In order to measure the loss in the arm cavities in reflection, we use the DC method described in T1700117.
It was not trivial to find free channels on the LSC rack. The least intrusive way we found was to disconnect the ALS signals DSUB9 (Attachment 1) and connect a DSUB breakout board instead (Attachment 2).
The names of the channels are ALS_BEATY_FINE_I_IN1_DQ for AS reflection and ALS_BEATY_FINE_Q_IN1_DQ for MC transmission. Actually, the script that downloads the data uses these channels exactly...
We misalign the Y arm (both ITM ad ETM) and start a 30 rep measurement of the X arm loss cavity using /scripts/lossmap_scripts/armLoss/measureArmLoss.py and download the data using dlData.py.
We analyze the data. Raw data is shown in attachment 3. There is some drift in the measurement, probably due to drift of the spot on the mirror. We take the data starting from t=400s when the data seems stable (green vertical line). Attachment 5 shows the histogram of the measurement
X Arm cavity RT loss calculated to be 69.4ppm.
We repeat the same procedure for the Y Arm cavity the day after. Raw data is shown in attachment 5, the histogram in attachment 6.
Y Arm cavity RT loss calculated to be 44.8ppm. The previous measurement of Y Arm was ~ 100ppm...
Loss map measurement is in order.
X Arm: 0.875 +/- 0.005
Y Arm: 0.869 +/- 0.006
The measured RIN of the arm cavity transmission when the PRFPMI is locked is ~10x in RMS relative to the single arm POX/POY lock. It is not yet clear to me where the excess is coming from.
Attachment #1 shows the comparison.
My speculation for the worse RIN is:
- Unoptimized alignment -> Larger linear coupling of the RIN with the misalignment
- PRC TT misalignment (~3Hz)
Don't can you check the correlation between the POP QPD and the arm RIN?
I agree, I think the PRC excess angular motion, PIT in particular, is a dominant contributor to the RIN. Attachments #1-#3 support this hypothesis. In these plots, "XARM" should really read "COMM" and "YARM" should really read "DIFF", because the error signals from the two end QPDs are mixed to generate the PIT and YAW error signals for these ASC servos - this is some channel renaming that will have to be done on the ASC model. The fact that the scatter plot between these DoFs has some ellipticity probably means the basis transformation isn't exactly right, because if they were truly orthogonal, we would expect them to be uncorrelated?
I guess what this means is that the stability of the lock could be improved by turning on some POP QPD based feedback control, I'll give it a shot.
- PRC TT misalignment (~3Hz)
Don't can you check the correlation between the POP QPD and the arm RIN
how bout corner plot with power signals and oplevs? I think that would show not just linear couplings (like your coherence), but also quadratic couplings (chesire cat grin)
I want to measure the transfer function of the arm cavities to extract the pole frequencies and get more insight into what is going on with the DC loss measurements.
The idea is to modulate the light using the PSL AOM. Measure the light transmitted from the arm cavities and use the light transferred from the IMC as a reference.
I tried to start measuring the X arm but the transmission PD is connected to the QPD whitening filter board with a 4 pin Lemo for which I couldn't find an adapter.
Could this be because of the PDA520 limited BWs? I tried playing with the PD gain/bandwidth switch but it seems like it was already set to high bandwidth/low gain.
In any case, the extracted pole frequency ~ 2.9kHz implies a finesse > 600 (assuming FSR = 3.9MHz) which is way above the maximal finesse (~ 450) for the arm cavities.
I disconnected the source from the AOM. But left the other two BNCs connected to the SR785. Also, TRY PD is still teed off. Long BNC cable is still on the ground.
when doing the AM sweeps of cavities
make sure to cross-calibrate the detectors
else you'll make of science much frivolities
much like the U.S. elections electors
I measured the cross-calibration of the two PDs on the PSL table.
I used the existing flip mounted BS that routes the beam into a PDA255, the same as in the IMC transmission.
I placed a PDA520, the same as the one measuring TRY_OUT on the ETMY table, on the transmission of the BS (Attachment 1).
I used the SR785 to measure the frequency response of PDA520 with reference to PDA255 (Attachment 2). Indeed, calibration is quite significant.
I calibrated the Y arm frequency response measurement.
However, the data seem to fit well to 1/sqrt(f^2+fp^2) - electric field response - but not to 1/(f^2+fp^2) - intensity response. (Attachment 3).
Also, the extracted fp is 3.8KHz (Finesse ~ 500) in the good fit -> too small.
When I did this measurement for the IMC in the past I fitted the response to 1/sqrt(f^2+fp^2) by mistake but I didn't notice it because I got a pole frequency that was consistent with ringdown measurements.
I also cross calibrated the PDs participating in the IMC measurement but found that the calibration amounted for distortions no bigger than 1db.
Ok, now I understand my foolishness. It should definitely be 1/sqrt(f^2+fp^2) .
From the last plot:
- Subtracting the offset of 0.0095, the modulation depth were estimated to be 0.20 for 11MHz, 0.25 for 55MHz
- Carrier TEM00 1.0, 1st order 0.01, 2nd order 0.05, 3rd order 0.002, 4th order 0.004
==> mode matching ~93%, dominat higher order is the 2nd order (5%).
Eric: now we have the number for the mode matching. How much did the cavity round-trip loss be using this number?
Using these numbers for both arms (Modulation takes away .2*.25 = 5% power, mode matching takes away 7% after that), I get the following from my data from March:
Xarm loss is 561.19 +/- 14.57 ppm
Yarm loss is 130.67 +/- 18.97 ppm
Obviously, the Xarm number looks very fishy, but its behavior was qualitatively very different when I took the data. ASDC would change from ~0.298 to ~0.306 when the Yarm was locked vs. misaligned, whereas the xarm numbers were .240 to .275.
In any case, I'll do the measurement again tomorrow, being careful with offsets and alignment; it won't take too long.
There are multiple methods by which the arm loss can be measured, including, but not limited to:
We found that the second method is extremely sensitive to errors in the ITM transmissivity. The first method was not an option for a while because the AOM (which serves as a fast shutter to cut the light to the cavity and thereby allow measurement of the cavity ringdown) was not installed. Johannes and Shubham have re-installed this so we may want to consider this method.
Most of the recent efforts have relied on the 3rd method, which itself is susceptible to many problems. As Yutaro found, there is something weird going on with ASDC which makes it perhaps not so reliable a sensor for this measurement (unfortunately, no one remembered to follow up on this during the vent, something we may come to regret...). He performed some checks and found that for the Y arm, POY is a suitable alternative sensor. However, the whitening gain was at 0dB for the measurements that Johannes recently performed (Yutaro does not mention what whitening gain he used, but presumably it was not 0). As a result, the standard deviation during the 10s averaging was such that the locked and misaligned readings had their 'fuzz' overlapping significantly. The situation is worse for POX DC - today, Eric checked that the POX DC and POY DC channels are indeed reporting what they claim, but we found little to no change in the POX DC level while misaligning the ITM - even after cranking the whitening gain up to 40!
Eric then suggested deriving ASDC from the AS110 photodiode, where there is more light. This increased the SNR significantly - in a 10s averaging window, the fuzz is now about 10 ADC counts out of ~1500 (~<1%) as opposed to ~2counts out of 30 previously. We also set the gains of POX DC, POY DC and ASDC to 1 (they were 0.001,0.001 and 0.5 respectively, for reasons unknown).
I ran a quick measurement of the X arm loss with the new ASDC configuration, and got a number of 80 +/- 10 ppm (7 datapoints), which is wildly different from the ~250ppm number I got from last night's measurement with 70 datapoints. I was simultaneously recording the POX DC value, which yielded 40 +/- 10 ppm.
We also discovered another possible problem today - the spot on the AS camera has been looking rather square (clearly not round) since, I presume, closing up and realigning everything. By looking at the beam near the viewport on the AS table for various configurations of the ITM, we were able to confirm that whatever is causing this distortion is in the vacuum. By misaligning the ITM, we are able to recover a nice round spot on the AS camera. But after running the dither align script, we revert to this weirdly distorted state. While closing up, no checks were done to see how well centered we are on the OMs, and moreover, the DRMI has been locked since the vent I believe. It is not clear how much of an impact this will have on locking the IFO (we will know more after tonight). There is also the possibility of using the PZT mounted OMs to mitigate this problem, which would be ideal.
Long story short -
GV Edit 8 Oct 2016: Going through some old elogs, I came across this useful reference for loss measurement. It doesn't talk about the reflection method (Method 3 in the list at the top of this elog), but suggests that cavity ringdown with the Trans PD yields the most precise numbers, and also allows for measuring TITM
As per Ignacio's request, I restored the arm locking.
- MC WFS relief
- Slow DC restored to ~0V
- Turned off DARM/CARM
- XARM/YARM turned on
- XARM/YARM ASS& Offset offloading
For both sidebands to be antiresonant in the arms, the first modulation frequency has to be:
f1 = (n + 1/2) c / (2*L)
where L is the arm length and c the speed of light. For L=38m, we pick to cases: n=3, then f1a = 13.806231 MHz; n=2, then f1b = 9.861594 MHz.
If we go for f1a, then the mode cleaner half length has to change to 10.857m. If we go for f1b, the MC length goes to 15.200m. A 2 meter change from the current length either way.
And the mode cleaner would only be the first of a long list of things that would have to change. Then it would be the turn of the recycling cavities.
Kind of a big deal.
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.
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.
I made some attempts to measure the current length of the arm cavities by using the mass-kicking technique.
However unfortunately I am running out my energy to complete the measurement,
so I will finish the measurement at some time today.
I still have to set an appropriate kick amplitude. Right now I am injecting AWG into ETMY_LSC_EXC at 0.2 Hz with amplutde of 400 cnts.
I guess it needs a little bit more amplitude to get more psuedo-constant velocity.
Volunteers are always welcome !
The procedure was well-described in entry #555 by Dr.Stochino.
Here is just an example of the time series that I took today showing how the time series looks like.
Why not just scan the Green laser to measure the arm lengths instead? The FSR of the arm is ~3.75 MHz and so all you have to do is lock the arm green and then sweep the PZT until the resonance is found at 3.75 MHz.
I managed to execute the first few transitions of locking the arm lengths to the laser frequency in the CARM/DARM basis using the IR ALS system 🎉 🎊 . The performance is not quite optimized yet, but at the very least, we are back where we were in the green days.
Over the week, I'll try some noise budgeting, to improve the performance. The next step in the larger scheme of things is to see if we can lock the PRMI/DRMI with CARM detuned off resonance.
Yesterday evening Nic and me were in the lab. The Mode Cleaner was unlocked, but after many attempt we could fix it and we did many scans of the Y arm cavity.
Today I was not able to keep the MC locked. Koji helped me remotely, and eventually the MC locked back, but after half an hour of measurements I had to stop.
I made some more scan of the Y arm though. I also tried to do the same for the X arm, but the MC unlocked before the measurement was finished. I'll try to come back in the night.
I see no evidence of anything radically different from my PSL table optical characterization in the IMC transmitted beam, see Attachment #1. The lines are just a quick indicator of what's what and no sophisticated peak fitting has been done yet (so the apparent offset between the transmission peaks and some of the vertical lines are just artefacts of my rough calibration I believe). The modulation depths recovered from this scan are in good agreement with what I report in the linked elog, ~0.19 for f1 and ~0.24 for f2. On the bright side, the ALS just worked and didn't require any electronics fudgery from me. So the mystery continues.
I like to ask someone to review the calculation on the wiki.
Last night (Oct 07), I ran armLoss script in order to obtain the latest numbers for the arm cavity loss.
Here is the summary
Measured arm reflectivity R_cav: 0.875 +/- 0.005
Estimated round trip loss L_RT: 157ppm +/- 8ppm
Estimated finesse F: 1213+/-2
Data Points: 34
Measured arm reflectivity R_cav: 0.869 +/- 0.006
Estimated round trip loss L_RT: 166ppm +/- 8ppm
Estimated finesse F: 1211+/-2
Data Points: 26
TE=10ppm, LE=L_RT/2, RE=1-TE-LE
TF=0.005, LF=L_RT/2, RF=1-TF-LF
rcav = -rF +(tF^2 rE)/(1-rF rE)
R_cav = rcav^2
F = pi Sqrt(rF rE)/(1-rF rE)
The second sideband is resonant in the arms for a cavity length of 37.9299m.
The nearest antiresonant arm lengths for f2 (55MHz) are 36.5753m and 39.2845m.
If we don't touch the ITMs, and we use the room we still have now on the end tables, we can get to 37.5m.
This is how the power spectrum at REFL would look like for perfect antiresonance:
And this is how it looks like for 37.5m:
Or, god forbid, we change the modulation frequencies...
We took the data for the new absolute length measurement of both arms, after the latest vent and move. We will analyze soonly. We had done a round of analysis, but then Koji pointed out that our data wasn't so clean because the whitening filters were on (and saturated the ADC). We now have the data (but not the analysis) for the better data with the WF off.
So our dirty-data preliminary number for the X arm is 37.73meters, which is 14cm different from our old length. We were supposed to move by ~20cm, so....either this measurement is bad because the data sucked (which it did), or we are 6cm off. Or both.
I'll do another analysis with the clean data for both arms later today/tomorrow.
After analyzing the cleaner data, I get the following:
Y_Length_long = 37.757 meters
X_Length_long = 37.772 meters
As stated in the wiki, the goal arm length was L = 37.7974 m for each arm.
So we're within 2cm for X, and within 4cm for Y.
According to Kiwamu's awesome tolerance calculation, we need to be within 2cm for each arm. Given that we started out 20cm wrong for X and 25cm wrong for Y, we're a lot closer now, even though we aren't meeting our Yarm requirement yet.
Probably some Optickle action is in order, to see what these new lengths give us in terms of sideband phase and other stuff.
If you want more digits on my calculated numbers (which are probably meaningless, but I haven't done a careful error analysis), in my directory ...../users/jenne/Xarm and ..../users/jenne/Yarm run Xarm_find_peaks_and_length.m and Yarm_find_peaks_and_length.m respectively. These will output the lengths.
Summary: After today's meeting, Gabriele and I looked into the arm loss situation, to see if we should really believe the losses that had been suggested by my previous measurements. We made some observations that we're not sure how to explain, and we're thinking about other ways to try and estimate the losses to corroborate previous findings.
We first looked to see if the ASS had some effective offset, leaving the alignment not quite right. Once ASS'd, we twiddled each arm cavity mirror in pitch and yaw to see if we could achieve higher transmission. We could not, so this suggested that ASS works properly.
We then looked at potential offsets in the Xarm loop. We found that an input offset of 25 counts increased the transmission, but only very slightly. With this offset adjusted, we confirmed the qualitative observation that locking/unlocking the xarm causes a much bigger change in ASDC than doing the same with the harm.
However, we noted that the ASDC data (which is the DC value of the AS55 RFPD) was quite noisy, hovering around 50 counts. Looking at the c1lsc model, we found that we were looking at direct ADC counts, so the signal conditioning was not so great. We went to the LSC rack and stole the SR560 that had been hooked up as a REFLDC offsetter, and used it to give ASDC a gain of 100, and a LP at 100Hz, since we only care about DC values. We then undid the gain in the input FM; and this calmed the trace down a fair bit. The effects due to each arm locking/unlocking was still consistent with previous observations.
At this point, we looked at the arm transmission and ASDC signals simultaneously. Normally, when misaligning a cavity, one would expect the reflected power to rise and the transmission to fall.
However, we saw that when misalignment the Yarm in yaw in either direction, or the Xarm in one direction, both the IR transmission and ASDC would fall. This initially made us think of clipping effects.
So, we checked out the AS beam situation on the AP table. On a card, the beam looks round as we could tell, and the beam spot on AS55 was nice and small. (We tweaked its steering a little bit in pitch to put it at the center of the "falling-off" points) The reflection and transmission falling effect remained.
At this point, we're not really sure what could be causing this effect. After the reflected beams recombine at the BS, the output path is common, so it's strange that this odd effect would be the same for both arms.
Lastly, we discussed other ways that we may be able to see if the Xarm really has ~500ppm loss. Since its transmission is ~1.4%, Gabriele estimated that we may be able to see a ~300Hz difference in the arm cavity pole frequency between the two arms, based on the modification of the cavity finesse due to loss. Since we don't currently have the AOM set up to inject intensity noise, we talked about using frequency noise injection to measure the arm cavity poles, though this would be coupled with the IMC pole, but this could hopefully be accounted for.
1) Turn off feedback to ETMY (the ETMY button on the LSC screen).
2) Put a 1 into the YARM->MC2 output matrix element on the LSC screen.
3) Turn off FM6 (comb), FM7 (0.1:10) on the MC2_MCL filter bank. This is to make the IOO-MCL loop more stable and to reduce the IOO-MCL low frequency gain.
4) Set the MC2-LSC gain to 0.5, turn the output ON, turn ON FM4 & FM5 & FM6 of the MC2-LSC filter bank.
5) Turn on the input of MC2-LSC and the arm should now lock.
6) After locking, set the MC2-MCL gain to zero. Hopefully with a few second ramp time.
(A comment by KA - c.f. this entry )
The interferometer is warming up!
I had some issues locking the IMC at first. It turned out that the MC3 side OSEM signal wasn't getting to the ADC. A satellite box sqush fixed it.
I touched up the PMC alignment; the best I could do is 0.75V, probably due to the AOM being in place.
I haven't touched the WFS offsets, but the current ones seem to be doing ok. I'll touch them up tonight when the seismic activity has calmed.
I made some changes to the state of the PZT/PC crossover gain in the mcdown script, resulting in the IMC catching lock quicker.
Thankfully, the tip tilt pointing stayed good during the upgrade. I barely had to touch the ETM alignment to lock the arms. ETMX is showing some errant motion, though...
ALS noise suppressed to 1KHz/rtHz. 1kHz RMS.
Plot 1: Scan of X arm by changing offset into Phase Tracker -> Xarm loop. Filter bank ramp time set to 120 s + using a 30 mHz low pass filter. IR beam is aligned to x arm, but not well.
Plot 2: ALS error signal with loop open (BLUE), closed with old filters (PURPLE), and with new, better boost (RED).
Plot 3: Bode plot of new boost (FM10), v. old, sad boost (1:50 pole:zero). RMS is now less than 1 kHz or ~50 pm. (in your face, Kiwamu!)
Changes made to the ALS servo:
ALS-TRX has been calibrated to read from 0-1 instead of counts in 1000 s. Calibration factor = 1/4500 = 0.00022
Old antiwhitening filter has been removed. Added LPF at 1000Hz to remove glitches at high frequencies.
No changes made.
FIlter FM5 modified. 1000:1 changed to 3000:1
5. Offset for ALS scan were given through C1ALS_OFFSETTER1 with LPF50m enabled.
The filter modules of the servo were:
Check PZT out range for ALS. Figure out what the deal is with ALS SLOW servos.
Add DQ channels for ALS.
Automatic ALS up script (enable and disable phase tracker included).
RMS is now less than 1 kHz or ~50 pm. (in your face, Kiwamu!)
Isn't this still a factor of 2 away from the limit in the paper?
My understanding is that that number is an in-loop evaluation of the loop so far (as the first step of the loop evaluation).
This is not what we can directly compare with the number in the paper.
Basically the entry 8741 is telling us that the new filter suppresses the error signal better than before.
That's clearly shown in the attachment 2.
Last week we vented and we cleaned the main optics of the arm cavities.
I measured the frequency of the cavity poles for both the arm cavities to see how they changed (see previous elog entry 2226). These the results:
fp_X = 1616 +/- 14 Hz
fp_Y = 1590 +/- 4 Hz
The Y arm cavity pole moved down by 130 Hz, whereas the X arm moved by only 34 Hz. I wonder if that is because Kiwamu spent much more time on cleaning ITMY than on any other optic.
dfmn = ----- * (m+n) * acos(sqrt(g1*g2))
I'm going to work on the X arm to measure the arm cavity finesse.
The idea is to measure the cavity transfer function to estimate the frequency of its cavity pole. That should be a more accurate measurement than that based on the cavity decay time.
I'm starting now and I'm going to inject a swept sine excitation on the OMC_ISS_EXC input cable laying on the floor nearby the AP table (see pic).
In orderf to do that I disconnected the cable from the OMC breakout box laying on the floor. I'm going to plug the cable back in as soon as I'm done.
Since I need to measure the transfer function between TRX and MC_TRANS_DC I picked off the beam going to RFAM PD to send it to a PDA255 photodiode (cannibalized from the AbsL's PLL) which I installed on the PSL table.
I centerd the beam on the PD and I was able to see the injected signal.
I think I'm ready to measure the transfer function.
Except for the RFAM PD everything is as before.
I'm gonna go grab dinner and I should be back to keep working on that in about one hour.
Since I need to measure the transfer function between TRX and MC_TRANS_DC I picked off the beam going to RFAM PD to send it to a PDA255 photodiode (cannibalized from the AbsL's PLL) which I installed on the PSL table.
Back from dinner. Taking measurements.
We had persistent frustration by occasional unlock during ASSing.
Today, I added triggers to the servo gains in order to elliminate this annoyance.
Each ASS servo gain slider is multiplied with the corresponding LSC Trigger EPICS channel (i.e. C1:LSC-iARM_TRIG_MON, where i=X or Y).
This has been done by ezcaread modules in RCG.
The model and screen have been commited to svn.
After Den's work with the ASS model this week, all of the channel names were changed (this wasn't pointed out in his elog....grrr), so none of the A2L scripts worked.
They are now back, however there is still some problem with the plotting that I'm not sure I understand yet. So, the measurement works, but I don't think we're saving the results and we certainly aren't plotting them yet.
I wanted to check where the spots are on the mirrors, to make sure Den's stuff is doing what we think it's doing. All of the numbers were within ~1.5mm of center, although Rossa keeps crashing (twice this afternoon?!?), so I can't copy and paste the numbers into the elog.
A near-term goal is to copy over Den's work on the Yarm to the Xarm, so that both arms will auto-align. Also, I need to put the set of alignment scripts in a wrapper, and have that wrapper call-able from the IFO Configure screen.
Also, while thinking about the IFO Configure screen, the "save" scripts weren't working (on Rossa) today, even though I just made them work a week or so ago. Rossa, at least, was unhappy running csh, so I changed the "save" script over to bash.
In either .../scripts/XARM or ...../scripts/YARM run either A2L_XARM or A2L_YARM.
The wrapper script will, like the MC script, open a striptool so you can monitor the lockin outputs, setup the measurement, run the measurement, including misbalancing coils on the optics for calibration, and then calculates the spot positions. It records the measurement in a log file in /data_spotMeasurements under each arm's directory. The wrapper script then runs the plotting script which reads the logfile, and plots all past measurements.
Here is that plot for the Yarm:
The first two points were measured within a few minutes of eachother, the third set of points was after input pointing adjustment during IFO alignment. Clearly the pointing that optimized the cavity transmission (trying to leave the test mass mirrors alone, and only moving TT1 and TT2) does not also give the best spot centering. I claim that this is a result of the arm being aligned to the green beam, which was never locked to the 00 mode when we were at air. This is a lesson learned....take the time to deal with the green beams.
I haven't finished debugging the scripts so that the measurement is fully automatic, like the MC, but I did measure the arm spot positions just now.
These numbers aren't especially precise....I just picked numbers off of a StripTool plot, but they give us a good idea of how very far off we are. Also, I don't know yet which way the signs go...I have to think about that in terms of the direction I mis-balanced the coils. It's the same convention as the MC though. You can see in the attached quad camera image (quadrants match the corners of the table) that these numbers aren't unreasonable.
EDIT: It occurs to me now, a little later, that it had been at least half an hour since I last realigned the cavities, so some of this apparent miscentering is due to the input pointing drift. That doesn't account for all of it though. Even when the cavities have very high transmitted power, the spots are visibly miscentered.