I specifically checked the specification before mounting the mirror. It says clearly "Arrow at the thinnest location pointing towards Side 1". I guess they just ignored it.

As for LO1, I mounted it without noticing the location of the arrow. Later, I checked and the ghost beam was horizontal so I left it as it is. Yeah, I guess I will remount the mirror. Also, what do we do with the pencil markings? It's not vacuum-compatible.

{Yehonathan, Anchal}

I released the AS1 wires from the winches, removed the adapter from the SOS tower, and removed the Lambda optic from the adapter. Attachment 1 shows the pencil markings on the optic before cleaning. I cleaned the pencil marking from the side of the optic with acetone using swabs until there were no pencil residues on the swab (attachment 2 shows the swab I used next to an unused swab). I was not able to remove the markings completely though (attachment 3).

I remounted the optic with the arrow rotated by 90 degrees counterclockwise.

We hang the adapter on the winches and adjust the height of the magnet and the adapter roll using the winches. We monitor the height of the adapter using a live stream from the Cannon camera. The camera's tilt was adjusted using straight features on the SOS tower. When we ran out of winch travel we adjust the height using the lower EQ stops and pull tight the wires. Attachment 4 shows the alignment of the side magnet with respect to the SOS tower and a side OSEM.

We checked the ghost beam trajectory and it looks much better (attachment 5)

We started realigning the OpLev. We realize that the height of the beam should be 5+14/32" = 5.437 by measuring the height of the screw holding the side OSEM from the table. The real height from the schematics is 5.425 We make the beam parallel with the table first using an iris and then the QPD.

Today, I balanced the counterweight. First using an iris, then by placing a QPD close to the SOS measuring the reflection from AS1. I locked the counterweight's set screw and the QPD Y readout looks good. Attachment 6 shows the QPD y readout near the beat node between pitch and pos. The node comes very close to zero which indicates that the pitch is balanced.

I measured the free-swinging motion using the QPD x and y axes. Attachment 7 shows the spectra of that motion. The major peaks are at 755mHz, 953mHz, and 1.05Hz.

I locked the EQ stops while retaining the XY alignment on the QPD and installed 5 green OSEMs. AS1 is ready for transfer into the vacuum chamber.

{Paco, Yehonathan, Anchal}

Today we suspended AS4 (E2000226-B). Anchal mounted Lambda Optic mirror with an RoC closest to AS4 in a thin optic mount. He noted that this optic as well as AS1 don't have a wedge angle. The specs claim that the wedge angle is 2 degrees what should have been clearly seen by inspecting the optic with a naked eye. All the ghost beam deflections probably come from the curvature of the mirror.

We did all the height and roll balancing using a camera (Attachment 1,2). We balanced that pitch of the adapter using a QPD not before we realigned the OpLev setup.

We measured the motion spectra (attachment 3). Major peaks are found at 755 mHz, 964 mHz, and 1.062Hz. I locked the counterweights setscrew and observed that the pitch balance doesn't change. I locked the EQ stops such that the alignment of the mirror remained the same by monitoring the QPD signals. I clamped the suspensions wires to the suspension block.

The only thing remaining is inserting the OSEMs.

{Paco, Yehonathan}

Today we suspended LO2 (E1800089) which Anchal has loaded into the thick optic adapter. Attachments 1,2 show the height and roll balance adjustments.

I realigned the opLev setup and balanced the suspended mass. We figured that if we use 2 counterweights we will be 1 short. We decided to use 1 mass at the back of the adapter. This has the additional advantage that the Viton tip on lower back EQ stop can touch it and act normally. The optic was successfully balanced in this way. Attachment 3 shows the motion spectra on the QPD. There are major peaks at 712 mHz, 854 mHz, 876 mHz, and 996 mHz. As expected using only 1 counterweight raised the center of mass and lowered the pitch resonance frequency. The optic was locked keeping the alignment fixed on the center of the QPD, OSEMs were inserted and the SOS tower was engraved.

We should apply some glue to the counterweight to prevent it from spinning on the setscrew.

There were several cases where the long EQ stops didn't perform as expected.

In one type of case, we used a counterweight at the front of the adapter but not in the back leaving a recess where the lower back EQ stop should touch.

In the other type, a recess in the thick optics adapter prevented the upper EQ stop from touching the adapter. In the first thick optic, the screw was screw barely scratched the recess' corner. In the second case, it didn't touch it at all.

In the last group meeting, we discussed using Peek screws (made out of plastic) to prevent metal on metal bumping when the EQ can touch the adapter and Peek nuts when it doesn't to increase its impact area.

Mcmaster has 1.5" long 1/4-20 screws (part number 98885A131) that will fit well in the OSEM plates. We can order 20 of those.

The biggest Peek nuts on Mcmaster however are not big enough (7/16" wide) to cover the entire bottom recess area which is 0.5" wide (they are good enough for the top recess area in the thick adapter optic design). Koji suggested that we can use a big Peek washer for that purpose that can be held between nuts. We should then order 10 Peek nuts (98886A813) and 1 package of 10 Peek washers (0.63" OD) (93785A600).

{Yehonathan, Paco}

{Paco, Yehonathan}

Today we suspended SR2 (E1800089) which Anchal has loaded into the thick optic adapter. Attachments 1,2 show the height and roll balance adjustments.

I realigned the opLev setup and balanced the suspended mass. Attachment 3 shows the motion spectra on the QPD. There are major peaks at 723 mHz, 832 mHz, and 996 mHz. I inserted OSEMs and tightened them in place. I adjusted the OSEM plates to make sure the magnets are at the center of the OSEMs, then I tightened the OSEM plates to the SOS tower.

The optic was locked keeping the alignment fixed on the center of the QPD.

Again, we should apply some glue to the counterweight to prevent it from spinning on the setscrew. Is there a glue other than EP30 that we can use?

Related: Peek nuts, screws and washers were ordered from Mcmaster.

{Yehonathan, Anchal, Paco}

In the cleanroom, we removed AS1 and AS4 from their SOS towers. We removed the mirrors from the adapters and put them in their boxes. The broken magnets were collected from the towers and their surfaces were cleaned as well as the magnet sockets on the two adapters and on the side block from where the magnets were knocked off.

We prepared our last batch of glue (more glue was ordered three days ago) and glue 2 side magnets and 2 face magnets. We also took the chance and apply glue on the counterweights on the thick optic adapters so there is no need to look for alternatives for now.

The peek screws and nuts were assembled on the thick optics SOS towers instead of the metal screws and nuts that were used as upper back EQ stops.

{Yehonathan, Anchal}

Came this morning, the gluing of the magnets was 100% successful. Side blocks, counterweights were assembled. We suspend AS4 and adjust the roll balance and the magnet height (attachments 1,2). OpLev was slightly realigned.

The pitch was balanced. We had to compensate for the pitch shift due to the locking of the counterweights. Once we got good pitch balance, the motion spectrum was taken (attachment 3). Major peaks are at 755mHz, 953mHz, 1040mHz.

Previous peaks were 755mHz, 964mHz, and 1.062Hz so not much has changed. We pushed back the OSEMs, adjusted OSEM plate and locked it tightly. We lock the EQ stops and transfer AS4 to the vacuum chamber in foil. We open the foil inside the chamber. No magnets were broken. Everything seems to be intact. We connect the OSEMs to CDS.

Today I suspended AS1. Anchal helped me with the initial hanging of the optics. Attachments 1,2 show the roll balance and side magnet height. Attachment 3 shows the motion spectra.

The major peaks are at 668mHz, 821mHz, 985mHz.

For some reason, I was not able to balance the pitch with 2 counterweights as I did with the rest of the thin optics (and AS1 before). Inserting the weights all the way was not enough to bring the reflection up to the iris aperture that was used for preliminary balancing. I was able to do so with a single counterweight (attachment 4). I'm afraid something is wrong here but couldn't find anything obvious. It is also worth noting that the yaw resonance 668mHz is different from the 755mHz we got in all the other optics. Maybe one or more of the wires are not clamped correctly on the side blocks?

The OSEMs were pushed into the OSEM plate and the plates were adjusted such that the magnets are at the center of the face OSEMs. The wires were clamped and cut from the winches. The SOS is ready for installation.

Also, I added a link to the OSEM assignments spreadsheet to the suspension wiki.

I uploaded some pictures of the PEEK EQ stops, both on the thick and thin optics, to the Google Photos account.

I was wondering whether I should take AS1 down to redo the wire clamping on the side blocks. I decided to take the OpLev spectrum again to be more certain. Attachments 1,2,3 show 3 spectra taken at different times.

They all show the same peaks 744mHz, 810mHz, 1Hz. So I think something went wrong with yesterday's measurement. I will not take AS1 down for now. We still need to apply some glue to the counterweight.

I picked up the PR2 mirrors (labeled M1, M2) from Anchel's table and took them to the cleanroom. By inspection, I spotted some dust particles on M1. I wasn't able to remove them with clean air so I decided to use M2 which looked much cleaner. I wasn't able to discern any wedge angle on the optic. I inserted the optic into a thin optic adapter. The optic is thicker than I expected so I use long screws for the mirror clamping. I expect that the pitch balance will shift towards the front of the mirror so I assembled only 1 counterweight for now. The side blocks with wires in them were installed.

I engraved the SOS and installed the winches on it. Paco came in and helped me to hang the optic. Looking at the wire hanging angle I realize that 1 counterweight at the front is not enough. I install a second counterweight at the back and observe that I can cross the balancing point.

I locked the EQ stops. Suspension work continues tomorrow...

I went to the Y end. The AUX laser was on Standby. I pushed the Standby button. The laser turned on and there was some green light. However, the controller displayed the message "CABLE?" which according to the manual means that the laser head is powered but there is no control over the laser (e.g. the control cable is disconnected). I turned off the controller and disconnected both the power and control cables. I put them back and turned the controller back on.

I pushed the Standby button, the laser turned on and this time the controller displayed the laserhead's state. I was able to change the current/temperature. The problem seems to be resolved.

{Yehonathan, Anchal, Paco}

Yesterday, Anchal and Paco removed AS1 from the vacuum chamber and moved it into the cleanroom. The suspension wires were cut and the AS1 optic was put on the table.

Two things were noticed:

1. One of the wires was not sitting inside the side block groove (attachment 1)

2. One of the face magnets was grossly tilted (attachment 2). Probably due to uneven polishing of the dumbbell.

We put new wires into the side blocks making sure they sit in their grooves and we removed the tilted magnet. A different, more straight magnet was picked from the remaining spare magnets. The dumbbell and adapter were cleaned from glue residues and a batch of glue was prepared.

In the process of gluing a different magnet was knocked off. We cleaned that magnet too. The 2 magnets were glued on the adapter.

Today I came and saw that the gluing failed completely. One of the magnets was completely away from its socket and the other one wasn't glued at all.

I prepared a new batch of glue and glued the two magnets.

Yesterday, I rebuilt the OpLev setup in the cleanroom in order to suspend AS1. It took me a while to find all the necessary parts but I found them in the end.

The HeNe laser was placed on the optical table and turned on. The beam was aimed to bounce off a folding mirror to the SOS tower.

The beam's height was controlled by the HeNe laser stage and made to be 5+14/32". The beam from the folding mirror was made parallel to the table, first with an iris and then with the QPD connected to a scope.

Preparing the SOS tower for the suspension I noticed that the wire clamp is scratched on both sides from previous suspensions. I discarded that wire clamp but couldn't find the spares. Time ran out and I had to stop.

Came this morning and saw that the IMC is unlocked.

Went into MC Lock screen and see that the watchdog is down and the PSL shutter is closed. I tried to open the shutter but nothing happened - no REFL signal or beam on the MC REFL camera .

Thinking this has something to do with the watchdog I upped the watchdog:

ezcawrite C1:SUS-MC2_LATCH_OFF 1

The watchdog on the MEDM screen became green but the shutter still seemed unresponsive. I went to the PSL table and made sure that the shutter is working. I opened the AS table and saw there no MC REFL beam anywhere.

Thinking that MC1 must be completely misaligned I opened the MC align screen to find that indeed all the alignment values has been zeroed! (attachment).

I burt restore c1iooepics from Mar 4th 00:19. Didn't help.

I try to burt restore c1susepics from Mar 1st 13:19. Still zero.

I try to burt restore c1susaux from Mar 1st 00:19 -> seems like alignment values have been restored.

I open the shutter. Beam is flying! MC Watchdogs tripped! I close the shutter. OK, I need to wait until the MCs are dampped enough. MC2 and MC3 have relaxed so I enable their watchdogs. MC1 is still swinging a bit. I turn on damping for MC1 as well.

MC locked immediately but the REFL is still high like 1.2. Is it normal?

I turn on the WFSs and the REFL went down to 0.3 nice. I run the MC WFS relief script.

{Paco, Yehonathan}

We tried to roughly balance the adapter with two counterweights at the front, like with the other thin optics using an iris. As before, we couldn't get the beam above the iris hole no matter how much we inserted the counterweights into the adapter. We noticed that one of the side blocks is actually the one where the clearance for the wire was made on the wrong side. So there was clearance on both the up and bottom sides of the side block (see attachment 1).

Could this be the cause of the balancing issue? Running out of ideas on how to fix it we gave it a try and replaced it with a spare side block. We also found that the wire on the other side block was kinked so we replaced the wire on this one as well.

After inserting new wires into the side blocks, we hung the adapter on the winches and the beam was above the iris aperture! How could this tiny amount of missing mass make this much difference?

We were able to roughly balance the adapter.

We then tried to balance the roll of the adapter but accidentally knocked off the side magnet 😫.

We usually glue several side magnets together and they all together support the metallic plate on which the magnets are magnetically attached to. This time we had only one side magnet to glue so instead of trying to glue the magnet vertically we are trying to glue it horizontally using a flat surface and a stage to clamp it (attachments 2,3).

BTW, the HeNe was not working when we came into the cleanroom. We realized it was the old HeNe that we already determined to be broken but there was no sign on it. I attached a "BAD" sign on it and replaced it with the new HeNe. The OpLeve beam was realigned. All of this happened before all the things described above

The gluing seemed to be successful. I assembled the side block with the magnet on the adapter. Paco helped me hang the adapter on the SOS tower.

The height and roll of the adapters were balanced (attachment 1,2).

The QPD was placed at the beam reflection. The beam was centered horizontally on the QPD and then measured vertically. The pitch DOF was balanced using the counterweights. The counterweight was locked. Balance was retained.

I tried to assemble the upper mirror clamp on the tower but for some reason, one of its tap holes was not able to accept screws. I gave it to Jordan for retapping. I measured the motion spectrum using the QPD connected to a scope (attachment 3).

Major peaks are at 668mHz, 942mHz, and 1029mHz.

I have made an editable draw.io diagram of the planned simplified BHD setup on the ITMY table (see attached).  10 pts = 1 inch.

This is very sketchy but easily adjustable since we are removing everything but the ITMY Oplev from that table.

{Yehonathan, Anchal}

We connected the c1auxey1 chassie to the different boxes (coil drivers, SAT amp, etc.) using DB9 cables and labeling them in the process. We ran out of 2.5 foot DB9 cables so we used 5 foot as a temporary solution.

The chassie was powered, but a two issues arised:

1. The Acromags didn't turn on.

2. When connecting the green laser shutter BNC cable, the power supply overloaded.

We took the chassie back to the bench. The wire that powers the Acromags was disconnected. We made a new longer wire and made sure it is not connected flimsily.

The issue with the BNC turned out to be a much deeper problem: The GND and EXC wires on the DIN rail connector were switched! Making the shield of the BNC to have high volatage compared to the shield of the green shutter causing current to overflow when the BNC was connected.

We switched back the EXC and GND wires. Not trusting the digital I/O tests that were done before due to this mistake we tested some of the I/Os using a spare coil driver. We tested both the inputs and the outputs and they all seemed to work.

Finally, we also noticed that the 2 RTS DB9s were wrongly female type so we switched them to males. We closed the lead on chassie and installed it back in the rack. We connected the cables and saw that the green shutter BNC cable was no longer shorting the power supply.

I begin modeling the initial BHD setup using Finesse. I started with copying C1_w_BHD.kat from the 40m/bhd repo and making changes to reflect the current BHD setup:

1. OMCs were removed.

2. Only 1 PD per BHD port was left.

3. Transmission of PR2 was changed to 2.2%. The PRG was calculated to be ~15.5.

4. Actual RoCs of new optics were dialed in (Yesterday me and Paco went into the cleanroom to measure the RoCs and they seem to match the datasheets).

Here's a table comparing the old (design?) RoCs with the new RoCs:

The changes looked quite alarming, especially for LO4 and AS3, so I wrote a script to calculate the mode matching between the LO and AS beams called AS_LO_ModeMatching.ipynb and pushed it into the repo. In the notebook a bright AS beam is created by creating a small asymmetry between the arms of ~ 0.003 degrees (~10pm). Amplitude detectors were put at the input ports of the BHD BS to calculate the fields in the AS and LO beams. In particular TEM00, TEM02 and TEM20 were measured for each beam.

The calculation shows that with the old RoCs the mode matching between the LO and AS beams is 99% while for the new RoCs it is ~ 50%.

Ok, it turns out these optics were purchased on purpose, as this elog shows. Jon considered building a mode-matching telescope with stock optics as an initial step before purchasing the custom optics (referred to as "design" optics in my elog).

I dialed in the new distances between the optics into the .kat file as described in this elog and pushed the changes to the repo. With the new distances, I got mode-matching of 87% for the full IFO and 89% for FPMI so there's probably no need to worry as the mode-matching with these optics was already designed.

Came this morning, opened the PSL and there was not even a beam on the MC REFL.

Looking at the big monitor it seems like the WFS signals went through the roof during the "auto-alignment" night session.

I restored the MC alignment from before the misalignment happen and wait for the SUS to damp. Once the RMS values went below 200 I enabled the watchdog and the coil outputs.

I opened the PSL shutter and the IMC locked immediately. I turned on the WFS servo and the MC REFL DC went down to 0.3. I run the WFS relief script.

I'm planning on simulating the BHD readout noise in a manner very similar to the ALS noise model using Simulink. I've made a sketch of the model for the longitudinal DOFs (attached). A model for ASC will be similar but with more measurement devices (OpLevs, QPDs, WFSs).

I'm not pretending to simulate everything in this diagram on the first go, it is just a sketch of the big picture.

I feel like there is an instability in my thought process on this. Before my tendency to try to scale and generalize this problem brings me to a full stop I will make small but quick progress.

First thing is to calculate the noise budget for a simple Michelson. The involved optics are:

• ITMX
• ITMY
• BS
• LO1
• LO2
• AS1
• AS4

all sensed with OSEMs and OpLevs only.

Things to fetch:

1. OSEM sensing noise. Where do I get the null stream (AKA butterfly mode)?

2. Oplev noise, look at the SUM channel (or this elog)

3. Actuation TF. Latest elog.

4. Coil driver noise. Going to take the HP supply curve from this elog.

5. Seismic noise + Seismic stack TF. Or maybe just take displacement noise from gwinc.

6. Laser noise. Still need to search.

7. DAC noise. Still need to search.

I have made a Simulink diagram to use in the MICH modeling (attachment) for the homodyne angle detection scheme. The model will be used for each optic separately and the noises will be combined in quadrature.

I gathered some more bits of info to fill the Simulink boxes. This is what I have so far:

I was finally able to set up a stable suspension model with the help of Yuta and I'm now ready to start doing some MICH noise budgeting with BHD readout. (Tip: turns out that in the zpk function in Matlab you should multiply the poles and zeros by -2*pi to match the zpk TFs in Foton)

I copied all the filters from the suspension MEDM screens into a Matlab. Those filters were concatenated with a single pendulum suspension TF with poles at [0.05e-1+1i, 0.05e-1-1i] and a gain of 4 N/kg.

I multiplied the OLTF with the real gains at the DAC/DAC/OSEMs/Coil Driver and Coils. I ignore whitening/dewhitening for now. The OLTF was calculated with no additional ad-hoc gain.

Attachment 1 shows the calculated open-loop transfer function.

Attachment 2 shows OLTF of ETMY measured last week.

Attachment 3 shows the step and impulse responses of the closed-loop system.

[Paco, Yehonathan]

We fixed the slow control over the green beam shutters.

At the Y arm the wrong BNC was connected to the shutter driver. We connected the correct BNC to the driver and switched the remote mode. The green Y shutter now works but in reverese, meaning that sending 1 to C1:AUX-GREEN_Y_Shutter closes the shutter and vice versa. This needs to be fixed.

At the X end the problem was a bit more complicated. Previously, the shutter was controlled by c1auxey. We figured that c1auxex has a lot of spare bio channels. We found an Acromag BNC front panel (with wires already soldered to the BNCs) lying around in the lab and installed it on the c1auxex Acromag chassie. We then connected the topmost BNC to channel 0 on XT1111A in the chassie. The BNC was connected to the green shutter driver on the X end.

EPIC channel was added to the c1auxex db file while it was commented out on the psl shutters db file. Modbus was restarted on c1auxex and c1psl. c1psl had to be burt restored to regain MC lock. Now the green X shutter works properly.

I made some progress in modeling MICH loop.

Putting all the LSC and SUS filters together with the MICH Finesse model I constructed an OLTF model and plot it with the measurement done by Paco and Yuta in this elog (attachment 1).

There are 2 unknown numbers that I had to adjust in order to fit the model with the measurement:

1. The SUS damping loop gain (found to be ~ 2.22), which seems to vary wildly from SUS to SUS.

2. The coil driver gain (found to be 45), which I should measure.

I find coil_driver_gain*SUS_damping_filter_gain by increasing it until the SUS damping loop becomes unstable.

The coil driver gain I find by making the measurement and model overlap.

However, there is one outstanding discrepancy between the measurement and the model: Paco and Yuta measure the MICH calibration to be 1.3e9 cts/m while my model shows it to be 1.3e10 cts/m, an order of magnitude larger.

Details

I should write down what I didn't include for completeness:

1. AA filters

2. AS55 input 60Hz comb filter

3. Violin filters

After discussing with Paco, we agreed that the discrepancy in the MICH calibration might come from the IQ mixing angle which for the IFO is not optimized, while in Finesse is set such that all the amplitude is in one quadrature.

The box was given to Juan Gamez (SURF)

{Yuta, Yehonathan}

For MICH noise budgeting we measure the input electronics noise which includes the AS55 RFPD, preamp, demod board, the whitening, and the AA filters, and the ADC noises. To do so we simply close the laser shutter and take the spectrum of C1:LSC-AS55_I_ERR_DQ and C1:LSC-AS55_Q_ERR_DQ shown in attachment 1.

Next, we measured the output electronics noise which includes the DAC, dewhitening and AI filters, and coil driver noises. We disabled the BS watchdog and went to 1X4 rack. We measured the spectrum of one of the lemo outputs on the BS coil driver module using an SR785. Attachment 2 shows the spectrum together with the SR785 dark noise.

{Yuta, Yehonathan}

We measured the AS55 demod board conversion from the amplitude of a 55MHz signal to a demodulated signal. We hooked the unused REFL55 LO into the PD input port on the AS55 demod board.

The REFL55 LO was measured to be 1.84 Vpp. The IQ outputs were: I = 0.86 Vpp, Q = 2.03 Vpp giving an amplitude of 2.205 Vpp. The overall conversion factor is sqrt(0.86**2+2.03**2)/(1.82/2)=2.422.

We also set to measure the loss in the RF cable from AS55 PD to the demod board on 1Y2. REFL55 was connected with a long BNC cable to the input of the cable under test. REFL55 at the input was measured to be 1.466 Vpp and 1.28 Vpp at the output signifying a transmission of 87.6%.

I calculated a noise budget for the MICH using AS55 as a sensor. The calculation includes closed-loop TF calculations.

The notebook and associated files can be found on https://git.ligo.org/40m/bhd/-/blob/master/controls/compute_MICH_noisebudget.ipynb.

Attachment 1 shows the loop diagram I was using. The equation describing the steady-state of the loop is

, where G is the adjacency matrix given by

First, the adjacency matrix G is constructed by stitching the small ABCDE matrices together. Once the inverse of (I-G) is calculated we can simply propagate any noise source to  and then calculate  to estimate the displacement of the optics. 

Attachment 2 shows the calculated noise budget together with Yuta's measurement.

All the input and output electronics are clumped together for now. Laser noise is irrelevant as this is a heterodyne measurement at 55MHz.

It seems like there is some mismatch in the calibration of the optical gain between the measurement and model. The missing noise at 3-30Hz could be due to angle-to-length coupling which I haven't included in the model.

I fixed some mistakes in the budget:

1. The BS pendulum resonance was corrected from 0.8Hz to 1Hz

2. Added missing X3 filter in the coil filters

3. Optical gain is now computed from MICH to AS55 instead of BS to AS55 and is calculated to be: 9.95e8 cts/m.

4. Coil driver gain is still unmeasured but it is found to be 1.333 to make the actuation calibration from BS to MICH match the measurement (see attachment 1).

Attachment 2 shows the resulting MICH OLTF.

Laser noise was added to the budget in a slightly ad-hoc fashion (will fix later): Yuta and I measured MC_F and computed MC_F*(Schnupp asymmetry)/(Laser frequency). Attachment 3 shows the updated noise budget.

the main laser noise coupling for a Michelson is because of the RIN, not the frequency noise. You can measure the RIN, in MC trans or at the AS port by getting a single bounce beam from a single ITM.

I measured the RIN by taking the spectrum of C1:MC_TRANS_SUMFILT_OUT and dividing it by the mean count on that channel (~13800 cts). Attachment 1 shows the result.

I updated the MICH AS55 noise budget but got a very low contribution (gold trace in attachment 2).

It seems too low I think. What could've gone wrong? Finesse calculates that the transfer function from laser amplitude modulation to AS55 is ~ 1.5e-9 at DC. If I turn off HOMs I get 1e-11 at DC, so this coupling is a result of some contrast defect. Should I include some RMS imbalances in the optics to account for this? Should I include it as a second-order effect due to MICH RMS deviation from zero crossing?

{Yuta, Yehonathan}

We went to the BS table to check the POP beam power. We first notice that the POP beam has a nice gaussan profile on the viewing card. We traced it the beam to the viewing port and measured the power there. Before measuring the power we misalign ITMY/ITMX to get rid of interferences. We measure the beam to be 205uW in both cases.

The expected power is

950 mW * 0.9 (IMC transmission?) * 5.637%(PRM) * 97.8%(PR2) * 50%(BS) * 98.6%(ITM) * 50%(BS)  * 2.2%(PR2) = 260uW

which is reasonably close to what we measure which confirms that PR2 transmission is around what we think it is.

This strengthen our suspicion that LO beam gets clipped somewhere.

We also improved the clipping on the POP camera by one of the beamsplitters along the beam path and the alignment to the POPDC PD (~100 cts before, ~ 1000 cts after).

I set out to test the actuation bandwidth of the 950nm laser. I hooked the laser to the output of the bias tee of PD testing setup. I connected the fiber coming out of the laser to the fiber port of 1611 REF PD.

The current source was connected to the DB9 input of the PD testing setup. I turned on the current source and set the current to 20mA. I measured with a fluke ~ 2V at the REF PD DC port.

I connected the AC port of the bias tee to the RF source of the network analyzer and the AC port of the REF PD to the B port of the network analyzer. Attachment 2 shows the setup.

I took a swept sine measurement (attachment) from 100kHz to 500MHz.

It seems like the bandwidth is ~ 1MHz which is weird considering the spec sheet says that the pulse rise time is 0.5ns. To make sure we are not limited by the bandwidth of the cables I looped the source and the input of the network analyzer using the cables used for the previous measurement and observed that the bandwidth is a few 100s of MHz.

{Yuta, Yehonathan}

We wrote a notebook found on Git/40m/measurements/LSC/FPMI/NoiseBudget/FPMISensitivity.ipynb for calculating the MICH, DARM (currently XARM), CARM (currently YARM) sensitivities in the FPMI lock which can be run daily.

The IN and OUT channels of each DOFs are measured at a certain GPS time and calibrated using the optical gains and actuation calibration measured in the previous post.

Attachment shows the results.

It seems like the UGFs for MICH and DARM (currently XARM) match the ones that were estimated previously (100Hz for MICH, 120Hz for DARM) except for CARM for which the UGF was estimated to be 250Hz and here seems to be > 1kHz.

Indeed one can also see that the picks in the CARM plot don't match that well. Calculation shows that at 250Hz OUT channel is 6 times more than the IN channel. Calibrations for CARM should be checked.

MICH sensitivity using REFL55 at high frequencies is not much better than what was measured with AS55.

DARM sensitivity at 10Hz is a factor of a few better than the single arm lock sensitivity.

Now it is time to do the budgeting.

I tried to lock the Y/X arms to take some noise budget. However, we noticed that TRX/Y were oscillating coherently together (by tens of percent), meaning some input optics, essentially PR2/3 are swinging. There was no way I could do noise budgeting in this situation.

I set out to debug these optics. First, I notice side motion of PR2 is very weakly damped .

The gain of the side damping loop (C1:SUS-PR2_SUSSIDE_GAIN) was increased from 10 to 150 which seem to have fixed the issue. Attachment 1 shows the current step response of  the PR2 DOFs. The residual Qs look good but there is still some cross-couplings, especially when kicking POS. Need to do some balancing there.

PR3 fixing was less successful in the beginning. I increased the following gains:

C1:SUS-PR3_SUSPOS_GAIN: 0.5 -> 30

C1:SUS-PR3_SUSPIT_GAIN: 3 -> 30

C1:SUS-PR3_SUSYAW_GAIN: 1 -> 30

C1:SUS-PR3_SUSSIDE_GAIN: 10 -> 50

But the residual Q was still > 10. Then I checked the input matrix and noticed that UL->PIT is -0.18 while UR->PIT is 0.39. I changed UL->PIT (C1:SUS-PR3_INMATRIX_2_1) to +0.18. Now the Q became 7. I continue optimizing the gains.

Was able to increase C1:SUS-PR3_SUSSIDE_GAIN: 50 -> 100.

Attachment 2 shows the step response of PR3. The change of the entry of the input matrix was very ad-hoc, it would probably be good to run a systematic tuning. I have to leave now, but the IFO is in a very misaligned state. PR3/2 should be moved to bring it back.

{Radhika, Paco, Yehonathan}

For educational purposes we update the XARM noise budget and add the POX11 calibrated dark noise contribution (attachment).

{Paco, Yehonathan}

BHD Optics box was put into the x-arm last clean cabinet. (attachment 5)

OSEMs were double bagged in a labeled box on the x-arm wire racks. (attachment 1)

SOS Parts (wire clamps, winches, suspension blocks, etc.) were put in a box on the x-arm wire rack. (attachment 3)

2"->3" optic adapter parts were put in a box and stored on the xarm wire rack. (attachment 3)

Magnet gluing parts box was labeled and stored on the xarm rack. (attachment 2)

TT SUS with the optics were stored on the flow bench at the x end. Note: one of the TT SUS was found unsuspended. (attachment 4)

InVac parts were double bagged and stored in a labeled box on the x arm wire rack. (attachment 2)

{Anchal, Yehonathan}

We came this morning and the IMC was misaligned. The IMC was realigned and locked. This of course changed the input beam and sent us down to a long alignment journey.

We first use TTs to find beam on BHD DCPD/Camera since it is only single bounce on all optics.

Then, PR2/3 were used to find POP beam while keeping the BHD beam.

Unfortunately, that was not enough. TTs and PRs have some degeneracy which caused us to lose the REFL beam.

Realizing this we went to AS table to find the REFL beam. We found a ghost beam that decieved us for a while. Realizing it was a ghost beam, we moved TT2 in pitch, while keeping the POP DCPD high with PRs, until we saw a new beam on the viewing card.

We kept aligning TT1/2, PR2/3 to maximize the REFL DCPD while keeping the POP DCPD high. We tried to look at the REFL camera but soon realized that the REFL beam is too weak for the camera to see.

At that point we already had some flashing in the arms (we centered the OpLevs in the beginning).

Arms were aligned and locked. We had some issue with the X-ARM not catching lock. We increased the gain and it locked almost immediately. To fix the arms gains correctly we took OLTFs (Attachment) and adjusted the XARM gain to 0.027 to make the UGF at 200Hz.

Both arms locked with 200 Hz UGF from:

From GPS: 1346713049
    To GPS: 1346713300

From GPS: 1346713380
    To GPS: 1346714166

HEPA turned off:
From GPS: 1346714298
    To GPS: 1346714716

[Paco, Yehonathan]

Summary:  We locked LO phase using the DC PD (A - B) error point without saturating the control point, i.e. not a "bang bang" control.

Some suspensions were improved so we figure we should go back to trying to lock the LO phase.

We misalign ETMs and lock MICH using AS55. We put a small MICH offset by putting C1:LSC-MICH_OFFSET = -80.

AS and LO beams were aligned to overlap by maximizing the BHD signal visibility.

BHD DCPDs were balanced by misaligning the AS beam and using the LO beam only.

We measure the transfer function between the DCPDs and find the coherence is 1 at 1 Hz (because of seismic motion) so we measure the ratio between them to be 0.3db.

AS beam is aligned again to overlap with the LO beam. For the work below, we use the largest MICH OFFSET we could impinge before losing the lock = +90. This has the effect of increasing our optical gain.

We started using the HPC LOCK IN screen to dither POS on the different BHD SUS. We first started with AS1 (freq = 137.137 Hz, gain = 1000). The sensing matrix element was chosen accordingly (from the demodulated output) and fed to the LO_PHASE; because this affected the AS port alignment this was of course not the best choice. We moved over to LO2 (freq = 318.75 Hz, gain = 1000) but for the longest time couldn't see the dither line at the error point (A-B).

After this we added comb60 notch filters at DCPD_A and DCPD_B input signals. We ended up just feeding the (A-B) error point to LO1, and trying to lock mid fringe, which suceeded without saturation. The gain of the LO_PHASE filter was set to 0.2 (previously 20; attributable to the newly unclipped LO beam intensity?), and again we only enabled FM4 and FM5 for this. After this a dither line at 318.75 Hz finally appeared in the A-B error point! To be continued...

Doing POX-POY noise measurement as a poor man's FPMI for diagnostic purposes. (Notebook in /opt/rtcds/caltech/c1/Git/40m/measurements/LSC/POX-POY/Noise_Budget.ipynb)

The arms were locked individually using POX11 and POY11. The optical gain was estimated to be by looking at the PDH signal of each arm: the slope was computed by taking the negative peak to positive peak counts and assuming that the arm length change between those peaks is lambda/(2*Finesse), where lambda = 1um and the arm finesse is taken to be 450.

Xarm peak-to-peak counts is ~ 850 while Yarm's is ~ 1100. This gives optical gains of 3.8e11 cts/m and 4.95e11 cts/m respectively.

Next, ETMX actuation TF is measured (attachments 1,2) by exciting C1:LSC-ETMX/Y_EXC and measuring at C1:LSC-X/YARM_IN1_DQ and calibrating with the optical gain.

Using these calibrations I plot the POX-POY (attachment 3) and POX+POY (attachment 4) total noise measurements using two methods:

1. Plotting the calibrated IN and OUT channels of XARM-YARM (blue and orange). Those two curves should cross at the UGF (200Hz in this case).

2. Plotting the calibrated XARM-YARM IN channels times 1-OLTF (black).

The UGF bump can be clearly seen above the true noise in those plots.

However, POX+POY OUT channel looks too high for some reason making the crossing frequency between IN and OUT channels to be ~ 300Hz. Not sure what was going on with this.

Next, I will budget this noise with the individual noise contributions.

{Yehonathan, Anchal}

Came this morning to find that all the BHD optics watchdogs tripped. Watchdogs were restored but the all ADCs were stuck.

We realized that the C1SU2 model crashed.

We run /scripts/cds/restartAllModels.sh to reboot all machines. All the machines rebooted and burt restored to yesterday around 5 PM.

The optics seem to have returned to good alignment and are damped.

I pushed a notebook and a Finesse model for comparing different LO phase locking schemes. Notebook is on https://git.ligo.org/40m/bhd/-/blob/master/controls/compare_LO_phase_locking_schemes.ipynb,

Here's a description of the Finesse modeling:

I use a 40m kat model https://git.ligo.org/40m/bhd/-/blob/master/finesse/C1_w_initial_BHD_with_BHD55.kat derived from the usual 40m kat file. There I added and EOMs (in the spaces between the BS and ITMs and in front of LO2) to simulate audio dithering. A PD was added at a 5% pickoff from one of the BHD ports to simulate the RFPD recently installed on the ITMY table.

First I find the nominal LO phase by shaking MICH and maximizing the BHD response as a function of the LO phase (attachment 1).

Then, I run another simulation where I shake the LO phase at some arbitrary frequency and measure the response at different demodulation schemes at the RFPD and at the BHD readout.

The optimal responses are found by using the 'max' keyword instead of specifying the demodulation phase. This uses the demodulation phase that maximizes the signal. For example to extract the signal in the 2 RF sideband scheme I use:

pd3 BHD55_2RF_SB $f1 max $f2 max $fs max nPickoffPDs

I plot these responses as a function of LO phase relative to the nominal phase divided by 90 degrees (attachment 2). The schemes are:

1. 2 RF sidebands where 11MHz and 55MHz on the LO and AS ports are used.

2. Single RF sideband (11/55 MHz) together with the LO carrier. As expected, this scheme is useful only when trying to detect the amplitude quadrature.

3. Audio dithering MICH and using it together with one of the LO RF sidebands. The actuation strength is chosen by taking the BS actuation TF 1e-11 m/cts*(50/f)**2 and using 10000 cts giving an amplitude of 3nm for the ITMs.

For LO actuation I can use 13 times more actuation strentgh becasue its coild drivers' output current is 13 more then the old ones.

4. Double audio dithering of LO2+MICH detecting it directly at the BHD readout (attachment 3).

Without noise considerations, it seems like double audio dithering is by far the best option and audio+RF is the next best thing.

The next thing to do is to make some noise models in order to make the comparison more concrete.

This noise model will include Input noises, residual MICH motion, and laser noise. Displacement noise will not be included since it is the thing we want to be detected.

Changed the BHD BS transmissivity to 0.56.

Demodulation Phases

As was noted before. The LO phase sensitivity plot vs LO phase from the previous elog shows the optimal sensitivity at each LO phase. That means that the optimal demodulation phase might change as a function of LO phase. Attachment 1 shows the previous plot and a plot showing the optimal modulation phase for some of the methods. When double demodulation is involved I optimize one modulation and show the optimal demodulation angle of the second. As can be seen, optimal audio demodulation angles don't change as a function of LO phase.

Additionally, as expected maybe, for the single RF sideband methods that nominally should not have worked at nominal LO phase (angle in which BHD Diff is most sensitive to MICH), the optimal demodulation angle changes quite a bit around the nominal LO phase.

Fixed demodulation angle

Attachment 2 shows the LO phase sensitivity in the single 55MHZ sideband method when we fix the demodulation angle. -23.88 is the demod angle optimal for nominal LO phase. 66.12 is 90 degrees away from that. -75.21 is the is the demod angle optimal for LO phase at the amplitude quadrature and 14.78 is 90 degrees away from that. It can be seen that fixing the demod angle can be mostly harmless.

Effect of MICH offset

The simulations were run with 0 MICH offset. Attachment 3 shows the LO phase sensitivity of the different methods when MICH offset is introduced together with the optimal demod angle. As expected the single RF SB methods are sensitive to this offset while the double demod methods are not since they are not relying on DC fields.

Fields at the BHD BS. More on this later.