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ID Date Author Type Category Subject
11843   Thu Dec 3 10:05:07 2015 yutaroUpdateOptical LeversCalibration of oplevs for ITMX/ETMX

I updated the figures. I think it's easier to read now.

11875   Fri Dec 11 16:16:36 2015 yutaroUpdateOptical LeversCalibration of oplevs for ITMX/ETMX

Based on calibration measurement I have done (elog 11785, 11831), I updated calibration factors of oplevs on medm screen as follows. Not to change loop gain oplev servo, I also changed oplev servo gain.

C1:SUS-ETMX_OL_PIT_CALIB, C1:SUS-ETMX_OL_PIT_GAIN

(45.1,16) => (200,3.5)

C1:SUS-ETMX_OL_YAW_CALIB, C1:SUS-ETMX_OL_YAW_GAIN

(85.6,8) => (222,3.0)

C1:SUS-ETMY_OL_PIT_CALIB, C1:SUS-ETMY_OL_PIT_GAIN

(26,-16) => (140,-3.0)

C1:SUS-ETMY_OL_YAW_CALIB, C1:SUS-ETMY_OL_YAW_GAIN

(31,-21) => (143,-4.5)

C1:SUS-ITMX_OL_PIT_CALIB, C1:SUS-ITMX_OL_PIT_GAIN

(110,8) => (122,7.2)

C1:SUS-ITMX_OL_YAW_CALIB, C1:SUS-ITMX_OL_YAW_GAIN

(81,-11) => (147,-6)

C1:SUS-ITMY_OL_PIT_CALIB, C1:SUS-ITMY_OL_PIT_GAIN

(159,15) => (239,10)

C1:SUS-ITMY_OL_YAW_CALIB, C1:SUS-ITMY_OL_YAW_GAIN

(174,-21) => (226,-16)

11881   Mon Dec 14 23:49:03 2015 ericqUpdateOptical LeversCalibration of oplevs for ITMX/ETMX
 Quote: Based on calibration measurement I have done (elog 11785, 11831), I updated calibration factors of oplevs on medm screen as follows. Not to change loop gain oplev servo, I also changed oplev servo gain.

After making sure that the upper UGFs were properly in place, I saved these settings to the SDF files. Thanks Yutaro!

12746   Mon Jan 23 15:16:52 2017 gautamUpdateOptical LeversETMY Oplev HeNe needs to be replaced

On the control room monitors, I noticed that the IR TEM00 spot was moving around rather a lot in the Y arm. The last time this happened had something to do with the ETMY Oplev, so I took a look at the 30 day trend of the QPD sum, and saw that it was decaying steeply (Steve will update with a long term trend plot shortly). I noticed the RIN also seemed rather high, judging by how much the EPICS channel reading for the QPD sum was jumping around. Attached are the RIN spectra, taken with the OL spot well centered on the QPD and the arms locked to IR. Steve will swap the laser out if it is indeed the cluprit.

12747   Mon Jan 23 17:24:26 2017 SteveUpdateOptical LeversETMY Oplev HeNe is running hot

ETMY He/Ne 1103P  body temp is  ~45 C The laser was seated loosely  in the V-mount with black rubber padding.

The enclosure has a stinky plastic smell from this black plastic. This laser was installed on Oct 5, 2016 See 1 year plot.

Oplev servo turned off. Thermocouple attached to the He/Ne

It will be replaced tomorrow morning.

 Quote: On the control room monitors, I noticed that the IR TEM00 spot was moving around rather a lot in the Y arm. The last time this happened had something to do with the ETMY Oplev, so I took a look at the 30 day trend of the QPD sum, and saw that it was decaying steeply (Steve will update with a long term trend plot shortly). I noticed the RIN also seemed rather high, judging by how much the EPICS channel reading for the QPD sum was jumping around. Attached are the RIN spectra, taken with the OL spot well centered on the QPD and the arms locked to IR. Steve will swap the laser out if it is indeed the cluprit.

12750   Tue Jan 24 17:52:15 2017 CraigUpdateOptical LeversETMY Oplev HeNe is replaced

Steve, Craig, Gautam

Today Steve replaced the ETMY He/Ne sr P919645 OpLev laser with sr P947049 and Craig realigned it using a new AR coated lenses.

Attached are the RIN of the OpLev QPD Sum channels.  The ETMY OpLev RIN is much lower than when Gautam took the same measurement yesterday.

Also attached are the pitch and yaw OLG TFs to ensure we still have acceptable phase margins at the UGF.

The last three plots show the optical layout of the ETMY OpLev, a QPD reflection blocker we added to the table, and green light to ETMY not being blocked by any changes to the OpLev.

Quote:

ETMY He/Ne body temp is  ~45 C The laser was seated loosely  in the V-mount with black rubber padding.

The enclosure has a stinky plastic smell from this black plastic. This laser was installed on Oct 5, 2016 See 1 year plot.

Oplev servo turned off. Thermocouple attached to the He/Ne

It will be replaced tomorrow morning.

 Quote: On the control room monitors, I noticed that the IR TEM00 spot was moving around rather a lot in the Y arm. The last time this happened had something to do with the ETMY Oplev, so I took a look at the 30 day trend of the QPD sum, and saw that it was decaying steeply (Steve will update with a long term trend plot shortly). I noticed the RIN also seemed rather high, judging by how much the EPICS channel reading for the QPD sum was jumping around. Attached are the RIN spectra, taken with the OL spot well centered on the QPD and the arms locked to IR. Steve will swap the laser out if it is indeed the cluprit.

12858   Wed Mar 1 08:28:04 2017 SteveUpdateOptical Leversoplev laser RIN

Gautam and Steve,

New JDSU 1103P HeNe oplev laser RIN was measured on the SP table with cover on.

This is the beginning of an effort to improve oplev laser noise.

12863   Thu Mar 2 13:59:04 2017 SteveUpdateOptical Leversoplev laser RIN

The laser got much better at low frequency as it warmed up. This laser is almost as good as the electronics?

Dark noise cal was the same today as it was 2 days ago.

12864   Thu Mar 2 17:58:45 2017 ranaUpdateOptical Leversoplev laser RIN

This measurement looks bogus - the difference between dark and not dark is not significant enough to believe. Need to figure out how to match better into the ADC range.

12866   Fri Mar 3 17:24:21 2017 SteveUpdateOptical Leversoplev laser RIN

Corrected oplev laser RIN plot at day 3

RXA:

1. to measure RIN, the lever arm should be really short, not long.
2. the beam should be 3x smaller than the active area of the diode
3. the specular beam should be dumped on a razor dump.
4. we need to make a summary page for HeNe laser testing so that we can see 24 hour specgrams of these things for ~3-4 lasers at the same time.
5. We should add specgram stuff for the existing HeNe SUM channels on the active OLs.

GV: The channel the PD Steve is using is hooked up to C1:ALS-FC_X_F_IN. As I found out today, there can be considerable RF pickup between the C1:ALS-FC_X_F_IN and C1:ALS-FC_Y_F_IN channels, which share a common 4-pin LEMO cable - this is because the rise time of the square wave output of the Wenzel dividers is <1us, so suitability of this particular channel for the RIN measurement set up has to be reconsidered. Perhaps we can use one of the six spare PEM channels over at 1X6.

12873   Wed Mar 8 15:28:37 2017 SteveUpdateOptical Leversoplev laser RIN

Gautam and Steve,

 Quote: Corrected oplev laser RIN plot at day 3 RXA: to measure RIN, the lever arm should be really short, not long. the beam should be 3x smaller than the active area of the diode the specular beam should be dumped on a razor dump. we need to make a summary page for HeNe laser testing so that we can see 24 hour specgrams of these things for ~3-4 lasers at the same time. We should add specgram stuff for the existing HeNe SUM channels on the active OLs. GV: The channel the PD Steve is using is hooked up to C1:ALS-FC_X_F_IN. As I found out today, there can be considerable RF pickup between the C1:ALS-FC_X_F_IN and C1:ALS-FC_Y_F_IN channels, which share a common 4-pin LEMO cable - this is because the rise time of the square wave output of the Wenzel dividers is <1us, so suitability of this particular channel for the RIN measurement set up has to be reconsidered. Perhaps we can use one of the six spare PEM channels over at 1X6.

We did the following:

1, switched data channel  from  C1:ALS-FC_X_F_IN to C1:PEM-MIC_1_OUT_DQ   Actual connection at 1X7 rack, input C17

Tested channel with 1Hz, 100 mV sine wave through DV

2, placed BS into the beam path so the reflected value on the PDA100A 0.1mW,  beam od ~1mm, beam path lenght 11 cm, gain 20dB 3.7Vdc

The full output of this 1103P 2.8 mW was saturating the PDA100A

Summery :finding it to be too good to be this good

12885   Tue Mar 14 09:08:11 2017 SteveUpdateOptical LeversETMX HeNe is dead

ETMX oplev laser is dead. It will be replaced this after noon. Sus damping recovered.

12888   Tue Mar 14 15:05:18 2017 SteveUpdateOptical Levershistory of ETMX oplev laser

 Quote: ETMX oplev laser is dead. It will be replaced this after noon. Sus damping recovered.

This 3 years old HeNe [ JDS 1103P, sn 351889 ]  has been dying for some time or just playing possum at age 1,126 days

I did not replace the ETMX oplev laser because I was unable to bring up the the C1ASC_ETMX_OPTLEV_SERVO  medm screen on laptops.

12890   Fri Mar 17 10:47:16 2017 SteveUpdateOptical Levers ETMX oplev laser replaced

JDSU 1103P. sn T8070866, made March 2007, output power 2.7 mW,  on pd 17,750 counts,

GV 17 March 3pm: I found the Innolight NPRO was off when I walked down to the X end earlier, possibly was accidentally tripped during the Oplev laser replacement. I turned it back on.

Quote:

 Quote: ETMX oplev laser is dead. It will be replaced this after noon. Sus damping recovered.

This 3 years old HeNe [ JDS 1103P, sn 351889 ]  has been dying for some time or just playing possum at age 1,126 days

I did not replace the ETMX oplev laser because I was unable to bring up the the C1ASC_ETMX_OPTLEV_SERVO  medm screen on laptops.

12932   Mon Apr 10 09:49:32 2017 SteveUpdateOptical Leversoplev laser RIN test planning

We are planning to test 3 identical 1103Ps RIN with  continous temp monitoring and control later.

Selected  temp sensor   Platinum RTD 1PT100KN1515CLA   or           RTD-830

Temp controller  with analoge output 0-10Vdc, CNi854  and external dc pulse driven  relay

 Criteria Thermocouple RTD Thermistor Temp Range -267°C to 2316°C -240°C to 649°C -100°C to 500°C Accuracy Good Best Good Linearity Better Best Good Sensitivity Good Better Best Cost Best Good Better

Order placed 4-12-17 for sensor  RTD-830,  controller CNi8-5-4 and relay  SSRL240DC25 = ~\$500.

Still need: fuse, fuse housing, on/off switch, female  AC receptical, chassy box and AC power cord.

12934   Mon Apr 10 14:21:57 2017 ranaUpdateOptical Leversoplev laser RIN test planning

I'm suspicious of this temperature sensor comparison. Usually, what they mean by accuracy is not the same as what we mean. I would not buy these yet. How about we just use what Caryn used several years ago (elog search) ?

PS  Steve LM34

12997   Wed May 17 18:08:45 2017 DhruvaUpdateOptical LeversBeam Profiling Setup

Andrew and I set up the razor blade beam profiling experiment for He-Ne lasers on the "SP" table.  Once I receive the laser safety training, I will make power measurements and fit it to an erfc curve from which I will calculate the gaussian profile of the beam. I'm attaching some pictures of the setup.

Least count of the micrometer - 2 microns

Laser  : Lumentum 22037130:1103P

Photodetector : Thor Labs PDA100A

13002   Mon May 22 10:53:02 2017 DhruvaUpdateOptical LeversBeam Profiling Results

 Quote: Andrew and I set up the razor blade beam profiling experiment for He-Ne lasers on the "SP" table.  Once I receive the laser safety training, I will make power measurements and fit it to an erfc curve from which I will calculate the gaussian profile of the beam. I'm attaching some pictures of the setup.  Least count of the micrometer - 2 microns  Laser  : Lumentum 22037130:1103P Photodetector : Thor Labs PDA100A

I had measured the y-profile of the beam of Friday at 5 axial locations and fit them to an erfc function using the lsqcurvefit function of MATLAB.

The results were as follows -

z(cm)    w (in)

4          0.0131

10        0.0132

15        0.0137

20       0.0139

25        0.0147

I left w in inches in the intensity plots as MATLAB gave more accurate fits for those values.

I converted these to S.I while making the spot-size vs z plot and the corresponding values in microns were

332.74, 335.28, 347.98, 353.06, 373.38.

On fitting these values to the formula for the spot size of a Gaussian beam, the beam waist came out to be 330.54 microns and the location of the beam waist was at z=-2cm, where z=0 marks the head of the laser.

TO-DO : Measure the spot size of the beam at more axial points to obtain a better fit.

Measure the x-profile of the beam.

Analyse the error in the spot sizes and corresponding error in the beam waist.

13006   Tue May 23 10:27:24 2017 DhruvaUpdateOptical LeversBeam Profiling Results

I have attempted to calculate the instrument error (micrometer least count) using the values of the spot size obtained by the least squares fitting method. This error is large towards the centre of the beam as the power varies significantly between adjecent markings of the micrometer. Using the new values of error obtained, I used the chi-square fitting minimisation method to further optimise the waist size.

The modified values are -

z(cm)    w (in)

4         0.0134

10        0.0135

15        0.0140

20        0.0142

25        0.0150

And the revised values for the beam waist and location are 338.63 microns and -2.65 cm respectively.

I will now try to use the chi-square stastitic to estimate the error in spot size.

13007   Tue May 23 15:22:04 2017 ranaUpdateOptical LeversBeam Profiling Results
1. Include several sources of error. Micrometer error is one, but you should be able to think of at least 3 more.
2. There should be an error bar for the x and y axis.
3. Also, use pdftk to put the PDFs all into a single file. Remove so much whitespace.
4. Google 'beautiful plots python' and try to make your plots for the elog be more like publication quality for PRL or Nature.
13008   Tue May 23 16:33:00 2017 SteveUpdateOptical LeversBeam Profiling Results

You may compare your results with this.

RXA: please no, that's not the right way

13018   Tue May 30 13:36:58 2017 SteveUpdateOptical LeversETMY Oplev HeNe is replaced

Finally I reallized what is killing the ETMY oplev laser. Wrong  power supply, it  was driving the HeNe laser by 600V higher voltage than recommended. Power supply 101T-2300Vdc replaced by 101T-1700Vdc ( Uniphase model 1201-1, sn 2712420 )

The laser head 1103P, sn P947049 lived for 120 days and it was replaced by sn P964431   New laser output 2.8 mW,  quadrant sum 19,750 counts

13021   Tue May 30 18:31:54 2017 DhruvaUpdateOptical LeversBeam Profiling Results

​Updates in the He-Ne beam profiling experiment.

1. I've made intensity profile plots at two more points on the z-axis. The additition of this plots hasn't affected the earlier obtained beam waist significantly.
2. I have added other sources of error, such as the statisitical fluctuations on the oscilloscope(which is small compared to the least count error of the micrometer) and the least count of the z-axis scale.
3. I have also calculated the error in the parameters obtained by fiiting by calculating the covariance matrix using the jacobian returned by the lsqcurvefit function in MATLAB.
4. I have also added horizontal error bars to all plots.
5. All plots are now in S.I. units

13053   Thu Jun 8 12:43:42 2017 DhruvaUpdateOptical LeversBeam Profiling Results

 Quote: ​Updates in the He-Ne beam profiling experiment. ​

New and improved plots for the He-Ne profiling experiment

Font size has been increased to 30.

The plots are maximum size (Following Rana's advice, I saved the plots as eps files(maximized) and converted them to pdf later).

There is a shaded region around the trendline that represents the parameter error.

Function that I fit my data to (should have mentioned this in my earlier elog entries)

$P = \dfrac{P_0}{2}\Bigg[1+erf\Big(\dfrac{\sqrt2(X-X_0)}{w}\Big) \Bigg]$

Description of my error analysis -

1. I have assumed a 20% deviation from markings in the micrometer error.

2. Using the error in the micrometer, I have calculated the propogated error in the beam power :

$\delta P = \sqrt{\dfrac{2}{\pi}}{P_0}\dfrac{\delta x}{w}\exp\Bigg({\frac{-2(X-X_0)^2}{w^2}}\Bigg)$

I added this error to the stastistical error due to the fluctuation of the oscilloscope reading to obtain the total error in power.

3. I found the Fisher Matrix by numerically differentiating the function at different data points $P_b$ with respect to the parameters $p_i =$  $P_0, X_0$ and $w$.

$F_{ij} = \sum_{b} {\frac{\partial P_b}{\partial p_i}\frac{\partial P_b}{\partial p_j}}$$\frac{1}{\sigma^2_b}$

I then found the covariance matrix by inverting the Fisher Matrix and found the error in spot size estimation.

13141   Tue Jul 25 02:03:59 2017 gautamUpdateOptical LeversOptical lever tuning thoughts

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.

13147   Fri Jul 28 15:36:32 2017 gautamUpdateOptical LeversOptical lever tuning thoughts

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.

 Quote: 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.

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:

$y_{meas}(s) = P(s)\sum_{i=1}^{3}d_{i}(s) + \sum_{k=1}^{4}n_{k}(s)$

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.

13156   Tue Aug 1 16:05:01 2017 gautamUpdateOptical LeversOptical lever tuning - cost function construction

### 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, $\left | \frac{1}{1+P(s)C(s)} \right |$. 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.

 Quote: An elog with a first pass at a mathematical formulation of the cost-function for controller optimization to follow shortly.

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.

13432   Thu Nov 16 13:57:01 2017 gautamUpdateOptical LeversOptical lever noise

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...

13433   Thu Nov 16 15:43:01 2017 ranaUpdateOptical LeversOptical lever noise

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.

13437   Tue Nov 21 11:37:29 2017 gautamUpdateOptical LeversBS OL calibration updated

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

BS Yaw       14  /  170    (old/new)     urad/counts

 Quote: 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.

13439   Tue Nov 21 16:28:23 2017 gautamUpdateOptical LeversBS OL calibration updated

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...

 Quote: Need to double check against OSEM readout during the sweep.

13441   Tue Nov 21 23:04:12 2017 gautamUpdateOptical LeversOplev "noise budget"

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.

13444   Wed Nov 22 05:41:32 2017 ranaUpdateOptical LeversOplev "noise budget"

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.

13448   Wed Nov 22 15:29:23 2017 gautamUpdateOptical LeversOplev "noise budget"

[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...

 Quote: 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.

13449   Wed Nov 22 16:40:00 2017 KojiUpdateOptical LeversOplev "noise budget"

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.

13451   Wed Nov 22 19:20:01 2017 ranaUpdateOptical LeversOplev "noise budget"

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)

13452   Wed Nov 22 23:56:14 2017 gautamUpdateOptical LeversOplev "noise budget"

## 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...

 Quote: 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)

13453   Thu Nov 23 18:03:52 2017 gautamUpdateOptical LeversOplev "noise budget"

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.

Quote:

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

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...

13484   Fri Dec 15 18:24:46 2017 ranaSummaryOptical LeversPRM

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.

13548   Mon Jan 15 17:36:03 2018 gautamHowToOptical LeversOplev calibration

## 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.

ETMX, Pitch 200 175 0.88
ETMX, Yaw 222 175 0.79
ITMX, Pitch 122 134 1.1
ITMX, Yaw 147 146 1
BS, Pitch 130 136 1.05
BS, Yaw 170 176 1.04
ITMY, Pitch 239 254 1.06
ITMY, Yaw 226 220 0.97
ETMY, Pitch 140 164 1.17
ETMY, Yaw 143 169 1.18

## 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).

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.

14345   Tue Dec 11 18:20:59 2018 gautamUpdateOptical LeversBS/PRM HeNe is dead

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?

14402   Tue Jan 15 18:16:00 2019 gautamUpdateOptical LeversETMY Oplev HeNe needs replacement

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.

 Quote: Finally I reallized what is killing the ETMY oplev laser. Wrong  power supply, it  was driving the HeNe laser by 600V higher voltage than recommended. Power supply 101T-2300Vdc replaced by 101T-1700Vdc ( Uniphase model 1201-1, sn 2712420 ) The laser head 1103P, sn P947049 lived for 120 days and it was replaced by sn P964431   New laser output 2.8 mW,  quadrant sum 19,750 counts
14404   Fri Jan 18 12:52:07 2019 gautamUpdateOptical LeversBS/PRM Oplev HeNe replaced

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.

14484   Mon Mar 18 17:06:12 2019 gautamUpdateOptical LeversITMY HeNe replaced

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).

 Quote: I'll ask Chub to replace it this afternoon.
14541   Mon Apr 15 10:20:44 2019 gautamUpdateOptical LeversBS Oplev PIT was oscillating

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...

14902   Fri Sep 20 11:39:04 2019 gautamUpdateOptical LeversETMX Oplev HeNe Dead

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.

14911   Sun Sep 29 16:08:25 2019 gautamUpdateOptical LeversETMX Oplev HeNe replaced

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.

 Quote: 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.
15079   Thu Dec 5 18:15:01 2019 gautamUpdateOptical LeversITM, PRM and BS Oplevs re-centered

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.

15607   Fri Oct 2 10:29:49 2020 gautamUpdateOptical LeversETMY, BS and ITMX HeNes degrading

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.

15717   Wed Dec 9 11:54:11 2020 gautamUpdateOptical LeversITMX HeNe replaced

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.

ELOG V3.1.3-