As we move towards the goal of controlling the temperature of the seismometer near the X end, I am setting up hardware and interfaces to provide sensor readout and heater control there.

Seismometer temperature sensor configuration:

• Yesterday night I removed the foam-insulated can off the seismometer to attach AD590 temperature sensors (we are using the TO-52 package).
• I attached one to curved surface of the can and another to the seismometer inside :

• The wires go through a hole in the concrete base and are long enough to reach the top of the whole enclosure. In the image of the can, the thick red wires to the right lead to the heater (a parallel combination of two resistances, which I measured to be 12.9 ohms), while the sensor is on the left. 
• I've used duct tape to secure the sensors, and ensured there is good contact and it doesn't wobble around. The wires are also taped down so that the sensor doesn't move a lot if someone tugs them.

There was already an older sensor attached to the seismometer, presumably from 5 years ago when Kira was working on this. This one used the 8-pin SOIC package, and in our experience it was hard to achieve and maintain good contact with this variety of the sensor because of the awkward shape, so this sensor was replaced. The new sensor circuit also has pin headers for connection, unlike the old one where the sensor was permanently soldered to a board.

I added a heater circuit to the seismometer, and moved the 4 channel temperature readout circuit that I built in there as well. The heater uses the lowermost power supply available on the 1x9 rack, visible below reading 24V -

• It was turned off prior to my using it, and there is no current drawn when I'm not using it - so I think it is not connected to any other device. I mentioned how I added DIN fuses to complete the circuit in another elog.
• The heater circuit is simply a MOSFET being operated as a switch, with a 2x scaled version of the DAC signal being fed into the gate, letting current through when the DAC is at 4.6V (30000 counts).
• The resistance of the heating pads is 13 ohms. This means we can achieve a max power of roughly 44W, which is delivered to the can that weighs 15 kg.

I also interfaced the temperature sensors to the BNCs we laid earlier, enabling readout through EPICS. This let us do a step response test where I heated the can for 4 hours and let it cool after that, letting us observe the temperature profiles - 

This plot is a lot noisier than the ones we got through the RPi ADC for the puck, and we think this is due to the EPICS system being worse. The noise is initially absolutely terrible, and then suddenly improves after around 1.5 hours. We are not sure what would cause this behaviour and will have to investigate.

The code for this is available at /users/Mayank/step.py

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?

See attachments

 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).

 more detailed instructions needed....

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.

P.S.

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).

I am currently working on getting the driver reinstalled on Donatella for the sensoray. An issue keeps arising that will not allow me to run "make" successfully in the unzipped driver folder. Will continue to remedy this.

This is why there is no light showing up on the device while plugged in. The computer does see the device, but does not show its model due to the inability for it to communicate without the driver.

-Alex

I have made little progress in getting the sensoray driver installed on Donatella. I have confirmed that it is indeed the reason why none of the hardware is working. I am now working through changes on a virtual machine that is running Scientific Linux to find something that may work. If no progress is made soon, I will ensure that software for a replacement video encoder is able to be installed before requesting we order one.

I have been looking at various replacements for the sensoray, and have found that the majority of new usb video encoders don't have drivers anymore and now just work through being embedded with video-capturing software. This means that the hardware must be used with a compatible video player such as VLC or OBS. VLC can natively be run with terminal commands, and because OBS is open source, there are packages that can be downloaded to use terminal commands to control the software as well. I am not sure to what extent the usb video encoder can then be controlled with these commands, but this seems to be the easiest method so far. I will finish picking which new unit we should purchase tomorrow, and order it through JC. 

I have confirmed the ability to install the sensoray drivers on Debian 11 in a virtual machine environment. I will do testing with the sensoray device on this tomorrow and if all works, begin working on code for capturing images. I will then test this out on Donatella once Tega finishes installing Debian across all computers in the coming week or so. 

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.

Alex

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.

Higher Harmonic Generation:

• We can follow the current system of ALS with using 1.5 times PSL frequency as AUX instead of second harmonic as 1 um is strongly absorbed in Si.
• To generate 1.5 times PSL frequency, three stages would be required.
  • SHG: Second Harmonic Generation mode matched to convert 2um to 1um. If we are instead making 2 um from 1um to start with, this stage will not be required.
  • SFG: Sum Frequency Generation mode matched to sum 2um photon and 1um photon to give 0.65 um photon.
  • DPDC: Degenerate Parametric Down Conversion mode matched to convert 0.65 um to 1.3 um (which would be 1.5 times PSL frequency).
• To compare, we can either convert pick-off from PSL to AUX frequency by doing the above 3 stages (Scheme II).
• Or we can just do SHG and SFG at PSL pick-off and do another SHG at AUX end (Scheme I) to compare the AUX and PSL both converted to 0.65 um (which would be 2 times AUX and 3 times PSL frequency).
• This method would have added noise from SHG, SFG and DPDC processes along with issues to be inefficiency of conversion.

Arbitrary AUX frequency:

• We can get away with using some standard laser near 1.5 um region directly as AUX. Most probably this would be 1550 nm.
• What's left is to devise a method of comparing 1.5 um and 2um frequencies. Following are two possible ways I could think of:

Using a frequency comb:

• Good stable frequency combs covering the wavelength region from 1.5 um to 2 um are available of the shelf.
• We would beat PSL and transmitted AUX separately with the frequency comb. The two beat note frequencies would be:
  
  
• Here, m1 and m2 represent the nearest modes (comb teeth) of frequency comb to PSL and AUX respectively.
• Carrier Envelope Offset frequency () can be easily generated by using an SHG crystal in front of the Frequency comb. This step is not really required since most of the modern frequency combs now comb with inbuilt zero stabilization.
• Mixing above beatnotes with would remove from them along with any noise associated with .
• Further, a Direct Digital Synthesis IC is required to multiply the AUX side RF signal by m1/m2. This finally makes the two RF signals to be:
  
  
• Which on mixing would give desired error signal for DFD as :
  
• This method is described in Stenger et al. PRL. 88, 073601 and is useful in comparing two different optical frequencies with a frequency comb with effective cancellation of all noise due to the frequency comb itself. Only extra noise is from the DDS IC which is minimal.
• This method, however, might be an overkill and expensive. But in case (for whatever reason) we want to send in another AUX at another frequency down the 40m cavity, this method allows the same setup to be used for multiple AUX frequencies at once.

Using a Transfer Cavity:

• We can make another more easily controlled and higher finesse cavity with a PZT actuator on one of the mirrors.
• In the schematic, I have imagined it has a triangular cavity with a back end mirror driven by PZT.
• Shining PSL from one side of the transfer cavity and employing the usual PDH, we can lock the cavity to PSL.
• This lock would require to be strong and wide bandwidth. If PZT can't provide enough bandwidth, we can also put an EOM inside the cavity! (See this poster from Simon group at UChicago)
• Another laser at AUX frequency, called AUX2 would be sent from the other side of the cavity and usual PDH is employed to lock AUX2 to the transfer cavity.
• So clearly, this cavity also requires coatings and coarse length such that it is resonant with both PSL and AUX frequencies simultaneously.
• And, the FSS for AUX2 needs to be good and high bandwidth as well.
• The transmitted AUX2 from the transfer cavity now would carry stability of PSL at the frequency of AUX and can be directly beaten with transmitted AUX from the 40m cavity to generate an error signal for DFD.
• I believe this is a more doable and cheaper option. Even if we want to do a frequency comb scheme, this could be a precursor to it.

_________________________

EditTue Jul 21 17:24:09 2020: (Jamie's suggestion)

Using Mode Cleaner cavity as Transfer Cavity:

• If we coat the mode cleaner cavity mirrors appropriately, we can use it to lock AUX2 laser (mentioned above).
• This will get rid of all extra optics. The only requirement is for FSS to be good on AUX2 to transfer PSL (MC) stability to AUX frequency.
• I've added suggested schematic for this scheme at the bottom.

Setting the record straight

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.

Trying to fix the unstable 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?

• I tried to think about it from different aspects. One thing is sure that  remains greater than 1 in all of the frequency region plotted for. This is also evident in the common-mode to residual noise transfer function which shows no oscillation peaks and is a clean mirror image of the open-loop transfer function (See Attachment 1, page 2).
• Another way is to look for the phase margin. This is a little controversial way of checking stability. For clarity, the open-loop transfer function I'm plotting does not contain the '-1' feedback in it. So the bad phase value at unity gain frequency is -180 degrees (or 180 degrees) for us. I've taken the difference from the closest side and got 76.2 degrees of phase margin.
• Another way I checked was by plotting a Nyquist plot for the open-loop transfer function. It is said that if the contour does not encircle the point '-1' in the real axis, then the loop would be stable even if the where is the frequency where phase lag becomes -180 degrees at the lowest frequency. For us, is at 1 Hz because of the test mass actuator pole. But I have verified that the Nyquist contour of the open-loop transfer function does not encircle '-1' point. I have not uploaded the Nyquist plot as it is not straight forward to plot. Because of large dc gain, it covers a large region and one needs to zoom in and out to properly follow what the contour is really doing. I didn't get time to make insets for it.

Is this close to reality?

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.

Additional Info:

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.

This is not a reply to comments given to the last post; Still working on incorporating those suggestions.

Trying out a better filter from scratch

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.

Few questions:

• What anti-aliasing filters are used in ALS?
• Is the digital delay fixed to a constant upper limit or is it left to change as per filters? I have already used a 470 us delay (modeled with Pade 4th order approximation).
• I could not find a good place where channel names are listed with corresponding meaning. Where can I find them?
• Is there a channel which keeps a record of lock status? In short, how do I find the in-lock times

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.

Few questions:

• What anti-aliasing filters are used in ALS?
• Is the digital delay fixed to a constant upper limit or is it left to change as per filters? I have already used a 470 us delay (modeled with Pade 4th order approximation).
• I could not find a good place where channel names are listed with corresponding meaning. Where can I find them?
• Is there a channel which keeps record of lock status? In short, how do I find the in-lock times

Additional Info:

The code used to calculate the transfer functions and plot them is in the repo 40m/ALS/noiseBudget

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 took photos and videos of the PSL table with lights on.
• I uncovered the AP table, took photos and video, and covered it back.
• I went to the X End table and took a video without opening the enclosure.
• Apart from flipping light switches, nothing else should have changed.

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.

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.

Basic controls and measurement 101 at 40m

• I learned a few things from Koji about how to align the cavity mirrors for green laser or IR laser.
• I learned how to use ASS and how to align the green end laser to the cavity. I also found out about the window at ETMX chamber where we can directly see the cavity mode, cool stuff.
• Koji also showed me around on how to use diaggui and awggui for taking measurements with any of the channels.

Preliminary measurement for calibration scheme

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.

• I sent excitation signal at 200 Hz, 250 Hz and 270 Hz at C1:SUS-ETMX_LSC_EXC channel using awggui with an amplitude of 100 cts and gain of 2.
• I measured corresponding peaks in the beatnote spectrum using diaggui.
• Page 1 shows the ASD data for the 4 measurements taken with Hanning window and averaging of 10.
• Page 2 shows close up Spectrum data for the 4 measurements taken with flattop window and averaging of 10.
• I converted this frequency signal into displacement by using conversion factor or .

If full interferometer had been locked, we could have used the DARM error signal output to calibrate it against this measurement.

Data

I used an Acromag XT1221 in CTN to play around with different wiring and see what works.  Following are my findings:

Referenced Single Ended Source (Attachment 1):

• If the source signal is referenced single ended, i.e. the signal is only on the positive output and the negative side is tied to GND on the source side AND this GND is also shared by the power supply powering the Acromag, then no additional wiring is required.
• The GND common to the power supply and the source is not required to be Earth GND but if done so, it should be at one point only.
• RTN terminal on Acromag can be left floating or tied to IN- terminal.

Floating Single Ended Source (Attachment 2):

• If the source signal is floating single-ended i.e. the signal is only on the positive output and the negative output is a floating GND on the source, the the IN- should be connected to RTN.
• This is the case for handheld calibrators or battery powered devices.
• Note that there is no need to connect GND of power supply to the floating GND on the source.

Differential Source (Attachment 3):

• If the source is differential output i.e. the signal is on both the positive output and the negative output, then connect one of the RTN terminals on Acromag to Earth GND. It has to be Earth GND for this to work.
• Note that you can no longer tie the IN- of different signals to RTN as they are all carrying different negative output from the source.
• Earth GND at RTN gives acromag a stable voltage reference to measure against the signals coming in IN+ and IN-. And the most stable voltage reference is of course Earth GND.

Conclusion:

• We might have a mix of these three types of signals coming to a single Acromag box. In that case, we have to make sure we are not connecting the different IN- to each other (maybe through RTN) as the differential negative inputs carry signal, not a constant voltage value.
• In this case, I think it would be fine to always use differential signal wiring diagram with the RTN  connected to Earth GND.
• There's no difference in software configuration for the two types of inputs, differential or single-ended.
• For cases in which we install the acromag box inside a rack mount chasis with an associated board (example: CTN/2248), it is ok and maybe the best to use the Attachment 1 wiring diagram.

Comments and suggestions are welcome.

Related elog posts:

40m/14841    40m/15134

Edit Tue Jan 26 12:44:19 2021 :

Note that the third wiring diagram mentioned actually does not work. It is an error in judgement. See 40m/15762 for seeing what happens during this.