Summary:

Currently, I am unable to engage the coil-dewhitening filters without destroying cavity locks. One reason why this is so is because the present Oplev servos have a roll-off at high frequencies that is not steep enough - engaging the digital whitening + analog de-whitening just causes the DAC output to saturate. Today, Rana and I discussed some ideas about how to approach this problem. This elog collects these thoughts. As I flesh out these ideas, I will update them in a more complete writeup in T1700363 (placeholder for now). Past relevant elogs: 5376, 9680. 

1. Why do we need optical levers?
   • ​​To stabilize the low-frequency seismic driven angular motion of the optics.
2.  In what frequency range can we / do we need to stabilize the angular motion of the optics? How much error signal suppression do we need in the control band? How much is achievable given the current Oplev setup?
   • ​​To answer these questions, we need to build a detailed Oplev noise budget.
   • Ultimately, the Oplev error signal is sensing the differential motion between the suspended optic and the incident laser beam.
   • What frequency range does laser beam jitter dominate the actual optic motion? What about mechanical drifts of the optical tables the HeNes sit on? And for many of the vertex optics, the Oplev beam has multiple bounces on steering mirrors on the stack. What is the contribution of the stack motion to the error signal?
   • The answers to the above will tell us what lower and upper UGFs we should and can pick. It will also be instructive to investigate if we can come up with a telescope design near the Oplev QPD that significantly reduces beam jitter effects (see elog 10732). Also, can we launch/extract the beam into/from the vacuum chamber in such a way that we aren't so susceptible to motion of the stack?
3. What are some noises that have to be measured and quantified?
   • Seismic noise
   • ​​Shot noise
   • Electronics noise of the QPD readout chain
   • HeNe intensity noise (does this matter since we are normalizing by QPD sum?)
   • HeNe beam pointing / jitter noise (How? N-corner hat method?)
   • Stack motion contribution to the Oplev error signal
4. How do we design the Oplev controller?
   • ​​The main problem is to frame the right cost function for this problem. Once this cost function is made, we can use MATLAB's PSO tool (which is what was used for the PR3 coating design optimization, and also successfully for this kind of loop shaping problems by Rana for aLIGO) to find a minimum by moving the controller poles and zeros around within bounds we define.

5. What terms should enter the cost function?

   • ​​In addition to those listed in elog 5376
   • We need the >10Hz roll-off to be steep enough that turning on the digital whitening will not significantly increase the DAC output RMS or drive it to saturation.
   • We'd like for the controller to be insensitive to 5% (?) errors in the assumed optical plant and noise models i.e. the closed loop shouldn't become unstable if we made a small error in some assumed parameters.
   • Some penalty for using excessive numbers of poles/zeros? Penalty for having too many high-frequency features.
6. Other things to verify / look into
   • ​​Verify if the counts -> urad calibration is still valid for all the Oplevs. We have the arm-cavity power quadratic dependance method, and the geometry method to do this.
   •  Check if the Oplev error signals are normalized by the quadrant sum.
   • How important is it to balance the individual quadrant gains?
   • Check with Koji / Rich about new QPDs. If we can get some, perhaps we can use these in the setup that Steve is going to prepare, as part of the temperature vs HeNe noise invenstigations.

Before the CDS went down, I had taken error signal spectra for the ITMs. I will update this elog tomorrow with these measurements, as well as some noise estimates, to get started.

Attachment #1 - Measured error signal spectrum with the Oplev loop disabled, measured at the IN1 input for ITMY. The y-axis calibration into urad/rtHz may not be exact (I don't know when this was last calibrated).

From this measurement, I've attempted to disentangle what is the seismic noise contribution to the measured plant output.

• To do so, I first modelled the plant as a pair of complex poles @0.95 Hz, Q=3. This gave the best agreement with measurement by eye, I didn't try and optimize this too carefully. 
• Next, I assumed all the noise between DC-10Hz comes from only seismic disturbance. So dividing the measured PSD by the plant transfer function gives the spectrum of the seismic disturbance. I further assumed this to be flat, and so I averaged it between DC-10Hz.
• This will be a first seismic noise model to the loop shape optimizer. I can probably get a better model using the GWINC calculations but for a start, this should be good enough.

It remains to characterize various other noise sources.

I have also confirmed that the "QPD" Simulink block, which is what is used for Oplevs, does indeed have the PIT and YAW outputs normalized by the SUM (see Attachment #2). This was not clear to me from the MEDM screen.

GV 30 Jul 5pm: I've included in Attachment #3 the block diagram of the general linear feedback topology, along with the specific "disturbances" and "noises" w.r.t. the Oplev loop. The measured (open loop) error signal spectrum of Attachment #1 (call it y) is given by:

If it turns out that one (or more) term(s) in each of the summations above dominates in all frequency bands of interest, then I guess we can drop the others. An elog with a first pass at a mathematical formulation of the cost-function for controller optimization to follow shortly.

Summary:

I've been trying to put together the cost-function that will be used to optimize the Oplev loop shape. Here is what I have so far.

Details:

All of the terms that we want to include in the cost function can be derived from:

1. A measurement of the open-loop error signal [using DTT, calibrated to urad/rtHz]. We may want a breakdown of this in terms of "sensing noises" and "disturbances" (see the previous elog in this thread), but just a spectrum will suffice for the optimal controller given the current noises.
2. A model of the optical plant, P(s) [validated with a DTT swept-sine measurement]. 
3. A model of the controller, C(s). Some/all of the poles and zeros of this transfer function is what the optimization algorithm will tune to satisfy the design objectives.

From these, we can derive, for a given controller, C(s):

1. Closed-loop stability (i.e. all poles should be in the left-half of the complex plane), and exactly 2 UGFs. We can use MATLAB's allmargin function for this. An unstable controller can be rejected by assigning it an extremely high cost.
2. RMS rrror signal suppression in the frequency band (0.5Hz - 2Hz). We can require this to be >= 15dB (say).
3. Minimize gain peaking and noise injection - this information will be in the sensitivity function, . We can require this to be <= 10dB (say).
4. RMS of the control signal between 10 Hz and 200 Hz, multiplied by the digital suspension whitening filter, should be <10% of the DAC range (so that we don't have problems engaging the coil de-whitening).
5. Smallest gain margin (there will be multiple because of the various notches we have) should be > 10dB (say). Phase margin at both UGFs should be >30 degrees.
6. Terms 1-5 should not change by more than 10% for perturbations in the plant model parameters (f0 and Q of the pendulum) at the 10% (?) level. 

We can add more terms to the cost function if necessary, but I want to get some minimal set working first. All the "requirements" I've quoted above are just numbers out of my head at the moment, I will refine them once I get some feeling for how feasible a solution is for these requirements.

For a start, I attempted to model the current Oplev loop. The modeling of the plant and open-loop error signal spectrum have been described in the previous elogs in this thread.

I am, however, confused by the controller - the MEDM screen (see Attachment #2) would have me believe that the digital transfer function is FM2*FM5*FM7*FM8*gain(10). However, I get much better agreement between the measured and modelled in-loop error signal if I exclude the overall gain of 10 (see Attachments #1 for the models and #3 for measurements).

What am I missing? Getting this right will be important in specifying Term #4 in the cost function...

GV Edit 2 Aug 0030: As another sanity check, I computed the whitened Oplev control signal given the current loop shape (with sub-optimal high-frequency roll-off). In Attachment #4, I converted the y-axis from urad/rtHz to cts/rtHz using the approximate calibration of 240urad/ct (and the fact that the Oplev error signal is normalized by the QPD sum of ~13000 cts), and divided by 4 to account for the fact that the control signal is sent to 4 coils. It is clear that attempting to whiten the coil driver signals with the present Oplev loop shapes causes DAC saturation. I'm going to use this formulation for Term #4 in the cost function, and to solve a simpler optimization problem first - given the existing loop shape, what is the optimal elliptic low-pass filter to implement such that the cost function is minimized? 

There is also the question of how to go about doing the optimization, given that our cost function is a vector rather than a scalar. In the coating optimization code, we converted the vector cost function to a scalar one by taking a weighted sum of the individual components. This worked adequately well.

But there are techniques for vector cost-function optimization as well, which may work better. Specifically, the question is  if we can find the (infinite) solution set for which no one term in the error function can be made better without making another worse (the so-called Pareto front). Then we still have to make a choice as to which point along this curve we want to operate at.

I disabled the OL loops for ITMX, ITMY and BS at GPStime 1194897655 to come up with an Oplev noise budget. OL spots were reasonably well centered - by that, I mean that the PIT/YAW error signals were less than 20urad in absolute value.

Attachment #1 is a first look at the DTT spectra - I wonder why the BS Oplev signals don't agree with the ITMs at ~1Hz? Perhaps the calibration factor is off? The sensing noise not really flat above 100Hz - I wonder what all those peaky features are. Recall that the ITM OLs have analog whitening filters before the ADC, but the BS doesn't...

In Attachment #2, I show comparison of the error signal spectra for ITMY and SRM - they're on the same stack, but the SRM channels don't have analog de-whitening before the ADC.

For some reason, DTT won't let me save plots with latex in the axes labels...

I bet the calibration is out of date; probably we replaced the OL laser for the BS and didn't fix the cal numbers. You can use the fringe contrast of the simple Michelson to calibrate the OLs for the ITMs and BS.

I calibrated the BS oplev PIT and YAW error signals as follows:

1. Locked X-arm, ran dither alignment servos to maximize transmission.
2. Applied an offset to the ASC PIT/YAW filter banks. Set the ramp time to something long, I used 60 seconds.
3. Monitored the X arm transmission while the offset was being ramped, and also the oplev error signal with its current calibration factor.
4. Fit the data, oplev error signal vs arm transmission, with a gaussian, and extracted the scaling factor (i.e. the number which the current Oplev error signals have to be multiplied by for the error signal to correspond to urad of angular misalignment as per the overlap of the beam axis to the cavity axis.
5. Fits are shown in Attachment #1 and #2.
6. I haven't done any error analysis yet, but the open loop OL spectra for the BS now line up better with the other optics, see Attachment #3 (although their calibration factors may need to be updated as well...). Need to double check against OSEM readout during the sweep.
7. New numbers have been SDF-ed.

The numbers are:

BS Pitch     15  /  130    (old/new)     urad/counts

The numbers I have from the fitting don't agree very well with the OSEM readouts. Attachment #1 shows the Oplev pitch and yaw channels, and also the OSEM ones, while I swept the ASC_PIT offset. The output matrix is the "naive" one of (+1,+1,-1,-1). SUSPIT_IN1 reports ~30urad of motion, while SUSYAW_IN1 reports ~10urad of motion.

From the fits, the BS calibration factors were ~x8 for pitch and x12 for yaw - so according to the Oplev channels, the applied sweep was ~80urad in pitch, and ~7urad in yaw.

Seems like either (i) neither the Oplev channels nor the OSEMs are well diagonalized and that their calibration is off by a factor of ~3 or (ii) there is some significant imbalance in the actuator gains of the BS coils...

Per our discussions in the meetings over the last week, I've tried to put together a simple Oplev noise budget. The only two terms in this for now are the dark noise and a model for the seismic noise, and are plotted together with the measured open-loop error signal spectra.

1. Dark noise
   • Beam was taken off the OL QPD
   • A small DC offset was added to all the oplev segment input filters to make the sum ~20-30 cts [call this testSum] (usually it varies from 4000-13000 for the BS/ITMs, call this nominalSum).
   • I downloaded 20mins of dq-ed error signal data, and computed the ASD, dividing by a factor of nominalSum / testSum to account for the usual light intensity on the QPD.
2. Seismic noise
   • This is a very simplistic 1/f^2 pendulum TF with a pair of Q=2 poles at 1Hz.
   • I adjusted the overall gain such that the 1Hz peak roughly line up in measurement and model.
   • The stack isn't modelled at all.

Some remarks:

• The BS oplev doesn't have any whitening electronics, and so has a higher electronics noise floor compared to the ITMs. But it doesn't look like we are limited by this lower noise floor anywhere..
• I wonder what all those high frequency features seen in the ITM error signal spectra are - mechanical resonances of steering optics? It is definitely above the dark noise floor, so I am inclined to believe this is real beam motion on the QPD, but surely this can't be the test-mass motion? If it were, the measured A2L would be much higher than the level it is adjudged to be at now. Perhaps it's some resonances of steering mirrors?
• The seismic displacement @100Hz per the GWINC model is ~1e-19 m/rtHz. Assuming the model A2L = d_rms * theta(f) where d_rms is the rms offset of the beam spot from the optic center, and theta(f) is the angular control signal to the optic, for a 5mm rms offset of the spot from the center, theta(f) must be ~1e-17 urad @100Hz. This gives some requirement on the low pass required - I will look into adding this to the global optimization cost.

For the OL NB, probably don't have to fudge any seismic noise, since that's a thing we want to suppress. More important is "what the noise would be if the suspended mirrors were no moving w.r.t. inertial space".

For that, we need to look at the data from the OL test setup that Steve is putting on the SP table.

[steve, gautam]

What is the best way to set this test up?

I think we need a QPD to monitor the spot rather than a single element PD, to answer this question about the sensor noise. Ideally, we want to shoot the HeNe beam straight at the QPD - but at the very least, we need a lens to size the beam down to the same size as we have for the return beam on the Oplevs. Then there is the power - Steve tells me we should expect ~2mW at the output of these HeNes. Assuming 100kohm transimpedance gain for each quadrant and Si responsivity of 0.4A/W at 632nm, this corresponds to 10V (ADC limit) for 250uW of power - so it would seem that we need to add some attenuating optics in the way.

Also, does anyone know of spare QPDs we can use for this test? We considered temporarily borrowing one of the vertex OL QPDs (mark out its current location on the optics table, and move it over to the SP table), but decided against it as the cabling arrangement would be too complicated. I'd like to use the same DAQ electronics to acquire the data from this test as that would give us the most direct estimate of the sensor noise for supposedly no motion of the spot, although by adding 3 optics between the HeNe and the QPD, we are introducing possible additional jitter couplings...

You may want to consult with the cryo Q people (Brittany, Aaron) for a Si QPD. If you want the same QPD architecture, I can look at my QPD circuit stock.

too complex; just shoot straight from the HeNe to the QPD. We lower the gain of the QPD by changing the resistors; there's no sane reason to keep the existing 100k resistors for a 2 mW beam. The specular reflection of the QPD must be dumped on a black glass V dump (not some flimsy anodized aluminum or dirty razor stack)

Do not turn on BS/ITMY/SRM/PRM Oplev servos without reading this elog and correcting the needful!

I've setup a test setup on the ITMY Oplev table. Details + pics to follow, but for now, be aware that

1. I've turned off the HeNe that is used for the SRM and ITMY Oplev.
2. Moved one of the HeNe's Steve setup on the SP table to the ITMY Oplev table.
3. Output power was 2.5mW, whereas normal power incident on this PD was ~250uW.
4. So I changed all transimpedance gains on the ITMY Oplev QPD from 100k to 10k thin film - these should be changed back when we want to use this QPD for Oplev purposes again. Note that I did not change the compensation capacitors C3-C6, as with 10k transimpedance, and assuming they are 2.2nF, we get a corner frequency of 6.7kHz. The original schematic recommends 0.1uF. In hindsight, I should have changed these to 22nF to keep roughly the same corner frequency of ~700Hz.
   I've implemented this change as of ~5pm Nov 23 2017 - C3-C6 are now 22nF, so the corner frequency is 676Hz, as opposed to 723Hz before... This should also be undone when we use this QPD as an Oplev QPD again...
5. I marked the position of the ITMY Oplev QPD with sharpie and also took pics so it should be easy enough to restore when we are done with this test.
6. I couldn't get the HeNe to turn on with any of the power supplies I found in the cabinet, so I borrowed the one used to power the BS/PRM. So these oplevs are out of commission until this test is done.
7. There is a single steering mirror in a Polaris mount which I used to center the spot on the QPD.
8. The specular reflection (~250uW, i.e. 10% of the power incident on the QPD) is dumped onto a clean razor beam stack. Steve can put in a glass beam dump on Monday.
9. Just in case someone accidentally turns on some servos - I've disabled the inputs to the BS, PRM and SRM oplevs, and set the limiter on the ITMY servo to 0.

Here are some pics of the setup: https://photos.app.goo.gl/DHMINAV7aVgayYcf1.None of the existing Oplev input/output steering optics were touched. Steve can make modifications as necessary, perhaps we can make similar mods to the SRM Oplev QPD and the BS one to run the HeNe test for a few days...

Here are a couple of preliminary plots of the noise from a 20minute stretch of data - the new curve is the orange one, labelled sensing, which is the spectrum of the PIT/YAW error signal from the HeNe beam single bounce off a single steering mirror onto the QPD, normalized to account for the difference in QPD sum. The peaky features that were absent in the dark noise are present here.

I am a bit confused about the total sum though - there is ~2.5mW of light incident on the PD, and the transimpedance gain is 10.7kohm. So I would expect 2.5e-3 mW * 0.4A/W * 10.7 kV/A ~ 10.7V over 4 quadrants. The ADC is 16 bit and has a range +/- 10V, so 10.7 V should be ~35,000 cts. But the observed QPD sum is ~14,000 counts. The reflected power was measured to be ~250uW, so ~10% of the total input power. Not sure if this is factored into the photodiode efficiency value of 0.4A/W. I guess there is some fraction of the QPD that doesn't generate any photocurrent (i.e. the grooves defining the quadrants), but is it reasonable that when the Oplev beam is well centered, ~50% of the power is not measured? I couldn't find any sneaky digital gains between the quadrant channels to the sum channel either... But in the Oplev setup, the QPD had ~250uW of power incident on it, and was reporting a sum of ~13,000 counts with a transimpedance gain of 100kohm, so at least the scaling seems to hold...

I guess we wan't to monitor this over a few days, see how stationary the noise profile is etc. I didn't look at the spectrum of the intensity noise during this time.

Today Angelina and I looked at the PRM OL with an eye towards installing a 2nd QPD. We want to try out using 2 QPDs for a single optic to see if theres a way to make a linear combination of them to reduce the sensitivity to jitter of the HeNe laser or acoustic noise on the table.

The power supply for the HeNe was gone, so I took one from the SP table.

There are WAY too many optics in use to get the beam from the HeNe into the vacuum and then back out. What we want is 1 steering mirror after the laser and then 1 steering mirror before the QPD. Even though there are rumors that this is impossible, I checked today and in fact it is very, very possible.

More optics = more noise = bad.

Summary:

I checked the calibration of the Oplevs for both ITMs, both ETMs and the BS. The table below summarizes the old and new cts->urad conversion factors, as well as the factor describing the scaling applied. Attachment #1 is a zip file of the fits performed to calculate these calibration factors (GPS times of the sweeps are in the titles of these plots). Attachment #2 is the spectra of the various Oplev error signals (open loop, so a measure of seismic induced angular motion for a given optic, and DoF) after the correction. Loop TF measurements post calibration factor update and loop gain adjustment to be uploaded tomorrow.

Motivation:

We'd like for the Oplev calibration to be a reliable readback of the optic alignment. For example, a calibrated Oplev would be a useful diagnostic to analyze the drifting (?) ETMX.

Details:

1. I locked and dither aligned the individual arms.
2. I then used a 60 second ramp time to misalign <optic> in {ITMX, ITMY, BS, ETMX, ETMY} one at a time, and looked at the appropriate arm cavity transmission while the misalignment was applied. The amplitude of the misalignment was chosen such that in the maximally misaligned state, the arm cavity was still locked to a TEM00 mode, with arm transmission ~40% of the value when the cavity transmission was maximized using the dither alignment servos. The CDS ramp is not exactly linear, it looks more like a sigmoid near the start and end, but I don't think that really matters for these fits.
3. I used the script OLcalibFactor.py (located at /opt/rtcds/caltech/c1/scripts/OL) to fit the data and extract calibration factors. This script downloads the arm cavity transmission and the OL error signal during the misalignment period, and fits a Gaussian profile to the data (X=oplev error signal, Y=arm transmission). Using geometry and mode overlap considerations, we can back out the misalignment in physical units (urad).

Comments:

1. For the most part, the correction was small, of the order of a few percent. The largest corrections were for the ETMs. I believe the last person to do Oplev calibration for the TMs was Yutaro in Dec 2015, and since then, we have certainly changed the HeNes at the X and Y ends (but not for the ITMs), so this seems consistent.
2. From attachment #2, most of the 1Hz resonances line up quite well (around 1-3urad/rtHz), so gives me some confidence in this calibration.
3. I haven't done a careful error analysis yet - but the fits are good to the eye, and the residuals look randomly distributed for the most part. I've assumed precision to the level of 1 urad/ct in setting the new calibration factors.
4. I think the misalignment period of 60 seconds is sufficiently long that the disturbance applied to the Oplev loop is well below the lower loop UGF of ~0.2Hz, and so the in loop Oplev error signal is a good proxy for the angular (mis)alignment of the optic. So no loop correction factor was applied.
5. I've not yet calibrated the PRM and SRM oplevs.

​Now that the ETMX calibration has been updated, let's keep an eye out for a wandering ETMX.

I found that the BS/PRM OL SUM channels were reading close to 0. So I went to the optical table, and found that there was no beam from the HeNe. I tried power-cycling the controller, there was no effect. From the trend data, it looks like there was a slow decay over ~400000 seconds (~ 5 days) and then an abrupt shutoff. This is not ideal, because we would have liked to use the Oplevs as a DC alignment reference during the ventI plan to use the AS camera to recover some sort of good Michelson alignment, and then if we want to, we can switch out the HeNe.

*How can I export PDF from NDscope?

Perhaps the ETMY Oplev HeNe is also giving up - the power has fallen by ~30% over 1 year (Attachment #2), nearly twice as much as ETMX but the RIN spectrum (Attachment #1, didn't even need to rotate it!) certainly seems suspicious. Some "nominal" RIN levels for HeNes can be found earlier in this thread. I can't close any of the EY Oplev loops in this condition. I'll double check to make sure I'm routing the right beam onto the QPD, but if the problem persists, I'll replace the HeNe. ITMX HeNe also looks to be near EOL.

I replaced the BS/PRM Oplev HeNe with one of the heads from the SP table where Steve was setting up the OL RIN/pointing noise experiment. The old one was dead. The new one outputs 3.2 mW of power, I've labelled it with this number, serial number and date of replacement. The beam comes out of the vacuum chamber for both the BS and PRM, and the RIN spectra (Attachment #1) look alright. The calibration into urad and loop gains possibly have to be tweaked. Since the beam comes out of vacuum, I say that we shouldn't open the BS/PRM chamber for this vent - we don't have a proper plan for the in-air layout yet, so we can add this to the list of to-dos for the next vent.

I think we are down to our last spare HeNe head in the lab - @Chub, please look into ordering some more, the ITMX HeNe is going to need replacement soon.

Oplev HeNe was replaced this afternoon. We did some HeNe shuffling:

1. A new HeNe was being used for the fiber illumination demo at EX. We took that out and decided to use it as the new ITMX HeNe. It had 2.6mW output at 632nm (measured with the Ophir power meter)
2. Old ETMY HeNe was used for fiber illumination demo.
3. Old ITMX HeNe was putting out no light - it will be disposed.

Attachment #1 shows the RIN and Attachment #2 and #3 show the PIT and YAW TFs with the new HeNe.

The ITMX Oplev path is still not great - the ingoing beam is within 2mm of clipping on a 2" lens used in the POX path, and there is a bunch of scattered red light everywhere. We should take the opportunity when the chamber is open to try and have a better layout (it may be tricky to optize without touching the two in-vacuum steering optics).

The AS spot on the camera was oscillating at ~3 Hz. Looking at the Oplevs, the culprit was the BS PIT DoF. Started about 12 hours ago, not sure what triggered it. I disabled Oplev damping, and waited for the angular motion to settle down a bit, and then re-enabled the servo - damps fine now...

While working on recovering interferometer alignment, I noticed that the ETMX Oplev SUM channel reported 0 counts. Attachment #1 shows the 200 day trend - despite the missing data, the accelerating downward decay is evident. I confirmed that there is no light coming out of the HeNe by walking down to EX. The label on the HeNe says it was installed in March 2017, so the lifetime was ~30 months. Seems a little short? I may replace this later today.

To facilitate POX locking investigations, I replaced this HeNe today with one of the spares Chub/Steve had acquired some time ago. Details:

• Part number: Lumentum 22037130 (1103P)
• Serial number: PA00836
• Manufacture date: 01/2019
• Power output: ~2.64 mW (Measured with Ophir power meter in the 632nm setting)
• Power received on QPD: ~0.37 mW = ~18700 cts (Measured with Ophir power meter in the 632nm setting)

The RIN of the sum channel with the Oplev servo engaged, along with that for the other core FPMI optics, in shown in Attachment #1. The ETMX HeNe RIN is compatible with the other HeNes in the lab (the high-frequency behaviour of the BS Oplev is different from the other four because the QPD whitening electronics are different).

Not sure what to make of the ETMY RIN profile being so different from the others, seems like some kind of glitchy behaviour, I could see the mean level of the ASD moving up and down as I was taking the averages in DTT. Needs further investigation.

The old / broken HeNe is placed i(nside the packaging of the abovementioned replacement HeNe) on Steve's old desk for disposal in the proper way.

*It looked like Steve had hooked up a thermocouple to be able to monitor the temperature of the HeNe head. I removed this feature as I figured if we don't have this hooked up to the DAQ, it isn't a really useful diagnostic. If we want, we can restore this in a more useful way.

In preparation for locking tonight, I re-centered the spots on the Oplev QPDs for the ITMs, BS and PRM after locking and running the dither alignment for the arms and also the PRMI carrier. In the past, DC coupling the ITM Oplevs helped the angular stability a bit, let's see if it still does.

Attachment #1 shows that the ITMX, ETMY and beamsplitter Oplev light levels have decayed significantly from their values when installed. In particular, the ETMY and ITMX sum channels are now only 50% of the values when a new HeNe was installed. ELOG search revealed that ITMY and ETMX HeNes were replaced with newly acquired units in March and September of last year respectively. The ITMX oplev was also replaced in March 2019, but the replacement was a unit that was being used to illuminate our tourist attraction glass fiber at EX.

We should replace these before any vent as they are a useful diagnostic for the DC alignement reference.

The ITMX Oplev (installed in March 2019) was near end of life judging by the SUM channel (see Attachment #1). I replaced it yesterday evening with a new HeNe head. Output power was ~3.25 mW. The head was labelled appropriately and the Oplev spot was recentered on its QPD. The lifetime of ~20 months is short but recall that this HeNe had already been employed as a fiber illuminator at EX and so maybe this is okay.

Loop UGFs and stability margins seem acceptable to me, see Attachment #2-#3.

As part of the hunt why the X arm IR transmission RIN is anomalously high, I noticed that the BS Oplev Servo periodically kicks the optic around - the summary pages are the best illustration of this happening. Looking back in time, these seem to have started ~Nov 23 2020. The HeNe power output has been degrading, see Attachment #1, but this is not yet at the point where the head usually needs replacing. The RIN spectrum doesn't look anomalous to me, see Attachment #2 (the whitening situation for the quadrants is different for the BS and the TMs, which explains the HF noise). I also measured the loop UGFs (using swept sine) - seems funky, I can't get the same coherence now (live traces) between 10-30 Hz that I could before (reference traces) with the same drive amplitude, and the TF that I do measure has a weird flattening out at higher frequencies that I can't explain, see Attachment #3.

The excess RIN is almost exactly in the band that we expect our Oplevs to stabilize the angular motion of the optics in, so maybe needs more investigation - I will upload the loop suppression of the error point later. So far, I don't see any clean evidence of the BS Oplev HeNe being the culprit, so I'm a bit hesitant to just swap out the head...

When both arms were locked, we found that ITMY optical lever was very off-center. This seems to have happened after the c1susaux rebooting we did in June 17th. I opened the ITMY table and realigned the OPLev beam to the center when the arm was locked. I repeated this process for BS, PRM and ETMY. I did PRM because I've known that we have been keeping its OpLev off. The reason was clear once I opened the table. The oplev reflection beam was hitting the PD box instead of the PD. After correcting, I was able to swithc on PRM opLev loops and saw normal functioning.

This was a mistake. When arms are locked, PRM is misaligned by setting -800 offset in PIT dof of PRM. The oplev is set to function in normal state not this misalgined configuration. I undid my changes today by switching off the offset, realigning the oplev to center and then restoring the single arm locked state. The PRM OpLevs loops are off now.

[yehonathan, anchal, paco]

Yesterday around 9:30 pm, we centered the BS, ITMY, ETMY, ITMX and ETMX oplevs (in that order) in their respective QPDs by turning the last mirror before the QPDs. We did this after running the ASS dither for the XARM/YARM configurations to use as the alignment reference. We did this in preparation for PRFPMI lock acquisition which we had to stop due to an earthquake around midnight

Late elog. Original time 08/02/2021 21:00.

I locked both arms and ran ASS to reach to optimum alignment. ETMY PIT > 10urad, ITMX P > 10urad and ETMX P < -10urad. Everything else was ok absolute value less than 10urad. I recentered these three.

Than I locked PRMI, ran ASS on PRCL and MICH and checked BS and PRM alignment. They were also less than absolute value 10urad.

I centered all the optical levers on ITMX, ITMY, ETMX, ETMY, and BS to a position where the single arm lock on both were best aligned. Unfortunately, we are seeing the TRX at 0.78 and TRY at 0.76 at the most aligned positions. It seems less power is getting out of PMC since last month. (Attachment 1).

Then, I tried to lock PRMI with carrier with no luck. But I was able to see flashing of up to 4000 counts in POP_DC. At this position, I centered the PRM optical lever too (Attachment 2).

Here is an early sketch of the MC table. 

We are supposed to have BS Oplev Beams. We don't like the shallow angle reflections (i.e. AOI>45deg).

The laser is too big but I suspect the other components are too small. So it'd be check the actual size of the components including the optical mounts that are missing on the figure so far.

Possibility to swap BS and ITMX tables:
BS table, which Tega said MC table, is 2ft x 4ft. The ITMX table is 3ft x 5ft and only the central 2ft x 4ft area is used. The area around the BS table is the narrowest for the east arm. We need at least (2+delta) ft of the hallway width so that we can move the instrument. I'm not yet sure if the ITMX table can be placed there without precise investigation.
 

I have updated the BS table using feedback from Koji and Paco and the attached pdf document is the latest iteration.

[Koij Hiroki]

While KA was working on the DAC issue, BS/PRM/SRM Oplev died.

It seems that the BS/PRM/SRM HeNe died and was replaced in 2019 (4yrs + 200 days ago) and 2021 Jan (2yrs + 209 days ago).

We have no energy to work on the HeNe replacement tonight. This needs to be done on Monday.

Tag: OPLEV oplev HeNe died dead

[JC, Paco]

We replaced the HeNe laser for the BS/PRM/SRM Oplev.

After we found a 1103P head replacement in a cabinet along the YARM, JC and I swapped the Oplev laser for BS/PRM/SRM. The head's form factor was the same luckily so we didn't struggle much to recover the QPD signals for BS and PRM. The final alignment aimed to restore ~ 5400 counts in BS (SUM_INMON) and ~ 3100 counts in PRM (SUM_INMON) when aligned.

[JC, paco]

This morning we noted most optics were tripped, probably as a result of a recent M>5 earthquake in the area (on Sun 08/20). Most optics were restored and damped nicely, except for ITMY.

PMC locked to HOM --> realigned and locked

We aligned PMC to maximize its transmission to ~ 0.670, after this IMC was locked and we engaged the WFS to recover the alignment.

ETMY oplev laser --> replaced aligned and locked

Most suspended optics were restored, but we noticed the OpLev sum on ETMY and ITMY were too low so we checked the lasers on both optics. The ITMY HeNe laser is on, but the one on ETMY is off. JC tested with a new laser head and the controller was determined to be good. Then, we tried resetting the previous one (labeled Oct 25 2020) but didn't have luck, so yet another HeNe laser died. We removed the old one and luckily our spare had the same form factor so it wasn't hard to recover the nominal alignment. After this we verified that the OPLEV loops on ETMY were working.

ITMY local damping --> still "stuck" or worse

The local damping on ITMY is not working properly. This puts it in a weird alignment state which is why we also don't see a large Oplev sum count on the QPD. The shadow sensor (OSEM) signals are all small, the available rms monitors are ~ 0.0, 0.1 mV, and kicking the optic around doesn't produce a corresponding OSEM signal, even when undamped. Therefore, we believe ITMY is either stuck (UR/LR) or worse. We tried the usual "shake" technique but didn't see any sensors being restored.

[JC, Koji-remote, paco]

ITMY stuck --> Shaken remotely and restored, ARMS aligned

With Koji's assistance we restored ITMY (it was stuck) and finished aligning both arms. Then JC centered the OpLevs for ETMs, ITMs and BS

ITMY camera blinking --> Replaced camera

JC checked the situation with our ITMYF (face) camera as the image seemed faulty and blinking. The issue this time was not in the power supply as has been before, but rather the CCD itself. After replacing the unit and aligning the ARM cavity, we redrew the marker "guides" on the control room screen for quick reference.

[Koji, Hiroki]

*This work was done on Aug. 11th.

We found a fiber-coupled visible diode laser (iFLEX-1000) in a shelf on YARM (Attachment 1 and 2).
This laser may be used as the replacement of the He-Ne laser for OPLEV.

[Koji, Manasa]

We measured the wedge angle of the G&H and LaserOptik mirrors at the OMC lab using an autocollimator and rotation stage.

The wedge angles:

G&H : 18 arc seconds (rough measurement)

LaserOptik : 1.887 deg

Reflectivity of AR surface of LaserOptik (SN6)

The first step measurements of R for AR surface. I am not convinced with the data....because the power meter is a lame detector for this measurement.

I'm repeating the measurements again with PDs. But below is the log R plot for AR surface.

R percentage

6000ppm @ 42 deg
3560ppm @ 44 deg
7880ppm @ 46 deg
4690ppm @ 48 deg

 Hours of struggle and still no data 

I tried to measure the AR reflectivity and the loss due to flipping of G&H mirrors

 With almost no wedge angle, separating the AR reflected beam from the HR reflected beam seems to need more tricks.

The separation between the 2 reflected rays is expected 0.8mm. After using a lens along the incident beam, this distance was still not enough to be separable by an iris.

The first trick: I could find a prism and tried to refract the beams at the edge of the prism...but the edges weren't that sharp to separate the beams (Infact I thought an axicon would do the job better..but I think we don't have any of those).

Next from the bag of tricks: I installed a camera to see if the spots can actually be resolved.

The camera image shows the 2 sets of focal spots; bright set to the left corresponding to HR reflected beam and the other from the AR surface. I expect the ghost images to arise from the 15 arcsec wedge of the mirror. I tried to mask one of the sets using a razor blade to see if I can separate them and get some data using a PD. But, it so turns out that even the blade edge is not sharp enough to separate them.

If there are any more intelligent ideas...go ahead and suggest! 

How about to measure the AR reflectivity at larger (but small) angles the extrapolate the function to smaller angle,
or estimate an upper limit?

The spot separation is

D = 2 d Tan(\phi) Cos(\theta), where \phi = ArcSin(Sin(\theta) * n)

D = 2 d Tan(\phi) Cos(\theta), where \phi = ArcSin(Sin(\theta) / n)         (<== correction by Manasa's entry)

\theta is the angle of incidence. For a small \theta, D is propotional to \theta.

So If you double the incident angle, the beam separation will be doubled,
while the reflectivity is an even function of the incident angle (i.e. the lowest order is quadratic).

I am not sure until how much larger angle you can use the quadratic function rather than a quartic function.
But thinking about the difficulty you have, it might be worth to try.

n₁Sin(\theta₁) = n₂ Sin(\theta₂)

So it should be

\phi = ArcSin(Sin(\theta) / n 

I did check the reflected images for larger angles of incidence, about 20 deg and visibly (on the IR card) I did not see much change in the separation. But I will check it with the camera again to confirm on that.

 Use the trick I suggested:

Focus the beam so that the beam size at the detector is smaller than the beam separation. Use math to calculate the beam size and choose the lens size and position. You should be able to achieve a waist size of < 0.1 mm for the reflected beam.

I, by chance, found  that my windows partition has Vision32 installed.
So I run my usual curvature characterization for the TT phasemaps.

They are found under this link
https://nodus.ligo.caltech.edu:30889/40m_phasemap/40m_TT/（requires: LVC credentials)
or
/cvs/cds/caltech/users/public_html/40m_phasemap/40m_TT

asc/ (ascii files) --> .asc files are saved in Wyko ascii format.
bmp/ (screen shots of Vision32)
mat/ (Matlab scripts and results)
opd/ (Raw binary files)

Estimated radius of curvature

Mirror / RoC from Vision32 / RoC from KA's matlab code
G&H "A" 0864 / -527.5 m / -505.2 m
G&H "B" 0884 / -710.2 m / -683.6 m
LaserOptik SN1 / -688.0 m / -652.7 m
LaserOptik SN2 / -605.2 m / -572.6 m
LaserOptik SN3 / -656.7 m / -635.0 m
LaserOptik SN4 / -607.5 m / -574.6 m
LaserOptik SN5 / -624.8 m / -594.3 m
LaserOptik SN6 / -658.5 m / -630.2 m

The aperture for the RoC in Vision32 seems a bit larger than the one I have used in the code (10mm in dia.)
This could be the cause of the systematic difference of the RoCs between these, as most of our mirrors
has weaker convex curvature for larger aperture, as seen in the figure. (i.e. outer area is more concave
after the subtration of the curvature)

I did not see any structure like Newton's ring which was observed from the data converted with SXMimage. Why???

I adjusted the focal length of the focusing lens and reduced the beam size enough to mask with the razor blade edge while looking at the camera and then making measurements using PD.

I am still not satisfied with this data because the R of the HR surface measured after flipping seems totally unbelievable (at around 0.45).

G&H AR reflectivity

R percentage

11 ppm @4 deg
19.8 ppm @6 deg
20 ppm @ 8 deg
30 ppm @ 20 deg

Is it possible to calculate astigmatism with your tools?  Can we get curvature in X/Y direction, preferably aligned with some axis that we might align to in the vacuum?