I've been thinking a bit about what the ideal cable length / delay time for the upgraded ALS beatbox should be. Here are some thoughts, but no conclusions yet.
If you're not running your beatbox mixer in compression, there are two competing effects when you change the cable length. At first, more delay gives better sensitivity, but this does not go on to infinity, because cable attenuation eventually kills your signal. It turns out that the ideal length can be derived to be whatever length gives you 20/ln(10) = -8.7dB of attenuation. Frank found this out in PSL ELOG 825, and I found an HP document that derives this (and other useful DFD math) to the wiki, here.
In PSL ELOG 826, Frank calculated this ideal length for a 160MHz carrier in various kinds of cables.
However, this is not the end of the story. In the case of the DFD, we actually benefit from operating the mixer in compression, as makes our sensitivity less sensitive to flucuations in the beat amplitude. In this situation, the HP doc states "For maximum sensitivity, more delay can be added until the signal level out of the delay line is 8.7dB below the phase detector (mixer) compression point." I'm not sure I really understand the logic behind this statement, though.
Lastly, Koji mentioned the fact that the splitter in the demod board does not split at exactly 90 degrees, making the trajectory in the IQ plane an ellipse. This means that if the beat signal is moving around the ellipse a lot, or even wrapping around it, we can suffer from some nonlinear signal conversion. Also, if the raw DFD sensitivity is very high, the free swinging mirrors will cause the signal to swing around faster than the phase tracker can keep up. This should be easy to avoid, however; I doubt we will use so much cable that the beat would move by so much.
I intend to take all of this into account when picking a cable length! Jessica is going to help us make a nice box for them, too.
This afternoon, I had a fruitful conversation with Rich Abbott about various kinds of cables.
I've sent an email to Steve to ask him to buy 2 x 50m LMR-195 cables, with male SMA connectors. Rich highly recommends these for their polyethylene insulation, which makes them less microphonic and less susceptible to thermal expansion, low loss, and multi-ply bonded foil shielding.
50m means that the peak to peak mixer output swing corresponds to about a MHz. 1nV of mixer output noise looks like ~6mHz frequency noise, for a Level 3 mixer appropriately driven. As a comparison, the lowest our in-loop green PDH error signals get is 0.1Hz/rtHz.
The cable attenuation should be around 4.2dB at 50MHz, and 7.3dB at 150MHz, according to the data sheet. Thus, we should not be in the regieme where we are losing sensitivity to the attenuation.
By my rough geometric estimation, these two should fit in the 2U box I got ahold of today fine. Jessica is designing the front panel.
We currently have ~30m of RG-408 and RG-142 as our delay lines.
I've been working on getting a working ALS up and running. Things are in a bit of a transient state right now; I'm off to softball and dinner, and will resume work tonight. There will be a more detailed ELOG then, but here are some quick notes:
The main thing left to do is to install the RF amplifiers at the PSL table and route the green beat signals over to the LSC rack. I fear that some investigation into the whitening filters will be neccesary to make the performance adequate, however.
Too sleepy to make full ELOG. Stay tuned.
Two 25dB amplifiers (with fins!) are living in the top shelf on the PSL table, inputs currently grounded. I broke out the fused 24V power from the AOM driver to power the two amps and the AOM driver. I used the POP55 and AS165 heliax cables to get their outputs to the LSC rack, through delay lines, into demod board.
Driving with -20dBm at 55MHz, the BEATX signal chain has about 60Hz RMS noise, which is about what I measured for driving the old beatbox with a marconi. High frequency noise is a much nicer shape, though. The BEATY signal didn't seem to be getting through, will double check soon.
Still old delay cables, not nicely shielded or isolated or anything. We'll have to pipe the monitor signal from the LSC rack over to the control room analyzer now.
Turns out the reason that the BEATY signal wasn't working is that one of the two RF amplifiers (both of which are model ZHL-32A), isn't amplifying. Voltage at the pins is fine, so maybe its just broken. When the ZHL-3As that Rana ordered arrive, I'll install those.
Switching the working amplifier between the two channels, and using a Marconi driving -20dBm (the Y green beatnote amplitude), the phase tracker output RMSs are 70Hz and 150Hz for X and Y, respectively, which isn't too exciting. There is enough whitening gain and filtering that I don't think ADC noise is an issue (The magnitude of the phase tracker Q is ~10kcounts after +6dB whitening gain).
The RMS in both channels mostly comes from a whole mess of 60Hz harmonics. I'll see what I can do by taking better care of the delay line cables, but it is kind of weird that this would be worse now, given that there was little care given to them before either.
Also, for now, so I don't have to lug the marconi around everywhere, I'm currently driving both channels of the demod board with a spare 55MHz LO output of the LSC LO distribution box, which ends up being a factor of 5 smaller phase tracker error signal, but the noise level is about the same as with the marconi.
I was having issues trying to get reasonable noise performance out of the aLIGO demod board as an ALS DFD. Terminating the inputs to the LSC whitening inputs did not show much 60Hz noise, and an RMS in the single Hz range.
A 60Hz line of hundreds of uV was visible in the power spectrum of the single ended BNC and double-ended DB25 outputs of the board no matter how I drove or terminated.
So, I tried out hooking up the ALS beatbox. It turns out to work better for the time being; not only is the 60Hz line in the analog outputs about ten times smaller, the broadband noise floor in the resultant beat spectrum when driven by a 55MHz LO on the LSC rack is a fair bit lower too. I wonder if this is due to not driving the aLIGO board LO at the +10dBm it expects. With the amplifiers and beat note amplitudes we have, we'd only be able to supply around 0 dBm anyways.
Here's a comparison of the aLIGO board (black) and ALS beatbox (dark green) driven with the 55MHz LO, both going through the LSC whitening filters for a resultant magnitude of 3kCounts in the I-Q plane. The RMS sensing noise is about 30 times lower for the beatbox. (Note, this is with the old delay cables. When we switch to the 50m cables, we'll win further frequency noise sensitivity through the better degrees->Hz calibration.) I'm very interested to see what the green beat spectrum looks like with this setup.
Not only is the 60Hz line smaller, there is simply less junk in the beatbox signal. I did not expect this to be the case.
There were some indications of funky status of the aLIGO board: channels 3 and 4 are totally nonfunctioning, so who knows what's going on in there. I've pulled it out, to take a gander if I can figure out how to make it suitiable for our purposes.
I'm a little mystified. Peeking inside the aLIGO demod board, I saw that the reason that two of the channels weren't working was that their power connectors weren't plugged in, so no real mystery there.
I hooked up the board at the electronics bench, and found the noise to be completely well behaved, in contrast to the measurements I made when it was in the LSC rack. I've taken it back out to the LSC rack, and given it the X beatnote, and it seems to be performing pretty well.
I switched between the aLIGO demod board and beatbox during the same lock / beat. The LSC board performs margnially better from 3-100 Hz. The high frequency noise comes from the green PDH loop (coherence is near one above a few hundred Hz), so we don't expect any difference there.
To me, the beatbox noise looks like there is a broad feature that is roughly the same level as the real cavity motion in the 10-100 Hz range. So, I think we should use the aLIGO board afterall, presuming the noise doesn't shoot back up when I remount it in the rack...
The ALS noise is getting low enough where our normal approach of measuring ALS sensing noise by simply taking the PSD of the signal when the arm is PDH locked is not quite valid anymore, as it is sensing the real cavity fluctuations. Doing a frequency domain coherent subtration of the PSDs suggests a sensing noise RMS of ~150Hz for ALSX.
When the X arm is locked on ALS, POX sees about 250Hz RMS out of loop noise, which isn't the greatest; however, I used to be happy with 500Hz. By eye, sweeping through IR resonance is smoother. The real test is to get the Y arm ALS running, and swing it through PRFPMI resonance...
Fair warning, the LSC rack area is not so tidy right now, the demod board is resting on a stool (but not in the way of walking down the arm). I'll clean this up tomorrow.
Jessica and I took 45 mins (GPS times from 1122099200 to 1122101950) worth of data from the following channels:
C1:IOO-MC_L_DQ (mode cleaner)
C1:LSC-XARM_IN1_DQ (X arm length)
C1:LSC-YARM_IN1_DQ (Y arm length)
and for the STS, GUR1, and GUR2 seismometer signals.
The PSD for MCL and the arm length signals is shown below,
I looked at the coherence between the arm length and each of the three seismometers, plot overload incoming below,
For the coherence between STS and XARM and YARM,
Finally for GUR2,
A few remarks:
1) From the coherence plots, we can see that the arm length signals are coherent with the seismometer signals the most from 0.5 - 50 Hz. This is most evident in the coherence with STS. I think subtraction will be most useful in this range. This agrees with what we see in the PSD of the arm length signals, the magnitude of the PSD starts increasing from 1 Hz and reaches a maximum at about 30 Hz. This is indicative of which frequencies most of the noise is present.
2) Eric did not remember which of GUR1 and GUR2 corresponded to the ends of XARM and YARM. So, I went to the end of XARM, and jumped for a couple seconds to disturb whatever Gurald was in there. Using dataviewer I determined it was GUR1. Anyways, my point is, why is GUR1 less coherent with both arms and not just XARM? Since it is at the end of XARM, I was expecting GUR1 to be more coherent with XARM. Is it because, though different arms, the PSD's of both arms are roughly the same?
3) Similarly, GUR2 shows about the same levels of coherence for both arms, but it is more coherent. Is GUR2 noisier because of its location?
Ignacio and I downloaded data from the STS, GUR1, and GUR2 seismometers and from the mode cleaner and the x and y arms. The PSDs along the arms have the most noise at frequencies greater than 1 Hz, so we should focus on filtering in that area. The noise levels start dropping at around 30 Hz, but are still much higher than is seen at frequencies below 1 Hz. However, because the spectra is so low at frequencies below that, Wiener filtering alone injected a significant amount of noise into those frequencies and did not do much to reduce the noise at higher frequencies. Pre-filtering will be required, and I have started implementing a pre-filter, but with no improvements yet. So far, I have tried making a bandpass filter, but a highpass filter may be ideal in this case because so much of the noise is above 1 Hz.
The refreshed ALS didn't look so bad today (elog forthcoming), so I decided to give PRFPMI locking a shot tonight. I was able to hold the PRMI while swinging through resonsance, but transitions to RF signals failed. Demod angles / whitening gains/ etc. etc. all need to be rechecked
Some little things here and there that got cleaned up...
ALS is not currently limited by the demod board or whitening electronics.
The noise budget in the green locking paper shows the main noise sources to be these two, plus the residual fluctuations of the green PDH loop.
So, one next step is AUX PDH noise budget.
However, I wonder how much of the low frequency noise can be explained by instability of the beat alignement on the PSL table, and how this might be quantified.
Yesterday, I put together a few measurements to asses whether the new demod board has moved us in the right direction. Specifically I measured the output of the phase tracker in the following states, adjusting the phase tracker gain to maintain a ~2kHz UGH (but no boost on):
Results: The beat frequency spectrum is above the measured demod board and whitening chassis/ADC noise at all frequencies. It's a little close at 10Hz.
One nice feature is that the beat spectra are far more similar to each other than they used to be. RMS noise is in the 300-400Hz range, which isn't mindblowing, but not terrible. On the order of 50 pm for each arm. Most of this comes from below 10Hz.
Another thing to note is that, when we switch in the 50m cables, we should win a fair bit of Hz/V gain and push down these noises futher. (We're currently using 30m cables.)
By looking at some coherences, we can attribute some of the noise when IR locked to both colors of PDH loops.
Specifically, the coherence with the Green PDH error implicates the residual frequency noise of the AUX laser above a few hundred Hz, whereas the feature from 20-50Hz is probably real cavity motion, not ALS sensing noise. Some of the 1-3Hz noise is from real suspension/stack resonances too.
If it turns out that we do want to push the demod board noise down further, we could think about increasing the RF amplification. Driving the board harder translates directly to better noise performance. The 60Hz harmonics aren't so exciting, but not the end of the world.
Data files are attached, if you're in to that sort of thing.
I followed my hunch, and the truth comes out.
I recalled that the aLIGO demod board has a handy DB9 output on the back panel for the detected power at the RF and LO inputs. I hooked this up into the BEATY ADC channels while checking the ALSX spectrum in IR lock.
This is assuredly the limiting factor in our ALS sensitivity.
Note: I'm calling the fluctuations of the beatnote amplitude "RF Amplitude RIN," to put things in reasonble units. I haven't looked up the board's conversion of dBm to V, but the LO should be around 0dBm in this measurement.
The coherence between the phase tracker output and the LO amplitude is significant over a broad range, mostly dipping where real cavity motion peeks up into the spectrum.
Also, the feature from 10-100Hz in the RIN spectrum is one I've often seen directly in ALS spectra when beatnote alignement is bad or the beatnote frequency is high, convincing me further that this is what's to blame.
So: what do we do? Is there anything we can do to make the green alignment more stable?
Notes from tonight's work:
I've explored the beatnote fluctuations a bit further.
First, I realized that we already had a channel than functions much like an RF level monitor: the phase tracker Q output. I verified that indeed, the Q signal agrees with the RF monitor signals from the demod board within the phase tracker bandwidth. This simplifies things a little.
I also found that the Y beat suffers a fair bit less from these effects; which isn't too surprising given our experience with the alignment stability.
One possible caveat to my earlier conclusions is that the beatnote amplitude could be fluctuating due to real RIN of the green light transmitted through the cavity. In fact, this effect is indeed present, but can't explain all of the coherence. If it did, we would expect the DC green PDs (ALS-TR[X/Y]) to show the same coherence profile as the RF monitors, which they don't.
The next thing I was interested was whether the noise level predicted via coherence was realistic.
To this end, I implemented a least-squares subtraction of the RF level signal from the phase tracker output. I included a quadratic term of the RF power, but this turned out to be insiginficant.
Indeed, using the right gain, it is possible to subtract some noise, reproducing nearly the same spectrum as the coherence based estimate. The discrepency at 1Hz is possible from 1Hz cavity RIN, as suggested by the presence of some coherence with TRX.
However, this is actually kind of weird. In reality, I would've expected the coupling of RF level fluctuations to be more like a bilinear coupling; changing the gain of the mixer, rather than directly introducing a linearly added noise component. Maybe I just discovered the linear part, and the bilinear coupling is the left over low frequency noise... I need to think this over a little more.
Jessica will soon ELOG about some measurements suggesting that the conductive connector-ized ALS delay line enclosure is the way to go, when considering crosstalk between the delay lines. It is currently mounted and hooked up on the LSC rack, though I need to make a bunch of new SMA cables now that I think a semi-permanent arrangement has been reached.
I did a rough re-calibration of the phase tracker output, since the increased cable delay changes the degree/Hz gain. This was done by fitting a line to a slow sawtooth FM of the SRS DS345's (1Hz rate, 10kHz deviation, 30MHz carrier). This resulted in the following calibration updates
Again, this is a rough calibration. Nevertheless, it is not so surprising we don't get the 50m/30m = 4.4dB increase we would expect just from the lengths; the (I presume) increased cable loss matters. Also, the loss' frequency dependance is an additional reason that the phase tracker calibration is not constant over all frequencies.
I took spectra with the arms in IR lock, but didn't see any real improvement beyond a possible dip in the floor from 100-200Hz. This doesn't surprise me too much, however, since I don't believe that we are currently dominated by electronic noises that this gain increase would help overcome.
Last week, Koji mentioned the ALS phase noise added due to the post-cavity table motion the arm-transmitted green beams experience before hitting the beat PD. I should estimate the size of this effect for our situation.
The mode cleaner FF static filtering is by no means done. More work has to be done in order to succefuly implement it, by the means of fine tuning the IIR fit and finding better MISO Wiener filters.
I have begun to look at implementing FF to the YARM cavity for several reasons.
1) Even if the mode cleaner FF is set up as best as we can, there will still be seismic noise coupling into the arm cavities.
2) YARM is in the way of the beam path. When locking the IFO, one locks YARM first, then XARM. This means that it makes sense to look at YARM FF first rather than XARM.
3) XARM FF can't be done now since GUR2 is sketchy.
I'm planning on using this eLOG entry to document my Journey and Adventures (Chapter 2: YARM) to the OPTIMAL land of zero-seismic-noise (ZSN) at the 40m telescope.
I took data from 1123495750 to 1123498750 GPS time (Aug 13 at 3AM, 50 mins of data) for C1:LSC-YARM_OUT_DQ, and all T240 and GUR1 channels.
Here is the PSD of the YARM_OUT, showing the data that I will use to train the FIR filter:
Coherence plots for YARM and all channels of T240 and GUR1 sesimometers are shown below. This will help determine what regions to preweight the best before computing FIR filter. They also show how GUR1 is back to work compared to those of elog:11457.
Plotte below are the resultant subtractions for YARM using different witness configurations,
The best subtraction happens with all the channels of both the GUR1 and T240 seismometers, but one gets just as good subtraction without using the z channels as witnesses.
Also, why is the T240 seismometer better at subtracting noise for YARM compared to what GUR1 alone can acomplish? Using only the X and Y channels for the T240 gave the third best subtraction(purple trace).
My plan for now is as follows:
1) Measure the transfer function from the ETMY actuator to the YARM control signal
2) Collect data for YARM when FF for MCL is on in order to see what kind of subtractions can be done.
In my last post I calculated the different subtractions (offline) that could be done to YARM alone just to get a sense of what seismometers were better witnesses for the Wiener filter calculation.
In this eLOG I show what subtractions can be done when the MCL has FF on (as well as Eric's PRC FF), with the SISO filter described on elog:11496.
The plot below shows what can be done offline,
What is great about this results is that the T240-X and T240-Y channels are plenty enough to mitigate any remaining YARM seismic noise but also to get rid of that nasty peak at 55 Hz induced by the MCL FF filter.
The caviat, I haven't measured the TF for the ETMY actuator to YARM control signal. I need to do this and recompute the FIR filters with the prefiltered witnesses in order to move on to the IIR converions and online FF!
Now that the updated ALS is stable, and the PRC angular FF is revived, I've been working on relocking PRFPMI. While the RMS arm fluctuations are surely smaller than they used to be, there is no noticible difference to the ears when buzzing around resonance, but this doesn't really mean much.
Frustratingly, I am not able to stably blend in any RF CARM error signal into the slow length control path (i.e. CARM_B). Bringing AS55 Q into DARM with the 20:0 integrator is working fine, but we really need to supress CARM to get anywhere. I'm not sure why this isn't working; poking around into the settings that were used when we were regularly locking didn't turn up any differences as far as I could tell. Investigations continue...
Some minor changes to the locking script were made, to account for the increased ALS displacement sensitivity from the longer delay line.
Since the ALS is now in a fairly stable state, I've updated the calibrated PSD template at /users/Templates/ALS/ALS_outOfLoop_Ref.xml, and added some coherence plots for some commonly coupled quantities (beat signal amplitude, IR error signal, green PDH error signal and green transmission).
Yesterday, Rana, Jessica and I measured the Transfer function from LSC-YARM-EXC to LSC-YARM-IN1.
The plot below shows the magnitude and the phase of the measured transfer function. It also shows the normalized standard error in the estimated transfer function magnitude; the same quantity can be applied to the phase, only in this case it is interpreted as its standard deviation (not normalized). It is given by
where is the ordinary coherence function and is the number of averages used at each point of the estimate, in the case here we used 9 averages. This quantity is of interest to us in order to understand how the accuracy of transfer function measurement affects the ammount of subtraction that can be achieved online.
Since this transfer function is flat from 1-10 Hz (out of phase by 180 deg), this means that we can apply our IIR wiener filters direclty into YARM without taking into account the TF by prefiltering our witnesses with it. Of course this is not the case if we care about subtractions at frequencies higher than 10 Hz, but since we are dealing with seismic noise this is not a concern.
The coherence for this transfer function measurement is shown below,
I've had 6 5min+ locks so far; arm powers usually hit ~125 for a recycling gain of about 7; visibility is about 75%
The locking script takes a little under 4 minutes to take you from POX/POY lock to PRFPMI if you don't have to stop and adjust anything.
At Koji's suggestion, I used digital REFL11 instead of CM_SLOW, which got me to a semistable lock with some RF, at which time I could check the CM_SLOW situtation. It seemed like the whitening Binary IO switch got out of sync with the digital FM status somehow...
I've been making the neccesary changes to the carm_cm_up script. I also added a small script which uses the magnitude of the I and Q signals to set the phase tracker gain automatically based on some algebra Koji posted in an ELOG some years ago.
The RF transition seems much smoother now, most likely due to the improved PRC and ALS stability. In fact, it is possible to hold at arm powers of >100 solely on the digital servos; I don't think we were able to do this before until the AO had kicked in.
Right now I'm losing lock when trying to engage the CARM super boost. I also haven't switched the PRMI over to 1F signals yet. Would be good to hook the SR785 back up for a loop TF, but I'll stop here for tonight since our SURFs are presenting bright and early tomorrow morning.
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
More PRFPMI locks tonight. Right now, it's been locked for 22+ minutes, though with the PRMI still on 3F signals. I think the MC2/AO crossover needs some reshaping; there's a whole bunch of noise injected into CARM around 600 Hz, which is where the two paths differ by 180deg. (Addendum: broke lock at ~27 minutes, 4:16AM)
For most of this lock, sensing matrix excitations have been running for daytime analysis.
The nominal IMC loop gain / EOM crossover were making the AO path very marginal. I've adjusted the nominal settings and autolocker scripts.
There was some weird behavior of X green PDH earlier... Broadband RIN seen in ALS-TRX, coherent with the DC output of the beat PD, so really on the light. I fiddled with the end setup, and it mostly went away, though I didn't intentionally change anything. Disconcerting.
I spent some time tonight chasing down the cause of huge RIN in the X green PDH transmitted light, which I had started seeing on Monday. This was preventing robust locking, since the ALS sensing noise was ~10x worse above 50Hz, thus making the AO transition much flakier (though, impressively, not impossible!)
I went down to the X end, and found that turning the laser diode current down by 0.1A (from 2.0 to 1.9) smoothed things out completely. Unfortunately, this causes the power to drop, from GTRX of 0.45 to 0.3, but the ALSX sensitivity is unchanged, as compared with the recenent "out of loop" template.
This also seems to have changed the temperatures of the good modes, as no beat was evident at the previously good temperature. Beats were found at +5400 and +10500 counts on the slow servo offset slider; I suspect the third lies around the edge of the DAC range which is why I couldn't uncover it. In any case, I've parked it at 10500 for now, and will continue locking; nailing it down more precisely and offloading the slider offset to the laser controller will happen during daytime work...
Got to a 40 minute lock tonight. All other locks broke because of me poking something.
I redid some sensing excitations, right after carefully measuring the CARM OLG at its excitation frequency, so I can get at the open loop PD response.
I also used a MCL feedforward filter of Ignacio's which did not inject any observable noise into the CARM error signal during PRFPMI lock. He will make some elog about this.
A day late but here it is.
Eric and I turned on my SISO MCL Wiener filter elog:11535 during his PRFPMI 40min lock. We looked at the CARM_IN and CARM_OUT signals during the lock and with the MCL FF on/off. Here is the spectra:
There has been some discussion here and there of using fiber coupled IR beats for ALS. A few weeks ago, and again today with Eric G, I poked around a bit with the fiber box Manasa set up for the FOL scheme.
Somehow, the IR beatnote is ~1000 times smaller than expected, both with the Thorlabs fiber coupled PD and a fiber coupled NF 1611.
In essence, after the fiber combiner, there is on the order of hundreds of uW each of PSL and AUX X IR light. The output of the fiber from each source looks nice and gaussian. The DC output of the 1611 indicates that it is seeing the right level of light. The green beatnote exists with good SNR at twice the IR beat frequency, so we know that the IR beat isn't some junky modes beating.
For the 1611, we would expect an RF signal of ~1mW*0.9A/W*700V/A -> .6V / +8dBm. Instead we see ~2mV / -40dBm.
Incidentally, there is some 20mV / -20dBm signal at ~400kHz, presumably from the green PDH modulation at ~200k.
(The level out of the thorlabs PD is similarly tiny; it doesn't have a DC output though, so we don't know the DC power that the active surface really sees. Not that I expect it to be much different, but the NF just makes it easier to estimate.)
The only things that should be able to cause the beat to be smaller than expected from the power levels are mode matching and polarization matching. All the fibers are single mode, so mode matching should be effectively 100%. Maybe somthing fishy is happening with the polarizations, but they'd have to be really maliciously close to orthogonal to cause this level of mismatch.
Maybe we just don't understand the splitter/combiners. Mysterious.
Maybe we just don't understand the splitter/combiners.
After an email from Eric G, I think this is the case.
If you read the text at Thorlabs about Fiber-Based Polarization Beam Combiners/Splitters, it suggests that these things take input beams both aligned to their slow axes, and outputs one field along the slow, and one orthogonal to it on the fast axis. Which is exactly what we don't want for a beat.
From the AFW website about our product, the POBC-64-C-1-7-2-25dB:
port1 slow axis -> port3 slow axis
port2 slow axis -> port3 fast axis
I was thinking that the "FOSC" product line (which is called a "coupler" instead of a "splitter/combiner") was what we wanted.
Koji brought to my attention that the 90/10 splitters we already have are of this line. So, I rigged a few up to shine a hopefully beating pair of fields on the fiber coupled thorlabs PD.
I was able to get ~80uW each of PSL and AUX X light on the PD, which produced a -10dBm beatnote! Thus, I think these FOSC splitters are indeed what we want.
I then threw this IR beatnote at our ALS signal chain. The beatnote was too big to throw through our ~+27dB RF amps, so I just sent the -10dBm over to the LSC rack.
The IR beat spectrum is somwhat noisier from 10-100Hz, but, more interesting, is that the sub-4Hz noise is identical in the two beats, and very coherent. This excludes ALS noise arising from anything happening in the green beat optics on the PSL table.
Obviously, the high frequency noise is largely the same and coherent too, but also coherent with the AUX X PDH control signal, so it is understood.
Thanks to some expertly timed coffee from Ignacio, I have been able to achieve indefnite locks of the DRMI, first on a 1F/3F mix (P:REFL11, S: REFL165, M:AS55), and then purely on 3F (P:REFL33, S:REFL165, S:REFL165). MICH is currently actuated on the ITMs.
I saved a snapshot of the current settings so I don't lose my settings. I think one thing that prevented earlier recipies from working is that whitening gains may have changed, which we don't typically note down when reporting input matrix settings
My current settings for 3F locking:
+30dB whitening gain, +136 demod phase
PRCL = 9 x I - 200 counts
+24dB whitening gain, +3 demod phase
SRCL = 1 x I, MICH = 5 x Q - 1000counts
MICH: G=-0.03; Acq FM4/5; Trig 2/3/6/9
PRCL: G=-0.003; Acq FM4/5; Trig 1/2/6/9
SRCL: G=0.2; Acq FM4/5; Trig 2/3/6/9
I've injected excitations into the control filter outputs via the LSC-FFC FMS (and notched the frequencies in the control filters themselves), and noted GPS times for offline sensing analysis. (Namely the 10 minutes following 1125398900)
Handing off to pure 3F was a little finicky at first, I needed to use some pretty large offsets in the MICH_B and PRCL_B FMs. (-1000 and -200 counts respectively). Once these offsets were found, the DRMI can acquire on 3F. Alignment is pretty important, too. Acquiring is much faster when the loop gains are "too high." i.e. I see a fair amount of gain peaking at ~300Hz. Nevertheless, things are stable enough as is that I didn't feel like digging into reducing the gains to quieter values.
Single mode coupler, 2x2, 1064nm +/-20nm, 50/50 ratio, 900micron loose tube jacket, Hi1060flex fiber, 1m fiber length, FC/APC connectors
Four of these items ordered yesterday from http://afwtechnologies.com.au/sm_coupler.html
I've now made a collection of sensing matrix measurements.
In all of the plots below, the radial scale is logarithmic, each grid line is a factor of 10. The units of the radial direction are calibrated into demod board output Volts per meter. The same radial scale is used on all plots and subplots.
I did two PRMI measurements: with MICH locked and excited with either the ITMS or the BS + PRM compensation. This tells us if our PRM compensation is working; I think it is indeed ok. I though I remembered that we came up with a number for the SRM compensation, but I haven't been able to find it yet.
The CARM sensing int he PRFPMI measurement has the loop gain at the excitation frequency undone. All excitations were simultaneously notched out of all control filters, via the NotchSensMat filters.
The angular scale is set to the analog I and Q signals; the dotted lines show the digitial phase rotation angle used at the time of measurement.
Nice going. I think the LLO / LHO scheme is to acquire on 1F and then cdsutils avg to get the 3F offsets. The thinking is that that 1F signals have less intrinsic offset than the 3F signals, so we want to be use digital offsets for the 3F locks.
We looked at the DRMI noise spectrum and chose new excitation frequencies such that the lines are lower in frequency than before (~300 Hz instead of 800 Hz) and also not in some noisy region.
New filters is saved and loaded for all LSC DOFs.
POP110 and POP22 demod angles were adjusted for DRMI lock.
Last week, I never achieved a fully 1F lock, REFL165 was used for SRCL. Tonight, we created input matrix settings for pure 1F locking, and did some signal mixing to reduce the PRCL to SRCL coupling. The PRCL to MICH coupling was already low, since AS55 is fairly insensitive to PRCL.
Similarly, for the 3F signals, some signal mixing of REFL33I and REFL165Q was used to reduce the PRCL to MICH coupling. The PRCL to SRCL coupling in REFL165 isn't too bad, so no compensation was done. Interestingly, in this setting, the 3F MICH and SRCL signals agree with the 1F signals on their zero crossing, so no offsets are needed. REFL33 I does need an offset, however, to match the REFL11I PRCL zero crossing.
The DRMI acquires faster with SRCL set to 165I. Once acquired, the 1F/3F can be made smoothly, and both settings are very stable. The sensing matrix in each setting is consistent with each other. (The PRCL and SRCL lines in AS55 change, but really I shouldn't even plot them, since they're not very coherent).
For some reason, these show a sign flip relative to last week's measurements. The relative angles are consistent, though.
Next up is finding the right coefficient for the SRM in the MICH output matrix, when actuating on the BS.
Just a heads up while I'm out for a bit: the delay line is currently installed in the 55MHz modulation path.
I'll be back later, and will revert the setup.
With the adjustable delay line box installed in the 55MHz modulation path, I've measured the PRMI sensing matrix as a function of delay / relative phase between the 11MHz and 55MHz modulations. The relative frequency difference of 44MHz tells us that this should be cyclical after ~23nsec of delay, but losses in the delay cable change this; see Koji's elogs about the modulation cancellation setup for details.
TL;DR: Nothing really changes, other than REFL33 optical gain. MICH/PRCL angles remain degenerate.
The results aren't so surprising. The demod angles for the 55MHz diodes don't even change, since the same 55MHz signal is used for the modulator and demodulators, so delaying it before the split should go unnoticed. Most of these measurements were made during the same lock stretch, PRCL on REFL11 I and MICH on AS55Q.
The only signals we would expect to change much are ones that have significant contriubtions from field products influenced by both modulations. None of the 1F PDs are like this, nor is REFL165. REFL33 is the odd man out, where the +44MHz field produced as a -11MHz sideband on the +55MHz sideband beats with the +11MHz sideband (and the same with the signs flipped). I made a simulation for the 40m poster at the March 2015 LVC meeting, but I don't think it ever made it to the ELOG.
Here are the results for the 0ns and 4ns cases, as an illustration of what changes (REFL33), and what doesn't (everything else). Again, these are calibrated to Volts out of the analog demod boards per meter of DoF motion.
So, since REFL33 is the only one really changing, let's just look at it by itself:
Qualitatively, the change in magnitude looks similar to the simulation result. The demod angles fall by some roughly linear amount. The angle difference is even more stationary than predicted there, though.
Tonight we noticed that the REFL_DC signal has gone bipolar, even though the whitening gain is 0 dB and the whitening filter is requested to be OFF.
We should check out the switch operation of several ofthe LSC channels in the daytime - where is the procedure for this diagnostic posted?
While investigating the BIO situation with the LSC machine and the iscaux2 processor last night, we wondered if maybe the Anti-Aliasing filters were mistakenly disabled. But why do we need these anyway?
Our ADCs digitize at 64 kHz and there is a digital lowpass in the IOP at 5 kHz before we downsample to 16 kHz. So mainly we're trying to prevent some aliasing at the 64 kHz IOP rate. But our analog AA filter is a 8th order ELP at 7570 Hz, so its overkill.
So, I propose that we bypas the AA via hardwiring the board and implement a 10 kHz pole in the whitening board (D990694) before the whitening by turning R127, etc. into a 0.1 uF cap. Along with the 100 Ohm series resistor, this will make a pole at ~15 kHz. Probably ought to check that the input resistor is metal film. Also, if we replace C158/C159, etc. with a 0.47 nF cap, we'll get 2 poles at 35 kHz to limit the higher frequencies from saturating.
About the analog CARM control with ALS:
We're looking at using a Sigg designed remotely switchable delay line box on the currently undelayed side of the ALS DFD beat. For a beat frequency of 50MHz, one cycle is 20ns, this thing has 24ns total delay capability, so we should be able to get pretty close to a zero crossing of the analog I or Q outputs of the demod board. This can be used as IN2 for the common mode board.
Gautam is testing the functionality of the delay and switching, and should post a link to the DCC page of the schematic. Rana and Koji have been discussing the implementation of the remote switching (RCG vs. VME).
I spent some time this afternoon trying to lock the X arm in this way, but instead of at IR resonance, just wherever the I output of the DFD had a zero crossing. However, I didn't give enough thought to the loop shapes; Koji helped me think it through. Tomorrow, I'll make a little pomona box to go before the CM IN2 that will give the ALS loop shape a pole where we expect the CARM coupled cavity pole to be (~120Hz), so that the REFL11 and ALS signals have a similar shape when we're trying to transition.
The common mode board does have a filter for this kind of thing for single arm tests, but puts in a zero as well, as it expects the single arm pole, which isn't present in the ALS sensing, so maybe I'll whip up something appropriate for this, too.
1. POP110 RF amps are powered from the cross connect. But that +15V block has GND connections that are not connected to the ground.
i.e. The ground potential is given by the signal ground. (Attachment 1)
This is caused by the misuse of the DIN connector blocks. The hod side uses an isolated block assuming a fuse is inserted.
However, the ground sides also have the isolated blocks
2. One of the POP110 RF cable has a suspicious shiled. The rigidity of the cable is low, suggesting the broken shield. (Attachment 2)
Fixed! I noticed that whitening gain changes weren't having any effect on CM_SLOW. I then checked REFL_DC, where this also seemed to be the case. Since the gain is controlled via VME machine, and whitening filter switching is controlled via RCG, I figured there must be something wrong with the board. I checked all of the DC PD signals, which share a whitening filter board, and they all had the same symptoms.
I went and peeked at the board, and it turns out the backplane cable had fallen off.
I plugged it in, things look ok.
Something odd is happening with the CM board. Measuring from either input to OUT1 (the "slow output") shows a nice flat response up until many 10s of kHz.
However, when I connect my idependently confirmed 120Hz LPF to either input, the pole frequency gets moved up to ~360Hz and the DC gain falls some 10dB. This happens regardless if the input is used or not, I saw this shape at a tee on the output of the LPF when the other leg of the tee was connected to a CM board input.
This has sabotaged my high bandwidth ALS efforts. I will investigate the board's input situation tomorrow.
As it turns out, our version of the common mode board does not have high input impedence. I think this is what is messing with the lowpass.
I added photos of the PCB to our 40m DCC page about this board: D1500308, wherein you can see that we have Revision B.
On the aLIGO wiki's CommonModeServo page, one finds that high input impedence was added in Revision E. At LIGO-D040180, one finds this was implemented via an additional dual AD829 instrumentation amplifier stage before the input amplification stage that exists on our board.
Also, I find that the boosts installed are the default 40:4k, 1k:20k, 1k:20k, 500:10k pole zero pairs. Given our 30-40kHz UGF for CARM thus far, maybe we would like to lower some of these boost corner frequencies, to actually be able to use them; so far we only use the first two.
We fixed the problematic DIN connectors on 1Y2, by swapping out the 3 DIN connector blocks that were of the wrong type (see attached image for the difference between the types appropriate for "Live" and "Ground").
Before doing anything, Eric turned the Wenzel multiplier off. We have not turned this back on.
Then we turned off the power supply unit at the base of 1Y2, removed the connectors from the rail, swapped out the connectors, reinstalled them on the rail, and turned the power supply back on. After swapping these out, we verified with a multimeter that between each pair of "Live" and "Ground" blocks, there was ~15V. We could now use the third unused pair of blocks to power the delay line phase shifter box, though for the moment, it remains powered by the bench power supply.
1. POP110 RF amps are powered from the cross connect. But that +15V block has GND connections that are not connected to the ground.
i.e. The ground potential is given by the signal ground. (Attachment 1)
This is caused by the misuse of the DIN connector blocks. The hod side uses an isolated block assuming a fuse is inserted.
However, the ground sides also have the isolated blocks
1. The delay-line box is now hooked up to the cross connect +15V supply.
2. The broken RF cable was fixed.
RG405 + SMA connector rule
- Don't bend the cable at the connector.
- Always use a cap on the connector. It is a part of the impedance matching.
- Use transparent shrink tube for strain relieving and isolation. This allow us to check the condition of the shield without removing the cover.