- I built another beat setup on the PSL table at the South East side of the table.
- The main beam is not touched, no RF signal is touched, but recognize that I was present at the PSL table.
- The beat note is found. The 3rd order sideband was not seen so far.
- A PLL will be built tomorrow. The amplifier box Manasa made will be inspected tomorrow.
- One of the two beams from the picked-off beam from the main beam line was introduced to the beat setup.
(The other beam is used of for the beam pointing monitors)
There is another laser at that corner and the output from this beam is introduced into the beat setup.
The combined beam is introduced to PDA10CF (~150MHz BW).
- The matching of the beam there is poor. But without much effort I found the beat note.
The PSL laser had 31.33 deg Xtal temp. When the beat was found, the aux laser had the Xtal temp of 40.88.
- I could observe the sidebands easily, with a narrower BW of the RF analizer I could see the sidebands up to the 2nd order.
The 3rd order was not seen at all.
- The beat note had the amplitude of about -30dBm. One possibility is to amplify the signal. I wanted to use a spare channel
of the ALS/FOLL amplifier box. But it gave me rather attenuation than any amplification. I'll look at the box tomorrow.
- Also the matching of two beams are not great. The PD also has clipping I guess. These will also be improved tomorrow
- Then the beat note will be locked at a certain frequency using PLL so that we can reduce the measurement BW more.
In the drawing, the FF path should actually be summed in after the Phase Tracker (i.e. after S_ALS). This means that the slow response of the phase tracker needs to be taken into account in the FF cancellation filter. i.e. D = -A_REFL * P_ALS * S_ALS. Since the Phase Tracker is a 1/f loop with a 1 kHz UGF, at 100 Hz, we can only get a cancellation factor of ~10.
So, tonight we added a 666:55 boost filter into the phase tracker filter bank. I think this might even make the ALS locking loops less laggy. The boost is made to give us better tracking below ~200 Hz where we want better phase performance in the ALS and more cancellation of the ALS-Fool. If it seems to work out well we can keep it. If it makes ALS more buggy, we can just shut it off.
Its time to take this loop cartoon into OmniGraffle.
We wanted to jump right in and see if we were ready to try the new "ALS fool" loop decoupling scheme, so we spent some time with CARM and DARM at "0" offset, held on ALS, with PRMI locked on REFL33I&Q (no offsets). Spoiler alert: we weren't ready for the jump.
The REFL11 and AS55 PDs had 0dB analog whitening, which means that we weren't well-matching our noise levels between the PD noise and the ADC noise. The photodiodes have something of the order nanovolt level noise, while the ADC has something of the order microvolt level noise. So, we expect to need an analog gain of 1000 somewhere, to make these match up. Anyhow, we have set both REFL11 and AS55 to 18dB gain.
On a related note, it seems not so great for the POX and POY ADC channels to be constantly saturated when we have some recycling gain, so we turned their analog gains down from 45dB to 0dB. After we finished with full IFO locking, they were returned to their nominal 45dB levels.
We also checked the REFL33 demod phase at a variety of CARM offsets, and we see that perhaps it changes by one or two degrees for optimal rotation, but it's not changing drastically. So, we can set the REFL33 demod phase at large CARM offset, and trust it at small CARM offset.
We then had a look at the transmon QPD inputs (before the dewhitening) for each quadrant. They are super-duper saturating, which is not so excellent.
We think that we want to undo the permanently-on whitening situation. We want to make the second stage of whitening back to being switchable. This means taking out the little u-shaped wires that are pulling the logic input of the switches to ground. We think that we should be okay with one always on, and one switchable. After the modification, we must check to make sure that the switching behaves as expected. Also, I need to figure out what the current situation is for the end QPDs, and make sure that the DCC document tree matches reality. In particular, the Yend DCC leaf doesn't include the gain changes, and the Xend leaf which does show those changes has the wrong value for the gain resistor.
After this, we started re-looking at the single arm cancellation, as Rana elogged about separately.
[SUCCESS] The 3f sideband cancellation seemed worked nicely.
- Beat effeciency improved: ~30% contrast (no need for amplification)
- PLL locked
- 3f modulation sideband was seen
- The attenuation of the 55MHz modulation and the delay time between the modulation source was adjusted to
have maximum reduction of the 3f sidebands as much as allowed in the setup. This adjustment has been done
at the frequency generation box at 1X2 rack.
- The measurement and receipe for the sideband cancellation come later.
- This means that I jiggled the modulation setup at 1X2 rack. Now the modulation setup was reverted to the original,
but just be careful to any change of the sensing behavior.
- The RF analyzer was returned to the control room.
- The HEPA speed was reduced from 100% (during the action on the table) to 40%.
Right before the PSL beam goes into the vacuum chamber, it goes through an AR-wedged plate.
This AR plate produces two beams. One of them is for the IO beam angle/position monitor.
And the other was usually dumped. I decided to use this beam.
A G&H mirror reflects the beam towards the edge of the table.
A 45deg HR mirror brings this beam to the beat set up at the south side of the table.
This beam is S-polarlized as it directly comes from the EOM.
The beam from the PSL goes through a HWP and some matching lenses before the combining beam splitter (50% 45deg P).
The AUX laser beam is attenuated by a HWP and a PBS. The transmitted beam from the PBS is supposed
to have P-polarization. The beam alignment is usually done at the PSL beam side.
The combined beam is steered by a HR mirror and introduced to Thorlabs PDA10CF. As the PD has small diameter
of 0.5mm, the beam needed to be focused by a strong lens.
After careful adjustment of the beam mode matching, polarization, and alignment, the beatnote was ~1Vpp for 2.5Vdc.
In the end, I reduced the AUX laser power such that the beat amplitude went down to ~0.18Vpp (-11dBm at the PD,
-18dBm at the mixer, -27dBm at the spectrum analyzer) in order to minimize nonlinearity of the RF system and
in order that the spectrum analyzer didn't need input attenuation.
The PD signal is mixed with a local oscillator signal at 95MHz, and then used to lock the PLL loop.
The PLL loop allows us to observe the peaks with more integration time, and thus with a better signal-to-noise ratio.
The signal from the PD output goes through a DC block, then 6dB attenuator. This attenuator is added to damp reflection
and distortion between the PD and the mixer. When the PLL is locked, the dominant signal is the one at 95MHz. Without this attenuator,
this strong 95MHz signal cause harmonic distortions like 190MHz. As a result, it causes series of spurious peaks at 190MHz +/- n* 11MHz.
10dB coupler is used to peep the PD signal without much disturbing the main line. Considering we have 6dB attanuator,
we can use this coupler output for the PLL and can use the main line for the RF monitor, next time.
The mixer takes the PD signal and the LO signal from Marconi. Marconi is set to have +7dBm output at 95MHz.
FOr the image rejection, SLP1.9 was used. The minicirsuit filters have high-Z at the stop band, we need a 50Ohm temrinator
between the mixer and the LPF.
The error signal from the LPF is fed to SR560 (G=+500, 1Hz 1st-order LPF). I still don't understand why I had to use a LPF
for the locking. As the NPRO PZT is a frequency actuator, and the PLL is sensitive to the phase, we are supposed to use
a flat response for PLL locking. But it didn't work. Once we check the open loop TF of the system, it will become obvious (but I didn't).
The actuation signal is fed to the fast PZT input of the AUX NPRO laser.
The ALS fool scheme is now diagrammed up in OmniGraffle, including its new official icon. The mathematica notebook has not yet been updated.
EDIT, JCD, 17Feb2015: Updated cartoon and calculation: http://184.108.40.206:8080/40m/11043
- PD response [Attachment 1]
The AUX laser temperature was swept along with the note by Annalisa [http://nodus.ligo.caltech.edu:8080/40m/8369]
It is easier to observe the beat note by closing the PSL shutter as the MC locking yields more fluctuation of the PSL
laser freuqency at low frequency. Once I got the beat note and maximized it, I immediately noticed that the PD response
is not flat. For the next trial, we should use Newfocus 1611. For the measurement today, I decided to characterize the
response by sweeping the beat frequency and use the MAXHOLD function of the spectrum analyzer.
The measured and modelled response of the PD are shown in the attachment 1. It has non-intuitive shape.
Therefore the response is first modelled by two complex pole pair at 127.5MHz with Q of 1, and then the residual was
empirically fitted with 29th polynomial of f.
- Modulation profile of the nominal setting [Attachment 2]
Now the spectrum of the PD output was measured. This is a stiched data of the spectrum between 1~101MHz and 99~199MHz
that was almost simultaneously measured (i.e. Display 1 and Display 2). The IF bandwidth was 1kHz. The PD response correction
described above was applied.
It obviously had the peaks associated with our main modulations. In addition, there are more peaks seen.
The attachment 2 breaks down what is causing the peaks.
From the measured peak height, we are able to estimate the modulation depths for 11MHz, 55MHz, IMC modulations, as well as
the relative phase of the 11MHz and 55MHz modulation. (It is not yet done).
- 3f modulation reduction [Attachment 3]
Now, the redcution of the 3f modulation was tried. The measured modulation levels for the 11MHz and 55MHz were almost the same.
The calculation predicts that the modulation for the 55MHz needs to be 1/3 of the 11MHz one. Therefore the attenuation of 9dB and 10dB
of the modulation attenuation knob at the frequency generation box were tried.
To give the variable delay time in the 55MHz line, EG&G ORTEC delay line unit was used. This allows us to change the delay time from
0ns to 63.5ns with the resolution of 0.5ns. The frequency of 55MHz yields a phase sensitivity of ~20deg/ns (360deg/18ns).
Therefore we can adjust the phase with the precision of 10deg over 1275deg.
The 3rd-order peak at 61.8MHz was observed with measurement span of 1kHz with very narrow BW like 30Hz(? not so sure). The delay
time was swept while measuring the peak height each time. For both the atteuation, the peak height clearly showed the repeatitive dependence
with the period of 18ns, and the 10dB case gave the better result. The difference between the best (1.24e-7 Vpk) and the worst (2.63e-6 Vpk)
was more than a factor of 20. The 3rd-order peak in the above broadband spectrum measurement was 6.38e-6 Vpk. Considering the attenuation
of the 55MHz modulation by 10dB, we were at the exact unluck phase difference. The improvement expected from the 3f reduction (in the 33MHz signal)
will be about 50, assuming there is no other coupling mechanism from CARM to REFL33.
I decided to declare the best setting is "10dB attenuation & 28ns delay".
- Resulting modulation profile [Attachment 4]
As a confirmation, the modulation profie was measured as done before the adjustment.
It is clear that the 3rd-order modulation was buried in the floor noise. 10dB attenuation of the 55MHz modulation yields corresponding reduction of the sidebands.
This will impact the signal quality for the 55MHz series error signals, particularly 165MHz ones. We should consider to install the Teledyne Cougar amplifier
next to the EOM so that we can increase the over all modulation depth.
When I finished my measurements, the modulation setup was reverted to the conventional one.
If someone wants to use the 3f cancellation setting, it can be done along with this HOW-TO.
The 3f cancellation can be realized by adding a carefully adjusted delay line and attenuation for the 55MHz modulation
on the frequency generation box at the 1X2 rack. Here is the procedure:
1) Turn off the frequency generation box
There is a toggle switch at the rear of the unit. It's better to turn it off before any cable action.
The outputs of the frequency generation box are high in general. We don't want to operate
the amplifiers without proper impedance matching in any occasion.
2) Remove the small SMA cable between 55MHz out and 55MHz in (Left arrow in the attachment 1).
According to the photo by Alberto (svn: /docs/upgrade08/RFsystem/frequencyGenerationBox/photos/DSC_2410.JPG),
this 55MHz out is the output of the frequency multiplier. The 55MHz in is the input for the amplifier stages.
Therefore, the cable length between these two connectors changes the relative phase between the modulations at 11MHz and 55MHz.
3) Add a delay line box with cables (Attachment 2).
Connect the cables from the delay line box to the 55MHz in/out connectors. I used 1.5m BNC cables.
The delay line box was set to have 28ns delay.
4) Set the attenuation of the 55MHz EOM drive (Right arrow in the attachment 1) to be 10dB.
Rotate the attenuation for 55MHz EOM from 0dB nominal to 10dB.
5) Turn on the frequency modulation box
For reference, the 3rd attachment shows the characteristics of the delay line cable/box combo when the 3f modualtion reduction
was realized. It had 1.37dB attenuation and +124deg phase shift. This phase change corresponds to the time delay of 48ns.
Note that the response of a short cable used for the measurement has been calibrated out using the CAL function of the network analyzer.
3f modulation cancellation theory http://nodus.ligo.caltech.edu:8080/40m/11005
3f modulation cancellation adjustment setup http://nodus.ligo.caltech.edu:8080/40m/11029
Receipe for the 3f modulation cancellation http://nodus.ligo.caltech.edu:8080/40m/11032
Modulation depth analysis http://nodus.ligo.caltech.edu:8080/40m/11036
I wonder if DRMI can be locked on 3f using this lower 55 MHz modulation depth. It seems that PRMI should be unaffected, but that the 3*f2 signals for SRCL will be too puny. Is it really possible to scale up the overall modulation depths by 3x to compensate for this?
This KTP crystal has the maximum allowed RF power of 10W (=32Vpk) and V_pi = 230V. This corresponds to the maximum allowed
modulation depth of 32*Pi/230 = 0.44. So we probably can achieve gamma_1 of ~0.4 and gamma_2 of ~0.13. That's not x3 but x2,
so sounds not too bad.
Then Kiwamu's triple resonant circuit LIGO-G1000297-v1 actually shows the modulation up to ~0.7. Therefore it is purely an issue
how to deliver sufficient modulation power. (In fact his measurement shows some nonlinearity above the modulation depth of ~0.4
so we should keep the maximum power consumption of 10W at the crystal)
This means that we need to review our RF system (again!)
- Review infamous crazy attn/amp combinations in the frequency generation box.
- Use Teledyne Cougar ampilfier (A2CP2596) right before the triple resonant box. This should be installed closely to the triple resonant box in order to
minimize the effects of the reflection due to imperferct impedance matching.
- Review and refine the triple resonant circuit - it's not built on a PCB but on a universal board. I think that we don't need triple
resonance, but double is OK as the 29.5MHz signal is small.
We want +28V supply at 1X1 for the Teledyne amp and the AOM driver. Do we have any unused Sorensen?
Based on the measured modulation profiles, the depth of each modulation was estimated.
Least square sum minimization of the relative error was used for the cost function.
-8th, -12th~-14th, n=>7th are not included in the estimation for the nominal case.
-7th~-9th, -11th~-15th, n=>7th are not included in the estimation for the 3f reduced case.
m_f1 = 0.194
m_f2 = 0.234
theta_f1f2 = 41.35deg
m_IMC = 0.00153
m_f1 = 0.191
m_f2 = 0.0579
theta_f1f2 = 180deg
m_IMC = 0.00149
(Sorry! There is no error bars. The data have too few statistics...)
I have measured very, very carefully the transfer function from pushing on MC2 to the Yarm ALS beatnote. In the newest loop diagram in http://nodus.ligo.caltech.edu:8080/40m/11030, this is pushing at point 10 and sensing at point 4.
Since it's a bunch of different transfer functions (to get the high coherence that we need for good cancellation to be possible), I attach my Matlab figure that includes only the useful data points. I put a coherence cutoff of 0.99, so that (assuming the fit were perfect, which it won't be), we would be able to get a maximum cancellation of a factor of 100.
This plot also includes the vectfit to the data, which you can see is pretty good, although I need to separately plot the residuals (since the magnitude data is so small, the residuals for the mag don't show up in the auto plot that vectfit gives).
If you recall from http://nodus.ligo.caltech.edu:8080/40m/11020, we are expecting this transfer function to consist of the suspension actuator (pendulum with complex pole pair around 1Hz), the ALS plant (single pole at 80kHz) and the ALS sensor shape (the phase tracker is an integrator, with a boost consisting of a zero at 666Hz and a pole at 55Hz). That expected transfer function does not multiply up to give me this wonky shape. Brain power is needed here.
Just in case you were wondering if this depends on the actuator used (ETM vs MC2), or IFO configuration (single arm vs. PRFPMI), it doesn't. The only discrepancy between these transfer functions is the expected sign flip between the MC2 and ETMY actuators. So, they're all pretty consistent.
Before locking the PRFPMI, I copied the boost filter (666:55) from the YARM ALS over to Xarm ALS, so now both arms have the same boost.
Things to do for ALSfool:
Wonkey shape: Looks like a loop supression. Your http://nodus.ligo.caltech.edu:8080/40m/11016 also suggests it too, doesn't it?
Dang it, yes. You're right. I should have caught that.
Since Dcpl and Srefl are both zero during the measurement (since it was an ALS lock), this is actually
So, I need to remove the effect of the ALS closed loop, to get the actual quantity I was looking for.
The cancellation went from ~10 dB to ~30 dB. This seems good enough. The new filter 'Comp1' is just constructed by eye. We then had to tune the filter module gain to a few %. Seems good enough for now, but we should really try to understand what it is and why it is the way that it is. In the above plot, the ORANGE trace is the old cancellation and the GREEN one is the new one. The filter TF is attached below - its not special, we made it by presing buttons in FOTON until the TF matched the measured TF of ALSY/LSC-MC_CTRL_FF_OUT.
I re-did the Mathematica notebook according to the most current diagram (note to daytime self: attach .nb file!!!), and found that the denominator has changed, such that plugging in the new D=-A_refl*P_als*S_als gives the same
full-system closed loop gain of
where is the open loop gain, and the * indicates either the REFL or ALS portions of the system.
I have also plotted some things with Matlab, although I'm a little confused, and my daytime self needs to spend some more time thinking about this.
In the actuators (both for REFL and ALS), I include a pendulum, the digital anti-imaging filters that let us go from the 16kHz model to the 64kHz IOP and the analog anti-imaging filters after the DAC. Note to self: still need to include violin filters here.
For the servo gains, I copy the filters that we are using from Foton, and give them the same overall gain multiplier that is in the filter bank. For the ALS going through the CARM filter bank, this is FMs 1, 2, 3, 5, 6 with a gain of 15. For the RF (actually, POY here) going through the MC filter bank, this is FMs 4, 5, 7 with a gain of 0.08.
For the plants of each system, since this is still single arm lock, I just include a cavity pole (80kHz for ALS, 18kHz for REFL).
In the sensors (both for REFL and ALS), I include the analog anti-aliasing as well as the digital anti-aliasing to allow us to go from the 64kHz IOP to the 16kHz front end system. For the ALS I also include in the sensor the closed loop response of the phase tracker loop (H/(1-H), where H is the open loop gain of the phase tracker). For both sensors, I also include a semi-arbitrary number to make the full single-loop open loop gain have a UGF of 200Hz. In the ALS sensor, I also include a minus sign to make the full open loop gain have the correct phase.
Here I plot the open loop gains of the individual single loops, as well as the open loop gain of the full system (Hals + Hrefl - Hals*Hrefl). I feel like I must be missing a minus sign in my ALS loop, but I don't know where, and my nighttime brain doesn't want to just throw in minus signs without knowing why. That will affect how the final ALSfool (blue trace) looks, so maybe it's not really as crazy as it looks right now.
Also, I was trying to explain to myself why we are getting the shape that we are in our measurements of the cancellation (http://nodus.ligo.caltech.edu:8080/40m/11041). But, I can't. Below are the plots of the transfer functions from either point 9 or 10 (before or after the G_refl) to point 5, which is the ALS error point. The measurement in elog 11041 should correspond to the blue trace here. For these traces, the decoupling is set to just (-A_refl), although there aren't any noticeable changes in the shape if I just set D=0. If we start with the assumption that D=0, the shape and magnitude are basically identical to this plot, and then as we make D=-A_refl P_als S_als, the transfer functions both go to zero.
So. Why is it that with no decoupling, the transfer function from 10 to 5 is tiny? Why do the shapes plotted below look nothing at all like the measured cancellation shape? Daytime brain needs to think some more.
Here is an updated cartoon, with the ALS sensor explicitly shown as the beatbox times the closed loop response of the phase tracker servo.
The most important transfer functions are written on the diagram. Others can be extracted from the attached Mathematica file (which corresponds to this diagram).
For tonight's experiment, I re-installed the delay line cable and changed the attenuation to 10dB for the 55MHz modulation.
I quickly locked the PLL and checked that the modulation is the ratio of the field strength between the worst (19ns) and best
case (28ns) is 31dB, that is ~35 times reduction.
The modulation setting was reverted.
Demod phase for REFL11/33/55/165 and AS55 were reverted to the previous numbers too.
I'm playing around with the lastest ALS fool feedforward on the Yarm, and I like what I'm seeing.
First, I verified that I could reproduce the TF shapes in ELOG 11041, which I was able to do with a gain of +9.3 and FMs 5 and 6 in the FF module.
Then, I locked the arm on ALS with full bandwidth, and on POY with the settings currently used the MC module, and took their spectra as references. (I put an excitation on the arm at 443Hz to line them up to the same arbitrary units.)
Then, with ALS at its usual 100Hz UGF and boosts on, the Fool path on, and the MC FM set to trigger on/off at 0.8/0.5, I slowly brought ALS towards zero offset and saw it pop right into resonance. I then manually triggered the PDH boosts.
Here are some spectra, showing that, with the Fool path on:
Once the PDH loop is running, the ALS loop can be switched out at the CARM FM output without much of an effect beyond a small kick.
However, looking at the loop shapes, maybe I got lucky here. I took the usual injection TFs at the MC FM, the CARM FM, and at ETMY, to get the overall OLG; all of them have >0.9 coherence pretty much everywhere except the first two points.
As desired, the PDH loop looks pretty normal.
I have no intuition about how the fooled CARM loop should look, since this is even more complicated than a two-loop system.
I don't currently know what is causing the odd feature in the overall at ~50Hz, and it spooked me out when I saw the multiple UGF crossings. The only thing I could think of happening there is the pole in the ALS phase tracker boost. I turned it off, and remeasured; the feature persists...
To wrap it up, here's something I think is pretty cool. Here's what happens when I give ETMY a 1000 count position step impulse (no ramp). [Here, CARM is on ALS with G=12, but only FM5 on]
Although the arm was controlled with IR before the kick, ALS maintained control. As soon as ALS brought the arm back towards resonance, the PDH loop picked it right back up.
Some random notes:
DTT data is attached, in case it's useful to anyone!
Koji raised a good question about the step response I wrote about. Namely, if the UGF of the ALS servo is around 100Hz, we would expect it to settle with a characteristic time on the order of tens of milliseconds, not seconds, as was seen in the plot I posted.
I claim that the reason for the slow response was the fact that the feedforward FM stayed on after the kick, despite the MC filter bank being triggered off. Since it has filters that look like a oscillator at 1Hz, the ringdown or exponential decay of this filter's output in response to the large impulsive output of the PDH control signal just before being triggered down would slowly push the ALS error signal around through the feedforward path.
Given this reasoning, this should be helped by adding output triggering to the FF filter that uses the MC trigger matrix row, as I wanted to do anyways. I have now added this into the LSC model (as well as DQ at 2kHz for the MC_CTRL_FF_OUT), and the impulse response is indeed much quicker.
In the following plot, I hit ETMY with a five sample, 5000 amplitude, impulse (rather than a step, as I did yesterday). The system comes back to PDH lock after ~40ms.
We tried several times tonight to engage the Fool path with the PRFPMI. No success.
First, we locked the arms on ALS, in CARM/DARM mode, and measured the cancellation ability, to make sure that the filter shape and gain was set correctly. For the PRFPMI, it was okay using the same shape as the single arm case, but the gain was +20.0. There might be a bit more cancellation to be gained if we adjust the shape at the ~1dB level, but we're already able to get 20dB of cancellation, so we decided that would be enough to give things a try. To get this much cancellation, we set the phase tracker loops to both have 2kHz UGFs, almost exactly. We should implement a UGF servo, or the amplitude method version of that as Koji suggested ages ago, so that the phase tracker is always at the same place.
I don't think that REFL 11 is seeing as much CARM as I expect. We ended up switching over to linearized REFL55 for our attempts. When we're close to zero CARM offset, the arms are constantly flashing through resonance, and we get the loud buzzing. REFL11 doesn't seem to see any of this, even though we should be close enough to see some PDH action. REFL55 does change as we get closer to resonance, so I think it's seeing some real CARM stuff.
We tried engaging the Fool, but I don't think it did anything too useful. We need to make an estimate of what we expect our gain of the REFL loop to be - or at least the sign.
The PRMI is still not stable enough. It keeps falling out of lock when we get to high-ish arm powers. Not good. More brain power tomorrow on the modulation cancellation issue.
Perhaps if things are stable at moderate arm powers, we can use an excitation to line up the ALS vs. REFL error signals, and then watch the noises of them change as we move around in CARM offset. This should tell us when the linearized REFL signal is quiet enough that it's worth triggering and trying to transfer over.
The last lockloss tonight, there was something funny going on, that we can't explain. Even though both arms were locked on the CARM/DARM combined ALS signals, beatx doesn't see the giant oscillation that causes carm to lose lock until much later. Fool was trying to do something, but that should affect both als individal signals in the same way. Mystery.
Last Wednesday we tried PRMI 3f modulation cancellation. Under the 3f modulation cancellation, the PRMI could not be locked
with REFL signals, and the PRCL signal was just barely sufficient to lock PRCL with help of AS55Q MICH.
- The PRCL signal level in REFL33 was reduced by factor of 20 compared with the conventional modulation setting.
=> The 3f modulation cancellation does not chage the level of 11/22MHz sidebands, it is expected that REFL33 signal
has no significant change of the signal level. But it does. If we change the relative phase between the modulations
at 11 and 55MHz, the signal level is recovered by factor of 5. Therefore something related to 55MHz modulation
(55MHz x 22MHz, or 44MHz x 11MHz) was contributing more than -11MHzx22MHz.
- Under the 3f demodulation cancellation, MICH signal in the REFL ports were extremely weak and there was
no hope to use it for any feedback control.
- WIth the PRMI locking by REFL33->PRCL and AS55Q->MICH, the sensing matrix was measured. All of the REFL
ports however, showed extremely degenerate sensing matrix between MICH and PRCL.
This was enough confusing to us, and we didn't draw any useful information from these. Here are some ideas to
investigate what is happening in out optical and electrical system.
- One approach is to use as simple optical setup as possible to inspect our sensing systems. For example,
we may want to try PRX/PRY/XARM/YARM cavities to check the functions of the REFL diodes and qualitatively characterize
the sensing chain (Optical gain [W/m], noise level, demodulation phase) so that we can compare these with
an interferometer seinsing model.
- Another approach is to change the mdulation setting more freely and empirically try to find the characteristic
of our optical/electrical systems. e.g. change the relative modulation phase and/or 55MHz attenuation, and try to understand
how 11-22, 11-44, 22-55, 0-33 pairs are contributing the signal.
I have used Optickle to model the effect of changing the phase between the 11MHz and 55MHz EOMs. Also, I have looked at what modulation order is most significant (we hope it's -22*11 and -11*22).
First, so that we can compare these numbers more directly to measurements, I have included in the model the fraction of light that gets to each PD. I'm assuming that the Faraday is about 80% transmissive, but I don't know what the true number is. Here's a cartoon showing the attenuation factors.
EDIT, 26March2015, JCD: REFL path updated. See elog 11172.
To model the change in relative phase between the 11MHz and 55MHz modultions, I have held the 11MHz EOM stationary, and moved the 55MHz EOM. Since I needed an actual distance, I used a conversion factor,
The sensing matrix was measured at 143Hz. It has been corrected from mevans-meters to Newtons as the denominator.
The big thing to notice here is that the PRCL magnitude is not changing by a factor of 20. More like a factor of 2. BUT, I have not yet included the fact that Koji also reduced the 55MHz modulation amplitude by a factor of 3.
As for the mysterious degeneracy of all the REFL PDs, I think we need to take a more careful measurement. It's possible that we were seeing the real thing for REFL33, but the others don't seem to change in degeneracy with relative modulation phase.
Why does it even matter for the 3*f1 signal what is going on with the f2 modulation? Well, it appears that we are definitely being dominated by the 44*11 and 55*22 components.
To check this, I restricted Optickle to various orders of modulation (ex. up to second order only includes the [-22, -11, 0, 11, 22] MHz components), and plotted them all. The change in the signal between one trace and another is the contribution from that extra modulation order. The traces are only minutely different between orders, after the 5th order. So, since they're all overlapping with the 5th order trace anyway, I don't plot them.
EDIT: to clarify, when I said "up to X order", I meant up to that order in 11MHz sidebands. Optickle is applying the 11MHz and 55MHz modulations in the same way every time, but then I specify up to what order to include in the summation of different contributions to the field at a given port. So, for the "up to 2nd order", I only include cross terms that come from [-22, -11, 0, 11, 22] MHz. For the next order, I only include terms that come from [-33, -22, -11, 0, 11, 22, 33] MHz, etc. So, there are no 55MHz effects when I'm only including contributions up to 2nd order (since there is a maximum cross beatnote of 44MHz), but starting with 3rd order, I do start to see signals in, for example, REFL55 and AS55, since I get terms from -22*33 and -33*22. The first order in 55MHz (i.e. 55MHz*Carrier) only starts to show up when I calculate "up to 5th order" and above, since that includes [-55, -44, -33, -22, -11, 0, 11, 22, 33, 44, 55] MHz.
What happens if I reduce the 55MHz modulation depth by 10dB? Since we are dominated by 55MHz-related signals, the signal at REFL 33 goes down. The maximum change we could have seen for the REFL33 PRCL signal (difference between max of blue trace and min of orange trace) is a factor of 27.
Where are we on the x-axis of these plots, and where was the maximal cancellation place that Koji found? I need to re-check that part of the code tomorrow, to make sure that I've included all of my contributions from different components of the (field* field) matrix.
But, the moral of the story for tonight is that at least for the REFL33 signal, it's actually plausible that the optical gain went down by a factor of 20, and that the MICH and PRCL signals were degenerate. I suspect that the total cancellation place that Koji found was somewhere around 175 degrees on the x-axis of these plots and that our nominal place is around 0 deg - around there, both the magnitude and the phase situations are possible simultaneously.
About 5-10 minutes ago I just put in the modulation cancellation setup according to the recipe in http://nodus.ligo.caltech.edu:8080/40m/11032
[Jenne, EricQ, Rana]
We spent this evening measuring and thinking about our 3f signals, and the effect of the modulation cancellation.
I reinstalled the delay line box, and reduced the modulation depth of the 55MHz signal, so that we are in the state of modulation cancellation - there should be almost no power at 33MHz injected into the vacuum. I carefully tuned the demod phase of the REFL 11, 33 and 55 MHz PDs, and locked the PRMI on REFL55 I&Q. The signal in REFL 165 was very tiny, so as best as I could tell, the demod phase that Koji found last week was correct.
Here is a little record of what the demodulation phases should be, for the "nominal" and "cancellation" configurations, so that we don't have to continually use the time machine.
Also, here is the locking recipe for REFL55 I&Q in the cancellation configuration:
With this setup, we measured the sensing matrix. MICH had an excitation at 370.123 Hz with 8,000 counts to -ITMX+ITMY (this is about 0.3nm for each ITM), and PRCL had an excitation at 404.123 Hz with 50 counts to PRM. For tonight, here is a PDF of the peaks in DTT. The data is saved in /users/Templates/LSC_error_spectra/SensMat_PRMI_24Feb2015.xml.
Rana has proposed to us an idea for why the REFL 33 signal should be dominated by the 55*22 contribution, rather than -11*22. Eric is in the process of checking this out with a Mist model to see if it makes sense. Here's the gist:
Our Schnupp asymmetry is small (3.9cm, IIRC), so the transmission of the 11MHz signal out the dark port is small. This means that the finesse of the PRC for 11MHz isn't so huge. On the other hand, since 55MHz is a higher frequency, the transmission out the dark port is larger and is a closer match to the PRM transmission, so the finesse of the PRC for 55MHz is higher.
Since the magnitudes of the fields at the reflection port are not changing significantly, our PDH signals are being created by the relative phase of something which is anti-resonant (ex. carrier or 22MHz for sideband lock) vs. something which is resonant (ex. 11MHz or 55MHz). Since the finesse of the 55MHz signal is larger, the accumulated phase change is greater, so we expect a larger slope to our PDH signals that involve 55MHz as compared to those that use 11MHz.
If we are comparing the contributions between -11*22 and 55*22, they both include the anti-resonant 22MHz. So, the difference in the signal strengths comes directly from the difference in phase accumulation due to the variation in the dark port transmission.
EricQ had a thought, and while I have enough awake brain cells to report the thought, we're going to have to revisit it when more of our brains are awake. In either case, the transmission through the dark port is small compared to the transmission of the ITMs, so why don't the ITMs dominate the finesse calculation, and thus are the 11MHz and 55MHz really getting that much of a difference in finesse? To be checked out.
If the reflectivity of the front mirror is fixed (=PRM reflectivity), the finesse increases when the reflectivity of the end
mirror (=Compond mirror reflectivity) increases. i.e. 11MHz has higher finesse, 55MHz has lower finesse.
If the reflectivity of the front mirror is fixed, the amplitude gain of the cavity is higher when the reflectivity of the end mirror increases. i.e. 11MHz has higher gain, 55MHz has lower gain
The Schnupp asymmetry is definitely not an important parameter (no need for Koji to explode). It only serves to give us a bigger Q phase signal slope, but is not significant for the I phase signals.
The main anomaly is that the REFL33 optical gain (W/m) seems to have been reduced so much by the phase and amplitude adjustment of the 55 MHz modulation signal. One guess is that the true 3f signal is being made not by the (2*f1 - (-f1)) beat, but by some higher order beat. In addition to the usual RF fields produced by the 2 modulations, we must consider that the sidebands on sidebands produce intermodulation fields just after the EOM and so the fields with which we interrogate the PRMI are more complicated.
Jenne's Optickle calculation today should show us what the sensing matrix contribution is from each pair of fields, so that we can have a sensing matrix signal budget.
And now I've removed the delay line, and am in the process of reverting the demod phases, etc.
I have measured the sensing matrix for the PRMI at the REFL photodiodes for both the nominal configuration and the 33MHz cancellation configuration. The nominal configuration measurements do not compare well with those from November (http://nodus.ligo.caltech.edu:8080/40m/10701) which makes me unhappy . Both sets of nominal config reported below are from today, after tuning the demod phases and making sure the MICH and PRCL loops looked the same as yesterday (esp. overall gain). The cancellation config data is from last night.
Note that the magnitude for each photodiode is referred to its own "PD counts". Since the electronics are different for each PD, and that is not taken into account here, you cannot directly compare an element from one PD to an element from another PD. What you can do (which is most of what we need right now) is compare all the different measurements for a single photodiode.
So, what I'm apparently seeing is that the magnitudes of the sensing matrix signals that are made using 55MHz (i.e. everything but REFL11) change when we go into the cancellation configuration, but the phases of the sensing elements do not change significantly. Also, I am apparently seeing that REFL11 and REFL33 only have about 3 degrees of separation between the MICH and PRCL signals no matter what configuration is used. This doesn't make a lot of sense, since we know that we can lock robustly on REFL33I&Q (it's been sitting there happily as I write this elog), so it seems crazy that we could actually be so degenerate. Also, at the bottom of the elog that I wrote in November 2014, I show a sensing matrix where both REFL11 and REFL33 have about 45 degrees of separation between the MICH and PRCL signals.
I don't think I'm doing anything too crazy here, particularly with the phase. For a given PD and given DoF, I find the magnitude of the peaks of the I and Q signals, and just do atan2(Q-signal, I-signal)*180/pi, and those are the numbers that go in the phase columns above.
Before taking my measurements, I tuned up the demod phases for the PRMI-only case. I think REFL11 may have previously been tuned for CARM when the arms were held with ALS, but I don't really recall. Anyhow, now all 4 REFL PDs are tuned for PRMI-only.
This was done while the PRMI was locked with REFL 55 I&Q.
EDIT, 26Feb2015: Last night I mixed up the REFL11 and REFL33 new demod phases. Bold are the corrected version. Also, note that REFL33 was formerly tuned for PRCL in PRFPMI, which may be why it changed by ~10 degrees.
15.3 +/- 0.3
Here's the recipe for locking REFL 55 I&Q in the nominal modulation configuration. It's the same as the REFL33 I&Q lock that I was using today, except that for the REFL33 version, the matrix elements are both unity.
I'm working on some more modelling investigations of this whole situation. The main thing I wanted to do was to look at the complex field amplitudes / IFO reflectivity to see how the PDH signal is affected by different field components.
I still have plenty more to do, but I got a result which I though I should share. In addition to Jenne's simulation, I also see that between our "nominal" and "canceled" states as defined in Kojis ELOG 11036, there is a factor of ~20 difference in the PRCL signal in REFL33.
The plots below are kind of like "PDH Signal Budgets" of the two states.
Specifically, the reason our gain gets reduced is that, in the "canceled" state, the 44*11 and 55*22 products conspire to weaken the signal by having a slope opposite to the -11*22 type products. In contrast, in our "nominal" case, all of the products slope together.
However, this also predicts that the nominal REFL33 is more sensitive to Carrier*33 than to the signal we desire, -11*22. The only reason it ever worked seems to be the biggest contriubutor, the unexpected 44*11!
The "residual" trace is the difference of REFL33 and the sum of the field products shown, to justify that all relevant products had been included.
The simulation that produced this was set up to create 4 orders of modulation at each EOM, with 3 orders of sidebands on sidebands. The demodulation phase was taken by lining up a PRM excitation entirely along I, as we would do on the actual instrument.
MIST Simulation files attached!
So, my previous post suggested that 44*11 products might be the dominating signals in our "nominal" setup. I suppose that this could be not so bad, since it's not too unlike -11*22; the 11MHz field couples into the PRC and reflects with a rapidly changing phase around PRC resonance, and 44MHz is antiresonant, so it is a good local oscillator for REFL33.
However, it then occured to me that my previous HOM analysis only looked at the 11MHz and 55MHz sidebands.
When extending this to all of the sidebands within 55MHz, I discovered a troubling fact:
With the IFO parameters I have, the second spatial order +22MHz and fourth spatial order +44MHz fields almost exactly coresonate with the carrier in the PRFPMI! (DR, too)
If this is true, then any REFL33 signal seems to be in danger if we have an appreciable amount of these modes from, say, imperfect modematching.
On the other hand, we've been able to hold the PRMI with REFL33 when ALS is "on resonance," so I'm not sure what to think. (As a reminder, this analysis is done with analytic formulae for the complex reflectivities of the arm cavities and coupled recycling cavities as a function of CARM, spatial order and field frequency. Source is attached.)
It seems the Y arm geometry is to blame for this.
Maybe we should try to measure/confirm the Y arm g-factor...
Ok... This is what I was afraid of, and it seems true.
i.e. the relation ship of the modulations for the 3f cancellation is making the PRCL signals cancel each other.
It agrees with Anamaria's analysis that 11x44 is the strongest component in aLIGO 27MHz signal.
00x33 has the order of
11x22 has the order of
11x44 has the order of
22x55 has the order of
Therefore 11x44 is inherently the strongest contribution at 33MHz.
(And then, of couse, the signal amplitudes have additional dependences on the reflectivity
and the gain of the IFO at each freq)
If we believe this result, it may be difficult to exploit the benefit of the signals under the 3f cancellation.
We probably have to go back to the original idea of cancelling the 3f modulation by adding 3f modulation.
(i.e. Produce 33MHz signal by freq tripling, add this signal to Kiwamu's box to elliminate 3f.)
No more illegal power supply at the LSC rack
The amplifiers are now being powered by the rack power supply through fuse blocks.
To make new connections, I shutdown the +/-15 V low noise power supplies. They were turned back ON after the work.
If so, or if not but you care about the signal that passes through these amplifiers, I suggest you remove this temporary power supply and wire the power from the rack power supplies through the fuse blocks and possibly use a voltage regulator.
In 24 hours, that power supply will be disconnected and the wires snipped if they are still there.
I have clarified my elog from last night to indicate that the sensing matrix in the "33MHz cancellation" configuration was measured with the PRMI held on REFL55 I&Q.
Also, I just re-read my control room notes from yesterday, and I typed the wrong demod phases into the table last night. The elog has been edited. Most significantly, the REFL33 demod phase did not change by 70 degrees. It did change by 10 degrees, but that is likely from the fact that it was formerly tuned for PRCL in PRFPMI, and last night I tuned it for PRCL in PRMI-only.
I have adapted one of Evan's python scripts into an ipython notebook for calculating our PRMI sensing matrix - the work is ~half done.
The script gets the data from the various PD channels (like REFL33_I) and demdoulates it at the modulation frequencies. At the moment its using just the sensing channels, but with the recent addition of the SUS-LSC_OUT_DQ channels, we can demod the actuation channels as well and not have to hand code the exc amplitudes and the basolute phase. Please ignore the phase for the moment.
The attached PDF shows the demod (including lowpass) outputs for a 2 minute stretch of PRMI locked on f2. Next step is to average these numbers and make the radar plots with the error bars. The script is scripts/LSC/SensingMatrix/PRMIsensMat.ipynb and is in the SVN now.
** along the way, I noticed that the reason this notebook hasn't been working since last night is that someone sadly installed a new anaconda python distro today without telling anyone by ELOG. This new distro didn't have all the packages of the previous one. I've updated it with astropy and uncertainties packages.
I've fixed the Radar plot making part, so that's now included too. The radial direction is linear, so you can see from the smearing of the blobs that the uncertainty is represented in the graphics due to each measurement being a small semi-transparent dot. Next, we'll put the output of the statistics on the plot: mean, std, and kurtosis.
One of the things that I looked at tonight was whether or not I could hold the PRMI on REFL165 at CARM offset of 0, and it turns out that I can. Hooray. The next step was having a look to see if it is actually less noisy than the REFL33 lock.
I calibrated REFL33 and REFL165 to meters (I have the data to do the same for 11 and 55, but haven't done so yet). This way, we can directly compare the signals from each PD.
I scanned between +3 and -3 CARM digital offset (which we think is about 1nm/count while held on ALS), with a ramp time of 10 seconds. I did this several times while the PRMI was locked on both REFL33 and REFL165. Here are the gps times for 8 examples where the PRMI did not lose lock during the sweep:
Here are screen shots from the first REFL33 sweep, and the first REFL165 sweep. DTT can't print 3 plots together, so I'll have to make this nicer later. The top plot is the error signals, calibrated to meters. The middle plot is the control signals, that need to be calibrated to Newtons. The bottom plot is the arm powers, so you can see roughly where we were in the sweep.
We'd like to see a MIST simulation, or perhaps e2e, to see what the predicted disturbance is for each of the error signals during the CARM resonance. We want to make sure that the loops are engaged for all of the degrees of freedom for the simulation.
Recipes for tonight:
REFL165 sometimes has a tough time catching lock by itself, but if you add either REFL33 or REFL55 error signals to the REFL165 signals, it'll catch, and then you can just remove the extra error signals. Also, it doesn't stay locked very robustly unless you include the PRCL FM1 boost.
Here are a bunch of PDFs of time series from last night's CARM sweeps. The y-axes are all calibrated (except for the TRX/TRY, which are just normalized to single arm power, as usual) to real units - meters for the error signals, and Newtons for the control signals. The y-axes for each plot are the same on all PDFs (ex, the control signal plot in the lower left has the same range for all cases) so that it is easy to compare directly.
The most striking thing is that while the PRMI is held on REFL33, the MICH control signal saturates as we go through arm resonance. If the PRMI is held on REFL165, there is no such problem. I think we're going to have a lot more luck keeping the PRMI on REFL 165.
Plots while held on REFL 33:
Plots while held on REFL 165:
Using PRX, I remeasured the relative actuation strengths of the BS and PRM to see if the PRM correction coefficient we're using is good.
My result is that we should be using MICH -> -0.2655 x PRM + 0.5*BS.
This is very close to our current value of -0.2625 x PRM, so I don't think it will really change anything.
The reason that the BS needs to be compensated is that it really just changes the PRM->ITMX distance, lx, while leaving the PRM-ITMY distance, ly, alone. I confirmed this by locking PRY and seeing no effect on the error signal, no matter how hard I drove the BS.
I then locked PRX, and drove an 804Hz oscillation on the BS and PRM in turn, and averaged the resultant peak heights. I then tried to cancel the signal by sending the excitation with opposite signs to each mirror, according to their relative meaured strength.
In this way, I was able to get 23dB of cancellation by driving 1.0 x PRM - 0.9416 * BS.
Now, in the PRMI case, we don't want to fully decouple like this, because this kind of cancellation just leaves lx invarient, when really, we want MICH to move (lx-ly) and PRCL to move (lx+ly). So, we use half of the PRM cancellation to cancel half of the lx motion, and introduce that half motion to ly, making a good MICH signal. Thus, the right ratio is 0.5*(1.0/0.9416) = 0.531. Then, since we use BS x 0.5, we divide by two again to get 0.2655. Et voila.
Better elog tomorrow - notes for now:
REFL165 for PRMI has been "a champ" (quote from Q). We're able to sit on ALS at average arm powers of 30ish. Nice.
Some ALSfool work - measured cancellation almost as good as single arm.
One time transitioned CARM -> normalized REFL55I
Many times did DARM -> normalized AS55Q, see lots of noise at 39ish Hz - may be coupling from MICH??
Arm ASC loops helped improve dark port contrast.
Note to selfs: Need to make sure DTT templates have correct freq ordering - must be small freqs to large freqs.
A slightly more coherent elog for last night's work.
All night, we've been using REFL165 to hold the PRMI. It's working very nicely. To help it catch lock, I've set the gain in the PRCL filter bank high, and then the *0.6 filter triggers on. The carm_cm_up script now will lock the PRMI on REFL165.
We had to reset the REFL165 phase after we acquired lock - it was -91, but now is -48. I'm not sure why it changed so significantly from the PRMI-only config to the PRFPMI config.
We measured the ALS fool cancellation with the arms held off resonance, at arm powers of a few. Although, they were moving around a lot, but the measurement stayed nice and smooth. Anyhow, we get almost as good of cancellation as we saw with the single arm (after we made sure that both phase trackers had the same UGF):
We were able to partly engage it one time, but we lost lock at some point. Since the frame builder / daqd decided that that would be just the *perfect* time to crash and restart, we don't have any frame data for this time. We can see up to a few seconds before the lockloss, while we were ramping up the RF PD loop gain though, and MICH was hitting the rails. I'm not sure if that's what caused the lockloss, but it probably didn't help.
The ALS fool gain was 22, and we were using FMs 4, 6 (the pendulum and Rana's "comp1"), the same filters that were used for the single arm case. The LSC-MC filter bank gain lost lock when we got to about 5.6 (we were taking +3dB steps).
We were using REFL55I/(TRX + TRY) as our CARM RF error signal. We were using REFL55 rather than REFL11 because we were worried that REFL11 didn't look good - maybe it was saturating or something. To be looked into.
Here's the striptool that was running at the time, since we don't have frame data:
At this point, since we weren't sure what the final gain should be for the RF CARM signal, and we could sit at nice high arm powers (arm powers of 30ish correspond to CARM offsets of about 50pm), we decided to try just a straight jump over to the RF signals.
The first time around, we jumped CARM to (-0.2)*REFL55/(TRX+TRY), but we only stayed lock for 1 or 2 seconds. That was around 1:55am.
We decided that perhaps it would be good to get DARM moved over first, since it has a much wider linewidth, so the rest of the trials for the night were transitioning DARM over to (0.0006)*AS55Q/(TRX + TRY). AS55 was saturating, so we reduced its analog gain from 18dB to 9dB and re-ran the LSC offsets. The MICH noise was pretty high when we were at low CARM offset, although we noticed it more when DARM was on AS55. In particular, there is some peak just below 40Hz that is causing a whole comb of harmonics, and dominating the MICH, PRC and DARM spectra. I will try to get a snapshot of that tonight - I don't think we saved any spectra from last night. Turning off DARM's FM3 boost helped lower the MICH noise, so we think that the problem is significant coupling between the two degrees of freedom.
After the first one or two tries of getting DARM to AS55, we started engaging the arm ASC loops - they helped the dark port contrast considerably. The POP spot still moves around, but the dark port gets much darker, and is more symmetric with the ASC on.
Much of tonight was spent fighting with ETMX. This time, ASC was definitely off, there was nothing coming out of the ASC filter banks except the static output of the ASS. I tried turning off the 1000 count POS offset, but I think that made it a little worse. I ended up putting the offset back.
It's a little confusing, since it sometimes moves when there is no LSC actuation. However, it definitely moves when there is some LSC actuation. I did a test where every time I enabled the IR arm locking and caught lock, I saw a step in the SUSPIT and SUSYAW error signals. Once lock was aquired, it would settle and stay somewhere. If I unlocked the cavity, there was no "undo" step - it just stayed where it was. I wasn't letting it sit long enough to see if it spontaneously moved during this test.
Here's a plot of this test. The only button I'm touching is the LSC enable button. ASC is off, ASS is frozen (DC values exist, but no dither, no feedback). This was done when the 1000 count POS offset was off. The steps are less bad when the offset is on.
In between fighting with the ETM, I was able to do several trials with the PRFPMI.
I was playing with CARM and ALS fool.
First, I used REFL55 normalized by the sum of the transmissions as the error signal for the MC filter bank and saw that REFL11 (as an out of loop signal) got much more smooth, and centered around zero. However, I wasn't able to get the same thing with REFL11. No matter the sign I used for the MC filter bank, the IFO would squeak (some high freq gain peaking I think), and then I'd lose lock. This was true whether I used REFL11 through the common mode board or just directly into the ADC.
Just now, I did one trial of switching DARM over to AS55Q, just to grab a spectra of the MICH noise that Q and I saw yesterday.
I'm a little confused by some delay that seems to exist between the "A" and "B" error signals (right after the LSC input matrix) and the _IN1 point of the servo filters. I didn't save the measurement (bad Jenne), but there's a ~40 degree difference between DARM_A_ERR/DARM_IN2 and DARM_IN1/DARM_IN2. I don't think there should be anything there. Anyhow, it makes the DARM loop measurements look funny. If you just look at, say, DARM_B_ERR/DARM_IN2, you'll think that there's no way that the loop will be stable. However, it will actually be fine.
For tomorrow, we should take the DARM loop measurement with much less actuation. As with last night, I blew the lock by trying to measure the DARM loop.
As discussed at today's meeting, we would like to (re)measure the Arm cavity lengths to ~mm precision, and their g-factors. Any arm length mismatch affects the reflection phase of the sidebands in the PRMI, which might be one source of our woes. Also, as I mentioned in a previous elog, the g-factors influence whether our 2f sidebands are getting pulled into the interferometer or not.
These both can be done by scanning the arm on ALS and measuring the green beat frequency at each IR resonance. (Misaligning the input beam will enhance the TM10 Mode content, and let us measure its guoy phase shift)
I started working on this today, but I have measurements to do, since at the time of today's measurements, I was fooled by the limits of the ALS offset sliders that I could only scan through two FSRs. Looking back at Manasa's previous measurment (ELOG 9804), I see now that more FSRs are possible.
Ways I will try to improve the measurement:
Just for kicks, here are scans from today.
Brief elog of my activities tonight:
I was able to transition the digitial CARM control to REFL11 through the common mode board a total of one time, lock broke after a few seconds.
My suspicion was that when we did this on Monday, we unintentionally had a reasonable DARM offset, which reduced the finesse enough to let us take linear transfer functions and hop over. So, tonight, I intentionally looked at transitioning to CM_SLOW at some DARM offset. Using DARM offset of a few times 0.1 really calms the "buzzing" down, and makes it fairly straightforward to measure linear CARM sensing TFs. However, the CARM optical plant seems to change a fair amount depending on the DARM offset, in such a way that I was not able to compensate well enough to repeatedly transition.
Before I did anything else tonight, I measured the ALS noise down to 0.1 Hz, as a benchmark of how things are behaving.
With the arms locked on POX/POY, the HZ calibrated ALS channels reported
Then, with the arms CARM/DARM locked on ALS, the PDH signals reported (using a line and the HZ channels for conversion)
Not bad! I roughly estimate this to mean ~90pm RMS CARM/DARM motion. (If X was as good as Y, it would be ~50pm...)
Some things I feel are worth noting:
Tomorrow, I'll post some transfer functions of the difference between the ALS and CARM plants that I measured.
I think we've seen this in simulations, but it's a little disheartening to see in real life. AS55Q looks like it flattens out pretty significantly right around the DARM=0 point.
Right now I have the arms held on ALS (CARM=-1*MC2, DARM=2*ETMY, as Q used last night), and the PRFPMI is on REFL165I&Q. I have set CARM to be as close to zero offset as I can (so I get all the usual buzzing), and then I'm sweeping the DARM offset between +3 and -3 counts (roughly +/-3nm) with a 3 second ramp and looking at normalized AS55Q. The channel called "DARM_B_ERR" is 0.006*AS55Q/(TRX + TRY). The arm transmissions, as well as the ASDC are plotted as well - ASDC is scaled to fit on the same axes as the transmissions.
Anyhow, here's the time series of the DARM sweeps. AS55 demod phase of -55 degrees seems to give the cleanest signal (within 5deg steps); this is the same phase that we've been using all week.
This has been done before:
Arm length measurements and g-factor estimates in 2012, but only with an accuracy of ~30 cm. However, Yuta was able to get many FSRs somehow.
We played around tonight with different possible ways of transitioning DARM to normalized AS55Q. Before each try, we would use ezcaservo (or just eyeball it) to make sure that the normalized RF signals had a mean of zero, so that we knew we were pretty close to zero offset in both CARM and DARM.
We tried something that is similar in flavor to Kiwamu's self-locking technique - we summed in some normalized AS55Q to the DARM error point (using the DoF selector matrix that I created a few weeks ago), and then tried to engage a little low frequency boost. We tried several times, but we never successfully made the transition.
In the end, we just did a direct transition over to normalized AS55Q, and lost lock after several seconds. The buzzing that we hear didn't change noticeably after the transition, which indicates that most of the noise is due to CARM (which makes since, since it has a much smaller linewidth). The problem with holding DARM is that occassionally we will have a CARM fluctuation that lets the arm power dip too low, and DARM's error signal isn't valid at low arm powers. So, we need to work on getting CARM stabilized before we will have a hope of holding on to DARM.
Here's the lockloss plot from that last lock:
Also this evening, I scanned back and forth over the CARM zero crossing while locked on ALS, to see what the RF error signals looked like. Normalized REFL55 seems to have much more high frequency noise near the edges of the linear range than does REFL11. Also, the REFL 11 signal is much larger. So, what I think I want to try to do is use ALS fool to lower the CARM noise by a bit, then make the DARM transition. Then, we can come back to CARM and ramp up the gain.
With these CARM sweeps, I think that I know the relative gain and sign between ALScomm and the normalized REFL signals, and the REFL signals versus the normalized versions. I think that 100*REFL11I/(TRX+TRY) gives the same slope at the zero crossing as just plain REFL11I. Same factor of 100 is true for REFL55I. The REFL11 slope is 20,000 times larger than the ALS slope, while the REFL55 slope is -500 times the ALS slope (note that REFL55 has a minus sign). We can probably trigger the Fool on when the arm powers are above 50, and trigger off when they're below 20. For the zero crossings, the REFL55 threshold should be about 20, and the REFL11 threshold should be about 500.
I also need to re-think the triggering logic for ALSfool. We probably don't want the zero crossing logic to be able to un-trigger the lock, just in case we get an extra noise blip. So, we want to trigger on with an AND, but only trigger off if the arm powers go too low. Also, the zero crossing logic should look at the normalized error signals, not the plain signals.
We need to modify the ALSwatch logic so that it doesn't look at EPICS values for the thresholding. There may be an updated filter module that includes a saturation monitor, but otherwise we can use the saturation monitor part that is in the OSC section of CDS_PARTS. We'll set the threshold on this to match the limiter in the filter bank. Then, if the filterbank output is constantly hitting the limiter, we should run the down scripts.