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ID Date Author Type Category Subject
2560   Wed Apr 21 16:45:01 2021 AidanLab InfrastructureCamerasTelephoto and macro lenses for smartphones

I purchased a set of telephoto and macro lenses for the lab. They're stored in the tool cabinet.

Attachment 1: IMG_2194.jpg
Attachment 2: IMG_2195.jpg
Attachment 3: IMG_2196.jpg
2559   Wed Apr 21 09:42:35 2021 Stephen, ranaThings to BuyCryo vacuum chamberReimagining QIL Cryo Vacuum Chamber

WIP log entry - working on getting all of our ideas down on the page, then will sort and elaborate.

We met to discuss a range of topics relating to the path ahead for the Cryo Vacuum Chamber. This reconsideration of the current state of things is necessary as the chamber needs to become the workhorse for PD characterization efforts soon, in addition to a range of other tests (large suspension tests will be conducted in a different chamber, yet to be designed)

1. Pumping station should be moved away from table, with long roughing lines perhaps coming down from above using some ceiling-mounted cable tray or similar.
2. Primary pumping line to chamber is large and overkill in terms of conductance. Can move to a longer, more flexible, smaller diameter connection (may need to adapt using CF zero-length flange).
3. Pumping of the external volume may be managed by valve arrangement and direct connection of both volumes to the turbo pump. Valve out the external volume once the pressure is low, and the pressure should hold well enough for conductive losses to be minimal.
4. Viewports seem suitable, no issues throughout. I learned about common coating behavior, namely that reflectance is generally at half the wavelength of transmission, so if I see a green reflection it suggests transmission at IR. Neat!
5. Cabling into chamber for temp sensors is pretty scary. Noticed the kink in the cable bundle caused by the flexible part of the cable extending longer than the grip of the connector's stiffening. Needs to be reassembled with the stiff cabling under the grip (could extend the grip, shorten the flexible leads, etc.) to avoid the kink.
6. External volume feels like a misdirected design.
1. For general case, consider mounting the cryocooler directly to chamber. Avoid losses related to thermal linkages from cold head of cryocooler to baseplate (diagram will be supplied, Rana was especially concerned that the V clamping arrangement didn't have adequate contact area in the line contacts)
2. For vibration-sensitive experiments, would be good to have a flexible bellows reducing vibrational energy through the vacuum skin, and flexible strap reducing vibrational energy to the baseplate.
3.  For contamination-sensitive experiments, would be necessary to implement a feedthrough as currently, but seems overkill for current slate of experiments.
4. The intention seems to have been to devise a scaleable solution that would work for Mariner, but we are currently very far from realizing that (cryocooler needs to be Stirling cycle for vibration, no validation yet of adequate thermal conductivity through the external volume to the baseplate, etc.)
7. Thermal straps with Mylar shielding is not the optimal implementation. Something like a rigid copper bar provides better conductivity, and can be shielded by G10 tubing with Aluminum metallization on the OD. If a flexible connection is required (for example, vibration isolation or positional uncertainty) a thermal strap may also be shielded in this sort of conduit.
8. Need to replace the V-groove copper connections with something with much more surface area. V-grooves are nearly 1D contact lines, so they are probably the mian cooling rate limiter at the moment. Need to get some new parts fabbed ASAP to continue working on this cryostat.
9. Vent valve should be a leak valve with a controlled, small conductance, perhaps backed by a filter. Want to allow slow, controlled venting.
10. Skyhook Crane should be on its cart wheels for easy relocation away from the experiments.
11. Skyhook should be replaced by a simple hoist mounted to ceiling of enclosure. Would require stiffening of some members of the enclosure's ceiling, but would permit easier access with fewer traffic jams.
12. Yellow solvent cabinet should be removed. Solvents should be stored under fume hood.
13. Would be great to get a stand (ie wire shelving, but heavy duty) which could hold the compressor, hold the pumping station, and provide a single location for any other items that need to be interfaced. All connections would be routed over the walkway via a run of cable rack.
14. Moving the chamber to the center of the table width would be helpful to opening up access to more ports. Currently located in a corner, such that only half of ports are accessible. This is an extra reason that the Sky Hook should be remounted to its wheeled base.
15. Should put all controllers on a rack, rather than consuming optical table space. All necessary serial comm cabling could then run to this singular location. We can use the rack next to the sink which has the NIM racks. The rack is completely unused right now. Need to get some rack parts to put some shelving in there.
16. Documentation of ongoing thoughts, design efforts, modifications, etc. can be contained at the wiki!
17. What is currently installed? Some insights from the wiki (ie gauges, pumps, viewports) should be elevated into a comprehensive diagram with a bill of materials or similar.
18. In the diagram need to note all instruments so that Radhika can include it in her work to interface with the DAQ. i.e. no more photos and screenshots to record data.
19. Where am I? A floorplan for this experiment (current and planned) would be worth some time, now that we are considering specific improvements.

Will sort the above into some sort of timeline (such as short term / long term).

Ruminations about the future chamber for suspension work:

1. Stirling cryocooler for vibration isolation
2. Straight-sided construction (i.e. rectangular prism) for more usable footprint inside
3. If vertical (i.e. lid) use counterbalance and hinge for easy opening without hoist or crane; might need side ports large enough for hand access, since the height will be prohibitive to reach all the way down to the baseplate.
4. Horizontal (i.e. door) might be preferred, especially with the volume in the form of a rectangular prism. This would allow access throughout the height of the chamber.
5. Usual vacuum vendors should be able to help bring the design from a sketch to a quote, so start those conversations.
2558   Tue Apr 20 09:43:49 2021 RadhikaSummary2um PhotodiodesLink to PD testing master doc

2557   Fri Apr 16 13:05:05 2021 AidanUpdate2um Photodiodes2um DC photodiode voltage weird drift with no incident power

We put the preamp output directly into a multimeter and observed the same fluctuating behavior as the DAC channel was changed.

We're bypassing the relay to see if that makes any difference. The old relay wiring (to be bypassed) is shown in the attached diagram. That didn't do anything.

We're looking at filtering the DC output by 5kHz to see if there are any resonances at higher frequencies that might go away. Changing SR560 output for AC path to DC and setting gain to 1 on that unit. Also changing gain in FM31 filter bank from 1E-3 to 1. The results are shown in the attached time series. The channels FM30 and FM31 see the same thing. The only difference is that FM31 goes through an SR560 with a 0.03Hz pole (6dB).

Success by bypassing the DAC bias voltage. We switched to a 300mV bias voltage from a function generator. Doing that removed the causal PD voltage drift induced by changing the laser diode current set voltage (see the last time series). So the issue is some weird coupling into the DAC bias voltage.

 Quote: [Aidan, Radhika, Nina] We noticed that the DC channel readout (FM30) of the JPL A1 photodiode is drifting around. What we observe with no light on the photodiode, is the DC output drifiting around. It gets particularly bad when we apply voltage to other DAC channels. For example, the attached plot shows the DC voltage from the photodiode as I change the set voltage to the laser diode driver. To be absolutely clear, the laser driver itself was completely powered off. I'm just varying the voltage going into the set point BNC connector on the back of it. For reference, the set up is: DAC (300mV bias) > relay > PD > relay  > FEMTO preamp (1000x gain) > ADC channel FM30

Attachment 1: IMG_2132.jpg
Attachment 2: FM30_31_w_latter_filtered.png
Attachment 3: IMG_2134.jpg
Attachment 4: bias_bypassed.png
2556   Fri Apr 16 11:27:01 2021 AidanUpdate2um Photodiodes2um DC photodiode voltage weird drift with no incident power

We noticed that the DC channel readout (FM30) of the JPL A1 photodiode is drifting around. What we observe with no light on the photodiode, is the DC output drifiting around. It gets particularly bad when we apply voltage to other DAC channels.

For example, the attached plot shows the DC voltage from the photodiode as I change the set voltage to the laser diode driver. To be absolutely clear, the laser driver itself was completely powered off. I'm just varying the voltage going into the set point BNC connector on the back of it.

For reference, the set up is:

DAC (300mV bias) > relay > PD > relay  > FEMTO preamp (1000x gain) > ADC channel FM30

Attachment 1: DC_2um_voltage_weirdness.png
2555   Wed Apr 14 09:49:38 2021 StephenDailyProgressCryo vacuum chamber

2021.04.14 StephenA

QIL Cryo vacuum chamber cooldown was not as successful under the new configuration (radiation shielded by cylindrical outer + inner shields, cold finger thermally strapped to baseplate).

--> Karthik's Si cantilever workpiece was stable at 240 K.

--> Cold Finger was stable at 200 K - there is significant thermal loss between the cold finger and the workpiece.

--> Inner shield was stable at 250 K - seems to be somewhat decoupled from the baseplate; not very satisfied with the current state of the shielding.

Will need to re-examine some of the connections, which were not optimal (especially the improvised dog clamped strap-baseplate interface). Fabricating an adapter piece for the thermal strap which will be bolted 4x on a 2" x 2" grid. Might also look into a new thermal strap which could interface with baseplate directly.

Also will need to consider options to decouple outer shield from inner, and double check that shield orientation has no other solution (hoping there's an answer to the question, why would outer shield be coupled to baseplate?)

Data - cooldown 20210408 (CSV = raw, XLSX = Stephen's plots) in Box Folder [Voyager\MarinerBox\CryoEngineering\CSVlogs]

Description - 6 day cooldown. Layout described in QIL/2552. The radiation shields were installed and thermal strap was connected to baseplate. The cryocooler was turned on/off at the start/end of the data collection, and the in-vac heater was not powered on at all.

Images -

1. IMG_8570 = starting conditions;
2. IMG_8585 = final conditions after 144 hours;

Plots -

1. cooldown_20210408_first_si_workpiece_with_shields_and_straps

Attachment 1: IMG_8570.JPG
Attachment 2: IMG_8585.JPG
Attachment 3: cooldown_20210408_first_si_workpiece_with_shields_and_straps.png
2554   Tue Apr 13 11:48:04 2021 RadhikaDailyProgress2um PhotodiodesOpened cryo chamber and removed old PD

I posted a video tutorial of the diode replacement.

https://dcc.ligo.org/G2100807

 Quote: 04/01/21 Aidan and I removed the old PD from the cryo chamber in order to start testing C3 (plan for tomorrow, 04/02). Steps: - Brought chamber up to room pressure, disconnected readout wires and vacuum pump. - Picked up chamber and placed it upside down on makeshift support stand (see pics). - Unscrewed mounting plate and 2 inner insulation plates to reveal mounted PD. - Had trouble unscrewing PD mount, since the screws were very close to the PD and we had to be careful not to slip and cause damage. Started with 2 side screws, then bottom (hardest), then top. - Successfully removed PD and put away. Placed chamber components back in place without bolting in. - Plan is to mount PD C3 in chamber tomorrow and begin testing.

2553   Fri Apr 9 16:23:22 2021 ranaDailyProgress2um PhotodiodesOpened cryo chamber and removed old PD

To aid in taking photos of these diodes, I put a USB microscope on Anchal's desk - you can grab it from there. I use it with mac Photo Booth, but it should be easy to use with any camera application.

Also, I recommend buying a macro lens(es) for cell phones from Amazon or B&H.  Label them with the QIL lab sticker so they don't disappear.

2552   Fri Apr 9 09:37:26 2021 StephenDailyProgressCryo vacuum chamberStarted cooldown with shielding, workpiece Si cantilever

2021.04.08, StephenA

Karthik had completed in-chamber alignment efforts during a prior visit. In air alignment also completed following viewport move.

--> posted demo video to ligo.wbridge QIL Cryostat HowTo Playlist.

1) Mounted RTDs to final positions - locations are Heater (cryo varnish+cigarette paper, pictured in IMG_8558 curing under weight of upsidedown bolt), Inner Shield (cryo varnish+cigarette paper, pictured in IMG_8559), Cold Finger (spring clamp), and Workpiece (spring clamp).

--> Final chamber layout may be viewed in IMG_8562

--> Note that Karthik's Si cantilever, mounted vertically in the right of the image, is NOT bolted down to the baseplate (just located on baseplate by dog clamps, held down via gravity). This will need to be investigated to enable workpiece cooling.

2) Installed radiation shield lids - no bolts to expedite the process and to see if there is any bulk motion during pumpdown and thermal cycling.

--> note that the lid for the outer radiation shield seems to interface with the current shield orientation perfectly; if there was a mismatch, it would point toward the inverted orientation being intended, but this seemed pretty definitive.

3) Installed the cryostat lid - final positioning and alignment made easier by teflon rails!

--> posted demo video to ligo.wbridge QIL Cryostat HowTo Playlist.

4) Pumped down - single button press to turn on pumping station.

--> note that it took about 1 hour for both gauges to reach a few mTorr.

5) Confirmed function of heater - set PID setpoint to 350 K and enabled outputs, observed temperature rise in heater RTD.

--> note that PID autotuning should be done at steady state with workpiece RTD, before enabling outputs again!

6) Turned on cryocooler - flip power lever and turn on green system switch.

--> start time was 10 am.

7) Started temperature datalogging to USB - press dull red indicator dot on upper right corner of CTC-100 once, and note that indicator is now bright red.

8) Remaining photos posted to the ligo.wbridge QIL Cryostat Photo Album

Attachment 1: IMG_8558.JPG
Attachment 2: IMG_8559.JPG
Attachment 3: IMG_8562.JPG
2551   Mon Apr 5 18:50:54 2021 RadhikaSummary Current PD testing schematic

I'm attaching my rough first draft of the QIL photodiode testing schematic. Please provide comments for fixes/improvement!

Attachment 1: QIL_PD_testing.jpg
Attachment 2: QIL_PD_testing.graffle
2550   Thu Apr 1 20:54:57 2021 RadhikaDailyProgress2um PhotodiodesOpened cryo chamber and removed old PD

04/01/21

Aidan and I removed the old PD from the cryo chamber in order to start testing C3 (plan for tomorrow, 04/02).

Steps:

- Brought chamber up to room pressure, disconnected readout wires and vacuum pump.

- Picked up chamber and placed it upside down on makeshift support stand (see pics).

- Unscrewed mounting plate and 2 inner insulation plates to reveal mounted PD.

- Had trouble unscrewing PD mount, since the screws were very close to the PD and we had to be careful not to slip and cause damage. Started with 2 side screws, then bottom (hardest), then top.

- Successfully removed PD and put away. Placed chamber components back in place without bolting in.

- Plan is to mount PD C3 in chamber tomorrow and begin testing.

Attachment 1: IMG_1834.jpg
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2549   Thu Apr 1 20:33:42 2021 StephenDailyProgressCryo vacuum chamberInstallation of Radiation Shields

2021.03.17, StephenA

1. Radiation Shields located (in TCS lab), unwrapped, fitted up.

Location in TCS Lab - IMG_8351

Removed lid and placed adjacent to chamber (cleared a little space, used 3 plastic flange covers to make nonmarring surface safe for lid and o-ring - IMG_8353

Fit up as installed - IMG_8354

Comments on the fit up - I looked at all of the apparent sources for insights into Rahul's original design intent - QIL elog 2276, DCC T1800308-v1, wiki for Cryo Vacuum Chamber. It appears that Rahul never decoupled the upper outer radiation shield from the cold plate, which seems like a strange omission. Chris and Raymond also appear to have been wrapping their heads around the intended layout, they came up with the fit up in QIL elog 2429 and sketch from QIL elog 2430. I will revisit their sketch at a future opportunity, but I went with something closer to 2429 as I was concerned about the height misalignments they described. Note that the height misalignment appears in Rahul's T1800308 CAD (see T1800308-v1 screenshot) so who knows what's "correct". I'll work on finalizing D2100310 CAD with radiation shield to capture the true current dimensions and fit up, to hopefully avoid such issues in the future.

2. Radiation Shields installed in Cryostat. Sequence was important here, as were a couple of improvised solutions to shortcomings of the existing parts.

Dog Clamps placed on bottom plate (to stand off bottom radiation shield bottom lid; not pictured, I think I placed some alumina washers on the dog clamps as well, not sure though anymore!). Also pictured are the usual PEEK legs for cold plate - IMG_8355

Bottom radiation shield bottom lid placed on dog clamps spacer, and bottom radiation shield cylinder placed on bottom lid - IMG_8356. Seems likely that the bottom radiation shield would be better configured upside-down.

Bolted cold plate down onto legs, with cold plate decoupled from bottom radiation shield - IMG_8357

Outer radiation shield installed and inner radiation shield installed (both needed to be tipped into place gingerly, but both cleared the cold finger cylinder with the flange removed. The heater also passed through the apertures successfully - IMG_8358

2x Alumina washers placed under outer radiation shield, inner radiation shield on cold plate - IMG_8374

Cold Flange reinstalled, though one of the brass SHCS was sheared - this was due to over torque, with 20 in*lb applied by mistake. Correct torque is 10 in*lb. The remaining 3 bolts were tightened to 10 in*lb. - IMG_8360

Top view of radiation shield apertures and cold plate grid - IMG_8375

Thermal strap interface to cold plate - dog clamps required due to strange spacing of clearance holes. - IMG_8376

Attachment 1: IMG_8351.JPG
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Attachment 4: t1800308-v1_cryostat_step_section_screenshot.png
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Draft   Thu Apr 1 20:28:26 2021 RadhikaDailyProgress2um PhotodiodesOpened cryo chamber and removed old PD

04/01/21

Aidan and I removed the old PD from the cryo chamber in order to start testing C3 (plan for tomorrow, 04/02).

Steps:

- Brought chamber up to room pressure, disconnected readout wires and vacuum pump.

- Picked up chamber and placed it upside down on makeshift support stand (see pics).

- Unscrewed outer and inner insulation plates.

2547   Thu Apr 1 18:42:55 2021 Stephen DailyProgressCryo vacuum chamberPreparations for Q measurements

2021.03.30, StephenA

1. Viewport swap to nozzle that is not occluded by cryo shield = complete. All bolts on both Active Ion Gauge and Viewport have been torqued gradually (about a half turn at a time, around the clock dial) until the conflat seal was metal-to-metal. Periscope on damped optical rod was rotated to make room for replacement.

Before viewport swap - IMG_8487

After viewport swap - IMG_8502

2. Cryo RTD repair = complete. Two RTDs had been damaged during prior mounting efforts by me. I was able to repair the clamped RTD at the single damaged solder joint. I was able to repair the former Al-Block RTD by replacing the RTD element, and making a new direct attachment (not preloaded, not varnished to the aluminum block anymore)

Materials and set-up for solder repair - IMG_8491

Repaired Clamp-2 RTD - IMG_8494

Damaged Al-Block RTD - IMG_8492 (note short length between kapton strain relief and aluminum block was not ideal, one lead had already fractured and the second soon followed at the slightest touch)

Remounted Clamp-2 RTD - IMG_8496 (RTD clamped at cold flange, strap is mounted)

Remaining Clamp-1 and Varnish RTDs are free - IMG_8497

Current readouts of CTC-100 controller, with repaired RTDs now behaving (note need to rename the Al-Block RTD) - IMG_8501

3. Next steps:

- Karthik to install clamps, align in-air relay, and confirm positition of radiation shield aperture.

- Remaining free RTDs to be mounted; current RTDs are mounted at Heater and Cold Flange, would be good to mount RTD at Work Piece/Clamp and at Outer Radiation Shield.

- Radiation shield lids to be installed (might be easiest to install Outer Radiation Shield RTD after installing lid)

- Mount lid, install bolts, pump down, turn on cryo cooler, the usual!

Attachment 1: IMG_8487.JPG
Attachment 2: IMG_8502.JPG
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2546   Wed Mar 31 11:06:53 2021 AidanComputingCymacsRebuilding the Cymacs software to compile models

[Aidan, Jon, Chris W, Ian]

Summary: We rebuilt the Cymacs C4TST today to get FM31_OUT into frames

Main points:

• Had to make this directory viewable on other machines by editing the /etc/exports file on FB4
• Had to update the /etc/fstab file on QIL-WS2 to give it the location of the new directory
• Had to manually mount the directoy on QIL-WS2 to avoid rebooting machine to achieve this (reboot will automatically mount the drive)
• C4TST model would not compile on FB4. Issue was two-fold:
• Compilation was done in wrong build directory
• Compilation was done using sudo
• To fix this, we had to clean up the compilation files from the failed sudo efforts
• sudo make clean-c4tst was run (didn't completely solve problem)
• Once the old compiles were cleaned up, we could compile models (not as sudo) in /opt/rtcds/rtbuild/
• Ran the following commands to manually restart the models (ignore the line numbers)
 1982  sudo /sbin/rmmod c4tst c4iop
1983  cd /opt/rtcds/caltech/c4/target/c4iop/scripts/
1984  ./startupC4rt
1985  cd ../../c4tst/scripts/
1986  ./startupC4rt
1987  systemctl start rts-awgtpman@c4iop.service
1988  cd .././../
1989  ls
1990  cd gds
1991  ls
1992  cd awgtpman_startup/
1993  ls
1994  ./awgtpman_c4iop.cmd
1995  ./awgtpman_c4tst.cmd
1996  systemctl restart daqd@standiop.service
1997  systemctl
1998  systemctl status daqd@standiop.service
1999  systemctl stop daqd@standiop.service
2000  sudo systemctl restart daqd@standiop.service


2545   Mon Mar 29 14:16:33 2021 AidanComputingCDSRebuilding C4TST with frames added to file

Added Simulink > Model-Wide Utilities > Model Info block to c4tst.mdl. Text inside that block is:

#DAQ Channels

FM31_OUT 16384

--

Now following https://nodus.ligo.caltech.edu:8081/QIL/2336

And it failed. See attached screenshot. Then I copied c4tst.mdl to the simLink directory. Compile still failed.

Attachment 1: Screen_Shot_2021-03-29_at_2.29.53_PM.png
2544   Thu Mar 25 12:28:19 2021 AidanComputingCymacsDAC channels not outputting voltages

Noticed that the DAC channels were not producing a corresponding output in the real world (I changed the Laser Current FM12 value and got not corresponding change on the laser diode driver display).

Sent the following to Chris: "Can you log into the QIL FB4 workstation to see if there is an issue with the DAC? I restarted the C4TST model last week and I don’t seem to have working DAC outputs anymore. The ADC channels still work and the model appears to be running. It just seems that I can’t output any voltages."

After observing that the "DK" (DACKILL) bit in the state word on the IOP status screen was red, the resolution to this was to restart the IOP and TST models.

Except access is denied by FB4

2542   Fri Mar 19 13:51:16 2021 AidanComputingGeneralActivated MATLAB on QIL-WS2

2541   Wed Mar 17 16:06:24 2021 aaronElectronicsEquipment transferborrowed filter

entered just before (Wed Mar 17 16:06:37 2021) to borrow a mini-circuits filter (SLP-100)

2540   Tue Mar 16 20:50:14 2021 AidanHowtoTutorial videoHow to operate the PD pumping station

I added a 7 minute video to the DCC that shows how to operate the HiCube 80 Eco pumping station.

LIGO-G2100609

Also added a "Tutorial video" category to the elog.

2539   Tue Mar 16 18:45:43 2021 AidanDailyProgress2um PhotodiodesCryo cool down and temperature rise - JPL PD LN2 (5 hours to rise)

I pumped the chamber down and added LN2 today. The pressure was slowly rising - it was about 20m Torr in the chamber when I added the LN2. Per Raymond's instructions, I added about a third of a container of LN2. This got the temperature down to about 89K (when I had 20W running in the heater). It stayed there for about 25-30 minutes.

I turned off the heater and left the LN2 to boil off. You could see the cloud coming out of the top (the plume height would increase proportionally to the heat in the heater).

Eventually the LN2 evaporated and the shield temperature started to increase back to room temperature. As of this post it is 282K (which took about 5 hours).

The PD thermistor is not currently registering. However, the temperature of the PD can be inferred from the shield temperature (see aLOG 2517).

The rate of increase in temperature was much faster than the previous test - see second time series. I wonder if the thermal mass of the shields in the Feb 2020 test was cooled down a lot more due to 5 hours at 80K in that test - thus reducing the overall ambient load on the inner shield ...

Attachment 1: Screen_Shot_2021-03-16_at_6.44.09_PM.png
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2538   Fri Mar 12 16:29:36 2021 AidanUpdate2um PhotodiodesVacuum needs leak checking

I pumped the small vacuum volume down but the pressure started rising as soon as I turned off the vacuum pump. Closing the main valve to the pump and the valve to the chamber did little to change the leak rate. So the main leak seems to be from the volume around the pressure gauge - best guess, the section and O-ring that I connected to the chamber yesterday.

Vacuum pressure was recorded from vacuum gauge to text file in Python (using pyserial). Haven't got this into EPICS just yet.

Attachment 1: Screen_Shot_2021-03-12_at_4.28.54_PM.png
2537   Fri Mar 12 08:31:05 2021 StephenHowtoCryo vacuum chamberQIL Cryo setup how to videos and photo album

Link to ligo.wbridge QIL Cryostat HowTo Playlist Cryostat on youtube - not super user-friendly as of yet, but populated with a couple of videos so far.

Link to ligo.wbridge QIL Cryostat Photo Album on google photos - not well curated, currently just a dump.

2536   Wed Mar 10 17:20:26 2021 AidanUpdate2um PhotodiodesPumped chamber down briefly - connected vacuum gauge to the computer
• I updated the SR560 settings for the AC Photodiode readout going into the ADC. They're now: AC coupled input, 2000x gain, low noise, 10kHz pole
• Updated the "undoSR560" filterbank for the FM31 input channel to undo the SR560 settings (zpk([0.15], [0], 7.5E-5)
• Unhooked oscilloscope from AC channel and switched SR560 to battery mode - no change in recorded spectra. I think I need to run the FEMTO preamp from a battery pack
• Hooked up the pumping station to the chamber and went through the pumping cycle described below. No major issues.
• Turned on the MKS vacuum gauge and hooked it into the serial port of QIL-WS1 to readback the vacuum pressure onto the DAQ. I want to transfer this across to QIl-NFS but there are no serial ports on that machine - will have to get a USB-Serial converter.
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2535   Thu Mar 4 16:12:26 2021 AidanHowto2um PhotodiodesJPL PD set up tutorial video

I recorded a 15 minute overview that describes the JPL PD set up and how to operate it. I'm in the process of embellishing the operation procedure (previous version can be found here: eLOG 2476).

2534   Thu Mar 4 13:37:51 2021 AidanUpdate2um PhotodiodesReference PD reading vs Power incident on viewport

I measured the power incident on the cryo chamber viewport and the reference PD reading to calibrate the incident power. Data is attached. Power meter head = S148C.

Attachment 1: POW_IN_vs_REF_PD.csv
﻿LD,REFPD,POWER
-0.4,0.207,0.314
-0.3,0.263,0.452
-0.2,0.32,0.589
-0.1,0.373,0.719
0,0.432,0.865
0.1,0.486,0.99
0.2,0.534,1.106
0.3,0.585,1.238
0.4,0.64,1.363

... 5 more lines ...
Attachment 2: IMG_1107.jpg
2533   Wed Mar 3 16:14:18 2021 AidanUpdate2um PhotodiodesBringing 2um PD testing back online

I ran the bright PD test on the photodiode currenlty in the vacuum chamber. The test was run at air and room temperature. I aligned the 2um laser onto the PD using the piezo mirror and the readout from the preamp. I then switched to the Keithley and ran the bright scan with the "runsweep.py" script. I actually ran the scan at multiple laser diode current settings by varying the control voltage into the diode driver. The change in response wrt control voltage looks linear but I need to run an analysis on it.

The data is stored in /home/controls/JPL_PD/data/20210303_bright_scans

Attachment 1: Screen_Shot_2021-03-03_at_4.11.47_PM.png
2532   Fri Feb 5 14:13:08 2021 ranaDailyProgressCryo vacuum chamberSuccessful cooldown in

is the x-axis in units of seconds? I think if we are clever, we should be able to look at a couple of the thermal time constants and figure out where the heat leaks are.

2531   Fri Feb 5 08:50:02 2021 StephenDailyProgressCryo vacuum chamberSuccessful cooldown, cold flange without shields

[updated with reference to data set, cleaner plot, images of chamber configuration]

StephenA, 2021.02.05

Data - cooldown 20210205 (CSV = raw, XLSX = Stephen's plots) in Box Folder [Voyager\MarinerBox\CryoEngineering\CSVlogs]

Description - 5.5 hour cooldown with data, which was then allowed to continue for a total of 96 hours (but data collection failed for the long stretch, except for a snapshot of the final state). The cold flange was below 100 K after 6 hours, and leveled off at about 80 K. The vacuum pressure was steady at 3 microTorr throughout, via a roughing line connecting the external and internal volumes (mitigated losses in external connection volume, with no areas dramatically cold to touch). The cold flange was radiating to room temperature surroundings, as the radiation shields were not installed. The cryocooler was turned on/off at the start/end of the data collection, and the in-vac heater was not powered on at all.

Images -

1. IMG_8047 = starting conditions;
2. IMG_8119 = intermediate conditions after 5.7 hours;
3. IMG_8148 = final conditions after 96 hours;
4. IMG_8232 markup = view of RTD locations on cold flange, with annotations;
5. IMG_8225 = top view of cold flange and chamber configuration without radiation shields.

Plots -

1. cooldown_20210205_uhv_coupled_cold_flange_first_segment - shows first 5.5 hours only;
2. cooldown_20210205_uhv_coupled_cold_flange_all_time - shows all 96 hours, with data points extracted from images.
Attachment 1: IMG_8047.JPG
Attachment 2: IMG_8125.JPG
Attachment 3: IMG_8148.JPG
Attachment 4: IMG_8232_markup.jpg
Attachment 5: IMG_8225.JPG
Attachment 6: cooldown_20210205_uhv_coupled_cold_flange_first_segment.png
Attachment 7: cooldown_20210205_uhv_coupled_cold_flange_all_time.png
2530   Fri Jan 29 10:14:16 2021 StephenDailyProgressCryo vacuum chamberRestarting Cryo vacuum chamber effort

StephenA, RaymondR remotely assisting (off payroll haha)

It seems that we won't likely receive the intended hand-off resources (especially of note is that Raymond can't seem to find the videos he made, wherein he guides through operations of the vacuum system and cryocooler). Raymond has been kind enough to support via Zoom as needed so that things can progress with some sort of guidance.

I'll stay on top of the lessons learned and dump these, along with photos and other resources in the log. I'll also make weekly visits with the intent of making continued progress.

1) What is the current state of the QIL Chamber?

- Raymond left the vacuum line shorting the "external volume" with the cross, cryocooler, etc. directly to the "main volume" because the losses between the "external volume" and the feedthrough entering the chamber were too large. The system was down in the e-5 torr range. Ref IMG_8019, with flex hose connecting bottom of 4-way cross to side of chamber.

- To implement this vacuum arrangement, there was a key component put aside on the table - IMG_8024 is a T which connects the roughing line to both the external volume and the main volume, using the flex line from IMG_8019 and the components pictured in IMG_8020.

- The copper feedthrough has all clamps attached, such that temperature measurements are being made on the adapter copper rod, which has a bolt pattern for thermal straps. Sensor names reflect current locations.

- We inspected everything (cryocooler connections, vacuum gauges, temperature logging), and pronounced it "ready to go" at the end of my work day.

2) What did I learn today?

- Cryocooler has only one setting, and temperature control must be engineered at the output using thermal contact, emissivity, etc.

- Formatting of USB is the main error that can befall the CTC100 datalogger. If the red dot in the upper right corner of the screen does not light up bright when it is tapped (this starts datalogging), then there is something wrong. Easy enough to test by removing the USB when the red dot is dim (datalogging paused) and checking whether there are log contents.

- Raymond's focus with the QIL chamber had been on answering the question, "can we cool down the cryocooler's connection (copper linkage which passes into the chamber) adequately?" He had never successfully obtained a cooldown that was below 150 K, and the primary limitation appeared to be related to high pressures in the "external volume".

3) What are we up to next?

- The next time I come in, I will be turning on the cryocooler and datalogging first thing, and I will hopefully have cooldown trends to share in the log.

- If those trends are > 150 K, I was advised that the next thing to do would be to bring the "external volume" out of the equation, and directly attach the cryocooler to the copper feedthrough linkage. This would be one way to demonstrate the least-lossy, best case scenario.

- If < 150 K, I am told that Karthik may be ready to move in for some measurements. If not, I would be interested in dropping in the suspended, shielded Silicon dummy (currently standing by) and seeing if we can measure a successful (< 150 K) cooldown on the Si mass.

4) Can we increase the height of the chamber?

I've shared lots of images related to the question of extending the height of the chamber. Here are my thoughts:

- Raw measurements - Ceiling = 0", Crane ~ -12",  Crane hook ~ -16", Chamber Lid ~ -26", Chamber Base ~ -40", Table ~ -43".

--> Not much space above the surface of lid, currently about 10" of range for a possible extension.

--> Actual useable range is less, due to real world limitations such as the height of lifting straps, interference with the angled crane arm, etc.

- It would require a clever solution to increase the crane height (spacer at base? extended height model?) or lower the lid height wrt the crane (position on lower table? lower the table on its leveling feet?) to buy a few more inches.

- Current allowed object height ~8" (could be extended to about 10" with modified PEEK spacers at base); would we benefit greatly from having a ~16" allowed object height? Or do we need to get more height out of this update?

- I need to follow up with my request for quote of an extension of ~10" height.

Attachment 1: CryoLabImages_QILelog2530.zip
Attachment 2: IMG_8019.JPG
Attachment 3: IMG_8024.JPG
Attachment 4: IMG_8020.JPG
2529   Mon Jan 25 16:05:43 2021 aaronMiscEquipment transfercantilever Qs to QIL

Mid afternoon I moved some equipment to QIL for making Q measurements of cantilevers, which Karthik is planning to do in the IR labs cryostat. See cryo elog for more information, photos attached to show location of equipment in QIL.

Attachment 1: 201A9C3E-1F44-4D03-B08F-F806B07011FA.jpeg
Attachment 2: 17B5D21C-CF37-41D3-95D6-F5C85596D728.jpeg
2528   Fri Jan 15 10:55:23 2021 PacoLab InfrastructureGeneralBorrow AG4395A

This morning I rolled the AG4395A from Adaptive Optics lab (labeled QIL, IP = 10.0.1.64) for use in Crackle. Will return after end using.

2527   Tue Dec 29 17:53:21 2020 ranaSummaryGeneralInstrument loan

doesn't seem so, but they sell this one:

http://www.nfcorp.co.jp/english/pro/mi/loc/m_lp/p_lp/lp_6016_01/index.html

which has a USB interface and pretty good voltage noise spectrum

2526   Tue Dec 22 15:20:14 2020 KojiSummaryGeneralInstrument loan

Is the reverse bias programmable? FEMTO has a bias trimmer on it. It's useful in the usual application, but for automation, the configuration of the input becomes cumbersome.

2525   Tue Dec 22 10:42:17 2020 ranaSummaryGeneralInstrument loan

I was thinking about getting this new current pre-amp from NF:

http://www.nfcorp.co.jp/english/pro/mi/loc/pre/ca5351/index.html

It seems to have a good noise performance and has a built in low pass filter and also a remote interface.

The FEMTO seems less fancy, but their noise performance is actually 2-3x better.

 Quote: FEMTO DLPCA200 low noise preamp (brand new) Keithley Source Meter 2450 (brand new) => Returned 11/23/2020 were brought to the OMC lab for temporary use.

2524   Wed Dec 16 22:08:36 2020 anchalUpdateEquipment transferTook delay line box, compressed nitrogen cylinder and lens to 2um (Crackle) lab
• Took a delay line box DB64 from QIL from the WOPO table to the 2um (formerly known as Crackle) lab. The box was marked Crackle on it.
• Took the compressed nitrogen cylinder for optics cleaning which was stored in Adaptive Optics lab.
• Took some lens from the cabinet in Adaptive optics lab.
• Took some other optics parts like pedestals, posts, lens mount etc.

See SUS_Lab/1877

2523   Tue Dec 8 11:27:51 2020 RadhikaMiscLab MonitoringParticle counter communication

I have a python script for communication with the Met One 227a particle counter, but it appears like I am not receiving a response from the device. I swapped out a few serial-USB cables, but this did not fix the issue. I suspect I am making an unsound assumption about the communication protocol so I am going back to basics and reading more on RS232 and ASCII commands.

2522   Fri Nov 20 18:49:43 2020 KojiSummaryGeneralInstrument loan

FEMTO DLPCA200 low noise preamp (brand new)

Keithley Source Meter 2450 (brand new) => Returned 11/23/2020

were brought to the OMC lab for temporary use.

2521   Fri Nov 20 18:47:42 2020 KojiSummaryGeneralPermenant exchange of TED200C(QIL) and TED200C(2umECDL)

I moved the brand new TED200C on the workbench to Crackle for 2um ECDL (permanently)
The TED200C temp controller used in the 2um PD test setup will stay there (permanently)

http://nodus.ligo.caltech.edu:8080/SUS_Lab/1851

2520   Thu Nov 19 16:25:30 2020 AidanUpdate2um PhotodiodesAgilis piezo mirror installed in JPL PD testing apparatus

Here's the python code I used to control this.

I incorrectly used the Move to Limit command ('1MV-3': axis 1, MoVe, negative direction, speed 3', where the speeds are given in the manual, see Section 4.7 in particular). Once this command is issued, the stage will keep moving until it receives the stop command. The JOG command would be more appropriate.

I confirmed a smooth change in the PD output as the beam translated across it.

 Quote: I installed the Agilis mirror before the lens and cryo-chamber. Used the USB interface to align the beam onto the PD. So we can control the alignment remotely now (or once I’ve properly connected the USB cable instead of today’s janky test connection).

Attachment 1: Screen_Shot_2020-11-19_at_4.27.17_PM.png
2519   Tue Nov 17 17:51:44 2020 AidanUpdate2um PhotodiodesAgilis piezo mirror installed in JPL PD testing apparatus

I installed the Agilis mirror before the lens and cryo-chamber. Used the USB interface to align the beam onto the PD. So we can control the alignment remotely now (or once I’ve properly connected the USB cable instead of today’s janky test connection).

Attachment 1: IMG_9401.jpg
Attachment 2: IMG_9400.jpg
2518   Wed Nov 11 16:09:22 2020 anchalSummaryECDLAUX wavelength finesee requirements in mariner - Added Excess Scatter Noise

An issue was raised with last calculation about the fact that our sensing of PDH signal isn't ideal and in the real world there is scattering, clipping extra adding excess noise in the PDH loop. This noise primarily comes by the intensity noise imparted on promptly reflected light from the cavity via various shaking optics etc on the table before it goes to the PDH reflection RF photodiode.

This noise's coupling to the PDH loop is identical to how shot noise of light couples into the PDH loop i.e.:

• Intensity noise of light is converted into voltage oise by the PDH photo diode.
• This is compared against the cavity finesse amplified real PDH error siganl at this stage.
• Therefore, in frequency noise, the affect of this intensity noise is smaller for higher finesse cavities since cavity finesse only amplifies the PDH signal anf not the scattering noise.

### Excess noise estimate

• I used this measurement taken in 40m with Koji to estimate this noise.
• This measurement contained a beatnote between IR coupled AUX light and the main laser IR pick-off when X-arm is locked to the main laser and AUX laser is locked to X-arm.
• So this noise measurement is an upper bound on the total noise in AUX laser frequency when it is locked to the X-arm.
• I compared this against the noise budget model for AUX PDH loop I have which uses the same control loop as the uPDH box used here.
• I found a bulge of excess noise below 100 Hz and it seemed to go done as 1/f^2 there. I was reminded by a chat I had with Rana and another professor sometime last year when Rana mentioned scattering noise showing up as "Scatter shelf" looking something like this.
• So I modeled excess noise as the difference between the noise budget and the measured noise with it extending after 100 Hz with the same roll off as in 10-100 Hz.

### Calibration noise budget

• I took the excess noise measured, converted it to W/rtHz by using current AUX PDH discriminant and photodiode gain, and normalized by the power (9.6mW) to get this noise in RIN/rtHz.
• Then I assumed that the same RIN would be imparted in the Mariner AUX loop and calculated excess intensity noise at the PDH loop by multiplying the above number with assumed 10mW of incident power to get it in W/rtHz.
• From here, I fed it to the same input as I feed the shot noise in the loop and calculated the effect in the overall noise budget.
• For high bandwidth and gain PDH loops required for calibration, this kind of noise would dominate up to a kHz before getting taken over by the residual laser frequency noise.
• I have again plotted cases for three choices of finesse/mirror transmission. If we used 99.95% reflectivity (1000 ppm transmission, finesse of 3140) we would be fine for most calibration lines except the one around 40 Hz. (assuming drive strength of 1e-13 m everywhere).
• Otherwise, if possible, we should go for higher finesse. Case (c) plotted here (Page 3), shows that for 99.995% reflectivity (100 ppm transmission, finesse of 31415), we will be fine in all over the range with cavity pole dropping to 63 Hz. This would be really nice of course, if it is possible.
• So we recommend HR coatings for 1418 nm in the Mariner to be 99.995% reflective (giving total power transmission of 100 ppm).
Attachment 1: AUX_Finesse_Study_With_ECDL.pdf
2517   Tue Nov 10 12:46:34 2020 AidanUpdate2um PhotodiodesJPL PD resurrection (cont.)

Looks like the temperature difference between the PD and the shield is relatively small. Even the transients when the heater is applied are order 5K.

This means that, for quick purposes, the shield RTD is a good proxy for the PD temperature.

The attached data is the difference between PD and shield RTD from circa 5th-6th February 2020.

 Quote: Okay - all the steps in the procedure of eLOG 2476 have been verified as working - with the exception of the RTDs in the chamber.  With regards to taking dark noise spectra at different biases and temperatures, looks like Raymond took spectra with biases of [50, 100, 200, 400, 600, 1000]mV. If no objections, I’ll stick to that number of measurements.  I’m a bit pushed for time with other stuff. I wonder if the shield RTD is sufficient to run tests on the system? I’ll go back through the data and see how reproducible the relationship between shield temperature and PD temperature is. If it is reliable then in the interests of time, I’m going to forgo re-installing the extra RTDs in the chamber just now.

Attachment 1: temperature_diff_shield_v_PD.png
2516   Mon Nov 9 15:49:58 2020 AidanUpdate2um PhotodiodesJPL PD resurrection (cont.)

Okay - all the steps in the procedure of eLOG 2476 have been verified as working - with the exception of the RTDs in the chamber.

With regards to taking dark noise spectra at different biases and temperatures, looks like Raymond took spectra with biases of [50, 100, 200, 400, 600, 1000]mV. If no objections, I’ll stick to that number of measurements.

I’m a bit pushed for time with other stuff. I wonder if the shield RTD is sufficient to run tests on the system? I’ll go back through the data and see how reproducible the relationship between shield temperature and PD temperature is. If it is reliable then in the interests of time, I’m going to forgo re-installing the extra RTDs in the chamber just now.

2515   Mon Nov 9 10:08:38 2020 anchalSummaryECDLAUX wavelength finesee requirements in mariner with 1418 nm ECDL (Preliminary)

I have a preliminary calculation to post here. This does not include noise sources from cavity fluctuations and main frequency noise. But it gives some idea about shot noise and frequency noise of AUX laser conttribution to the noise in calibration.

### What's included?

• I have put in measured PZT transfer function of ECDL at ANU upto 25 kHz. Above this point, they did not measure it so I couldn't make it artificially dirty. I just assumed 1/f roll off above (which is definitely incomplete picture).
• I have included phase effects of cavity FSR in the loop by adding the resonance features as mentioned here.
• I have added attempted resonance compensation for 1kHz and 2kHz features in the PZT after fitting the data with poles and zeros and iverting them.
• 10mW of incident AUX light is assumed on the arm cavity.
• Total 10 mW of combined power at 709 nm is assumed to fall on the beatnote. So AUX light would be frequency doubled for this beatnote.

### What's left?

• Need to add seismic noise and other measured excess noise that come from the cavity motion.
• Need to add laser frequency noise of main laser, however, it must be small since it is locked to mode cleaner.
• Need to add digital delay of Red Pitaya or whatever filter would be used for PZT resonance compensation.
• Need to model PZT transfer function of ECDL above kHz properly. ANU replied that they can't measure it for higher frequencies due to lack of time.
• Need to do time domain stability analysis. This I haven't been able to do as I have just been using python-controls package as black box to compute impulse and step responses of state space systems. When simply adding easured transfer function data, I couldn't create the state stpace representation for the system. I tried to fit multiple resonances above 2 kHz but couldn't really capture the magnitude of the response well. Maybe I can just assume higher harmonics of the 1kHz and 2kHz resonances?

### Attachments:

• Page 1 is the measured PZT transfer function fo 1900 nm ECDL from ANU along with the modeled 1/f roll off after 25 kHz.
• Page 2 is the open loop transfer function of the AUX PDH loop for the three different finesse cases studies. Note that the blue curve is hidden beneate the other two curves. Before objections came, I know this is unreal and incomplete, but I have to start somewhere.
• Page 3 is the calibration noise budget with different colors showing the three finesse cases.
• This is also incomplete but we can takeaway what the shot noise contribution would look like and initiate a dialogue about the integration time chosen (which is 100s here), the SNR aim chosen (1000 here) and what drive strength would be good enough.
• From notes of Craig and Gautum, I think we can drive the mirrors at 0.1pm ampltude in the calibration band. From my first and only calibration measurement in 40m, I could drive upto a pm without loosing lock in the cavities.
• But that was a simple single arm lock and full ifo lock must be more sensitive. So is 0.1 pm drive strength good enough or do we want to aim lower?
Attachment 1: AUX_Finesse_Study_With_ECDL.pdf
2514   Fri Nov 6 12:45:21 2020 AidanUpdate2um PhotodiodesPhotodiode testing recovery status

Embellished Chris's PD MEDM screen a bit to illustrate controls in a diagram. The representation of the RELAY SWITCH between the Keithley and the SR560 is a bit off - I think the transimpedance amplifier is switched out as well.

Also - Keithley bright PD sweep output is attached.

 Quote: Quick update, more detailed update to follow. Laser is working Photodiode sweep with the Keithley shows a sensible dark Current v Voltage plot (when laser is off) - indicating that PD wiring is still intact Laser was aligned onto photodiode (although it took a while aligning to find the signal) Ran a sweep with the Keithley and the laser on - saw another sensible bright Current v Voltage plot (more current than in the dark case) DAQ control still works DAC output is directly providing (unfiltered) 200mV bias Still to do: Get the SR785 plugged back in Get an SR560 inserted between DAC output and PD bias to low pass filter Investigate why the laser current set point is so noisy Sort out RTD situation inside the chamber Miscellaneous stuff

Attachment 1: MEDM.png
Attachment 2: PD_sweep.png
2513   Fri Nov 6 08:14:58 2020 AidanUpdate2um PhotodiodesPhotodiode testing recovery status

Quick update, more detailed update to follow.

• Laser is working
• Photodiode sweep with the Keithley shows a sensible dark Current v Voltage plot (when laser is off) - indicating that PD wiring is still intact
• Laser was aligned onto photodiode (although it took a while aligning to find the signal)
• Ran a sweep with the Keithley and the laser on - saw another sensible bright Current v Voltage plot (more current than in the dark case)
• DAQ control still works
• DAC output is directly providing (unfiltered) 200mV bias

Still to do:

• Get the SR785 plugged back in
• Get an SR560 inserted between DAC output and PD bias to low pass filter
• Investigate why the laser current set point is so noisy
• Sort out RTD situation inside the chamber
• Miscellaneous stuff
2512   Thu Nov 5 11:20:45 2020 anchalSummaryECDLTrue PDH Error signal TF and including FSR effects in approximated models

If we use ECDL for auxiliary frequency in 40m and hope to stabilize it up to 1 MHz with digital compensation of PZT, it is important to take into account any phase effect of the nearby FSR at 3.97 MHz. This should ideally be included in the Input Mode Cleaner loop considerations as well. These effects would be more prominent in longer cavities like aLIGO and LISA where FSR is very low and should we attempt to stabilize a laser lock beyond cavity's FSR.

I did a no assumptions calculation for getting a general transfer function fo PDH error signal in units of [W/Hz] assuming 1 W of incident power. This calculation would soon be uploaded here. I'll put down here primary results.

For incident field on a Fabry-Perot cavity (with fsr of $\nu_{fsr}$), reflected electric field transfer function (unitless) is given by:

$\dpi{300} \tiny R(\omega) = \frac{-r_1 + (r_2^2 + t_2^2) r_2 e^{-i \omega/\nu_{FSR}}} {1 - r_1 r_2 e^{-i \omega/\nu_{FSR}}}$

Then, PDH error signal for a modulation frequency of $\Omega$ at a modulation index of $\Gamma$, in units of [W/Hz] (i.e. error signal power per Hz of error in laser frequency from cavity resonance) is given by:

$\dpi{300} \tiny H_{\nu 2P}(\omega) = -\frac{i \pi P_0 J_0(\Gamma)J_1(\Gamma)}{\omega} \Bigl(R(\Omega)R(\omega) - R(0)R(\Omega + \omega) + R^*(\Omega)R(\omega) - R(0)R^*(\Omega - \omega)\Bigr)$

after demodulation and low pass filtering. Note this transfer function is a complex quantity as it carries phase information of the transfer function too. The real signal is obtained by multiplying this signal at $\omega$ with $e^{i \omega t}$ and taking the real value of the product.

Having done this, we can see how in the real PDH error signal, there is a low pass at cavity pole, given by $- \frac{\nu_{fsr}}{2\pi}log(r_1 r_2)$ and a notch every fsr. The notch creates a zig-zag in the phase of the tranfer function and has a HWHM same as cavity pole. After this point, I just fitted a ZPK model to the transfer function to obtain a empirically derived model for PDH error signal transfer function. Apart from the cavity pole, this model needs to have resonance and antiresonance features present at each FSR with resonance having a linewidth of cavity pole while anti-resonance having a linewdth of $\pi/\nu_{fsr}$. Here's how the ZPK model would look like:

\dpi{200} \begin{aligned} z:&\, \pi/\nu_{fsr} \pm n \nu_{fsr} j \\ p:&\, f_p \pm n \nu_{fsr} j, f_p \end{aligned}

I've attached my notebook where I did the fitting analysis and the overlap plot of real PDH error signal TF and the modelled approximation.

Attachment 1: PDHErrorTFModel.pdf
Attachment 2: PDHTFforACavity.ipynb.zip
2511   Wed Oct 28 14:05:19 2020 ranaSummaryECDLEffects of chosen AUX finesse and source on Calibration requirements
 Quote: would be easier to achieve there with higher laser powers and higher cavity finesse. But I haven't attempted that here as we do not know our NPRO PZT's resonance features yet.

I don't know why it would be easier to have higher finesse with longer arms. Something about beam size???

The NPRO PZT TF's are all in the 40m elog - there are many measurements of TF made over the past 10 years. Its like Raiders of the Lost Ark - you have to believe its there while searching.

ELOG V3.1.3-