For the splitting, I recommend not to use a splitter.

Instead, you can use a -10 or -20 dBm bi-directional coupler. You send the -10 dBm signal to the EOM amp, and you can fill up the needed power for the LO mixer. Also the "bi" nature of the coupler means that you can check for reflected power to diagnose if you are having impedance mis-match. Since you don't have an isolation amplifier in your setup, its important to make sure that reflections from one leg don't go back into the oscillator and disturb the other leg. Or maybe your oscillator box has an isolation amplifier between the oscillator and the splitter?

UPDHv3 box (serial 17142) is bogus. While retrieving values of some of the components to plug into working zero model, saw the VGA stage is bypassed by a previously unnoticed hack. Verified this by taking TF and not seeing any changes with respect to the gain knob (shown below are zero's model TFs suggesting a tunable UGF from ~ 10 Hz to 1 kHz), so this box is not good for a standalone servo.

As suggested a few meetings ago, made a quick and dirty lock using a single SR560 and took measurement of something* CLTF (SR560 gain = 10) below. New goal is to find a decent replacement, for which decided to use RedPitaya's python API "pyRPL". Just using the GUI out of the box can also lock the cavity relatively quickly but neither method results in longer than 1 minute lock... so took one step back to polish the pdh error signal.

* Something = Use SR785 TF measurement with source on Ch1, and to B input in SR560. The SR560 in (A-B) mode, and demodulated signal connected to A. The loop was closed with the SR560 output driving the PZT, and Ch2 of SR785. Wouldn't call this CLTF...

Error signal

Upon closer inspection the error signal seems to vary quite significantly on the scope (scanning @ 2 Hz), sometimes completely flipping its sign even though it always triggers on the same side of the ramp (see attachment for video, along with some neck excersise).

This might be the same behaviour from before, whereby the demodulated signal might still be "riding" a low-freq componen which can't be compensated with the LO (Marconi's carrier resolution = 1 Hz). Using the 10 MHz external Rb reference doesn't change anything. It seems that even with the coupler, reflections may be entering the mixer... 

Adding a LP filter (BLP-1.9+) right at the mixer output solves this for good. Even using 36 MHz LO vs anything else doesn't make a difference so this explains the previous issue. Moving back to lock using stable err signal.

For reference, the LO carrier is set to 36.000 MHz, +7 dBm (so the EOM is driven with an estimated +30 dBm well below the saturation or damage threshold +40 dBm).

DOPO locked

Achieved a good lock for pretty much all of the afternoon today. The laser ran at 937 mA current, the optimal gain on SR560 was found to be 50, with a LP cutoff at 300 Hz (12 dB/oct rolldown). The 300 Hz cutoff supresses most of the nasty 8 kHz noise (and harmonics) which I can hear with enough gain. Source still to be determined.

Locking update

The plan during these past few days has been to have fast control loop of the cavity (locked to laser using PZT, which succeeded using SR560s), and slow control loop where the laser temp. actuator is fed back the integrated PZT input to follow the long term cavity drift. For that, have been messing around with the high-level (GUI) API of PyRPL, with basically no success. In fact currently the RedPitaya cannot even replace the SR560 fast controls, which probably has to do with the +- 1 Volt limits on the RP input/output.

Another issue is that any loop gain depends on the REFL power, which will be at some point slowly ramped up to cross the OPO operating threshold, and while there is a (PBS + HWP) knob on how much light is hitting the RFPD, the lock is not yet good enough to keep up with the slow human action.

First light from nonlinear conversion

WIth the cavity locked, and under ~ 220 mW of pump (right before the cavity, i.e. 1.3 Amps of current on the driver), noticed a tiny green dot coming from within the crystal oven. This is pretty great news in terms of phase matching, but not necessarily so in terms of the right parametric conversion process (understanding is that SHG is easier to attain even with single pass). See tiny green spot as caught using phone camera in the attachment.

Test long term stability of the DOPO cavity lock; The cavity remained resonant overnight (start ~ 8 PM yesterday) and lost around 11 AM today. It might be good enough to approach lock point manually using laser temp. control and then engage the fast loop. In any case, today will set up an acromag channel for this. Configured "XT1541-2um-SlowDAC" to 10.0.1.47

- Noticed that the cavity transmission peaks @ 1064 nm were much wider than originally estimated by the dopo cavity design notebook suggesting a lower Finesse. So using the PDH error signal, and knowing the EOM sidebands are at 36 MHz estimated the current DOPO cavity linewidth to be 19.5 MHz, well in excess of the target 10.4 MHz.

- Updated the crystal AR coating specs from Raicol (R < 0.3% @ 1064/2128), but more importantly, I included the absorption coefficient of KTP, alpha=0.005/cm (often quoted as < 0.01 / cm) into the roundtrip loss and the design now gives 17.97 MHz. So, given the uncertainty in the absorption coefficient of the NL crystal, and all the coatings in the experiment, this adjustment might be enough to explain this observation.

With Aidan's assistance, I borrowed

• WiDy SWIR camera (Pembroke WiDy SWIR 640U-A) from the QIL (Attachment 1)
• Heimann thermopile sensor (HTPA80x64d) with microUSB-A and ethernet cables from the Adaptive optics lab (Attachment 2)

for ~ 2 um imaging in the Crackle lab.

Drew some new mounting scheme for the DOPO cavity; main revisions with respect to the current mount are -->

• Side mounts for both mirrors (instead of vertical)
• Both mirror mounts are the same (3-axis polaris K1) so both mirrors need to be attached properly
• Improved access to align crystal using newport 9031 (6 axis displacement mount), which is crucial to make the DOPO fields co-resonant

Attachment 1 illustrates the design; shows three views of the same assembly.

Concerns: mechanical noise from side mounted mirrors ... for this, there could be a solid piece which makes a rigid connection between the two mirrors (that's why they are upside down) and perhaps between the two tall posts (so S-shaped as viewed from the top)? Still working on this.

Heimann (HTPA80x64d) thermopile array;

- First test to grab frames was done in my personal Win10 machine, with no success. Either I was unable to configure the server correctly, or the software "ArraySoft" is not supported in Win10. Upon contacting Heimann, I received instructions to update to a newer version but was warned that it's just a new GUI, nothing really changed from v1 --> v2. So didn't even bother.

- Instead, wrote a simple python-socket UDP server to catch the video stream. Most trouble happened when using temperature mode (command "K"). The client streams a bunch of zeros... My guess is that this unit does not have an internal temperature calibration, and one could in principle be uploaded but we probably don't care. Streaming in raw voltage mode (command "t") works well, as shown by the sample frame shown in Attachment 1.

- After recovering the CTN Win7 laptop from Radhika, I gave "ArraySoft" another change just to know the frames I was getting in python were not bogus. For this I pointed a 532 nm laser pointer straight to the sensor and got an image shown in Attachment 2. The key difference is the processing of the video stream. Attachment 1 is a single frame, while Attachment 2 is the average of 30 frames with no offsets present. 

- Another issue present during this task was a faulty USB connection. Sometimes moving the sensor around would interrupt the stream (power lost). I carefully removed the case and exposed the TO-39 package and surrounding electronics to inspect and search possible failures but after seeing none, I swaped the USB power cable with my portable battery charger and had a more robust operation... So I dumped the old USB cable, and will get a new one.

- Since this one was borrowed from TCS lab, I placed an order for another one which will be set up permanently in the lab. Hopefully this will be enough for the OSA.

[Paco]

- Have been investigating 316 Hz noise in the control signal for the DOPO lock. Here is a list of some things that have been ruled out, mostly electrical:

  - EOM power supply --> noise still present in DOPO transmission
  - RFPD DC out --> no funky ground loops with scope (also looking at demod signal in different channel), noise still visible in transmission
  - RFPD power supply --> noise still visible in transmission...
  - Pump laser intensity (upstream pickoff) --> not a great test because pickoff optics are also on the optical table..
  - 2 x SR560s --> No effect after bypassing
  - Marconi --> same result as with anything in the loop after RFPD demod

- Things left to rule out:
  
  - Fume hood exhaust fan ** highly suspected, my phone's own cheap-o microphone power spectrum shows peaks at 316.5 Hz (!) when near the exhaust fan
  - NPRO temp controller fan --> phone audio spectrum shows line noise (60 Hz) mostly, and also 188 Hz... need to test further independently of the fume hood...

In ruling out the 6-axis translation mount on the DOPO cavity, I removed the PPKTP crystal + oven temporarily but still saw the noise. Since the resonator was no longer stable without the crystal, I needed to bring the mirrors closer and realign the output coupler from scratch. 

Restored DOPO cavity with crystal, alignment. MM efficiency ~ 35%... still optimizable.

Another Heimann Sensor / Boston Electronics delivered to Paco.
This unit (purchased May 2020/ / Delivered Aug 5th, 2020) has a FZ-Si window on it.
We don't know how it is.

[Paco, Anchal, Ian, Yehonathan]

Today, in preparation for the optical table to come, we vented the big crackle jar using the vent valve near the gauge. We detached the roughing pump and covered the bellows and pump connections with clean aluminum foil. We then proceeded to move several instruments, including some other pumps, a compressor, a couple of power supplies, power cords, the HeNe laser, misc. material blocks, and boxes with bearings and springs into the cage. The next operation required for us to displace the table is to lift the jar from the top and carefully dismantle the Crackle experiment and store it away somewhere.

Questions: where to store mostly?

Today the DOPO v0 got disassembled to make way for the optical table swap. Most components have been stored in the white cabinet's bottom panel.

[Radhika, Paco]

Today we fired up the 1418 nm ECDL and attempted initial adjustment of the aspheric lens. The design follows D2100115 which is a copy of the 2 um ECDL so we just changed the diode, the grating flexure angle, and the aspheric + flexure assembly and we are good to go. Radhika removed the 1900 nm aspheric flexure and we mounted the new collimating assembly which uses a f=3.1 mm (NA = 0.69) lens. At the beginning we had to feed over 300 mA of current to be able to see a beam (which was still diverging) so we had to free the flexure completely and align by hand to find the nominal positioning for a collimated beam. We lost a 2-56 screw in the process, but the final assembly is still in progress. The plan to follow is:

• Finalize flexure alignment
• Insert grating
• Characterize ECDL emission

[Radhika, Paco]

We tweaked the flexure alignment until we had a nominally collimated beam (~2 mW @ 250 mA of diode current) through the output aperture in the ECDL housing. We noted that the collimated beam is off-centered on that circular aperture along the horizontal (yaw) angle. After this, Radhika installed the ECDL grating and we hooked up the fiber output onto a InGaAs PD to monitor the power output. We tweaked the alignment of the grating (mostly yaw) to try and see a change in the power output to indicate optical gain in the diode, but saw no changes. We observed a change in the PD photocurrent as a function of the diode current in the absence of the grating (no optical feedback) which is indicative of ASE. We measured this level to be ~ 140 mV at 200 mA of current; with no observed threshold. In conclusion, we still need to refine our grating alignment to provide gain on the diode and observe lasing at the nominal 1450 nm wavelength.

[Paco]

Worked for a few hours to get the aspheric properly aligned. The procedure is quite finnicky, as the four 2-56 flexure screws have too much game and the fine thread setscrew that adds tension is too constrained. Anyways, it generally goes like this:

1. With the grating out of the way, and the 2-56 screws slightly loose, move the aspheric flexure until a collimated beam (as round as possible) exits the centered round aperture in front of the SAF chip.
2. Very carefully tighten the fine threaded setscrew in place to register the aspheric alignment.
3. Check that the desired beam hasn't changed
4. Insert grating careful not to touch the aspheric flexure (again, the mechanical registration is not great!)
5. Manually rotate the grating while monitoring the power at the fiber output until non-ASE light appears. This is quite sensitive to alignment/angle and the better the mode is matched back into the fiber, the easier it is to find the right position.
6. Fix the grating flexure.
7. Slightly tweak the grating mirror knobs by hand to maximize said power (careful to avoid saturating PD)

After this, I installed a second amplified InGaAs detector, hooked up the unbalanced MZ beamsplitter output into the two PDs, adjusted the gains to equalize the output voltages and then hooked the two signals to the A and B inputs of an SR560 in "A-B" mode. The output (gain 1) was good enough to feed back in the HV PZT amplifier input modulation which allowed the MZ to lock mid-fringe. The lock is rough, as the balanced homodyne signal retains a tiny offset due to imperfect balancing... Attachment 1 shows the setup, including a typical scope trace after coarse current tuning (Ch1 and Ch2 in yellow and blue represent the photocurrents in the two MZ ports in the absence of feedback).

Indeed, scanning the nominal PZT voltage broke the lock, potentially after crossing a mode hopping region.

Tasks to be done:

• Power characterization, including RIN, emission vs current.
• Emission spectrum characterization; using the homemade spectrograph (grating + lens + camera) as the PZT is scanned
• MZ feedback loop characterization, including optimizing the balancing stage
• Self-homodyne phase noise spectrum

Next, as was suggested during yesterday's group meeting, we will transition into a self-heterodyne setup (with an AOM which I have yet to check out in the QIL).

[Radhika, Paco]

In order to transition the ECDL laser noise characterization to a heterodyne setup, we needed to test the AOM (acousto-optic modulator). We wanted to drive the AOM at 80MHz using the Marconi signal generator. Since the AOM has a max driving power of 600 mW, we determined that if we run the Marconi at max output power (13dBm), we saturate the AOM through a variable attenuator and a 5W amplifier. The detailed setup is in Attachment 1.

As we scanned the AOM RF input power, we monitored the mean of the 0th and 1st order power outputs using 2 amplified photodiodes on the scope. Attachment 2 plots the results of the scan; although we noticed the 0th order dropping, we did not see evidence of diffraction in the 1st order. Our suspected theory is that the lost power from the 0th order is due to thermally-driven attenuation inside the AOM (we do not know what is inside the AOM, so this is purely speculative). The next thing we want to try is to add a DC power level to the AOM RF input, but we will double check with Aidan.

[radhika, paco]

We changed the setup to use a low power amplifier rather than the 5W amp from last time. The updated schematic is in Attachment 2. This is in part because 5W is an overkill to drive a fiber AOM which is known to saturate at 0.6 mW of RF input, but also because working with lower power active elements is easier and considerably safer. We dropped the 5W amp. in Rana's office last Friday, and got a ZHL-3A-sma. This little guy gives a max power output of 29.5 dBm (~890 mW) which should be more than enough while using the Marconi as our source (max output +13 dBm).

We hooked the amplifier to the load (AOM) without any couplers or attenuators in between, powered it with +24 VDC and quickly repeated a scan of the source power level while to see any sign of diffraction in the PDs. The result is in Attachment 2. We were a little bit disappointed that there appeared to be no diffraction, so next we tried scanning the RF frequency (it was nominally at 80 MHz) around and we finally succeeded in seeing some diffraction at 95 MHz! Paco thinks the internal fiber coupling made for the design wavelength (2004 nm) is suboptimal at 80 MHz and ~1.4 um wavelength. Therefore, to couple the 1st order back into the fiber, we need to shift the RF frequency to restore the diffraction angle at the cost of potentially not driving the optimal efficiency. An interesting observation made at the same time we saw 1st order light was that the power seemed to drift very slowly (-1%/min), which may have to do with some thermal drift inside the crystal... Our plan is to make a complete characterization of the diffraction efficiency at 1.4 um, and also investigate the slow intensity drifts as a function of RF input. The goal is not so much to understand and fix this last one, but to be able to operate the setup at a point where things are stable for a low frequency, frequency noise measurement.

[paco, nina]

We started rebuilding the DOPO in the lab even though the new optical table hasn't arrived. For this reason, we are using a 1 ft x 3 ft x 0.5 in anodized aluminum breadboard which we can then attach handles to move the setup. This also makes the prototype's footprint smaller. The first thing we did as usual was settle on a beam height. The beam height used before was ~ 3in (~ 75mm), and since the EOM, Faraday Isolator, and RFPD are nominally at that height from the breadboard, the only thing we had to fix was the pump laser head. The bare height is 55 mm, so we stacked two 9 mm thorlabs bases together, bolted them down to the breadboard and then mounted the NPRO laser head on the top. Finally, using a level we secured it to the breadboard using the three points and long 1/4-20 screws while being careful as we didn't want to flex the head too much.

Next up is aligning the laser to the EOM and Faraday Isolator. For this, we will use the beam profiles measured late last year. Another task ahead is to implement the new mount for the cavity.

[Paco, Radhika]

When previously trying to characterize the AOM, we had noticed no 1st order diffraction when operating at 80 MHz, but significant diffraction at 95 MHz. This motivated us to take measurements while sweeping across both RF drive frequency and Marconi drive power. For frequency, we swept from 80-120 MHz in steps of 1 MHz. For power, we swept across [3, 0, -3] dBm (3 dBm is max power before saturating AOM). We took our measurements of 0th and 1st order signal using an oscilloscope.

Contour plots of the 0th and 1st order signals can be seen in Attachments 1 and 2, respectively. Peak 1st order diffraction seems to occur at ~106 MHz. Using this AOM for a beat note measurement, the frequency difference would be greater than intended, which could lead to a weaker beat note signal. 

*Bonus: Today we moved the ECDL setup off the cryostat table and onto the other table. These measurements were taken after the move. 

Should measure the S-matrix using a bi-directional coupler.

[Paco, Radhika]

Today we tried to pick up from [1920] by repeating the sweep measurements across RF frequency, at 3 dBm (max power). We noticed that the 0th order signal would dip around the expected value, consistent with the plot in [1920]. However, there was no signal from the 1st order. Clearly diffraction was occurring as seen by the dip in 0th order, but nothing was coming out of the 1st order port. We spent some time debugging by swapping the photodetector inputs / playing with the PD gains / performing power cycles, but got no insight into the issue. 

We suspected the 1st order fiber coming out of the AOM might be damaged, since it loops around fairly tightly. After giving it more slack, we still saw no signal. We wanted to test the fiber, so we took an unused output of the 50-50 beamsplitter and fed it into the 1st order port, effectively running the AOM in reverse. We hooked up the input and 0th order ports to the photodiodes and did not observe any signal. From here we were more convinced that the 1st order fiber may have seen some damage. 

For next steps, we can still use the existing fiber setup to take measurements of relative intensity noise (RIN), using the 0th order output of the AOM. I plan to do this in the next few days. Meanwhile, Paco is looking into ordering parts for a free space setup. We found a free-space AOM at 1064nm that seems promising, and we will work to transition the setup accordingly. 

I took a relative intensity noise (RIN) measurement of the ECDL, by feeding the 0th order output of the AOM to the SR785. The RF power driving the AOM was set to 0 dBm. The RIN at 1 Hz is about 3x10^-5, which is consistent with informal measurements we took on 08/13. From my understanding this noise looks pretty low, which is good. I will consult with Paco and add more discussion or conclusions, if any.

[Paco,  Radhika]

Uninstalled the fiber AOM and temporarily removed the third fiber 2x2 port beamsplitter. We are now using this free-space AOM. Then, I managed to launch one of the outputs of the second fiber beamsplitter into free space using a F220APC-1550 fixed collimator. The beam clears the  AOM aperture nicely and lands on the other side.

This AOM operates at a RF frequency of 35 MHz, so we set up a sweep on the Marconi to cover a window of 35 MHz +- 15 MHz. Using an IR detector card, we looked for evidence of 1st-order diffraction (from the setup geometry, the 1st order beam should have been visibly discernable). We first scanned the AOM across yaw but did not notice diffraction. Then, Paco lowered the height of the fixed collimator and we repeated scanning across yaw. We eventually saw the beam "jump" - diffraction! We adjusted yaw until we recovered both 0th and 1st order beams, at 50/50 intensity.

In summary, the free-space AOM works and we have managed to see 1st order diffraction. Next steps will be to quantitatively measure this diffraction while sweeping across RF frequency and power.

[Paco, Radhika]

We had previously noticed that the ECDL laser power seemed weaker compared to when we originally set it up and tested it. Today Paco opened it up and tweaked the grating inside to obtain a max power of 3 mW. This way, we could better resolve the 0th and 1st order beams coming out of the AOM.

Since we don't yet have a lens to send the collimated 1st-order beam to fiber, we connected a power meter to detect the beam and hooked it up to the oscilloscope. We noted peak diffraction at around 38.5 MHz (rough estimate). Using the inverse relationship between laser wavelength and the RF frequency , and the fact that the AOM is designed to operate at 1550 nm at 35 MHz, we calculated that the ECDL wavelength should be ~1409 nm. Of course this is a rough estimation, but it is a quick validation that we are indeed operating near 1418 nm. 

[Paco, Radhika]

Last Friday we received a new lens to direct the AOM 1st-order beam from free space into a fiber cable. We mounted the lens and connected a fiber cable into the photodiode, and tried to align the lens and see a jump in the oscilloscope. We were not able to do so and wrapped up for the day.

Today we continued aligning the lens with the fine adjustment on the mount, and eventually saw signal on the scope! Hooray, done with free space. We then prepared for eventually taking a heterodyne beat note measurement and hooked up the appropriate inputs/outputs to the beamsplitters. We added in the 50-50 beamsplitter that takes in the 1st order diffracted beam along with the beam from the delay line as inputs. We passed one of the outputs to the photodiode and had to retweak the freespace-to-fiber lens until we recovered signal on the scope, and we saw the beatnote signal.

Next, while Paco is out of town I will continue to work towards making a frequency noise measurement. We made a roadmap today:

I will demodulate the beat note using a mixer and a 35 MHz LO sourced from the Marconi. The result will be a 2f cosine term, along with a much lower frequency term which encloses the frequency noise information. This will be passed through a low-pass filter to get rid of the first high-frequency term. The remaining time-domain signal will be passed to the SR785 to obtain a spectra of the frequency noise. Calibration will need to be performed to obtain the right units for the spectra, Hz²/Hz (or Hz/rtHz). 

Attachment 1 is a diagram of the current setup for measuring ECDL frequency noise. Since the last update, I have fed the beat note signal to a mixer, using a 35 MHz LO sourced from the Marconi. The resulting demodulated signal is passed to a low-pass filter, removing the 2f sinusoidal term (any trace of the frequency difference) and leaving behind a low-frequency term containing frequency noise information of the original beam (accumulated over the length of delay line).

I took spectra of the resulting signal using the SR785 (Attachment 2). Note that these units are still in V/rtHz, since the signal has not been calibrated to the appropriate units for frequency noise, Hz/rtHz. Finding the calibration term will involve study of delay line frequency discrimination. 

[Paco, Radhika]

Beatnote recovery

Restarted ECDL characterization last Friday. After some lab cleanup, and beatnote amplitude optimization we borrowed Moku Lab from Cryo lab to fast-track phase noise measurements. Attachment #1 shows a sketch of our delayed self-heterodyne interferometer. The Marconi 2023A feeds +7 dBm to a  ZHA-3A amplfier which shifts the frequency of the laser in one of the arms using a free space AOM. The first order is coupled back into a fiber beamsplitter to interfere with a 10 m delay line beam.

Improved beatnote detection

The 38.5 MHz beatnote was barely detectable before when using PDA20CS2 because at unity (lowest) gain stage, the bandwidth was only 11 MHz... We instead switched to an FPD310-FC-NIR type which has a more adequate high-frequency response. Attachment #2 shows the beatnote power spectrum taken with Moku Lab spectrum analyzer. The two vertical lines indicate (1) the heterodyne beatnote frequency and (2) the "free spectral range" indicating the actual delay in the MZ arms, which is calibrated to  = 9.73 m (using 1.46 for n, the fused silica fiber index).

Phase meter and freq noise calibration

We then tried using the phase meter application on the Moku. The internal PLL automatically detected the 38.499 MHz center frequency and produced an unwrapped RF phase timeseries (e.g. shown in Attachment #3). The MZ interferometer gives an AC signal

oscillating at , i.e. the angular beatnote frequency. The delay (calibrated above) characterizes the response of the MZ relating the RF phase noise spectrum to the optical phase noise spectrum. The RF phase obtained through the phase meter has a fourier transform

So the optical phase spectral density is related to the rf phase spectral density by a transfer function   Then, the RF & optical phase power spectral densities are related by   or

Then, because the instantaneous laser frequency is ,  in fourier domain  the frequency and phase PSDs are related by the magnitude square of this transfer function like

Following this prescription, we compute an estimate for the frequency noise ASD (square root of the PSD) shown in Attachment #4. The frequency noise estimated by this method has several contributions and *does not* necessarily represent the free-running ECDL frequency noise.

Next steps

• Noise budgeting (experiment)
• Control loop (open/closed) models

Following Koji's request, I took some time to clear the area surrounding the crackle chamber so it can be migrated to the former TCS lab.

I moved the DOPO setup which was sitting on a breadboard for easy transportation (Attachment #1) and placed into the other table in the lab. Attachments #2-3 shows the cleared area. Several instruments from the DOPO experiment still remain around the other side of the crackle chamber, if they need to be relocated I can move them as well.

See http://nodus.ligo.caltech.edu:8080/Mariner/121

How to move the large engine hoist through the narrow door

See http://nodus.ligo.caltech.edu:8080/Mariner/122

I returned the Gooch & Housego R26035-2-1.55-LTD AOM (SN 216939) from DOPO table

I assembled the second ECDL with a 19XX SAF gain chip (all pins remain disconnected), and a 14XX nm angle facet plate with a grating.

Attachments #1-4 show the final state of the housing along with the thorlabs current and temperature drivers. The remaining spare components (including screws, angle facet plates for the gratings and two SAF gain chips, one for each wavelength) are stored in a single box in the right top cabinet on the north end.

I attempted a matlab simulation for closing the loops on the triple suspensions. The system (hsts) was imported from https://git.ligo.org/jenne-driggers/SUS.
The active damping filters were read for the PR3 optic at LLO. 
There were a few glitches which were accounted for. 

- The complex pairs of poles and zeros differed ever so slightly in their conjugate parts. An example is given in Attachment 3. The real and imaginary values were rounded off to the 10th place
- The closed loop system was unstable. Brett suggested accounting for the electronic gains which were obtained from https://svn.ligo.caltech.edu/svn/sus/trunk/HSTS/L1/MC1/SAGM1/Results/2012-05-08_1700_L1SUSMC1_M1_ALL_TFs.pdf. A factor of 34.1 was accounted for all DOFs.
- The matlab version uses connect for closing the loops. Even after accounting for the gain, there were a few RHP poles (smaller now). 
- A simulink version for the system was created which essentially did the function of "connect" in matlab and it was simulated. This closed loop system was stable. A comparison of the Bode plots (m1->drive->DOF -------> m1->disp->DOF) is given in Attachment 1.
(forgot to wrap the phase for the Bode plots, will change to the corrected version tomorrow)
- The step responses fo the simulink plant for (m1->drive->DOF -------> m1->disp->DOF) were generated (Attachment 2).

Next Steps:

- Need to check for the inconsistencies between the two simulation methods
- Can proceed to add ports for noise inputs
- Need to verify the current electronic gain calibrations at LLO for the optic (if available)
 

Closer look at the damping siutation.
tldr: Corrected electronics gain, Signs of filters flipped, AND THE CORRECT MODEL (HLTS) WAS USED.

- I looked around the SVN to find the recent electronics gain. To begin with, the gains applied previous to the simulation were incorrect (MC1, 2012, e_k = 34.1). This was corrected to (PR3, 2023, e_k = 1.5404): https://svn.ligo.caltech.edu/svn/sus/trunk/HLTS/L1/PR3/SAGM1/Results/2023_07_20_1100_L1SUSPR3_M1_ALL_TFs.pdf

- With the new gain, the system was unstable. I wanted to check the closed loop behavior of each individual filter to see where the instabilty was rising from. I checked the matlab and simulink simulations for each filter turned on individually and flipping signs of feedback sequentially.

(0 = unstable, 1 = stable)

While writing this elog, I realized that PR3 was a large suspension and I needed to use the HLTS model instead of the HSTS model. I updated the model and poof, the closed loop system is stable. Matlab and simulink agree with each other now so I'll proceed with using Matlab now.

 (m1->drive->DOF -------> m1->disp->DOF) 
- Bode Plots: Attachment 1
- Step Responses: Attachment 2

Jeff Kissel: 

In short — above 10 Hz, the number that’s typically tossed around verbally for displacement noise floor is 5e-11 [m/rtHz].

Using this (with the assumption that it is white), I proceeded to estimate the displacements in M3 DOFs due to the sensor noise in M1 DOFs. The OSEM configuration/dimensions were obtained from https://dcc.ligo.org/DocDB/0002/D070447/002/D070447-v2_HLTS_OSEMLeverArmMeasurements.pdf

The noise in each DOF was estimated by taking the quadrature sum of the contribution of each sensor used for the DOF measurement (eg, for L, since there are 2 sensors measuring the same value, it would be  . 
I looked at the OSEM2EUL matrix for PR3 to get the change of basis. The matrix elements for the angular basis agrees with the small angle approximations using geometry to obtain the magnitudes (Attachment 1).
Attachment 2 has the individual as well as the combined ASD for the estimate Range [0.5 - 100 Hz]

Pages 1,2 are the positional and rotational ASD of m3 displacements respectively taking the quadrature sum of the sensor noise contribution to each DOF in m1.
Figures 4-9 are arragned showing the contribution of the individual DOFs of m1 to displacements of m3.
Noticable sensor noise in m1_disp were observed for the following:

m3_L ------> m1_L, m1_P
m3_T ------> m1_T, m1_R
m3_V ------> m1_V
m3_Y ------> m1_Y
m3_P ------> m1_L, m1_P
m3_R ------> m1_T, m1_R

I made some changes to the plots
tldr:
 - collected the linear and angular DOFs sensor noise contributions from M1 in a single plot
- added some meaningful titles and legends
- changed the limits of the plots

The plots were zoomed in with the following limits.
- x-limits [0.5 - 20Hz]
- y-limits [1e-18, 5e-9]

The x-limits were with the following rationale:
lower limit -> arbitarily chosen to incorporate the peaks <1Hz
upper limit -> the detection band begins from 10Hz, with the spectrum rolling off steeply and being 3 orders of magnitude lower than the peaks

The y-limits were with the following rationale:
lower limit -> arbitarily chosen, do not know the floor which is significant for detection. 5 orders of magnitude seemed reasonable
upper limit -> All peaks lower than 5e-9, thus no need of space above it.

A velocity damping filter with of the following form: zpk(0, 2*pi*[-20, -20], 100) was applied to the Length DOF while the remaining filters were set to 0. The following Attachements are comparisons betwen the Active Filters at Sites vs Velocity Damping Filter

Attachment 1 shows the Bode Plot comparisons

Attachment 2 shows the Ringdown Period comparisons

Attachment 3 shows the ASD comparisons for M3_L

With the following goals:

- 10x lower noise magnitude than the current filter design in the 10-20Hz range
- 10s ringdown time 

After gaining some intuition from Brett, I started designing the filter. The New design current contains the following components
- Velocity damping: zpk(0, [10, 10], 1e6). Zero at 0 to prevent feeding back the DC offset, Poles at 10Hz to low pass freqeuncies over 10Hz, Gain arbitarily chosen to achieve feedback.
- Notches at 10Hz and 15Hz balancing the magnitude to prevent sufficient loss in phase margin
- Bump at 0.745Hz to suppress the peak at the same frequency
 

The comparisons are made with just having the Longitudinal DOF filter active in the loop between the filter used at the site and the new design

Attachment 1 shows a comparison of the ASD and the ringdown times.
Attachment 2 shows a comparison of the filter designs
Attachemnt 3 is the open loop transfer function for M1 (L DOF) with the minimum stability margins. The current phase margin is ~22 degrees (Aiming for 30 degrees)
Attachment 4 shows a comparison of the OLTFs for M1 (L DOF). 

This in no way acheives the objectives, the next steps for tuning are:
- Tune parameters for the current filter design
- Add lead/lag compensators
- Compare the performance between notches and elliptical filters 

I took the Thorlabs PZT driver to the 40m for use with the BHD OMC.

PR3 Livingston Filter Design (LONGITUDINAL DOF)

tldr: meeting noise reduction requirements, meeting ringdown time requirements, doing okay on the phase margins (~20 degrees for the minimum stability margins)

Filter Design:

1. zpk(0, pair(-2*pi*100, 75), -1e6) (Damp the longitudinal DOF on M1)
2. bump(0.75,30, 10) (Damp the natural frequency of M1 at 0.75Hz)
3. bump(3.2, 5, 1.5) (I was trying to get some damping on the 3Hz peak, although it didn't improve the impulse response as compared to without the bump, it recovered 10 degrees of phase margin and improved the noise performace by a factor of ~5).
3. notch(10,10,7)*notch(15,10,7)*notch(20,10,7)*notch(10,10,5)*notch(15,10,5)*notch(20,10,5) (Suppress the noise from 10->20Hz)
The UGF was then set at 3Hz.

Attachment 1 shows a comparison of the ASD and the ringdown times.
Attachment 2 shows a comparison of the filter designs
Attachemnt 3 is the open loop transfer function for M1 (L DOF) with the minimum stability margins. The current phase margin is ~20 degrees 
Attachment 4 shows a comparison of the OLTFs for M1 (L DOF). 

PR3 Livingston Filter Design (PITCH DOF)

tldr: meeting noise reduction requirements, meeting ringdown time requirements, doing okay on the phase margins (~20 degrees for the minimum stability margins)

Filter Design: Using the same design philosophy for the longitudinal DOF, I proceeded to design the pitch damping loop

1. zpk(0, -2*pi*pair(100, 75), -1) (Damp the pitch DOF on M1)
2. bump(0.66, 15, 9) (Damp the natural frequency of M1 at 0.75Hz)
3. bump(3.75, 5, 3.5) (Recreating the accidental bump from the longitudinal filter, adding a small bump slightly ahead of where I want to set my UGF does well for the noise performance).
3. notch(10,12,8)*notch(15,12,5)*notch(20,12,7)*notch(10,12,5)*notch(15,12,7) (Suppress the noise from 10->20Hz, used a single notch at 20Hz exploiting the natural roll off of the system)
The UGF was then set at 3.62Hz.

Attachment 1 shows a comparison of the ASD and the ringdown times.
Attachment 2 shows a comparison of the filter designs
Attachemnt 3 is the open loop transfer function for M1 (P DOF) with the minimum stability margins. The current phase margin is ~21 degrees 
Attachment 4 shows a comparison of the OLTFs for M1 (P DOF). 

PR3 Trial Filter Design

tldr: PR3 (Livingston, HLTS) filters redesigned with the following goals.

• Lower noise by a factor ~10 in the 10-30Hz bandwidth
• Achieve similar ringdowns for the impulse response
• Have phase margins of ~30 degrees and gain margins of ~3dB

This was the first time I’ve designed damping filters => I’ll be verbose in the explanation. The ASD shows noise reductions at 10Hz and 20Hz between the site filter design and the trial filter design (assuming that the natural roll off makes 30Hz noise low enough). The filters were designed individually (with the remaining DOFs damping turned off).

The basic philosophy for designing filters for each DOF has been the following. 

• Velocity damping with a zero at 0 and a pair of complex poles. The zero at 0 is what gives viscous damping and the pole pair provides filtering of noise and gives a finite actuation size. Having complex part for the poles helps recover some phase margin while inducing some oscillations in the signal (which is assumed to be okay).
• Instead of manually setting the gain for the filters, the UGF for open loop transfer function can be manually set where we want it to be. (For newbies like me, the bode plot crosses the the 0dB or unity gain mark multiple times, the UGF is the frequency where it crosses that point the last time. The intuition behind setting the UGF is as follows:
  1) For feedback, all frequencies are fed back to the system, however the ones above the 0dB mark in the OLTF are significant.
  2) Mechanical resonances can usually be at any frequency, however in this case, the first order modes usually occur at lower frequenices. 
  3) Mechanical systems naturally roll off (magnitude steadily decreases) at higher frequencies
  4) From 3), you can set the UGF at higher frequencies for faster damping (more gain on the peaks), but that will also lead feeding back the sensing noise into the system. Thus, it’s a tradeoff.
• Once a UGF has been picked appropriately, add bumps at the frequencies where the peaks are not high enough in the OLTF. This ensures they are damped well.
• Add notches in places where you want a reduction in sensor noise entering the system by looking at the ASD of the sensor noise on M1 to Test Mass displacement.
• Add elliptical filters (bandstop in this case) for aggressive filtering of the modes that have considerable peak in the noise ASD due to bounce and roll (stretching of the wire) at higher frequencies. This means that there is virtually no damping on these modes (with the goal being to reduce the sensor noise at these modes). 6th order filters are used usually and in this case, the attenuation used was 40dB for the filter. (This is specifically for the vertical and roll modes, the metric was to do slightly better on the noise performance than the sites at these ringup frequencies).
• Most SISO loops after all this had decent gain margin but sucked with phase margin. Adding a small bump in front of the UGF (f0 + df) helps recover sizable phase margin at the cost of some gain margin. 

With these following toolsets, here’s the filter design for each DOF. (The electronics gain has not been accounted for here, it must be added to the filter design). The filters work with negative feedback with a gain of 1. 

The pdf has been arranged as following for each DOF. (Comparisons are between the site filter design and the trial filter design (need to come up with a better terminology for this)).

• First figure shows a comparison of the sensor noise ASD (DOF sensor noise on M1 to DOF Displacement on M3) in the bandwidth of interest and ringdown periods for the impulse response. (FOR Longitudinal DOF, the impulse input was ground, for the remaining DOFs, the input was at M1). 
• Second figure shows a comparison of the filter designs in the bandwidth of interest
• Third figure shows the open loop transfer function (P(s)*C(s)) with phase and gain margins
• Fourth figure shows a comparison of the open loop transfer functions (P(s)*C(s)).

DATA ACQUISITION (FILTER MODULES) FROM LIVINGSTON

tldr: the filter files (active modules) for all the triple suspensions at Livingston were obtained.

The previous time I obtained the damping filters from the site, it was very messy and manual. I spent some time cleaning the scripts to get the filter module switch status, gains and so forth along with the filters.
Here's random bits of information and some progress update along the way.

- A handy place to obtain channel names: https://cis.ligo.org/

- To obtain the filter module switch status, you look at the channel name ending in SWSTAT which returns a decimal value. You convert it to binary and look at the first 10 digits from the right. 0-OFF, 1-ON.

- I was having trouble using gwpy to get the filter status from the Livingston site. The error looked as follows:
NDSWarning: unique NDS2 channel match not found for 'L1:SUS-PR3_M1_DAMP_T_SWSTAT' warnings.warn(error.split('\n', 1)[0]

I needed a machine that could use foton (obtain filter files) and nds2utils(obtain filter module status ON/OFF) on the same machine.
Paco advised against installing nds2utils on the 40m PCs so I used the conn method from nds2. Here's a nice script that shows how to use the method -> https://git.ligo.org/40m/measurements/-/snippets/164
(Be careful, chann_buffers is a shared_pointer. For example, if you pass a list of 6 channels and want to access the data from the 3rd channel, you use

my_data = chann_buffers[2].data

It has multiple methods if you want to access things like time, etc.)

- Instead of connecting to the 40m host, I connected to the Livingston (L1) host. Here's a list of hosts for future reference.

(None, ('nds.ligo.caltech.edu', 31200)),
('H1', ('nds.ligo-wa.caltech.edu', 31200)),
('H0', ('nds.ligo-wa.caltech.edu', 31200)),
('L1', ('nds.ligo-la.caltech.edu', 31200)),
('L0', ('nds.ligo-la.caltech.edu', 31200)),
('V1', ('nds.ligo.caltech.edu', 31200)),
('C1', ('nds40.ligo.caltech.edu', 31200)),
('C0', ('nds40.ligo.caltech.edu', 31200)),
('K1', ('k1nds0.kagra.icrr.u-tokyo.ac.jp', 8088)

- For the triple suspensions at Livingston, I will be obtaining the data for the following triple suspensions (https://dcc.ligo.org/DocDB/0033/T1100073/004/T1100073-v4_suspensions_by_chamber.pdf)
from DAQSVN (The document mentions IMC1, IMC2, IMC3 but since its from 2013, I'm hoping that the naming convention has changed for them to MC1, MC2, MC3)
 

HLTS - PR3, SR3
HSTS - PRM, PR2, SRM, SR2, MC1, MC2, MC3

The connection kept timing out yesterday intermittently , I'll try again today.

ERRNO: read_server_response_wait: Timed out: errno: 62 - Timer expired

Triple Suspension DAMP_OUT Spectrum (Livingston)

tldr: DAMP_OUT spectra for all triple suspensions at Livingston. For each DOF, in the 10-30Hz range, the feedback signals with comparatively large magitudes (visually) are as follows:

L: PRM, MC2
T: PR3, MC2
V: PRM, MC1, MC2, MC3
Y: MC2
P: PRM, SR3, MC1, MC2, MC3
R: PRM, PR3, SR3, MC1, MC2, MC3
 

Method:
- Damping filters were obtained for each optic from svn: l1:filter files in python using foton and exported them to matlab
- The timeseries for each optic were obtained for DAMP_IN1 channels (256Hz) using gwpy for the time interval of 1 hour (1382893218, 1382896818). The asd were calculated from the timeseries with a window length of 8s and overlap of 4s. This asd along with it's corresponding freqeuncies were exported to matlab
- Since there was no way to change the frequency resolution over which it was calculated in gwpy, it was interpolated in matlab using the interp1 function to match the frequency resolution that was used in matlab.
- The DAMP_OUT spectra was thus calculated as [*DAMP_OUT = (Damping_Filter)x(*DAMP_IN1)]

Triple Suspension Test Mass Displacement Spectrum (Livingston)

[WIP] (need to think some more about what's happening differently between the small and large suspensions)

tldr: Test Mass Displacement spectrum for all triple suspensions at Livingston for true (seismic_motion + sensor_noise).
 

Method:
- DAMP_IN1 ASD was obtained as described in the previous elog.
- The sum of seismic motion and sensor noise can be estimated such that, seismic_motion(s) + sensor_noise(s) = (1 + OLG)*DAMP_IN1.
- This ASD is then given as an input to the damping filters (DOF_IN) and propogates through to the test mass displacement (m1_disp) which is shown in the plots
 

we only care about PRM, PR2, PR3, BS, SR3, SR2, SRM, so you should eliminate all other optics from the plots.

Then just plot the HSTS to get started and we can see how they compare.

The spectra for damp out for the following optics
- small triples {PR2, PRM, SR2, SRM} (Attachment 1)
- large triples {PR3, SR3} (Attachment 2)
- beamsplitter {BS} (Attachment 3)

For pwelch(), I was not taking into account the DC offset which is why I was getting funky plots (spectral leakage). I corrected for it, no more interpolation anymore!

Edit: I have changed the yaxis label for all plots to counts/rHz for now since I do not know the exact calibration

Can you explain how you calibrated the control signals into displacement units? It would be fine to start if you could just plot them with the y-axis in counts/rHz for now, unless you have good confidence in the calibration.

Small Triple Suspension Trial Filter Design

tldr: From the Damp_OUT spectrum of the small suspensions, PRM had a relatively larger magnitude in the 10-30Hz bandwidth. I thus choose to design filters for them using PRM as a reference.
While designing for PRM, I used the site noise asd (DAMP_IN*(1 + OLG)) given as input to the controller instead of using white noise while desinging them.
The design objectives that were met are as follows:

• Lower noise by a factor ~10 in the 10-30Hz bandwidth or better
• Achieve similar ringdowns for the impulse response
• Have phase margins of ~30 degrees and gain margins of ~3dB or better

The same design principles were used for PRM as for PR3 (explained in this elog). Here’s the filter design for each DOF. (The electronics gain has not been accounted for here, it must be added to the filter design). The filters work with negative feedback with a gain of 1. 
Note: I had an an interesting time designing the Roll Mode, bumping the peaks to increase damping on them after setting the UGF at 2.75 would increase the ringdown period. I reverted to using a similar overall "magnitude" that they used at the sites and added notches for reducing noise in the required bandwidth. The new filter still better on the ringdown (15 seconds lesser) so I'll leave it at that for now and make changes if required later. 

The pdfs have been arranged as following for each DOF. (Comparisons are between the site filter design and the trial filter design (need to come up with a better terminology for this)).

• Attachment 1 shows a comparison of the sensor noise ASD (DOF sensor noise on M1 to DOF Displacement on M3) in the bandwidth of interest and ringdown periods for the impulse response. (FOR Longitudinal DOF, the impulse input was ground, for the remaining DOFs, the input was at M1). 
• Attachement 2 shows a comparison of the filter designs in the bandwidth of interest
• Attachment 3 shows the open loop transfer function (P(s)*C(s)) with phase and gain margin (minimum stability margins)
• Atttachment 4 shows a comparison of the open loop transfer functions (P(s)*C(s)).