I was trying to lock PRMI+2 arms, but I'm losing patience with the MC losing lock. It's mostly well-behaved, but every now and again it'll lose lock, and it always seems to be just when I've gotten to a delicate part. Anyhow. I don't think anything needs fixing.
I am not able today to acquire PRMI lock with REFL 165 I&Q, nor am I able to follow Koji's prescription to transition to 165 from 55. However, I am able to transition to, or just straight-up aquire on, REFL 33 I&Q. The new ASC is awesome.
Before trying to lock with REFL165, I redid it's demod phase, by actuating on the PRM while locked on REFL 55, and minimizing the PRCL signal in the Q-phase. Old 165 phase was -83.5 degrees, new is -79.5 degrees.
I then tried the procedure of misaligning the PRM, acquiring ALS lock of both arms, finding the arm IR resonances, and moving the arms off resonance. Then I restored the PRM, and locked the PRMI on sidebands using REFL33 I&Q.
Something was a little weird and unexpected: If the arms were not far, far off resonance, there was a large MICH offset, so that AS was bright, and POP was pretty dark. If I moved the arms farther from resonance, POP would come back to the "normal" brightness, which was ~460 counts on POP110I. When POP was dark-ish, POP110I was about 150, although this number could be changed by moving the arms around. This number was not dependent on PRM alignment. Also, PRCL was not yet starting to resonate the carrier, because POPDC stayed at its low sideband-lock level of about 90 counts (vs. 2,000+ for a PRMI-only carrier lock), and POP was visually dark on the camera.
While playing with PRMI + 2 ALS arms is entertaining for the evening, I don't yet have a plan for dealing with the changing CARM optical plant for IR signals situation. That's in progress in the daytime.
We started out this evening by locking the PRMI, on different REFL PDs, just to make sure that we could. We were able to acquire lock (without having to transition) on either REFL55, REFL33 or REFL165 (I&Q phases for each). By looking at the transfer function between REFL 55 I and REFL 165 I, I determined that the phase of REFL165 needed to be -135.5, not +44.5, so that the gains were the same sign in all REFL PDs (this is in reference to Koji's elog 9777). The time to acquisition is much shorter with REFL55 than with the other 2 photodiodes. I'm not sure right now why this is, but it's pretty consistent, and even more so when the arms are held off resonance. It is also easy to acquire lock with REFL 55 and then transition to either of the 3f diodes.
MICH gain = 2.0. FMs 4, 5 always on. FMs 2, 3, 6, 7, 9 triggered with 35 up, 2 down. Trigger servo on POP110I with 100 up, 10 down.
PRCL gain = -0.020. FMs 4, 5 always on. FMs 2, 3, 6, 9 triggered with 35 up, 2 down. Trigger servo on POP110I with 100 up, 10 down.
PRCL ASC triggered with 50 up, 10 down. Both pitch and yaw servos had FMs 1, 9 always on, and the FM 6 boost turned on by hand after lock acquired. Pitch gain = -0.023, yaw gain = -0.027.
If using REFL55, PRCL = I = 1 in the input matrix, and MICH = Q = 1 in the input matrix.
If using REFL33, PRCL = I = 3 in the input matrix, and MICH = Q = 3 in the input matrix.
If using REFL165, PRCL = I = 0.15 in the input matrix, and MICH = Q = 0.1 in the input matrix.
We then moved on to trying to do PRMI + 2 arms, but have been plagued a little bit by locklosses, which may be due to the high seismic from the few large and many small Chilean aftershocks.
Settings: With the PRM misaligned, we acquired ALS lock in a CARM/DARM fashion. The Xarm servo was our "darm" proxy, with +1*ALSX and +1*ALSY. The Yarm servo was our "carm" proxy, with -1*ALSX and +1*ALSY. The opposite signs from what you might expect is from our having placed the 2 aux laser frequencies on opposite sides of the PSL. Our DARM (xarm) was actuating +1 on ETMX and -1 on ETMY. Our CARM (yarm) was actuating +1 on MC2. Then we put in a CARM offset of ~60 counts.
We then realigned the PRM, and locked the PRMI (settings as above). It is much easier to acquire lock with REFL55 than it is with REFL33 or REFL165. Also, when we are locking the PRMI with REFL55, the AS port is dark, and we see bright sideband resonance at the POP port, as we normally expect. However, as with last night, when we lock the PRMI with either of the 3f PDs, we see a very bright AS port, and a dark POP port. Tonight, putting in a MICH offset did not seem to make a significant difference. As with last night, moving the arms farther away from resonance removed this MICH offset. I am sure that this is not an artifact of the photodiode dark offsets not being set properly, since I rechecked those carefully.
We have not come to any interesting conclusions about PRMI+2 arms tonight, since we have started losing the MC every few minutes (maybe seismic-related?).
Much more success later!
We've had the arms locked on ALS and held off resonance for more than 2 hours now. That's good. We've done several trials of locking the PRMI and trying to reduce the ALS CARM offset. Once we were able to get quite close, but we never achieved zero CARM offset.
One big finding is that when the TRX and TRY are about 0.1 in the coupled-cavity case, we start to see real length information in the 1/sqrt(trans) signals. Q will edit this elog, or reply to it, with the data that he has collected.
So, here's the basic: "We reduced the CARM offset and saw more TRY" plot.
As Jenne mentioned, we suspected that we were seeing real displacement information in the sqrtInv signals. (We had incidentally hard switched to the transmon QPDs for all of this)
Here's a 2d-histogram of the ALS CARM error signal vs. the sqrtInv CARM signal (i.e. 1/sqrt(TRX) + 1/sqrt(TRY))
This is exactly the shape we expect, which is cool. You can see where we stepped the offsets, too. It looks like the signal gets into it's good linear range when ALS CARM was about -20, which is when TRY was a little under 0.1, which seems pretty early and potentially useful.
Also, here are snapshots of what REFL11_I and sqrtInv CARM were doing in the last five seconds of time in the above plot, which was shortly before we made the offset push that broke the PRMI lock. If you look really closely, maybe you can convince yourself that there is some common information in them...? It's hard to say. In any case, there is definitely CARM pdh action happening.
I re-centered beams on several PDs and a camera including :
AS55, ETMY_QPD, TRY and ETMYT_CCD.
The most important one was AS55.
When I was locking each arm I found that the error signal from AS55 was very coupled to the angular motion of the arms.
I checked the beam on the AS55 RFPD and found the beam on the edge of the photo diode. This is possibly because Valera and I had been touching the input beam alignment.
At that time the DC signal from AS55 without aligning PRM and SRM was about 5 mV.
Adjusting the beam position by a steering mirror brought the DC signal up to 20 mV.
Then the lock of each arm became more stable.
We consider the astigmatism effects of the stock options. The conclusions are:
1. For the AS path, the stock should work fine for the phase-one of BHD, if we could tolerate a few percent MM loss. The window for length adjustment to achieve >99% MM for both s and t is only 1 mm for 1% RoC error (compared to ~ 1 cm in the customized case).
2. The LO path seemed tricky. As LO3 & LO4 are both significantly curved (RoC<=0.5 m), the non-zero angle of incidence makes the astigmatism quite sever. For the t-plane the nominal MM can be 0.98, yet for the s-plane, the nominal MM is only 0.72. We could move things around to achieve a MM ~ 0.85, which is probably fine for the phase-one implementation but not long term.
Attachments 1-3 are for the AS path; 4-6 are for the LO path.
1 & 4. Marginalized MM distribution for the AS/LO paths. Here we assumed 5 mm positional error and 1% fractional RoC error. Due to the astigmatism, the nominal s-plane MM is only 0.72 for the LO path.
2 & 5. Scattering plots for the AS/LO paths. We color coded the points as the following: pink: MM>0.99; olive: 0.98<MM<=0.99; grey: MM<=0.98. For the AS path, MM is mostly sensitive to the AS1 RoC and can be adjusted by changing AS1-AS3 distance. For the LO path, the LO3 RoC and LO3-LO4 distance are most critical for the MM.
3 & 6. Assuming +- 1% AS1 (LO3) fractional RoC error, how much can we compensate for it using AS1-AS3 (LO3-LO4) distance. For the AS path, there exists a ~ 1 mm window where the MM for s and t can simultaneously > 99%. For the LO path, the best we can do is to make s and t both ~ 85%.
For the initial phase of BHD testing, we recently discussed whether the mode-matching telescopes could be built with 100% stock optics. This would allow the optical system to be assembled more quickly and cheaply at a stage when having ultra-low loss and scattering is less important. I've looked into this possibility and conclude that, yes, we do have a good stock optics option. It in fact achieves comprable performance to our optimized custom-curvature design [ELOG 15357]. I think it is certainly sufficient for the initial phase of BHD testing.
It turns out our usual suppliers (e.g., CVI, Edmunds) do not have enough stock options to meet our requirements. This is for two reasons:
However I found that Lambda Research Optics carries 1" and 2" super-polished mirror blanks in an impressive variety of stock curvatures. Even more, they're polished to comprable tolerances as I had specificied for the custom low-scatter optics [DCC E2000296]: irregularity < λ/10 PV, 10-5 scratch-dig, ROC tolerance ±0.5%. They can be coated in-house for 1064 nm to our specifications.
From modeling Lambda's stock curvature options, I find it still possible to achieve mode-matching of 99.9% for the AS beam and 98.6% for the LO beam, if the optics are allowed to move ±1" from their current positions. The sensitivity to the optic positions is slightly increased compared to the custom-curvature design (but by < 1.5x). I have not run the stock designs through Hang's full MC corner-plot analysis which also perturbs the ROCs [ELOG 15339]. However for the early BHD testing, the sensitivity is secondary to the goal of having a quick, cheap implementation.
The following tables show the best telescope designs using stock curvature options. It assumes the optics are free to move ±1" from their current positions. For comparison, the values from the custom-curvature design are also given in parentheses.
The AS relay path is shown in Attachment 1:
The LO relay path is shown in Attachment 2:
I've created a new tab in the BHD procurement spreadsheet ("Stock MM Optics Option") listing the part numbers for the above telescope designs, as well as their fabrication tolerances. The total cost is $2.8k + the cost of the coatings (I'm awaiting a quote from Lambda for the coatings). The good news is that all the curved substrates will receive the same HR/AR coatings, so I believe they can all be done in a single coating run.
Can you describe the mode matching in terms of the total MM? Is MM_total = sqrt(MM_vert * MM_horiz)?
MM_total = (MM_vert + MM_horiz) / 2.
The large astigmatic MM loss in the LO case is mainly due to the strong LO4 curvature (R=0.15m) with a 10 deg AOI. I looked again at whether LO1 could be increased from R=5m to the next higher stock value of 7.5m, as this would allow weaker curvatures on LO3 and LO4. However, no, that is not possible---it reduces the LO1-LO2 Gouy phase separation to only 18 deg.
There is, however, a good stock-curvature option if we want to reconsider actuating LO4 instead of LO2 (attachment 1). It achieves 99.2% MM with the OMCs, allowing positions to vary +/-1" from the current design. The LO1-LO4 Gouy phase separation is 72 deg.
Alternatively, we could look at reducing the AOI on LO3 and LO4 (keeping LO1-LO2 actuation).
Hmm? T1300364 suggests MM_total = Sqrt(MM_Vert * MM_Horiz)
Using the updated AOI's for the LO path: (4.8, 47.9, 2.9, 4.5) deg for (LO1, LO2, LO3, LO4), we obtain the following results.
First two plots are scattering plots for the t and s planes, respectively. Note that here we have changed to 0.5% fractional RoC error and 3 mm positional error. We have also changed the meaning of the colors: pink:MM>0.98; olive 0.95<MM<=0.98, and grey MM<=0.95. It seems that both planes would benefit statistically if we make the LO3-LO4 distance longer by a few mm.
We also consider how much we could compensate for the MM error in the last plot. We have a few mm window to make both planes better than 0.95.
We've begun assembling the new c1psl Acromag chassis based on Yehonathan's final pin assignments. So far, parts have been gathered and the chassis itself has been assembled.
Yehonathan is currently wiring up the chassis power and Ethernet feedthroughs, following my wiring diagram from previous assemblies. Once the Acromag units are powered, I will help configure them, assign IPs, etc. We will then turn the wiring over to Chub to complete the Acromag to breakout board wiring.
I began setting up the host server, but immediately hit a problem: We seem to have no more memory cards or solid-state drives, despite having two more SuperMicro servers. I ordered enough RAM cards and drives to finish both machines. They will hopefully arrive tomorrow.
RTFE. Where did the spares go?
I found them, thanks. After c1psl, there are 4 2GB DIMM cards and 1 SSD left. I moved them into the storage bins with all the other Acromag parts.
I finished pre-wiring the PSL chassis. I mounted the Acromags on the DIN rails and labeled them. I checked that they are powered up with the right voltage +24V and that the LEDs behave as expected.
I configured the Acromag channels according to the Slow Controls Wiki page.
We started testing the channels. Almost at the beginning we notice that the BIO channels are inverted. High voltage when 0. 0 Voltage when 1. We checked several things:
1. We checked the configuration of the BIOs in the windows machine but nothing pointed to the problem.
2. We isolated one of the BIOs from the DIN rail but the behavior persisted.
3. We checked that the voltages that go into the Acromags are correct.
The next step is to power up an isolated Acromag directly from the power supply. This will tell us if the problem is in the chassis or the EPICs DB.
I isolated a BIO Acromag completely from the chassis and powered it up. The inverted behavior persisted.
Turns out this is normal behavior for the XT1111 model.
For digital outputs, one should XT1121. XT1111 should be used for digital inputs.
Slow machines Wiki page was updated along with other pieces of information.
I replaced the XT1111 Acromags with XT1121 and did some rewiring since the XT1121 cannot get the excitation voltage from the DIN rail.
I added an XT1111 Acromag for the single digital input we have in this system.
I don't think this is an accurate statement. XT1111 modules have sinking digital outputs, while XT1121 modules have sourcing digital outputs. Depending on the requirement, the appropriate units should be used. I believe the XT1111 is the appropriate choice for most of our circuits.
You're right. We had the right idea before but we got confused about this issue. I changed all the XT1121s to XT1111 and vice versa. We already know which channels are sourcing and which not. Updated the wiring spreadsheet. The chassis seems to work. It's time to pass it over to Chub.
The ETM assembly has moved forward a couple of steps. We have completed the following:
1) Positioning the guide rod and wire stand-off on both the ETMs (5 and 7)
2) The magnets had to be cleaned with an acetone wash as they had touched the plastic Petri-dish (not cleaned for vacuum).
3) The magnets and the Al dumb-bells have been glued together and left to cure in the gluing fixture.
4) The guide-rod and wire stand-offs have also been glued to the optic and left to cure for 24 hrs.
JD: As you can see in my nifty status table, we are nearing the end of the suspension story.
JD: As you can see in my nifty status table, we are nearing the end of the suspension story.
We are going to try (but can't guarantee) to get ETMX to Bob for baking by Friday at lunchtime, that way we can re-suspend it on ~Monday, and place it in the chamber. Then we could potentially begin Green arm locking next week. Steve has (hopefully!!) ordered the spring plungers for ETMY. The receiving and baking of the spring plungers is the only current delay that I can foresee, and that only is relevant for one of the optics.
We (who is going to be in charge of this?) still need to move the SRM OSEMs & cables & connectors to the ITMY chamber from the BS chamber.
This morning I assembled LO3, LO4 and AS3 (all mirrors) onto polaris K1 mounts. The mounts stand as per this elog, on 4.5" posts with 0.5" Al spacers to match the beam heigth of 5.5". I also assembled ASL by adding a 0.14" Al spacer, and finally, recycled two DLC mounts (from the XEND flowbench) and posts to mount the 2 inch diameter beamsplitters BHDBS and AS2 (T=10%). I stored the previous 2" optics in the CVI and lambda optic cases and labeled appropriately.
This afternoon I epoxied the guiderod and wire standoff to the new PRM. I also epoxied the magnets that Suresh picked out to the dumbbell standoffs. We'll let them all cure over the weekend, and then I'll glue the magnets to the optic on ~Monday.
Notes about the epoxy:
Previously, we had been using the "AN-1" epoxy, which is gray, with a clear hardener. Bob recommended we switch to "30-2", which is clear with clear, and has been chosen for use in aLIGO. Both were vacuum approved, but the 30-2 has gone through ~2 months of testing at the OTF (Optics Test Facility?) over in Downs under vacuum, to check the level of outgassing (or really, non-outgassing).
The 30-2 is less viscous than the AN-1, and it takes less glue to do the same job, so we should keep that in mind when applying the epoxy. When I put the glue next to the guiderod and standoff, it got wicked along the length of each rod, which is good. I can't reach the whole length of the rod with my glue applicator because the fixture holding them in place blocks access, so the wicking is pretty handy.
I've also added the updated version of my Status Table for the suspensions.
The day before yesterday, I was cleaning a flow bench in the clean room.
I found that one SOS was standing there. It is the SRM suspension.
I thought of the nice idea:
- The installed PRM is actually the SRM (SRMU04). It is 2nd best SRM but not so diiferent form the best one.
==> Use this as the final SRM
- The SRM tower at the clean room
==> Use this as the final PRM tower.
==> The mirror (SRMU03) will be stored in a cabinet.
- The two SOS towers will be baked soon
==> Use them for the ETMs
This reduces the unnecessary maneuver of the suspension towers.
I aligned both the X and Y end green to the arms.
[Koji, Manasa, Jenne]
The Y arm was locked in IR, and we saw flashing in the Xarm (Gautam had the Xarm for green work when we began). I checked IPANG, and the beam was beautifully unclipped, almost perfectly centered on the first out of vacuum mirror. I aligned the beam onto the QPD.
We then swapped out the MC Y1 that we use at low power, and replace the usual 10% BS, so that we wouldn't crispy-fry MC REFL. Manasa adjusted the half wave plate after the laser, to maximize the power going toward the PMC. We relocked the PMC, and see transmission of ~0.84, which is at the high side of what we usually get. The beam was aligned onto MC REFL and centered on the WFS, and the MC was locked at nominal power. Koji tweaked up the alignment of the MC, and ran the WFS offset script. I aligned beam onto POP QPD and POP110 coarsely (using a flashing PRC, not a locked PRM-ITMY cavity, so the alignment should be rechecked). The arms have both been locked and aligned in IR....the green beams need to be steered to match the current cavity axis.
The AS beam, as well as REFL and POP, are all coming out of the vacuum nicely unclipped.
Notes: When Koji was aligning the SRM to get the SRC cavity roughly aligned (the AS flashes all overlapping), we noticed that there is some major pitch-yaw coupling. Serious enough that we should be concerned that perhaps some connector is loose, or an actuator isn't working properly. This should be checked.
Moral of the story: Coarse alignment of all mirrors is complete after pump-down and we have IR locked and aligned to both arms at nominal power.
Still to do:
* Restore PRM, align beam onto the REFL PDs.
* Lock PRM-ITMY cavity, align beam onto POP PDs.
* Align AS beam onto AS55.
* Recenter all oplevs.
* Recenter IPPOS and IPPANG at nominal power.
* Start locking!!
I have a concern about the SRM suspension. The yaw alignment bias produces huge pitch coupling.
This could be a connector issue or the rubbing of the mirror on the EQ stops.
We have the photos of the magnets and they were not touching the OSEMs.
Someone left the arms aligned, and the LSC engaged, so the arms have been locked almost continuously for several days hours. The trend below is for 4 days hours. What is most impressive to me is that we don't see a big degredation in the transmitted power over this time.
EDIT: Okay, I got excited without paying attention to units. It was only several hours, which is not too unusual. Although the lack of transmission degredation is still unusual. However, this may be due to improved oplevs? I'm not sure why, but we're not seeing (at least in this plot) the degredation to ~0.7 after an hour or so.
Since last Friday either the arms or the PRC can't lock.
The montors show the beam flashing on the end mirrors, but the cavity can't get locked. The error signal looks fine. I suspect a computer problem.
Also PRC can't lock. SPOB is suspiciously stuck at about -95. Although that's not a fixed number, but covering the by hand the SPOB PD on the ITMY table doesn't change the number. I check the DC output of the photodetector and it is actually seen the beam.
Suspecting computer problems started after last Thursday's IP switch, I rebooted the frame builder, c1dcuepics, c1daqctrl and all the front ends. I then burtrestored to February 1st at 1:00 am.
Before I burtrestored c1iscepics, SPOB had gone back to more typical numbers around 0, as it usually read when PRC wasn't locked.
But burtrestoring c1iscepics, return it to the -95 of earlier.
Burterestoring to other times or dates didn't solve the problems.
Koji and I started poking around, trying to understand what was going on. At first, we thought it might be related to a computer error, as it seemed.
Fortunately, Rob stopped by and explained that the boost stage of the filter comes under c1lsc control, and will be turned on or off depending on the power in the arms. Although if you turn it off, it will remain off, it just if its manually selected on, it may go on or off.
Similarly, the output from the Xarm filter bank to the ETMX filter input will be turned on or off depending on the power in the arm.
Anyways, the locking trouble turns out to be due to no RF sidebands at 33 MHz. The output of the Marconi was unplugged. I don't know who, or why did it, but I've plugged it in for now, so we can lock the arms. Let us know if you need in unplugged. Thanks.
The IFO is ready for 3F DRMI comissioning
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.