Do you guys have timestamps for when you started/ended the test? I have been trying to take some long-term RAM data but keep getting foiled by stuff (this test, RTS upgrade, switching of RAMmon channels, etc.)
To test it, we are shaking all of the suspension biases +/-1.0 with a script.
The hysteresis test has been aborted.
All of the suspensions have accumulated unexpectedly big DC biases of about 5 from their nominal points.
The test was from: 2011-12-17 09:48 to 11:49 (UTC).
This corresponds to the period from 2011-12-17 01:48 to 3:49 (PST).
This morning I found a frequency generator connected to something on the PSL table sitting on the blue step next to the sliding doors.
Is anyone using it? Has it been forgotten there? If that's the case, can the interested person please take care of removing it?
Last night there was an activity for a calibratuon work, which I helped. I can take care of the FG.
ELOG is back up and running after last Friday's disk-crash-a-thon. SVN is still a work in progress. Jenne and I are now restarting computers and such.
Please just try rebooting the vxworks machine. I think there is a key on the card or create that will reset the device. These machines are "embeded" so they're designed to be hard reset, so don't worry, just restart the damn thing and see if that fixes the problem.
This is what I remember doing all the time when Rob was around, but with all the new computers, I forgot whether or not this was allowed for the slow computers.
Anyhow, I went down there and keyed the crate, but auxey isn't coming back. I'll give it a few more minutes and check again, but then I might go and power cycle it again. If that doesn't work, we may have a much bigger problem.
I went and keyed the crate again, and this time the computer came back. I burt restored to Nov 10th. ETMY is damping again.
+ I found that burtrestore for the ETMX DC coil forces were not functional.
=> currently ETMX's "restore" and "mislalign" buttons on the C1IFO_ALIGN screen are not working.
=> According to the error messages, something seemed wrong on c1auxex, which is a slow machine controlling the DC force.
Suresh pointed out the oddity that all of the EX, EY slow channels were showing white boxes on the medm screens on all of the workstations except Rosalba. I don't know why Rosalba seemed to be working, but whatever. I'm not 100% sure that Rosalba was even working properly....I shutdown ETMX and ETMY's watchdogs before we went to boot computers, but when I came back to the control room the 2 optics were rung up anyway. Turning back on the watchdogs, the optics immediately began to damp happily.
Since Kiwamu had trouble with the slow channels for EX, we decided to key some crates.
We keyed the c1auxey, and c1auxex crates, waited a few seconds, and then things looked okay in medm-land. I looked at the "View Backup" for ETMX, and there were no values for the DC sliders, so since the arms are both flashing right now, I did a "save", and then confirmed that I can misalign and restore the optic. However, since I didn't fully align/lock the cavity, the saved value for right now shouldn't be fully trusted. We might have to manually align the cavity using the BS.
I was trying to characterize the AM/PM response of the X end laser. I tried to measure the AM response first, as follows:
Attachment #1 shows the measured AM response. It differs qualitatively in shape from the earlier measurements reported in this elog and on the wiki below the 100kHz region.
It looks like some of the features may have shifted in frequency. The previous measurement results can be found in /users/OLD/mott/PZT/2NPRO, can you plot the two AM measurements together?
There were a number of directories in /users/OLD/mott/PZT/2NPRO, I've used the data in Innolight_AM_New. Also, I am unsure as to what their "calibration" factor is to convert the measured data into RIN, so I've just used a value of 0.8, with which I got the plot to match up as close as possible to the plot in this elog. I also redid the measurement today, given that the laser parameters have changed. The main difference was that I used an excitation amplitude of +15dBm, and an "IF Bandwidth" of 30Hz in the parameter files for making these measurements, which I chose to match the parameters Mott used. There does seem to be a shift in some of the features, but the <100kHz area seems similar to the old measurement now.
Having put the PD back in, I also took measurements of the RIN with the input to the laser PZT terminated. There is no difference with the Noise Eater On or OFF!
The PDA photodetectors are DC coupled, so you cannot use them to go directly into the analyzer. Must use the DC block so that you can reduce the input attenuation on the B channel and then lower the drive amplitude.
Good policy for TF measurements: drive as softly as you can and still measure in a reasonable amount of time, but no softer than that.
The beat setup has been made on the PSL table. The BS and the PD were setup. The beat was found at 29.42degC and 50.58degC for the PSL and AUX crystal temperatures, respectively.
We are ready for the EOM test. I have instruments stacked around the PSL table. Please leave them as they are for a while. If you need to move them, please contact with me. Thanks.
A picked-off PSL after the main modulator was used as the PSL beam. This was already introduced close to the setup thanks to the previous 3f cancellation test ELOG 11029. The AUX beam was obtained from the transmission of 90% mirror. Both paths have S polarization. The beams are combined with a S-pol 50% BS. The combined beam is detected by a new focus 1GHz PD.
The PSL crystal temp (actual) was 50.58degC. The AUX crystal temp was swept upward and the string beat was found at 50.58degC. After a bit of alignment, the beat strength was -18dBm (at 700V/A RF transimpedance of NF1611) .
Attached are final details of the phase-locked loop (PLL) implementation we'll use for slaving the AUX 700 mW NPRO laser to the PSL.
The first image is a schematic of the electronics used to create the analog loop. They are curently housed on an analyzer cart beside the PSL table. If this setup is made permanent, we will move them to a location inside the PSL table enclosure.
The second image is the measured transfer function of the closed loop. It achieves approximately 20 dB of noise suppression at low frequencies, with a UGF of 50 kHz. In this configuration, locks were observed to hold for 10s of minutes.
this doesn't make much sense to me; the phase to frequency conversion (mixer-demod to PZT ) should give us a 1/f loop as Johannes mentioned in the meeting. That doesn't agree with your loop shape.
How about give us some more details of the setup including photos and signal/power levels? And maybe measure the LB1005 TF by itself to find out what's wrong with the loop.
Below is analysis of measurements I had taken of the AUX-PSL PLL using an SR560 as the servo controller (1 Hz single-pole low-pass, gain varied 100-500). The resulting transfer function is in good agreement with that found by Gautam and Koji (#13848). The optimal gain is found to be 200, which places the UGF at 15 kHz with a 45 deg phase margin.
For now I have reverted the PLL to use the SR560 instead of the LB1005. The issue with the LB1005 is that the TTL input for remote control only "freezes" the integrator, but does not actually reset it. This is fine if the lock is disabled in a controlled way (i.e., via the medm interface). However, if the lock is lost uncontrollably, the integrator is stuck in a garbage state that prevents re-locking. The only way to reset this integrator is to manually flip a switch on the controller box (no remote reset). Rana suggests we might be able to find a workaround using a remote-controlled relay before the controller.
Per Steve's instructions, we did the following:
The AUX laser is down to 5.4 mW output power
What's worse, because we wanted those fast switching times by the AOM for ringdowns, I made the beam really small, which
When going though the labs with Koji last week I discovered a stash of modulators in the Crackle lab. Among them there's an 80 MHz AOM with compact driver that had a modulation bandwidth of 30MHz. The fall time with this one should be around 100ns, and since the arm cavities have linewidths of ~10kHz their ringdown times are a few microseconds, so that would be sufficient. I suggest we swap this or a similar one in for the current one, make the beam larger, and redo the fiber modematching. That way we may get ~3mW onto the AS table.
I think I want to use AS110 for the ringdowns, so in the next couple days I'll look into its noise to get a better idea about what power we need for the arm ringdowns.
I worked a bit on recovering the DRMI locking again tonight. I decided to shutter the AUX laser on the PSL table at least until I figured out the correct locking settings. As has become customary now, there was a cable in the AS beampath (leading from the AS55 DC monitor to nothing, through the enclosure side panel, it is visible in Attachment #3 in this elog) which I only found after 30mins of futility - please try and remove all un-necessary cables and leave the AS beampath in a usable state after working on the AS table! In the end, I got several short (~3mins) stretches in tonight, but never long enough to do the loop characterization I wanted to get in tonight, probably wrong gains in one or more of the loops. In the last 30 minutes, the IMC has been frequently losing lock, so I am quitting for now. The AUX laser remains shuttered.
To facilitate the 1um MZ frequency stabilization project, I decided that the AUX laser was a better candidate than any of the other 3 active NPROs in the lab as (i) it is already coupled into a ~60m long fiber, (ii) the PSL table has the most room available to set up the readout optics for the delayed/non-delayed beams and (iii) this way I can keep working on the IR ALS system in parallel. So we moved the end of the fiber from the AS table to the SE corner of the PSL table. None of the optics mode-matching the AUX beam to the interferometer were touched, and we do not anticipate disturbing the input coupling into the fiber either, so it should be possible to recover the AUX beam injection into the IFO relatively easily.
Anjali is going to post detailed photos, beam layout, and her proposed layout/MM solutions later today. The plan is to use free space components for everything except the fiber delay line, as we have these available readily. It is not necessarily the most low-noise option, but for a first pass, maybe this is sufficient and we can start building up a noise budget and identify possible improvements.
The AUX laser remians in STANDBY mode for now. HEPA was turned up while working at the PSL table, and remains on high while Anjali works on the layout.
Attachment #1 shows the schematic of the experimental setup for the frequency noise measurement of 1 um laser source.
AUX laser will be used as the seed source and it is already coupled to a 60 m fiber (PM980). The other end of the fiber was at the AS table and we have now removed it and placed in the PSL table.
Attachment # 2 shows the photograph of the experimental setup. The orange line shows the beam that is coupled to the delayed arm of MZI and the red dotted line shows the undelayed path.
As mentioned, AUX is already coupled to the 60 m fiber and the other end of the fiber is now moved to the PSL table. This end needs to be collimated. We are planning to take the same collimator from AS table where it was coupled into before. The position where the collimator to be installed is shown in attachment #2. Also, we need to rotate the mirror (as indicated in attachment #2) to get the delayed beam along with the undelayed beam and then to combine them. As indicated in attachment #2, we can install one more photo diode to perform balanced detection.
We need to decide on which photodetector to be used. It could be NF1801 or PDA255.
We also performed the power measurement at different locations in the beam path. The different locations at which power measurement is done is shown attachment #3
There is an AOM in the beam path that coupled to the delayed arm of MZI. The output beam after AOM was coupled to the zero-order port during this measurement. That is the input voltage to the AOM was at 0 V, which essentially says that the beam after the AOM is not deflected and it is coupled to the zero-order port. The power levels measured at different locations in this condition are as follows. A)282 mW B)276 mW C)274 mW D)274 mW E)273 mW F)278 mW G)278 mW H)261 mW I)263 mW J)260 mW K)131 mW L)128 mW M)127 mW N)130 mW
It can be seen that the power is halved from J to K. This because of a neutral density filter in the path of the beam
In this case, we measured a power of 55 mW at the output of the delayed fiber. We then adjusted the input voltage to the AOM driver to 1 V such that the output of AOM is coupled to the first order port. This reduced the power level in the zero-order port of AOM that is coupled to the delayed arm of the MZI. In this case we measured a power of 0.8 mW at the output of delayed fiber.
We must be careful about the power level that is reaching the photodetector such that it should not exceed the damage threshold of the detector.
The power measured at the output of undelayed path is 0.8 mW.
We also must place the QWP and HWP in the beam path to align the polarisation.
As discussed at the meeting earlier this week, we will use some old *MOPA* channels for interfacing with the PLL system Jon is setting up. He is going to put a sketch+photos up here shortly, but in the meantime, Koji helped me identify a channel that can be used to tune the temperature of the Lightwave NPRO crystal via front panel BNC input. It is C1:PSL-126MOPA_126CURADJ, and is configured to output between +/-10V, which is exactly what the controller can accept. The conversion factor from EPICS value to volts is currently set to 1 (i.e. EPICS value of +1 corresponds to +1V output from the DAC). With the help of the wiring diagram, we identified pins 3 and 4 on cross-connect #J7 as the differential outputs corresponding to this channel. Not sure if we need to also setup a TTL channel for servo ENABLE/DISABLE, but if so, the wiring diagram should help us identify this as well.
The cable from the DAC to the cross-connect was wrongly labelled. I fixed this now.
(Analisa, Keerthana, Sandrine)
So far we tried four different techniques to scan the AUX laser. They are,
1. Scanning the marconi frequency to sweep the central frequency of the AUX laser.
2. Sweeping the side band frequency of the AUX laser by providing RF frequency from the spectrum analyser.
3. Double demodulation technique.
4. Single demodulation technique.
Now we are taking all the scan data with the help of Single demodulation technique.
Not much progress today with the AUX cavity scans. I've determined there still are some alignment issues.
At the start of today a large AUX/PSL beat note was visible on the AS110 sensor, at a similar power as where we left off last night (-60 dBm). Proceeding from there, I attempted to reproduce Johannes' measurement of the cavity transmission resonances. I misaligned the X-arm, locked the Y-arm cavity, and scanned the AUX RF offset approximately 8 MHz in 2 kHz steps. This should have swept through two FSRs, but nothing was visible.
Further inspection revealed that none of the PSL light was making it back to through the AUX fiber to the PSL table. I take this to mean that the beam seen earlier on AS110 was the ITMY reflection, and that the AUX injection axis was no longer reaching ETMY. I also found that the AUX beam size just after the 90/10 beasmsplitter looks anomolously large. Maybe a lens was recently changed? In any case, the mode-matching looks like it is going to need to be readjusted.
We did swap a lens as discussed in elog 13968, but they both had f=100mm specified, the difference being one was AR-coated for 1064 and bi-convex, while the other one was plano-convex and had a different coating. The reason for the large beam spot was something else: The fiber wasn't sitting in the coupler properly. When reconnecting the fiber after taking it out make sure to align the key on the fiber end with the notch in the coupler before tightening. After discovering this the following was done:
Before leaving I restored the XARM alignment. SRM remains misaligned, LSC off. Alignment shouldn't change drastically over night, so I suggest when picking this work up tomorrow to directly look for the beats after phaselocking AUX and PSL
At some point we want to change the AUX injection on the AS table to interfere less with the normal interferometer path, and avoid 10/90 beamsplitters which produce a fair amount of ghosting. The plan is to replace the 99/1 BS whose reflection goes to AS110 and AS55, while the transmission goes to the AS camera, with a 90/10 BS as shown in the attachment. This results in ~10% less light on the PDs compared to the pre-AUX era. Between this BS and the AS camera there will be a second 90/10 BS that sends the AUX light into the IFO, so we end up with marginally less AUX power into the IFO and the same PSL power on the AS cam. We're short optics, so this has to wait until two new beamsplitters arrive from CVI.
[Jon, Gautam, Johannes]
Jon spent some time trying to align the AUX beam to the SRC today, I got to the game kind of late so maybe others can add more detail.
The AUX beam that is reflected by the SRM looks terribly misshapen - it is quite elongated in vertical direction. Unfortunately I didn't snap a picture of it - anybody? It seemed at first as if this could be clipping - but after confirming the alignment of the AUX beam with the PSL output beam with aligned SRM, a slow dither of the SRM just moved the ugly pattern on the AS camera with no change to its shape - so clipping is unlikely. I'm now thinking that this is just the output beam of the fiber coupler after propagating ~15 meters to the SRM and back - even though this aspheric lens triplet coupler is supposed to be super-duper. I found that if I loosen the fiber slightly and pull it back just a bit at least the spot on the AS camera becomes nice and round - so maybe the fiber just doesn't sit well in this collimator? Not sure why that would be. I checked the fiber tip with the microscope, and while there was some gunk present, the central region and the core were clear (still cleaned using the fiber cleaning kit, which got rid of the debris). Either way, before switching to a different collimator I think we should give the Guoy phase measurement a shot - after all there was plenty of RF signal present on both AS110 and the PDA10CF placed at the YEND.
Looking for rogue beams on the AS table, I started placing some beam dumps. There was one particularly strong source of stray beams - a lens that was labeled with KPX094AR.33_F100. It became apparent after alignment efforts to the IFO had moved the AUX beam signifcantly off-center on this lens. According to the label it should have an AR coating for 1064nm, however judging by the amount of reflected light, it was certainly NOT AR-coated for 1064nm. I replaced it with a bi-convex f=100mm lens with confirmed AR-behavior.
The AUX laser is currently shuttered.
Per our Wednesday meeting, some items to work on are
These telescopes will be used to mode match//couple the dumped SHG light from both PSL and AUX (Y-Arm) lasers into PM fibers for use in FOL.
Using the waist measurements I made yesterday (29/7/14) as seed waists, I used a la mode to design coupling telescopes.
These are designed to match the output mode of the fibers with collimators.
ALM files are attached in .zip file.
Once the fibers are coupled, I will continue in assembling the Y-Arm FOL setup, using fiber coupled beam combiner and photodiodes.
I will also do the same procedure for the X-Arm, access permitting.
With the Y end laser, I was able to lock the PLL with a lower actuation range (1.6MHz/V), and with the PSL in both the free-running and MCL locked configurations.
I took spectra (attached) with the same actuation range (3.2 MHz/V) for the AUX X+PSL and AUX Y+PSL combinations (PSL shutter closed) just to keep things consistent. It looks like there is hardly any difference between the two combinations - could the apparent factor of 3 worse performance of the X end laser have been due to different actuation ranges on the Marconi?
I've not managed to take a spectrum for the proposed replacement Lightwave laser on the PSL table, though with Eric's help, I've managed to find the beatnote (at a temperature of 53.0195 degrees). I had to do some minor alignment tweaking for this purpose on the PSL table - the only optics I touched were the ones in the pink beam path in attachments 1 and 2 in this elog (the setup used to make the measurement is also qualitatively similar to attachment 3 in the same elog, except for the fact that we are feeding back to the Marconi and not the laser - a detailed sketch with specific components used will be put up later). I'll try and measure the frequency noise of this laser as well over the weekend and put up some spectra.
With regards to possibly switching out the Lightwave on the PSL table for the (faulty?) Innolight at the X end, I've verified the following:
It remains to characterize the beam coming out from the Lightwave laser and do a mode matching calculation to see if we can use the same optics currently in place (with slight rearrangement) to realize a satisfactory mode-matching solution - I've obtained a beam profiler to do this from Liyuan and have the software setup, but have yet to do the beam scan - the plan is to do this on the SP table, but we've put off moving the Lightwave laser off the PSL table until we (i) establish conclusively that the X end laser is malfunctioning and (ii) check the frequency nosie of the Lightwave relative to the Aux lasers currently at the ends.
The area around the Marconi is in a little disarray at the moment with a bunch of cables, SR560s, analyzers etc - I didn't want to disconnect the measurement setup till we're done with it. I have however turned both IR beat PDs on the PSL table off, and have reconnected the Marconi output to the Frequency Generation Unit and have set the carrier back to 11.066209MHz, +13dBm.
Here are some results from measuring the PSL / AUX Y beat.
With the Y end laser, I was able to lock the PLL with a lower actuation range (1.6MHz/V), and with the PSL in both the free-running and MCL locked configurations. (In the latter, I had to do a bit of human-turning-knob servo to keep the control signal from running away). I also took a spectrum with the marconi detuned from the beat frequency, to estimate the noise from the PD+mixer+SR560.
It looks like the AUX X laser is about 3 times noisier than the Y, though the Y laser looks more like a 10^5 noise-frequency product, whereas I thought we needed 10^4.
Gautam is investigating the PSL / AUX PSL beat with Koji's setup now.
Unless this is the limit from the way you guys set up the PLL, it seems like there's no difference between the two lasers that's of any import. So then the locking problem has been something else all along - perhaps its noise in the X-PDF lock somehow? PDH box oscillations?
I'm super excited about this new frequency readback, but I'm not sure that it's reliable yet. Without touching any settings, the readback is currently saying 78.6MHz, and is changing slightly (as is the FSS Slow Temp), so something is working. However, the beatnote as measured on the spectrum analyzer is 158.2MHz. So, either the calibration or the tracking or something isn't quite finished being tuned yet.
It's going to be super awesome when we have this though!!
As Jenne pointed out, this is still not fully tuned.
For example, I found today that the frequency counter requires more power at the input >= -20dBm to measure frequency inputs < 40 MHz. Since the RFPD gives ~ -40dBm at its output, the ~20dB gain amplifier will not be enough to measure low frequencies in case the beat power at the PD drops (which is very much possible when the alignment drifts or things move around on the PSL table). So I am shopping for an RF amplifier with higher gain to replace the current one. In the meantime, I will test the PID loop for set point frequency > 40MHz.
I have also observed the frequency difference between the measured frequencies on FC and the spectrum analyzer. I am not sure where this comes from as yet.
At this point, the FC readout is only reliable enough to find a beatnote that is lost on the spectrum analyzer.
I worked around the PSL table today.
The Y+PSL output from the optical fiber module for FOL was fed to the input of the Thorlabs FPD310.
200uW of incident light on the RFPD gave an RF signal of -70dBm as measured on a spectrum analyzer.
I swapped the beam splitter along the PSL path so that the incident power on the RFPD is ~1.5mW (Maximum incident power that the PD can tolerate is ~2mW).
This RF signal generated was - 43dBm which is still small for the input to the frequency counter module.
I checked this with a function generator and found that the frequency counter requires around -25 to -30 dBm at the input.
I plan to use the Minicircuits ZFL-1000ln that is on the IOO rack but not being used (This was used for green beatnote amplification but is not required/used anymore) to amplify the RF signal to the frequency counter.
If anyone has any objections to using this amplifier for the frequency counter, let me know.
I plan to use the Minicircuits ZFL-1000ln that is on the IOO rack but not being used (This was used for green beatnote amplification but is not required/used anymore) to amplify the RF signal to the frequency counter.
If anyone has any objections to using this amplifier for the frequency counter, let me know.
The above mentioned amplifier has been used to amplify the input to the frequency counter. The frequency counter is now getting an input that it can read.
I have done an edit to the ALS medm screen and the PSL and AUX Y laser beat frequency is now readable.
Last week, Eric and I noticed that the green transmission levels at the PSL table seem much lower now than they did a month or two ago. To investigate this, I attempted to reproduce a power budget for the X endtable setup - see the attached figure (IR powers measured with calorimeter, green powers measured with Ophir power meter). A summary of my observations:
It is worth noting that two years ago, the IR power from the AUX-Y laser was ~280 mW, so we should still be getting "enough" green power for ALS?
Plan C finally worked. We have 1.454mW of AUX X light at the PSL table (2mW incident on the fiber coupler).
Attached is the layout of the telescope.
What I did:
I stuck in Lens 1 (f=200mm) and measured the beam width after the focus of the lens at several points. I fit the data and calculated the beam waist and its position after this lens.
I used the calculated waist and matched it with an appropriate lens and target (fiber coupler) distance. I calculated the maximum coupling efficiency to be 77% for Lens 2 with f=50mm and the fiber coupler placed at 20cm from the waist of Lens1. I was able to obtain 72% coupling after putting the telescope together.
I locked the arms, ran ASS and brought back GTRX to its usual optimum value of ~0.5 counts after closing. We also have the X arm beatnote on the spectrum analyzer.
There are still a couple of things to fix. The rejected beam from the beam sampler has to be dumped using a razor blade.
I'll finish up the beat / frequency noise parts of the diagnosis tomorrow later, but I've done some investigation of the AUX X laser RIN.
I placed a PDA255 at one of the rejected beams from the PBS on the downstream side of the IR faraday, making sure the power didn't saturate the PD. I measured the RIN on a SR785, and simultaneously looked at the signal on a 100MHz scope.
The RIN has a very strong dependence on the laser diode current, and no noticable dependence on the crystal temperature or the presence of the PDH modulation / temperature control cables. Here are some traces, note that "nominal" current up until recently was 2.0A.
When adjusting the diode current, a peak beings to appear in the tens of kHz, eventually noticible in the DC power trace on the scope. The point at which this occurs is not fixed.
At all times, I saw a strong intensity fluctuation at around 380-400kHz on the scope whose amplitude fluctuated a fair amount (at least 75mVrms over Vdc=6.5V, but would often be 2 or 3 times that).
I didn't look at the frequency noise while doing this, because the WiFi at the X end was too slow, I'll do more tomorrow in the daytime.
Once again, the transmitted X green beam was showing enormous intensity fluctuations (50x higher than normal). Last month, I reduced the AUX X laser current from 2.0A to 1.9A, which I thought had fixed it somehow.
However, when I sent to the end to check it out today, I found the SR560 which is there to amplify the green PDH error signal before being sent to the AA board was overloading. Not so surprising, since the error signal was similarly noisy as the transmitted light.
I turned the SR560 gain down, and, after relocking, the transmitted light was stable. I've turned the AUX X laser current back up to 2.0A, it's previous nominal value, and the green transmitted light is still stable.
I'm a little mystified that the 560 could intefere with the loop, since it is not in the feedback path. Could it be that when it is overloading, it sends garbage backwards out of the inputs? But even then, its input is not connected to the real error point, but the buffered monitor port. Could it be interfering via the power line?
Before, I had hesitated adding gain to the PDH board's monitor point for DAQ purposes, because the motivation of the port is to provide a 1:1 version of the real error signal, and I didn't want to add gain to the AA board, because we normally don't have gain in those boards, and I didn't want to surprise future people. The SR560 was meant to be temporary, but as often happens, it was forgotten. Now, I think I will add gain to the error monitor buffer stage of the PDH boards.
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...
We set up a measurement of the AUX X laser AM today. Some notes:
Attachment #1 shows a preliminary scan from tonight - we looked at the region 10kHz-10MHz, with an IF bandwidth of 100Hz, 16 averages, and 801 log-spaced frequencies. The idea was to get an idea of where some promising notches in the AM lie, and do more fine-bandwidth scans around those points. Data + code used to generate this plot in Attachment #2.
Rana points out that some of the AM could also be coming from beam jitter - so to put this hypothesis to test, we will put a lens to focus the spot more tightly onto the PD, repeat the measurement, and see if we get different results.
There were a whole bunch of little illegal things Rana spotted on the EX table which he will make a separate post about.
I am running 40 more scans with the same params for some statistics - should be done by the morning.
I borrowed the HP impedance test kit from Rich Abbott today. The purpose is to profile the impedance of the NPRO PZTs, as part of the AUX PDH servo investigations. It is presently at the X-end. I will do the test in the coming days.
Update 12:00 21 Sep: Attachment #3 shows schematically the arrangement we use for the AM measurement. A similar sketch for the proposed PM measurement strategy to follow. After lunch, Steve and I will lay out a longish BNC cable from the LSC rack to the IOO rack, from where there is already a long cable running to the X end. This is to facilitate the PM measurement.
Update 18:30 21 Sep: Attachment #4 was generated using Craig's nice plotting utility. The TF magnitude plot was converted to RIN/V by dividing by the DC voltage of the PDA 55 of ~2.3V (assumption is that there isn't significant difference between the DC gain and RF transimpedance gain of the PDA 55 in the measurement band) The right-hand columns are generated by calculating the deviation of individual measurements from the mean value. We're working on improving this utility and aesthetics - specifically use these statistics to compute coherence, this is a work in progress. Git repo details to follow.
There are only 23 measurements (I was aiming for 40) because of some network connectivity issue due to which the script stalled - this is also something to look into. But this sample already suggests that these measurement parameters give consistent results on repeated measurements above 100kHz.
TO CHECK: PDA 55 is in 0dB gain setting, at which it has a BW of 10MHz (claimed in datasheet).
Some math about relation between coherence and standard deviation of transfer function measurements:
--- relation to variance in TF magnitude. We estimate the variance using the usual variance estimator, and can then back out the coherence using this relation.
--- relation to variance in TF phase. Should give a coherence profile that is consistent with that obtained using the preceeding equation.
It remains to code all of this up into Craig's plotting utility.
Brief summary of tonights work:
Our "requirement" for the end laser is as follows: We expect to (and have in the past) achieved ALS sensitivity of 1Hz/rtHz at 100 Hz. If the end PDH loop is 1/f from 100Hz-10kHz, then we have 40dB of supression at 100Hz, meaning the free running AUX laser noise should be no more than 100Hz/rtHz at 100Hz.
So, if we expect both the PSL and AUX lasers to have this performance when free running, we would get the green curve below. We do not.
I'll post more details about the exact currents, temperatures and include calibrated plots for the >30kHz range later. Here's the OLG for kicks.
Here is some of the promised data. As mentioned, changing diode current and crystal temperature didn't have much effect on the frequency noise spectrum; but the spectrum itself does seem too high for our needs.
At each temperature, we started measuring the spectrum at 1.8A, and stepped the current up, hoping to reach 2.0 A.
At 47.5 C, we were able to scan the current from 1.8 to 2.0 A without much problem. At 49.0C, the laser mode would hop away above 1.95A. At 50.4C it would hop away above 1.85A. The spectra were not seen to change when physically disconnecting the PZT actuation BNC from the rear of the laser.
The flattening out at the upper end is likely due to the SR560 output noise. I foolishly neglected to record the output spectrum of it, but with the marconi external modulation set to 3.2MHz/V, the few Hz/rtHz above 20k translates to a signal on the order of uV/rtHz, which seems reasonable.
Data and code attached.
The next step is to compare this data with the same measurement with the PSL and the AUX laser on the PSL table (or the end Y laser). If these show a lot lower noise level, we can say 1) the x-end laser is malfunctioning and 2) the y-end and AUX laser on the PSL are well low noise.
We set out to lock a marconi to the IR fiber beat of PSL + AUX X to measure some frequency noise, and failed.
In short, the Marconi's 1.6MHz max external FM isn't enough oomph to stabilize the PLL error signal. It's actually evident on the Agilent that the beat moves around a few times more than that, which I should've noticed sooner... We could briefly "lock" the PLL for a few tenths of a second, but weren't able to get a spectrum from this.
We also tried using the digital phase tracker temperature servo for some help at ~DC; this worked to the extent that we didn't have to twiddle the Marconi carrier frequency to stay on top of the fringes as the beat wandered, but it didn't otherwise stabilize the beat enough to make a difference in locking the PLL.
I suppose one more thing to try is to lock the PSL laser itself to each AUX laser in turn via PLL, and look for different / excess noise.
The Green and IR beat electronics are a in a little bit of disarray at the moment, but it's not like anyone else is going to be using them for the time being...
The problem here is that the MC displacement noise is leading to large frequency excursions of the PSL beam. Options
Turning on the MCL path (in addition to the MCL FF we always have on) let me lock the PLL for multiple seconds, but low frequency excursions still break it in the end. I was able to briefly observe a level of ~50Hz/rtHz at 1kHz, which may or may not be real. Tomorrow we'll send the PLL control signal to MC2, which should lock it up just fine and give us time to twiddle laser diode current, measure the PLL loop shape, etc.
We learned about the auxillary laser control loop, and then went into the lab to identify the components and cables represented by our transfer functions. We connected to the SR785 inside the lab so that we can use it to insert noise next time, and measure the output in various parts of the control loop.
Here I describe efforts to cancel the AUX laser PZT mechanical resonances from ~200 kHz-400kHz. While these may not be the resonances we end up wanting to suppress, I chose this region as an exercise because it contains the most significant peaks.
The PZT transfer measurement was taken on 09/06 by myself and Anchal. The Moku:Go outputted a swept-sine (1kHz - 1MHz) I sent to the AUX laser PZT. The beat note between the AUX and frequency-doubled PSL was sent to the DFD, and the I and Q channels were routed back as input to the Moku:Go. We also took a calibration transfer function of the Moku:Go, sending output 1 to inputs 1 and 2.
Almost all of the signal was present in the I channel, so I proceeded to use the I data for fitting/next steps. After normalizing the measured frequency response by the calibration measurement (and adjusting for the calculated time delays in the loop - see ), I fit the resulting data using vectfit [Attachment 1]. I supplied the function with n_poles=16, which in reality fit for 16 complex pairs of poles. This complexity of fit was not necessary to capture the 3 prominent peaks, but would likely be needed to fit any of the more heavily-damped resonances.
I chose to invert all fitted poles between 200 kHz and 367 kHz and the corresponding fitted zeros. The result of this filter applied to the original frequency response data can be seen in Attachment 2, where the blue-shaded region contains the inverted poles/zeros. In total, 9 pairs of poles and 9 pairs of zeros were inverted.
This looks really good to me. Rather than fully invert the plant, what we would like to do is now design a filter which allow this loop to have a high UGF and a high gain below 1 kHz. Anchal and Paco probably have gain requirements for this loop in the ALS-CAL paper they are writing. The loop would have the cavity transfer function, as well as the demod electronics for the green PDH loop.
In addition to the gain requirements, we would also like to have a phase margin > 30 deg, and a gain margin of > 10 dB.
We went into 40m to identify where XARM PDH loop control elements are. We didn't touch anything, but this is to note we went in there twice at 10 AM and 11:10 AM.