I set up the spectrum analyzer to make the WFS head RF transfer function measurement (V/W) on WFS1. I placed the Jenne laser on the AP table, along with the reference PD power supply, laptop, and laser power supply. The Agilent output AM modulates the laser; the reference PD is again NewFocus 1611, with its AC output sent to Agilent's R channel and DC output sent to an oscilloscope;
I closed the PSL shutter while checking for a location to place the breadboard, and opened it while writing this. Headed back to Cryo to pick up the large incandescent bulb we'd borrowed over the summer.
I measured the RF response of the fiber-coupled NewFocus 1611, calibrating out the cable delay. The laser current was set to 20.0 mA, and the RF power going into the splitter was -10 dBm. The DC voltage was 1.87 V, and Gautam and I measured the power from the fiber at 344uW.
Something still looks very wrong -- the PD is supposed to be flat out to 1GHz, and physical units pending, need food.
The fiber-coupled PD seems to have a factor of ~1.5 difference in responsivity compared to the free-space PD. There are some differences in the two ways I made the measurement that I don't yet understand.
I measured relative responsivities of the fiber and free coupled NewFocus 1611 PDs (scaled by the Jenne AM transfer function).
I made the measurement in two ways, see attachment three. In attachment one, I show the response for separately measuring the two PDs relative to a pickoff of the source (two-port thru calibration). In attachment two I measure the relative responses directly, without picking off a reference (three-port calibration). I scaled the transfer functions by their DC voltages; both PDs have transimpedances of 700 V/A.
However, there are some clear differences in the response (overall factor of 0.5dB offset that may be explained by a miscalibrated DC level; apparent periodicity in attachment 1) that I don't yet understand.The free path of the non-fiber PD is ~5-6 inches, which accounts for the ~45 degrees of phase advance of the fiber relative to free coupled PD signal. (12.7cm / (c / 300 MHz) * 360 degrees ~ 45 degrees)
Mon Oct 7 14:51:53 2019. I closed the PSL shutter to measure the WFS head responsivity.
I made a thru calibration as in this elog, treating laser, reference PD, and WFS RF output as a three-port device. The DC current supplied to the laser is 20.0 mA in all cases. The Agilent spectrum analyzer supplies a -10 dBm excitation to Jenne laser's AM port, and A/B is measured with 20dB attenuation on each input port. Results are in /users/aaron/WFS/data/191007/. The calibration had 100 averages, all other measurements 32 averages; other parameters found in the yml file, same folder as the data.
I normalized the result by the difference between the dark and bright DC levels of each segment.
Mon Oct 7 17:29:58 2019 opened PSL shutter.
I simulated this circuit with zero, but haven't gotten the results to match the measurements above.
I installed nds2 on donatello with yum, but still can't import nds2.
I installed nds2 again, this time successfully with
conda install -c conda-forge python-nds2-client
% Eqn 41 of
% "Doppler-induced dynamics of fields in FabryĖPerot
% cavities with suspended mirrors", Malik Rakhmanov (2000).
% read in ringdown timeseries:
at = importdata('tek00000.csv');
The backup of /cvs/cds (which runs as a cron job on fb40m; see /cvs/cds/caltech/scripts/backup/000README.txt)
has been down since fb40m was rebooted on March 3.
I was unable to start it because of conflicting ssh keys in /home/controls/.ssh .
With help from Dan Kozak, we got it to work with both sets of keys
( id_rsa, which allows one to ssh between computers in our 113 network without typing a password,
and backup2PB which allows the cron job to push the backup files to the archive in Powell-Booth).
It still goes down every time one reboots fb40m, and I don't have a solution.
A simple solution is for the script to send an email whenever it can't connect via ssh keys
(requiring a restart of ssh-agent with a passphrase), but email doesn't seem to work on fb40m.
I'll see if I can get help on how to have sendmail run on fb40m.
Ever since July 22, the backup script that runs on fb40m has failed to ssh to ldas-cit.ligo.caltech.edu to back up our trend frames and /cvs/cds.
This was a new failure mode which the scripts didn't catch, so I only noticed it when fb40m was rebooted a couple of days ago.
Alex fixed the problem (RAID array was configured with the wrong IP address, conflicting with the outside world), and I modified the script ( /cvs/cds/caltech/scripts/backup/rsync.backup ) to handle the new directory structure Alex made.
Now the backup is current and the automated script should keep it so, at least until the next time fb40m is rebooted...
After the mini boot fest that Jenne did today, I checked whether that fixed the overflow issues we yesterday prevented the alignemnt of the arms.
I ran the alignment script for the arms getting 0.85 for TRX and 0.75 for TRY: low values.
After I ran the script ,C1SUSVME1 and C1SUSVME2 started having problems with the FE SYNC (counter at 16378). I rebooted those two and fix the sync problem but the transmitted powers didn't improve.
Are we still having problem due to MC misalignment?
I've now also trended the MOPA output power for the last 200 days to check a possible correlation with the FSS reflected power. See attachment.
The trend shows that the laser power has decayed but it seems that the FSS reflected power has done it even faster: 30% drop in the FSS vs 7% for the MOPA in the last 60 days (attachment n.2).
My attempt to passively measure the transfer function of the foam failed fantastically.
As it turns out, the room temperature fluctuations inside the PSL box reach the 1 mK/rHz noise floor of the AD590 (or maybe the ADC) at ~1-2 mHz. Everything at higher frequencies is noise.
So to see what the foam is doing we will have to do something smarter - we need a volunteer to disable the RC temperature servo from the EPICS screen and then cycle the PSL table lights every hour in the morning.
We'll then use our knowledge of the Laplace transform to get the TF from the step responses.
more detailed instructions needed....
In my calculation of the digital filters of the optical transfer functions the carrier light is resonant in coupled cavities and the sidebands are resonant in recycling cavities (provided that macroscopic lengths are chosen correctly which I assumed).
Carrier and SB (f2) shouldn't be resonant at the same time in the SRC-arms coupled cavity. No additional filtering of the GW signal is wanted.
The SRC macroscopic length is chosen to be = c / f2 - rather than = [ (n+1/2) c / (2*f2) ] - accordingly to that purpose.
I calculated the frequency of the double cavity pole for the 40m SRC-arm coupled cavity.
w_cc = (1 + r_srm)/(1- r_srm) * w_c
where w_c is the arm cavity pole angular frequency [w_c = w_fsr * (1-r_itm * r_etm)/sqrt(r_itm*r_etm) ]
I found the pole at about 160KHz. This number coincides with what I got earlier with my optickle model configured and tuned as I said in my previous entry. See attachments for plots of transfer functions with 0 and 10pm DARM offsets, respectively.
I think the resonance at about 20 Hz that you can see in the case with non-zero DARM offset, is due to radiation pressure. Koji suggested that I could check the hypothesis by changing either the mirrors' masses or the input power to the interferometer. When I did it frequency and qualty factor of the resonance changed, as you would expect for a radiation pressure effect.
This gave me more confidence about my optickle model of the 40m. This is quite comforting since I used that model other times in the past to calculate several things (i.e. effects of higher unwanted harmonics from the oscillator, or, recently, the power at the ports due to the SB resonating in the arms).
We started a vacuum work in this morning. And still it's going on.
Although the last night the green team replaced a steering mirror by an 80% reflector on the PLS table, the beam axis to the MC looks fine.
The MC refl beam successfully goes into the MCrefl PD, and we can see the MC flashing as usual.
We started measuring the distance of the optics inside the vacuum chamber, found the distance from MC3 to MMT1(curved mirror) is ~13cm shorter than the design.
We moved the positions of the flat mirror after the Faraday and the MMT1, but could not track the beam very well because we did not completely lock the MC.
Now we are trying to get the lock of the MC by steering the MC mirrors.
Kevin suceeded in locking it !!
Here we trended also the PMC and the MZ. The drop in the PMC happens at the same rate as the MOPA's.
That let us think that the FSS transmitteed power has gone down because of the reference cavity progressive misalignment to the laser beam.
We need to adjust that alignment sometime.
The drop in the NPRO output power (upper row, 3rd plot: Ch10 C1:PSL_126MOPA_126MON) accompained an increase of "fuzziness" in PMCTRANSPD and both coincided in time with the day we tempoarirly removed the flap from the laser chiller's chiller (July 14 2009).
The frame builder is timed from the Symmetricom GPS card now, which is getting the IRIGB timecode from the freq. distribution amplifier (from the VME GPS receiver card).
I have adjusted the GPS seconds to match the real GPS time and the DTT seems to be happy: sweeping MC2 MCL filter module produces nice plot.
Test points are working on SUS.
Excitations are working on SUS.
I am leaving the frame builder running and acquiring the data.
I started my attempt on noise budgeting of ALS by going back to how Kiwamu did it and adding as many sources as I could find up till now. This calculation is present in ALS_Noise_Budget notebook. I intend to collect data for noise sources and all future work on ALS in the ALS repo.
The noise budget runs simulink through matlab.engine inside python and remaining calculations including the pygwinc ones are done in python. Please point out any errors that I might have done here. I still need to add noise due to DFD and the ADC after it. For the residual frequency noise of AUX laser, I have currently used an upper limit of 1kHz/rt Hz at 10 Hz free-running frequency noise of an NPRO laser.
I've added 4 proposed schemes for implementing ALS in voyager. Major thing to figure out is what AUX laser would be and how we would compare the different PSL and AUX lasers to create an error signal for ALS. Everywhere below, 2um would mean wavelengths near 2 um including the proposed 2128nm. Since it is not fixed, I'm using a categorical name. Same is the case for 1um which actually would mean half of whatever 2 um carries.
I found out an error I did in copying some control model values from Kiwamu's matlab code. On fixing those, we get a considerably reduced amount of total noise. However, there was still an unstable region around the unity gain frequency because of a very small phase margin. Attachment 3 shows the noise budget, ALS open-loop transfer function, and AUX PDH open-loop transfer function with ALS disengaged. Attachment 4 is the yaml file containing all required zpk values for the control model used. Note that the noise budget shows out-of-loop residual arm length fluctuations with respect to PSL frequency. The RMS curve on this plot is integrated for the shown frequency region.
Adding two more poles at 100 Hz in the ALS digital filter seems to work in making the ALS loop stable everywhere and additionally provides a steeper roll-off after 100 Hz. Attachment 1 shows the noise budget, ALS open-loop transfer function, and AUX PDH open-loop transfer function with ALS disengaged. Attachment 2 is the yaml file containing all required zpk values for the control model used. Note that the noise budget shows out-of-loop residual arm length fluctuations with respect to PSL frequency. The RMS curve on this plot is integrated for the shown frequency region.
But is it really more stable?
For that, we'll have to take present noise source estimates but Gautum vaguely confirmed that this looked more realistic now 'shape-wise'. If I remember correctly, he mentioned that we currently can achieve 8 pm of residual rms motion in the arm cavity with respect to the PSL frequency. So we might be overestimating our loop's capability or underestimating some noise source. More feedback on this welcome and required.
The code used to calculate the transfer functions and plot them is in the repo 40m/ALS/noiseBudget
Attachment 5 here shows a block diagram for the control loop model used. Output port 'Res_Disp' is used for referring all the noise sources at the residual arm length fluctuation in the noise budget. The open-loop transfer function for ALS is calculated by -(ALS_DAC->ALS_Out1 / ALS_DAC->ALS_Out2) (removing the -1 negative feedback by putting in the negative sign.) While the AUX PDH open-loop transfer function is calculated by python controls package with simple series cascading of all the loop elements.
## Cavity Pole
This is not a reply to comments given to the last post; Still working on incorporating those suggestions.
Rana suggested looking first at what needs to be suppressed and then create a filter suited for the noise from scratch. So I discarded all earlier poles and zeros and just kept the resonant gains in the digital filter. With that, I found that all we need is three poles at 1 Hz and a gain of 8.1e5 gives the lowest RMS noise value I could get.
Now there can be some practical reasons unknown to me because of which this filter is not possible, but I just wanted to put it here as I'll add the actual noise spectra into this model now.
Yes, that loop was unstable. I started using the time domain response to check for the stability of loops now. I have been able to improve the filter slightly with more suppression below 20 Hz but still poor phase margin as before. This removes the lower frequency region bump due to seismic noise. The RMS noise improved only slightly with the bump near UGF still the main contributor to the noise.
For inclusion of real spectra, time delays and the anti-aliasing filters, I still need some more information.
Related Elog post with more details: 40m/15587
The only two PZT Phase modulation transfer function measurements I could find are 40m/15206 and 40m/12077. Both these measurements were made to find a good modulation frequency and do not go below 50 kHz. So I don't think these will help us. We'll have to do a frequency transfer function measurement at lower frequencies.
I'm still looking for ALS PDH loop measurements to verify the model. I found this 40m/15059 but it is only near the UGF. The UGF measured here though looks very similar to the model prediction. A bit older measurement in 2017 was this 40m/13238 where I assume by ALS OLTF gautum meant the green laser PDH OLTF. It had similar UGF but the model I have has more phase lag, probably because of a 31.5 kHz pole which comes at U7 through the input low pass coupling through R28, C20 and R29 (See D1400293)
If the green laser is not being used, can I go and take some of these measurements myself?
Koji recommended that I can add whitening filters to suppress ADC noise easily. I added a filter before ADC in ALS loop with 4 zeros at 1.5 Hz and 4 poles at 100 Hz and added a reversed filter in the digital filter of ALS. This did not change the performance of the loop but significantly reduced the contribution of ADC noise above 1 Hz. One can see ALS_controls.yaml for the filter description. Please let me know if this does not make sense or there is something that I have overlooked.
Now, the dominant noise source is DFD noise below 100 Hz and green laser frequency noise above that. For DFD noise, I used data dating back to Kiwamu's paper. The noise contribution from DFD in the model is lower than the latest measured ALS noise budget post on elog. I'll look further into design details and noise of DFD.
Code, data, and schematics
I entered 40m today at around 1:20 pm and left by 1:45 pm. I entered 104 through the machine shop entry. I did the following:
I entered 40m today at around 1:10 pm and left by 1:50 pm. I entered 104 through the machine shop entry. I took top view single picture photos of ITMY, BS, AP, ITMX, ETMX and ETMY tables. The latest photos will be put here on the wiki soon.
Andrew made a battery-powered 0.7 nVrtHz input-referred noise pre-amplifier for gain of 200. That might help you.
we'd need a preamp with better than 1nV/rtHz to directly measure the noise I guess.
RXA: 0.7 nV is OK if you're not interested in low noise measurements. Otherwise, we have the transformer coupled pre-amp from SRS which does 0.15 nV/rHz and the Rai Weiss FET amp which has 0.35 nV for high impedance sources.
I went to 40m yesterday at around 2:30 pm and Koji showed me how to acquire lock in different arms and for different lasers. Finally, we took a preliminary measurement of shaking the ETMX at some discrete frequencies and looking at the beatnote frequency spectrum of X-end laser's fiber-coupled IR and Main laser's IR pick-off.
We verified that we can send discrete frequency excitation signals to ETMX actuators directly and see a corresponding peak in the spectrum of beatnote frequency between fiber-coupled X-end IR laser and main laser IR pickoff.
If full interferometer had been locked, we could have used the DARM error signal output to calibrate it against this measurement.