
In order to stabilize the green PDH servo, we held the arm using the IR PDH and aligned the end-green to the X arm.
We see 2 TEM00-like modes and one oblong TEM00+TEM01 mode that can lock to the cavity. It is not clear to me as yet as to how to differentiate between these 2 TEM-00 like modes and how we should decide between them.
One of the TEM00-like mode is strongly matched to the arm cavity. Normalized GTRX measures 0.6 counts. The other TEM00-like mode is weakly matched to the cavity. Normalized GTRX measures 0.12 counts. This might be the reason why Jenne and Masayuki were seeing a lower beat amplitude. Camera images are shown below.
On another note, we found that an oblong mode (looks like a TEM00+TEM10 mode) also locks to the cavity. The mode looks weird in that, only one half of the mode is seen moving due to seismic noise and the other part does not. I am not sure how I can describe this...so here is a 10 second video of how this mode looks like.
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.
I wrote the script to scan the cavity using ALS until it finds IR resonance . This script is (script)/ALS/ALSfindIRresonance.py I attached the time series of the C1:ALS-OFFSETTER and IR transmission of XARM when the script was working.
When you start this script, it start rough scan. It steps the offset of the C1:ALS-OFFSETER with ramp time, and for each step it checks the value of C1:LSC-TR. At rough scan, one step is 0.1 count. When IR transmission become larger than threshold, this script start fine scan. In fine scan, this script steps the offset by 0.01 for the range of 2. For each step, C1:LSC_TR value is measured, and after fine scan it set the offset to the value which had the maximal C1:LSC-TR.
I put new button 'Scan %ARM' to the ALS screen. You can run this script by pushing that button.
For PRMI + 2arm, we tried to make the ALS control noise better. As this entry we had huge 60 Hz comb noise in PDH loop of YARM.
So we tried to figure out the problem and fix it.
We checked which power supply the staff in Y-end are connected to, and change some of them to connect to 1Y4 AC power supply from wall AC. What we changed was
1.Main end laser
3.Green REFL PD
We checked error signal of PDH control and compared before and after. The 60 Hz peak get better from -80 dBVpk to -90 dBVpk. Also I attached the plot of XARM, privious YARM (the data of Yesterday night), and current YARM ALS in-loop noises. The RMS of ALS in-loop noise of Y-arm get better by factor of 2. However, even the 60 Hz comb noise get better than before, RMS get worse by comb noise.
We would like to make these noise better at least until these noises don't affect to RMS, so we should continue to check.
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.
In 2arms + MICH configuration, residual motion of the cavity will couple with MICH signal. When cavity length change, the reflectivity of cavity also change. And that cause the phase shift in reflected light. That phase shift is detected in MICH signal. When we try to lock the DRMI + arm, that coupling will be problem for lock acquisition. For practice to estimate that coupling, I estimated the coupling between the cavity motion and the AS55Q signal.
What I did
- Measurement steps
I did the same measurement as that of this entry. For the estimation below steps are needed. The detail of each step will be written below.
--Measurement and calibration of the AS55Q error signal with MICH + 2arms locked by ALS control
--Measurement of the ALS in-loop noise and estimation of residual motion of the cavities.
--Calibration of the coupling from residual arm motion to AS55Q signal
- Calibration of the AS55Q signal
1. Sensor gain estimation
We used the same method as the previous entry,
We excited the BS at 580 Hz with a given amplitude (Vin). We enabled the notch filter at 580 Hz in the LSC MICH servo. We measured the peak height (Verr) of the AS55Q error signal. We used the actuator response (A_bs) of BS measured in this entry.
We can get the sensor gain (H) of AS55Q in unit of count/m
H = ------- -------
By this calculation H = 4.2e+07.
2. Fitting of OLTF for the MICH loop
We measured the OLTF of the MICH loop. Modelled OLTF is fitted into the measurement data. That modelled OLTF includes the actuator response of BS, the MICH servo filters, DAI,DAA,AI,AA filters, the TF of sample and hold circuit. (About DAI, DAA filters and S/H circuit please read this entry. About AI,AA filters please read this entry) Also I put time-delay into that OLTF. I estimated that time-delay and the gain of OLTF by fitting. The time delay was 311usec.
3. Estimation of the MICH free running noise
With modeled OLTF, I estimated the MICH free running noise.
Estimation of the coupling from residual cavity motion to AS55Q signal
The ALS in-loop noise data has the unit of Hz/rHz (disturbance of the cavity resonant frequency). By multiplying L_arm/f_laser we can convert the unit to m/rHz (disturbance of the cavity length) .
I used the same coupling constant between residual motion of cavity and MICH noise as this entry. For estimation of the coupling constant, we excited ETMs and measured the TF from excitation signal to AS55Q error signal. I assumed the cavity pole as 4000 Hz. The result is discussed below
ALS in-loop noise include the sensor noise. in high frequency region the in-loop noise is dominated by the sensor noise. So in this region in-loop noise does not mean actual residual motion of the cavity. And this sensor noise pushes the mirror. So we have to estimate the actual motion of the cavity by multiplying the servo transfer function of the control in this region.
I made 2 plots. Both include the MICH free running noise and estimated coupling noise from both arms. In one plot, for estimation of the coupling I multiplied only coupling constant to calibrated in-loop noise of the ALS loop. In another plot, I multiplied coupling constant and OLTF of ALS loop in order to estimate the actual motion of the cavity. If the 3 curves are coincide in first plot, that means the ALS in-loop noise is same as the residual cavity motion in that region and the MICH free running noise is dominated by coupling from residual cavity motion. If those curves are coincide in second plot, that means the ALS in-loop noise is sensor noise in that region.
Above 40 Hz, the 3 curves are totally in coincident in first plot. On the other hand in second plot the 3 curves look similar in this region. That may mean above 40 Hz the ALS noise are dominated by sensor noise and MICH free running noise is dominated by the coupling from residual cavity motion. Also in the region between 10 Hz and 40 Hz, the MICH free running noise seems to be dominated by coupling from cavity motion.
In second plot, the coupling from cavity motion is overestimated. It's possibly because of overestimation of coupling constant, but I'm not sure.
Koji mentioned that we should measure the residual motion of the cavity by using POX and POY. Now the ALS is much more stable than before, so I think we can easily do the measurement again with out of loop measurement. That will be more strait forward measurement.
Control signal measurement of end PDH control
The Yarm ALS wasn't robust. Yesterdays night, we found that suspension kicked by something and that was the reason why the end PDH control lost lock. To make sure that the PDH loop itself is robust, I measured control signals of End PDH loops. When the gain inclease, the peak at UGF appeared and become unstable. Both arms does not seems unstable before the peaks appear.
Xarm PDH servo gain optimization
I optimized the x end PDH servo gain with measuring OLTF. Now the servo gain is 5.0. UGF is around 10 kHz and phase margin is 40 degree.
Also I measured out of loop noise. I locked the arm using IR PDH, and measure the ALS error signal. The high frequency noise become better.
I calibrated the ALS-OFFSETTER output.
I measured the FSR of cavity in unit of counts. That was 395 counts. Our cavity FSR is 3.8 MHz, so 1 count of the OFFSETTER output is 9.7 kHz.
I calibrated the ALS-OFFSETTER output.
I measured the FSR of cavity in unit of counts. That was 395 counts. Our cavity FSR is 3.8 MHz, so 1 count of the OFFSETTER output is 9.7 kHz.
Really? What cavity length did you use in the calculation?
My job right now is to characterize the green PDH loops on each arm. Today, Jenne took me around and pointed at the optics and electronics involved. She then showed me how to lock the green beams to the arms (i.e. opening the shutters until you hit a TM00 shape on the transmitted beam camera). Before lunch, the y arm was easiest to lock, and the transmitted power registered at around 0.75.
After lunch, I took a laptop and SR785 down to the y end station. I unhooked the PDH electronics and took a TF of the servo (without its boost engaged, which is how it is currently running) and noise spectrum with the servo input terminated.
I then set up things a la ELOG 8817 to try and measure the OLTF. However, at this point, getting the beam to lock on a TM00 (or something that looked like it) was kind of tough. Also, the transmitted power was quite a bit less than earlier (~0.35ish), and some higher order modes were higher than that (~0.5). Then, when I would turn on the SR785 excitation, lock would be lost shortly into the measurement, and the data that was collected looked like nonsense. Later, Koji noted that intermittent model timeouts were moving the suspensions, thus breaking the lock.
We then tried to lock the x arm green, to little success. Koji came to the conclusion that the green input pointing was not very good, as the TM00 would flash much less brightly than some of the much higher order modes.
Tomorrow, I will measure the x arm OLTF, as it doesn't face the same timeout issue that is affecting the y arm.
Yesterday, made a slew of measurements on the X-arm when locked on green. By tweaking the temperature loop offset and the green input PZT pointing, I was able to get the transmitted green to around 1.0. The PDH board gain was set to 4.0. I had trouble making swept sine measurements of the OLTF; changing the excitation amplitude for different frequency ranges would result in discontinuities in the measured TF, and there was only a pretty narrow band around the UGF that seemed to have reasonable coherence.
So, I used the SR785 as a broadband noise generator and measured the TF via dividing the spectra in regions of coherence. Specifically, I used the "pink noise" option of the SR785. I also used a SR560 as a low pass to get enough noise injected into the lower frequency range to be coherent, while not injecting so much into the higher frequencies that the mode hopped while measuring.
The servo board TF was easily fitted to a 4th order zpk model via VFIT, but I'm having trouble fitting the OLTF. (There is a feature in the servo TF that I didn't fit. This is a feature that Zach saw [ELOG 9537], and attributed to op amp instability) Plots follow. Also, while these need to be calibrated to show the real noise spectrum of the cavity motion, I'm attaching the voltage noise spectra of the error and control signals as a check that electronics/PD noise isn't dominating either signal.
What a such long pain we suffered.
After more than three years from Kiwamu's discovery, the PDH box 50kHz oscillation issue was finally solved.
This "weird peak at 50kHz" was caused by the oscillation of the voltage regulator (ON's MC7912).
As it imposed common noise almost everywhere, it screwed up transfer function measurements
like EricQ saw recently.
The negative voltage regulator (79XX) tends to get unstable than the positive counter parts (78XX).
The oscillation was removed by adding 22uF electrolytic capacitor between the output pin (pin3) and the ground pin (pin1) of MC7912.
This is indeed more than 20 times of the specification you can find in the data sheet.
With the newly repaired PDH board, I spent some time with the x arm green PDH loop. I found it SO MUCH EASIER to measure the OLTF by injecting before the servo, instead of after it. (i.e. I added a swept sine from the SR785 to the mixer output (error signal) before the servo input). This is likely because the error signal is much flatter. I used a 10mV excitation across the whole frequency range (30-100kHz).
Here's the OLTF. I'm working on fitting it and breaking it up into its constituent TFs, then making a rudimentary noise budget.
OK, the next question will be "why the loop bandwidth is so miserable?"
In other words, what is preventing us to have the bandwidth of 20~30kHz?
Your breaking down will give us the answer.
I'm not as good as a fit, but it seems that there is a loop delay of about 30 microseconds, looking at the high frequency phase. This might explain the limitation on the BW. Eric should be able to get the delay out of the fit with some care.
There was 0.2 mW green at the X end.
The doubling oven temp was changed from 37.5 to 36 degrees C
Power at green shutter 3 mW The alignment was not touched.
I found that the X end SLOW control was left on for ~15days. The output of the filter had grown to ~2e7.
This yielded the laser temperature pulled with the maximum output of the DAC.
This was the cause of the power reduction of the X end SHG; phase matching condition was changes as the wavelength of the IR was changed.
Once the SLOW output was reset, the green REFL was reduced from 4000cnt to 1800cnt.
Y arm green: Nothing much was disturbed. I touched the steering mirrors and brought GTRY from 0.2 to 0.9.
X arm green: The PDH lock was not very stable mostly because of the low power in green. I changed the oven temperature for the doubler to 36.4 corresponding to maximum green power. GTRX increased from 0.1 to 0.9
Both the X and Y arm green alignment were tuned on the PSL table to their respective beat PDs.
The PSL green shutter was not responding to the medm buttons. I found the PSL green shutter set to 'local' and 'N.O' (these are switches in the shutter controller). I do not see any elog and not sure as to why the controller was even touched in the first place. I set the shutter controls to 'remote' and 'N.C'.
The X and Y arms were locked successfully using ALS and the arms could be scanned and held to support IR resonance.
The same procedure as in elog 9219 was followed. In-loop noise was measured to be between 200-300 Hz rms for the lock.
ALS settings for the lock
X arm : FM 2, 3, 5, 6, 7, 8, 10 Gain = 11.0
Y arm : FM 2, 3, 5, 6, 7, 8, 10 Gain = 10.0
Nice restoration. We eventually want to make transition of the servo part from ALS to LSC model for the further handing off to the other signals.
Please proceed to it.
ETMX green power at shutter 3.6 mW at 36.35 C doubler crystal temp. [ Innolight IR settings 2.0 A, 40.83 C, 500 mW before Faraday 1/2 plate ]
ETMY green power at shutter 0.75 mW at 35.8 C doubler crytal temp. [ NPRO IR settings 1.82A, 231 mW_ display, DT 21 C, DTEC +1V, LT 40 C, LTEC 0.1V, T +41.041 ]
Alex, 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- pvc, slit corrugated loom tubing " 1.5" ID
The fiber is not protected between 1Y1 and 1Y4
The X -arm fiber is in the high cable tray and it has has coupler mounts.
The Y -arm fiber is in the low cable tray and it has no coupler mounts.
The fibers are only protected at entering and exiting the trays.
We have only 68 ft spare 1.5" ID protective plastic tubing.
Nic, Jenne, EricQ, and Koji should describe the demonstartion of CESAR achieved tonight.
Q and I have started to use the ALS slow servo for the end aux lasers while locking the arms using ALS. The servo prevents us from hitting the limits of the PZT range for the end lasers and a better PDH locking.
But keeping the servo ON causes the slow output to drift away making it hard to find the beat note everytime the arm loses lock. The extensive beat note search following the unlock can be avoided by clearing history of the slow servo.
Q and I have started to...
To check the basolute frequency stability of the old monochrome HP 8591E RF Spectrum analyzer that we're using for the ALS beat readout, I hooked its 10 MHz reference output (from its rear panel) into the A channel of the SRS SR620 frequency counter. The SR620 is locked to the FS 720 Rubidium clock via the 10 MHz connections in their rear panels.
So, we can assume that this is a good absolute readout. It reads 9.999860.7 +/- 0.3 Hz. So its 139.1-139.4 Hz lower than 10 MHz. The +/- 0.3 is just a slow drift that I see over the course of 10 minutes.
So, let's say that the analyzer is low by 10 ppm, so the arm length estimates are short by ~0.4 mm. A negligible correction, so there's no need to use atomic clocks to measure our arm lengths.
Green light power decreased from 3 mW to 1 mW at the ETMX-ISCT shutter. More later.
It is actually very tricky to measure the green power at the output of the doubling crystal as the IR often leaks into the measurement.
I checked the green beam powers on the X/Y/PSL tables.
CONCLUSION: There is no green beam which exceeds 5mW anywhere in the 40m lab.
Note: The temperature of the doubling crystal at the X end was optimized to have maximum green power. It was 36.3degC and is now 36.7degC.
When the angles of the wave plates are optimized, we have 539mW input to the doubling crystal.
With the Xtal temperature of 36.7degC, where the green power is maximized, the power right after
the harmonic separator (H.S.) was 9.6mW.
Xtal temp 36.7degC ~~~
Xtal temp 36.7degC
If we believe these 4.69mW and 4.54mW are purely from the green, we have 4.8mW right after the H.S.
This corresponds to the conversion efficiency of 1.6%/W (cf. theretical number 2%/W)
By disabling the heating of the crystal, we can reduce the green light by factor of 60. But still the reading right after the H.S. was 5.3mW
Xtal temp 29.2degC ~~~
Xtal temp 29.2degC
Naively taking the difference, the green beam right after the H.S. is 4.4mW.
In either cases, the green power right after the oven is slightly less than 5mW.
When the angles of the wave plates are optimized, we have 287mW input to the doubling crystal.
With the Xtal temperature of 36.0degC, where the green power is maximized, the power right after
the harmonic separator (H.S.) was 0.86mW.
Xtal temp 36.0degC ~~~
Xtal temp 36.0degC
When the temperature was shifted to 39.2degC, the reading after the H.S. was 70uW. Therefore the contamination by the IR is small
in this setup and we can believe the above reading in 70uW accuracy. This 0.86mW corresponds to the conversion efficiency of 1.2%/W.
The incident IR is 80mW. We have 170uW after the H.S., which corresponds to the conversion efficiency of 2.6%/W. Maybe there is some IR contamination?
From the vacuum chamber total 1mW of green is derivered when both arms are locked and aligned.
Thus the total green power at the PSL table is less than 5mW.
The Y arm green transmission had come down to 0.3 and the green steering mirrors on the Y end table required some minor alignment adjustments to bring back transmission to around 0.75 counts.
We've been having trouble tuning the ALS DIFF matrix. Trying to see if the MC2 EXC can be cancelled in ALS DARM by adjusting the relative gains in ALSX and ALSY Phase Tracker outputs.
There's a bunch of intermittent behavior. Between different ALS locks, we get more or less cancellation. We were checking this by driving MC2 at ~100-400 Hz and checking the ALS response (with the ALS loops closed). We noticed that the X and Y readbacks were different by ~5-10 degrees and that we could not cancel this MC2 signal in DARM by more than a factor of 4-5 or so. In the middle of this, we had one lock loss and it came back up with 100x cancellation?
Attached is a PDF showing a swept sine measurement of the ALSX, ALSY, and DARM signals. You can see that there is some phase shift between the two repsonses leading to imperfect cancellation. Any ideas? Whitening filters? HOM resonance? Alignment?
So far today, I've been working with the Y-end green PDH locking. Using a SR560 to roll off the AG4395A output to take a loop measurement at the servo output, I measured the following OLG, and inferred the CLG from it. The SR560 really helped it getting good coherence without introducing a big offset that changes the optical gain, thus distorting the loop shape, etc. etc.
You would think this loop looks pretty good, 10k UGF, and 45 degrees of phase margin, gain peaking is sane, and pretty smooth slope. But, the thing still was flipping out of lock while I measured this.
I suspect shenanigans at >100k. This is motivated by the fact that I've seen some big noise in the error signal around 150k. I don't have a good noise plot right now, because I'm trying to get a scheme going where I stitch together a bunch of 1 decade spectra from the 4395, but the noise floor isn't consistent across each patch (even though the attenuation stays the same, and I confirmed I'm in "noise" mode). I'm working on a loop measurement up there, too, but I haven't been able to get the right filter/amplitude settings yet.
So, even though this plot is not totally correct (read: wrong and bad), I include it just for the sake of showing the big honking spike of noise at ~150K.
[ Rana, Jenne]
We remeasured the Yend PDH box.
When we first started, the green couldn't hold lock to the arm - it kept flickering between modes. Changing the gain of the PDH box (from 7.5 to 6.0) helped.
We measured a calibration, from our injection point to our measurement point.
The concept was that we'd take the mixer output, and put that into an SR560, and put the swept sine injection into the other input port of the '560, and use A-B. So, for this calibration, we left A unplugged, and just had the RF out of the 4395 going to input B of the '560. The 600 Ohm output of the '560 went to the error point input on the PDH box (during normal operation the mixer output is connected directly to the error point input). The SR560 was set to gain of 1, no filtering. I don't recall if we were using high range or low noise, but we tried both and didn't really see a difference between them.
We had the 4395 take that calibration out, and then we measured the closed loop gain up to 1 MHz. (Same measurement setup as above, but we connected the mixer out to the input of the SR560 to close the loop, and made sure we were locked on a TEM00 green mode.) Rana used an ipython notebook to infer the open loop gain from our measurement. Our conclusion is that we don't have nearly enough gain margin in our loop. We found the PDH box gain knob at 7.5, and we turned it down to 6.0, but the loop is still pretty borderline. We used the high impedance active probe to measure the error point monitor, since we aren't sure that that point can drive a 50 Ohm load.
We also measured the error point spectra and the control point spectra. Unfortunately, the saved data from the analyzer (no matter what is on the screen) comes out in spectrum, not spectral density. So, we need to check our conversion, but right now to get from Watts power to Volts, we do sqrt(50 ohm * data). We then need to get to spectral density, and right now we're just dividing by the square root of the bandwith that is reported in the .par file. This last step is the one we want to especially check, by perhaps putting some known amount of noise (from an SR785?) into the 4395, and checking that our calibration math returns the expected noise spectrum.
What still needs to be done is to calibrate this into Hz/rtHz. To do this, we were thinking that we should look at the error point on a 'scope while the cavity is flashing.
Anyhow, here is the uncalibrated error point spectrum. Purple is a measurement up to 30kHz, with 30Hz bandwidth. Blue is a measurement up to 300kHz with 300Hz bandwidth. The gain peaking schmutz above 10kHz sucks, and we'd like to get rid of it. We also see the same peak at ~150kHz that Q saw earlier today. We were using the high impedance probe here too.
We have the data for the control point (all the data files are in /users/jenne/ALS/PDHloops/Yend_18Aug2014), but we haven't plotted it yet.
Things that need doing:
* (JCD) Think about this box's purpose in life. What kind of gain do we need? Do we need more / less than we're currently getting? NPRO freq noise is 1/f and is 10kHz/rtHz at 1Hz (this is from a plot of an iLIGO NPRO from Rana's thesis, but it's probably similar). Talk to Kiwamu; the noise budget in the paper seems to indicate that we had some kind of boost on or something. Also, if we need much more gain than we already have, we'll definitely need a different box, maybe the PDH2 box that they have over in WBridge.
* (EQ, priority 1) Measure and calibrate error point noise down to lower freq for both arms. What could we win by putting in a boost? If the residual noise is high, maybe the laser isn't good at following arm, so beatnote isn't good length info for the arm, and we can't succeed.
* (EQ, priority 2) Measure TF of PDH box, and a separate measurement of the Pomona box that is between the mixer and the error point - is that eating a bunch of phase? It's already an LC circuit which is good, but do we really want a 120kHz lowpass when our modulation frequency is roughly 200kHz? Ask ChrisW - he worked on one of these with Dmass.
* (EQ, priority 2ish) Measure TF of Xend PDH loop (unless you already have one, up to ~1MHz).
* (JCD) Make DCC tree leaf for PDH box #17. Take photos of box.
Heading to dinner, going to come back for more green fun, but here's a quick update:
Xarm Peak-to-Peak of the PDH signal in the mixer output is about 70mV when GTRX was about 0.4. The sideband-generating function generator has an output of 2V (forgot to note rms or pp)
Yarm Peak-to-Peak of the PDH signal in the mixer output is about 640uV when GTRX was about 0.71. The sideband-generating function generator has an output of 0.091V (forgot to note rms or pp)
The Yarm signal thus correspondingly has a waaay noisier trace. I would've had scope plots to show here, but the scope freaked out about how large my USB drive capacity was and refused to talk to it >:|
This suggests to me that our modulation depth for the Yarm may be much too small, and may be part of our problems with it.
Here is a plot of last night's data with both the control and the error point on the same plot, in Volts. Q is still working, so I don't have a calibration number yet to get these to Hz.
Note in the control spectrum that we have very significant 60Hz lines.
EDIT: I also added a new branch to the DCC Document Tree, and 2 leafs (one for each end). Here's the ALS PDH servo branch: E1400350
It's not so impressive yet, but here's a plot that shows (a) Rana's guess for laser frequency noise, (b) The inferred in-loop version of that noise, (c) The CARM linewidth FWHM, translated to Hz.
For (b), I take the loop that Rana and I measured last night, and I assumed that it continued on forever as 1/f toward low frequency. Then I do 1/(1+G) to get the closed loop version of the loop (which is a measurement with an artificial line tacked on the end), and multiply this with the laser freq noise, which is also totally artificial.
For (c), I do df/f = dL/L, with f = c/lambda_green, since the rest of the plot is meant to be in green frequency units.
This is my beginnings of trying to come up with a requirement for our green PDH boxes. We weren't very clear in the MultiColor paper about the nitty-gritty details (obviously), but then Kiwamu didn't expand on those details in his thesis either. He talks a lot more about the design considerations for the digital ALS loop, which isn't what I want today. I will send him an email to see if he had any notes that didn't make it into his thesis.
Reasonable amounts of time were spent bending the AG4395 to my will; i.e. figuring out the calibration things Jenne and Rana did, finding the right excitation amplitude and profile that would leave the light steadily locked, and finding the right GPIB incantation for getting spectra in PSD units instead of power units. I'm nearing completion of a newer version of AG4395 scripts that have proper units, and pseudo-log spectra (i.e. logarithmically spaced linear sweeps)
Here is too many traces on one plot showing parts of the OLTF for the x green PDH. One notable omission is the PD response (note to self:check model and bandwidth). The servo oddly seems to have a notch around 100k. My calibration for the CLG injection may not have been perfect, instead of flattening out at 0dB, I had 2dB residual. I tried to correct for it after the fact, assuming that certain regions were truly flat at 0dB, but I want to revisit it to be thorough. I found some old measurements of the Innolight PZT PM response, which claims to be in rad/V, and have included that on the plot.
In the end, the mixer and PZT response make it look like getting over 10kHz bandwidth may be tough. Even finding a good higher modulation frequency to be able to scoot the LP up would leave us with the sharp slope in the PZT phase loss, and could cause bad gain peaking. Maybe it's worth thinking about a faster way of modulating the green light?
Tomorrow morning, I'll calibrate all the noise spectra I have into real units. These include:
However, looking at the floors, it occurs to me that I may have left the attenuation on the input too high, in an effort to protect the input the PDH box, which rails all the time when not locked to a 00 mode, sometimes even with the input terminated or open. It's kind of a pain that the agilent makes it really hard to see the data when you're in V/rtHz mode, because I should've caught this while measuring :/
I used a scope to capture a pdh signal happening, which will let me transform the mixer output into cavity motion. The control signal goes to the innolight PZT with a ~1MHz/V factor. Here are the uncalibrated plots, for now.
A MIST simulation tells me that the green pdh horn-to-horn displacement is about 1.2nm, or ~18kHz. I used this, along with the scope trace attached to the previous post, to calibrate the mixer output at 193419 Hz per V. (EDIT: I was a little too hasty here. What I'm really after is the slope of the zero crossing, which turns out to be almost exactly twice my earlier naïve estimate. See later post for correct spectra)
For the control signal, I assumed a flat Innolight PZT PM response of 1MHz/V. ( Under 10kHz, it is indeed flat, and this is the region where the control signal is above the servo output noise in yesterday's measurements)
Here are all of the same spectra from last night, with the above calibrations.
Going off Jenne's earlier plot, it looks like the in-loop error signal RMS is ten times bigger than the CARM linewidth.
I calibrated the control signal from Volts to Hz using the rough PZT calibration of 5MHz/V for the Yend NPRO.
For the error signal, Q said that the Yarm PDH peak-to-peak height was about a factor of 100 smaller than the Xarm, so I used a calibration of 1.9e7 Hz / V.
Then, from Q's Mist simulation including the high Xarm loss, and the plot that he posted in the control room, the CARM linewidth looks like it is about 2pm. This is the number that I have included on today's plot. Note though that yesterday I was using a linewidth of about 30pm, which I got from an Optical simulation about a year ago. I do not know why these numbers come out an order of magnitude different! The CARM linewidth is actually about 20 pm. Both Q and I failed at reading log-x plots yesterday. I have corrected this, and replotted.
Anyhow, here's the Yarm noise spectra calibrated plot:
I have emailed Kiwamu, but haven't heard back from him yet on what the original design considerations were, if he remembered us ever using a boost, etc. What this looks like to me is that we need to do some serious work to get the noise down. Maybe fixing the gain peaking and triggering the boost will get us most of the way there?
I remeasured all of the noise spectra again today, making sure the input attenuation was as low as it could safely be. I also got a snap of the y green PDH signal; it's fairly larger than I saw the other day, which is good. I used this to calibrate the error signal voltage spectra.
Here are the noise traces for each arm. During these measurements GTRX was about .6, GTRY about 1.0 The Yarm noise doesn't look so good: the error signal is just barely above the mixer+lowpass output noise, and the RMS is plauged by 60Hz lines. (Is this related to what we see in IR TRY sometimes?)
Here are the arms error signals compared directly:
We spent time trying to relieve the Yend green PDH of it troubles.
We realized that the mixer in the PDH setup (mini circuits ZAD-8+), wants 7dBm of LO to properly function. However, we use one function generators output, through a splitter, to give signals to the laser PZT and the mixer LO.
We don't want 7dBm of power hitting the laser PZT, though. The summing node that adds the servo output to the sideband signal was supposedly designed to do some of this attenuation. Rana measured that 10Vpp out of the function generator resulted in 20mVpp on the fast input to the NPRO, after the summing node. Hence, the 0.09V setting was only resulting in something like 0.2mV hitting the PZT. The PZT has something like 30 rad/V PM response, meaning we only had ~0.006 rad of modulation.
Now, the function generator is set to 2 Vpp, meaning 4 mVpp hitting the PZT, meaning ~0.12 radians of modulation. The mixer is now getting +7dBm on its LO, and the PDH traces look much cleaner. However, the PDH error signal is now something like 100mVpp, which is much bigger than the PDH board is designed for, so there is now a 10dB attenuator between the reflection PD DC block and the RF input to the mixer.
Here are screenshots of the Inmon channel (which has a gain of ~20) showing a sweep through some PDH signal, and the error signal while in green lock. Huge 60Hz harmonics are still observed.
Regarding these 60Hz issues, we need to make sure that we remove all situations where long BNCs are chained together with barrel connectors, or Ts are touching other ones. We also should glue or affix the pomona summing box to the shelf, so that its not just laying on the floor.
The concrete next step is to go fiddle with things, and see if we can get the 60Hz noise to go away, then measure the PDH loop and noises again. Hopefully, this should make the ALS much more reliable.
I found that the barrel of one the BNC to BNC connectors used for getting the output of the PDH servo box to the laser controller was touching the ETMY chamber. When I held it away, all of the 60Hz harmonics disappeared from the mixer output spectrum; this was pretty repeatable. This inspired me to replace the refl PD and PZT signal cables (which were 2 and 3 cables stitched together, respectively) with 20' long BNCs. I also cleaned up a lot of the routing of signal and power cables in the little rack, and moved the big T->DC Block->Attenuator combo off of the panel mount, because I didn't like how it was wiggling. It and the summing pomona box are sitting on top of the PDH box and function generator, instead of hanging freely.
All of the 60Hz harmonics were banished afterwards, and the green locked happily.
This required me touching the Y end table, to remove the old cable and its cable ties, and putting the new one in. I don't think I did anything immediately apparently bad; the green and IR transmissions both are within nominal ranges.
I haven't had luck measuring the CLG yet, which I wanted to do to get and set the UGF before measuring the noises. However, here is a scope trace of the in-lock error signal, which compares quite favorably to the trace posted in the previous post; the scope indicates that the signal has 1/3 of the RMS that it did before I replaced the cables.
I hope to measure up the current status after I get back from dinner.
Yesterday I measured the spectra and OLTF of the Y-Arm green PDH, after the LO touch-up and 60Hz hunt from last week. I also went to lower frequencies with the SR785, but forgot to take some of the background spectra down there, so I don't have the full breakdown plots yet. Nevertheless, here is the improvement in the PDH error signal:
I also measured the OLTF (SR785 injection at the error signal, Auto level ref 5mV at channel 2, 10mV/s source ramping, 50mV max output)
As you can see, we have tons of phase margin. Flipping the local boost switch had no visible effect on the OLTF; we should change it to something that puts this surplus of phase to good use, and squash the error signal even more. Putting an integrator at 5kHz should still leave about 45 degrees phase margin at 10k. I've started making a LISO model of the PDH board from the DCC drawing, and then I'll inspect the boards individually to make sure I catch the homegrown modifications.
Data, and code used to generate the plots is attached.
Quick post of plots and data; I'll fill in more detail tonight.
TL;DR: I pulled both green PDH boxes and made LISO models, compared TFs and noise levels.
Pictures of X and Y boards, respectively
TF comparison to LISO. (Normalized to coincide at 1Hz)
Noise comparison to LISO
All data, EAGLE schematics, LISO source and plots in the attached zip.
I had noticed in the past, that the digital control signal monitor for the X end would saturate well before the ADC should saturate (C1:ALS-X_SLOW_SERVO_IN1, which is from the "output mon" BNC on the box). It turns out that there is some odd saturation happening inside the box itself.
In this scope trace, the servo input is being driven with a 0.02Vpp, 0.1Hz sine wave, gain knob at 1.0. This is bad.
Evan and I poked around the board, and discover that for some reason currently unknown to us, the variable gain amplifier (AD8336) can't reach its negative rail, despite the +-12V arriving safely at its power supply pins.
I also realized that the LF356 in the integrator stage in this box had been replaced with a LT1792 by Kiwamu in ELOG 4373. I've updated my schematic, and will upload both boxes' schematics to the DCC page Jenne created for them. (D1400293 and D1400294)
I've been having trouble locking the X - green for the past few hours. Has there been some configuration change down there that anyone knows about?
I'm thinking that perhaps I need to replace the SHG crystal or perhaps remove the PZT alignment mirrors perhaps. Another possibility is that the NPRO down there is going bad. I'll start swapping the Y-end NPRO for the X-end one and see if that makes things better.