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ID Date Author Type Category Subject
15087   Mon Dec 9 19:19:04 2019 YehonathanUpdatePSLAOM first order beam alignment

{Yehonathan, Rana}

In order to setup a ringdown measurement with perfect extinction we need to align the first order beam from the AOM to the PMC instead of the zeroth order.

We connected a signal generator to the AOM driver and applied some offset voltage. We spot the first order mode and align it to the PMC. The achieved transmitted power is roughly as it was before this procedure.

Along the way few changes has been made in the PSL table:

1. Some dangling BNCs were removed.

2. Laser on the south east side of the PSL table was turned off.

3. DC power supplies were removed (Attachment 1 & 2). The rubber legs on the first one are sticky and leave black residue.

4. The beam block that orginally blocked the AOM high order modes was raised to block the zeroth order mode (Attachment 3).

5. The unterminated BNC T junction (Attachment 4 - before picture). from the PMC mixer to the PMC servo was removed.

However, we are currently unable to lock the PMC on high gain. When the gain is too high the PZT voltage goes straight to max and the lock is lost.

Attachment 1: 20191209_193112.jpg
Attachment 2: 20191209_193203_HDR.jpg
Attachment 3: imageedit_2_7551928142.gif
Attachment 4: imageedit_3_5863650538.gif
15089   Tue Dec 10 01:24:17 2019 YehonathanUpdatePSLAOM first order beam alignment

 However, we are currently unable to lock the PMC on high gain. When the gain is too high the PZT voltage goes straight to max and the lock is lost.

Just realized that the diffracted beam is frequency shifted by 80MHz. It would shift the PZT position in the PMC lock acquisition, wouldn't it?

15090   Tue Dec 10 13:26:46 2019 YehonathanUpdatePSLAOM first order beam alignment

nvm the PZT can scan over many GHz.

Quote:

 However, we are currently unable to lock the PMC on high gain. When the gain is too high the PZT voltage goes straight to max and the lock is lost.

Just realized that the diffracted beam is frequency shifted by 80MHz. It would shift the PZT position in the PMC lock acquisition, wouldn't it?

15091   Tue Dec 10 15:17:17 2019 YehonathanUpdatePSLPMC is locked

{Jon, Yehonathan}

We burt-restored the PSL and the PMC locked immediately.

The PMC is now locked on the AOM first order mode.

15092   Tue Dec 10 18:27:22 2019 YehonathanUpdatePSLPMC is locked

{Yehonathan, Jon}

We are able to lock the PMC on the TEM00 mode of the deflected beam.

However when we turn off the driving voltage to the AOM and back on the lock is not restored. It get stuck on some higher order mode.

There are plethora of modes present when the PZT is scanned, which makes us believe the cavity is misaligned.

To lock again on the TEM00 mode again we disconnect the loop (FP Test point 1), find a TEM00 mode using the DC output adjust and close the loop again.

15093   Wed Dec 11 15:01:57 2019 JonSummaryPSLPMC cavity ringdown measurement

[Jon, Yehonathan]

We carried out a set of cavity ringdown measurements of the PMC. The 1/e decay time scale is found to be 35.2 +/- 2.4 (systematic) μs. The statistical error is negligible compared to the systematic error, which is taken as the maximum absolute deviation of any measurement from the average value.

To make the measurement, we injected the first order deflection beam of an 80 MHz AOM, then extinguished it quickly by cutting the voltage offset to the AOM driver provided by an RF function generator. A 100 MHz oscilloscope configured to trigger on the falling voltage offset was used to sample the cavity in transmission as sensed by a PDA55. We found the detector noise of the DC-coupled output of the 35.5 MHz REFL PD to be too high for a reflection-side measurement.

Further loss analysis is forthcoming.

Attachment 1: IMG_0101.jpg
15094   Wed Dec 11 15:29:17 2019 YehonathanUpdatePSLPMC is locked

Make sure to measure the power drop of the beam downstream of the AOM but before the PMC. Need to plot both together to make sure the chop time is much shorter than the 1/e time.

15096   Thu Dec 12 19:20:43 2019 YehonathanUpdatePSLPMC cavity ringdown measurement

{Yehonathan, Rana, Jon}

To check whether we laser is being shut fast enough for the ringdown measurement we put a PD55 in the path that leads to the beat note setup. The beam is picked off from the back steering mirror after AOM and before the PMC.

@Shruti the PD is now blocking the beam to your setup.

As before, we drive the AOM to deflect the beam. The deflected beam is coupled to the PMC cavity. We lock the PMC and then shut the beam by turning off the output of the function generator that provides voltage to the AOM driver.

We measure the transmitted light of the PMC together with the light that is picked off before the PMC. In Attachment 1, the purple trace is the PMC transmission, the green trace is the peaked-off beam and the yellow trace is the function generator signal.

Rana was pointing out that the PDs, the function generator and the scope were not carefully impedance matched, which could lead to erroneous measurements. He also mentioned that the backscattered beam was too bright which might indicate that the PMC is oscillating. To remedy this we lowered the gain of the PMC lock to ~8.

We repeat the measurement after setting all the components to 50ohm (attachment 2). We then realize that the BNC T junction connected on the function generator is splitting the signal between the 50ohm AOM driver and 1Mohm oscilloscope channel which causes distortions as can be seen. We remove the T junction and get a much cleaner measurement (see next elog).

It seems like either the shutting speed or the PDs are only slightly faster than the PMC. I also check the AOM driver RF output fall time doing the same kind of measurement (attachment 3).

We suspect the PDs' bandwidth is to blame (although they are quoted to have 10MHz bandwidth).

In any case, this is fast enough for the IMC and arm cavities whose lifetime should be much longer than the PMC.

I will post an elog with some numbers tomorrow.

Attachment 1: IMG_0105.jpeg
Attachment 2: TEK00001.PNG
Attachment 3: 20191212_151642.jpg
15097   Fri Dec 13 12:28:43 2019 YehonathanUpdatePSLPMC cavity ringdown measurement

I grab the data we recorded yesterday from the scope and plot it in normalized units (remove noise level and divide by maximum). See attachment.

It can be seen that the measured ringdown time is ~ 17us while the shut-off time is ~12us.

I plan to model the PD+AOM as a lowpass filter with an RC time constant of 12us and undo its filtering action on the PMC trans ringdown measurement to get the actual ringdown time.

Is this acceptable?

Attachment 1: Ringdown_InitialProcess.pdf
15098   Mon Dec 16 18:19:42 2019 shrutiUpdatePSLPMC cavity ringdown measurement : beat-note disruption

I have removed the PD55 + ND filter attached to it (see Attachment) and placed it next to the oscilloscope, after disconnecting its output and power supply. The post is still in place.

I did see the beat after that.

 Quote: {Yehonathan, Rana, Jon} To check whether we laser is being shut fast enough for the ringdown measurement we put a PD55 in the path that leads to the beat note setup. The beam is picked off from the back steering mirror after AOM and before the PMC. @Shruti the PD is now blocking the beam to your setup.

Attachment 1: IMG_0040.jpg
15099   Tue Dec 17 00:23:28 2019 YehonathanUpdatePSLMapping the PSL electronics

I added to the PSL wiring list the ioo channels and the laser shutter (See attached pdf for an updated list).

The total channel numbers for now:

 ai 57 ao 13 bi 1 bo 36

I counted each mbbo as 1 bo but I am not sure that's correct.

Still need to allocate Acromags.

Attachment 1: PSL_Wirings_-_Sheet1_(2).pdf
15100   Tue Dec 17 18:05:06 2019 YehonathanUpdatePSLMapping the PSL electronics

Updated the channel list (Attached):

1. Removed the MC steering mirror PZT channels

3. Recounted the mbbos correctly

4. Allocated Acromags:

 Model Purpose No. Spare channels XT1221 ai 7 11 XT1541 ao + src bo 2 9 ao XT1121 src bo 2 4 XT1121 sink bo 1 4

I think we can start wiring.

Attachment 1: PSL_Wirings_-_Sheet1_(3).pdf
15102   Tue Dec 17 20:45:30 2019 ranaUpdatePSLPMC cavity ringdown measurement

idk - I'm recently worried about the 'thermal self locking' issue we discussed. I think you should try to measure the linewidth by scanning (with low input power) and also measure the TF directly by modulating the power via the AOM and taking the ratio of input/output with the PDA55s. I'm curious to see if the ringdown is different for low and high powers

 Quote: I plan to model the PD+AOM as a lowpass filter with an RC time constant of 12us and undo its filtering action on the PMC trans ringdown measurement to get the actual ringdown time. Is this acceptable?

This is an ole SURF report on thermal self-locking that may be of use (I haven't read it or checked it for errors, but Royal was pretty good analytically, so its worth looking at)

15103   Fri Dec 20 18:33:21 2019 YehonathanUpdatePSLMapping the PSL electronics

Final (hopefully) PSL channel list is attached with allocated Acromag channels. Wiring spreadsheet coming soon.

Current Acromag count:

 AI 8 AO 2 BIO 4 Number of channels 8*8+2*8+4*16=144 Number of wires 144*2=288

Attachment 1: PSL_Wirings_-_Channel_List.pdf
15104   Mon Dec 23 19:30:20 2019 YehonathanUpdatePSLMapping the PSL electronics

15105   Fri Dec 27 15:01:02 2019 YehonathanUpdatePSLPMC cavity ringdown measurement

I measured PMC ringdowns for several input powers. I change the input power by changing the DC voltage to the AOM.

First, I raise the DC voltage to the AOM from 0V and observe the signal on the picked off PD. I see that at around 0.6V the signal stops rising. The signal on the PD is around 4V at that point so it is not saturated.

Up until now, we provided 1.5V to the AOM, which means it was saturated.

I measured ringdowns at AOM voltages of 0.05, 0.1, 0.3, 0.5, 1 volt by shutting off the DC voltage to the AOM and measuring the signal at the PMC transmission PD and the picked off PD simultaneously for reference.

Attachment 1 shows the reference measurement for different AOM voltages. For low AOM DC voltages, the response of the AOM+PD is slower.

Attachment 2 shows the PMC transmission PD measurements which barely change as a function of AOM voltage but shows the same trend. I believe that if the AOM+PD response was much faster there would be no observable difference between those measurements.

Attachment 3 shows PMC transmissions and references for AOM voltages 0.05V and 1V. It seems like for low AOM voltages we are barely fast enough to measure the PMC ringdown.

I fitted the 0.3V ringdown and reference to a sum of two exponentials (Attachment 4).

The fitting function is explicitly a * nm.exp(-x/b) +c* nm.exp(-x/d) +e

For the PMC transmission I get:

a = 0.21
b = 3.64 (us)
c = 0.69,
d = 39.62 (us)
e = 2.0e-04

For the reference measurement:

a = 0.34
b = 4.97 (us)
c = 0.58
d= 31.22 (us)
e = 1.11e-03


I am still not able to do deconvolution of the ref from the measurement reliably. I think we should do a network analyzer measurement.

Shruti, the PD is again in your beam path.

Attachment 1: PDAOMResponse.pdf
Attachment 2: PMCTransmission.pdf
Attachment 3: RingdownsAndRefs.pdf
Attachment 4: TwoExponentialFitAOM0.3V.pdf
15106   Fri Dec 27 16:26:11 2019 YehonathanUpdatePSLPMC Linewidth measurement

I try to measure the linewidth of the PMC by ramping the PMC PZT.

I do it by connecting a triangular shape signal to FP Test 1 on the PMC servo front panel (I know, it is probably better to connect it to DC EXT. next time.) and turn the servo gain to a minimum.

Attachment 1 shows the PMC transmission PD as the PZT is swept with the EOM connected and when it is disconnected. It shows the PMC over more than 1 free spectral range.

For some reason, I cannot seem to be able to find the 35MHz sidebands which I want to use to calibrate the PZT scan. I made sure that the EOM is driven by a 35MHz signal using the scope. I also made sure that the PMC cannot to lock without the EOM connected.

I am probably doing something silly.

Attachment 1: PMCTransmissionSpectra.pdf
15107   Tue Dec 31 03:03:02 2019 gautamUpdatePSLPMC cavity ringdown measurement

When I was looking at this, the AOM shutdown time was measured to be ~120 ns, and while I wasn't able to do a ringdown measurement with the PMC (it'd just stay locked because at the time i was using the zeroth order beam), the PMC transmission decayed in <200 ns.

15108   Wed Jan 1 04:53:11 2020 gautamUpdatePSLMapping the PSL electronics

For the IMC servo board, it'd be easiest to copy the wiring scheme for the BIO bits as is configured for the CM board (i.e. copy the grouping of the BIO bits on the individual Acromag units). This will enable us to use the latch code with minimal modifications (it was a pain to debug this the first time around). I don't see any major constraint in the wiring assignment that'd make this difficult.

15109   Wed Jan 1 14:14:00 2020 YehonathanUpdatePSLPMC Linewidth measurement

Turns out the 35MHz sidebands are way too weak to resolve from the resonance when doing a PZT scan.

I connect the IFR2023B function generator on the PSL table to the EOM instead of the FSS box and set it to generate 150MHz at 13dbm.

To observe the resulting weak sideband I place a PDA55 at the peak-off path from the transmission of the PMC where there is much more light than the transmission of the PMC head mirror. Whoever is using this path there is a PD blocking it right now.

I do a PZT scan by connecting a triangular signal to the EXT DC on the PMC servo with and without the EOM (Attachment 1). A weak sideband can clearly be spotted now.

Using the above 150MHz sideband calibration I can find the roundtrip time to be 1.55ns.

I take a high-resolution scan of a resonance peak and fit it to a Lorentzian (Attachment 2) and find a roundtrip loss of 1.3%.

Using the above results the cavity decay time is 119ns.

We should investigate what's going on with the ringdown measurements.

Attachment 1: 150MHzSideBandCreation.pdf
Attachment 2: LinewidthMeasurment.pdf
15110   Wed Jan 1 16:04:37 2020 YehonathanUpdatePSLMapping the PSL electronics

Done.

Quote:

For the IMC servo board, it'd be easiest to copy the wiring scheme for the BIO bits as is configured for the CM board (i.e. copy the grouping of the BIO bits on the individual Acromag units). This will enable us to use the latch code with minimal modifications (it was a pain to debug this the first time around). I don't see any major constraint in the wiring assignment that'd make this difficult.

15111   Mon Jan 6 15:36:55 2020 JonUpdatePSLAssembly underway for c1psl upgrade

[Jon, Yehonathan]

We've begun assembling the new c1psl Acromag chassis based on Yehonathan's final pin assignments. So far, parts have been gathered and the chassis itself has been assembled.

Yehonathan is currently wiring up the chassis power and Ethernet feedthroughs, following my wiring diagram from previous assemblies. Once the Acromag units are powered, I will help configure them, assign IPs, etc. We will then turn the wiring over to Chub to complete the Acromag to breakout board wiring.

I began setting up the host server, but immediately hit a problem: We seem to have no more memory cards or solid-state drives, despite having two more SuperMicro servers. I ordered enough RAM cards and drives to finish both machines. They will hopefully arrive tomorrow.

15112   Mon Jan 6 16:07:12 2020 gautamUpdatePSLAssembly underway for c1psl upgrade

RTFE. Where did the spares go?

 Quote: I began setting up the host server, but immediately hit a problem: We seem to have no more memory cards or solid-state drives, despite having two more SuperMicro servers. I ordered enough RAM cards and drives to finish both machines. They will hopefully arrive tomorrow.
15113   Mon Jan 6 19:05:09 2020 not gautamUpdatePSLAssembly underway for c1psl upgrade

I found them, thanks. After c1psl, there are 4 2GB DIMM cards and 1 SSD left. I moved them into the storage bins with all the other Acromag parts.

Quote:

RTFE. Where did the spares go?

 Quote: I began setting up the host server, but immediately hit a problem: We seem to have no more memory cards or solid-state drives, despite having two more SuperMicro servers. I ordered enough RAM cards and drives to finish both machines. They will hopefully arrive tomorrow.
15114   Tue Jan 7 18:51:51 2020 JonUpdatePSLNew c1psl server assembled

I've assembled a new SuperMicro rackmount machine to replace c1psl. It is currently set up on the electronics bench.

• OS: Debian 10.2
• Hostname: c1psl1 (will become c1psl after installation)
• IP: 192.168.113.54 (registered in the martian DNS)
• Network drive mount point set up (/cvs/cds), which provides all the EPICS executables.
15115   Fri Jan 10 14:21:19 2020 YehonathanUpdatePSLc1psl reboot

PSL controls on the sitemap went blank. Rebooted c1psl. PSL screens seem normal again.

15116   Fri Jan 10 19:48:46 2020 yehonathanUpdatePSLAssembly underway for c1psl upgrade

{Yehonathan, Jon}

I finished pre-wiring the PSL chassis. I mounted the Acromags on the DIN rails and labeled them. I checked that they are powered up with the right voltage +24V and that the LEDs behave as expected.

Attachment 1: 20200110_194429.jpg
Attachment 2: 20200110_194516_HDR.jpg
15118   Mon Jan 13 16:05:18 2020 yehonathanUpdatePSLAssembly underway for c1psl upgrade

{Yehonathan, Jon}

I configured the Acromag channels according to the Slow Controls Wiki page.

We started testing the channels. Almost at the beginning we notice that the BIO channels are inverted. High voltage when 0. 0 Voltage when 1. We checked several things:

1. We checked the configuration of the BIOs in the windows machine but nothing pointed to the problem.

2. We isolated one of the BIOs from the DIN rail but the behavior persisted.

3. We checked that the voltages that go into the Acromags are correct.

The next step is to power up an isolated Acromag directly from the power supply. This will tell us if the problem is in the chassis or the EPICs DB.

15119   Mon Jan 13 23:30:53 2020 YehonathanSummaryPSLChanges made since Gautam left

As per Gautam's request, I list the changes that were made since he left:

1. The AOM driver was connected to a signal generator.

2. The first order beam from the AOM was coupled into the PMC while the zero-order beam is blocked. We might want to keep this configuration if the pointing stability is adequate.

3. c1psl got Burt restored to Dec 1st.

4. Megatron got updated.

Currently, c1susaux seems unresponsive and needs to be rebooted.

15120   Tue Jan 14 17:16:43 2020 yehonathanUpdatePSLAssembly underway for c1psl upgrade

{Yehonathan, Jon}

I isolated a BIO Acromag completely from the chassis and powered it up. The inverted behavior persisted.

Turns out this is normal behavior for the XT1111 model.

For digital outputs, one should XT1121. XT1111 should be used for digital inputs.

Slow machines Wiki page was updated along with other pieces of information.

I replaced the XT1111 Acromags with XT1121 and did some rewiring since the XT1121 cannot get the excitation voltage from the DIN rail.

I added an XT1111 Acromag for the single digital input we have in this system.

15124   Wed Jan 15 10:12:46 2020 gautamUpdatePSLAssembly underway for c1psl upgrade

I don't think this is an accurate statement. XT1111 modules have sinking digital outputs, while XT1121 modules have sourcing digital outputs. Depending on the requirement, the appropriate units should be used. I believe the XT1111 is the appropriate choice for most of our circuits.

 For digital outputs, one should XT1121. XT1111 should be used for digital inputs.
15125   Wed Jan 15 14:10:28 2020 JonConfigurationPSLNew EPICS database for C1PSL + C1IOO

### Summary

I have completed the new EPICS channel database for the c1psl and c1ioo channels (now combined into the new c1psl Acromag machine). I've tested a small subset of channels on the electronics bench to confirm that the addressing and analog channel calibrations are correct in a general sense. At this point, we are handing the chassis off to Chub to complete the wiring of the Acromag terminals to Dsub feedthroughs. At the 40m meeting today, we identified Feb. 17-22 as a potential window for installation in the interferometer (Gautam is out of town then). Below are some implementaton details for future reference.

### Analog channel calibration for Acromag

For analog input (ai) channels, the Acromag outputs raw values ranging from +/-30,000 counts, but the EPICS IOC interprets the data type as ranging from +/-2^15 = 32,768. Similarly, for analog output (ao) channels, the Acromag expects a drive signal in the range +/-30,000 counts. To achieve proper scaling, Johannes had previously changed the EGUF and EGUL fields from +/-10 V to +/-10.923 V. However, changing the engineering fields makes it much harder for a human to read off the real physical I/O range of the channel.

A better way to achieve the correct scaling is to simply set the field  ASLO=1.09225 (65,536 / 60,001) in addition to the normal EGUF and EGUL field values (+/-10 V). Setting this field forces a rescaling of the number of raw counts that works as so (assuming a 16-bit bipolar ADC or DAC, as are the Acromags):

OVAL = (RVAL * ASLO + AOFF + 2^15) * (EGUF - EGUL) / 2^16 + EGUL

In the above mapping, OVAL is the value of the channel in engineering units (e.g., V) and RVAL is its raw value in counts. It is not the case that either the ASLO/AOFF or EGUF/EGUL fields are used, but not both. The ASLO/AOFF parameters are always applied (but their default values are ASLO=1 and AOFF=0, so have no effect unless changed). The EGUF and EGUL parameters are then additionally applied if the field LINR="LINEAR" is set.

This conversion allows the engineering fields to remain unchanged from the real physical range. The ASLO value is the same for both analog input and output channels. I have implemented this on all the new c1psl and c1ioo channels and confirmed it to work using a calibrated input voltage source.

15126   Wed Jan 15 15:04:31 2020 gautamUpdatePSLPMC Linewidth measurement

For the ringdowns, I suggest you replicate the setup I had - infrastructurally, this was quite robust, and the main problem I had was that I couldn't extinguish the beam completely. Now that we have the 1st order beam, it should be easy.

15127   Wed Jan 15 16:08:40 2020 not gautamUpdatePSLAssembly underway for c1psl upgrade

You're right. We had the right idea before but we got confused about this issue. I changed all the XT1121s to XT1111 and vice versa. We already know which channels are sourcing and which not. Updated the wiring spreadsheet. The chassis seems to work. It's time to pass it over to Chub.

Quote:

I don't think this is an accurate statement. XT1111 modules have sinking digital outputs, while XT1121 modules have sourcing digital outputs. Depending on the requirement, the appropriate units should be used. I believe the XT1111 is the appropriate choice for most of our circuits.

 For digital outputs, one should XT1121. XT1111 should be used for digital inputs.
15131   Fri Jan 17 21:56:22 2020 YehonathanUpdatePSLAOM first order beam alignment

Today I noticed that the beam reflected from the PMC into the RFPD has a ghost (attachment) due to reflection from the back of the high transmission beam splitter that stirs the beam into the RFPD.

The two beams are focused into the RFPD.

In the past, the ghost beam was probably blocked by the BS mirror mount.

I put an iris to block the ghost beam.

Attachment 1: 20200117_174841.jpg
15132   Fri Jan 17 22:11:19 2020 YehonathanUpdatePSLRingdown measurements

I prepare for the ringdown measurement of the PMC according to Gautam's previous experiments.

1. I assembled the needed PDs and power supplies, lenses, beamsplitters and optomechanics needed for the measurement.

2. I surveyed the laser power with an Ophir power meter in the different parts of the experiment. All the measurements were done with the AOM driver excited with 1V DC.

For the PMC reflection, we chose to split off the beam that goes into the reflection camera. The power in that beam is ~ 0.11mW when the PMC is locked and 2.1mW otherwise.

For the PMC transmission, we chose to split the beam that is transmitted through the second steering mirror after the PMC. The power in that beam is 2mW.

For the peak off before the PMC, we chose to split the beam that goes into the fiber coupler. That path contains also the other AOM diffraction orders: 2.26mW in the 0th order beam, 6.5mW in the 1st order beam, 0.14mW in the 2nd order beam.

3. I placed a 10% beam splitter in the peak-off path such that 90% still goes into the fiber coupler (Attachment 1). I place a lens and PDA255 to measure the peak-off (Attachment 2).

It's getting late, I'll continue with the PD placements on Tuesday.

Attachment 1: 20200117_192455.jpg
Attachment 2: 20200117_192448.jpg
15133   Mon Jan 20 12:16:50 2020 gautamUpdatePSLPMC input reverted to AOM zeroth order beam

Summary:

1. The input beam to the PMC cavity was changed back to the zeroth order beam from the AOM.
2. The PMC was locked and nominal transmission levels were recovered.
3. The AOM driver voltage was set to 0V DC.
4. A razor beam dump was placed to catch the first (and higher order) beams from the AOM (see Attachment #1), but allow the zeroth order beam to reach the PMC cavity.
5. Some dangling cabling was cleared from the PSL enclosure.

Details

• HEPA turned to 100% while work was going on in the PSL enclosure.
• Input power to the PMC cut from ~1.3 W to ~20 mW using the first available HWP downstream of the laser head, before any realignment work was done.
• Next, the beam dump blocking the undeflected zeroth order beam was removed.
• Triangle wave was applied to the PZT servo board "EXT DC" input to sweep the cavity length to make the alignment easier.
• After some patient alignment, I could see a weak transmitted beam locked to some high order mode, at which point I increased the input power to 200mW, and did the fine alignment by looking at the mode shape of the transmitted beam.
• Once I could lock to a TEM00 mode, I bumped the power back up to the nominal 1.3W, I fine tuned the alignment further by minimizing PMC REFL's DC level.
• Dialled the power back down (using HWP) for installation of the beam block to catch the AOM's first (and higher order) beams.
• Checked that the reflected beam from the PMC cavity is well centered on the PMC REFL PDH photodiode. The ghost from the AR coating of the high-T beamsplitter is blocked by the iris installed by yehonathan on Friday.
• The beam was a little low on the PMC REFL CCD camera - I raised the camera by ~1cm.
• With the beam axis well matched to the PMC, I measured 1.33 mW going into the cavity, and 1.1 W transmitted, so $T_{\mathrm{PMC}} \approx 83 \, \%$. Whatever loss numbers we extract should be consistent with this fact.
• HEPA turned back down to 30% shortly after noon.

Note that for all the alignment work, only the two steering mirrors immediately upstream of the PMC cavity were touched.

Attachment 1: IMG_8362.JPG
15134   Mon Jan 20 15:11:20 2020 gautamUpdatePSLPMCT photodiode grounding issue

For a few days, I've noticed that the PSL overview StripTool panel shows PMC transmission and FSS RMTEMP channels with variation that is too large to be believable. Looking at these signals on an oscilloscope, there was no such fuzziness in the waveform. I ruled out flaky connections, and while these are the only two channels currently being acquired by the temporary Acromag setup underneath the PSL enclosure, the Acromags themselves are not to blame, because once I connected a function generator to the Acromag instead of the PMC transmission photodiode, both channels are well behaved. So the problem seems to be with the PMC transmission photodiode, perhaps a grouding issue? Someone please fix this.

Attachment 1: PMCT_anomaly.pdf
15135   Mon Jan 20 20:20:36 2020 gautamUpdatePSLPMC servo checkout

Summary:

The PDH discriminant of the PMC servo was measured to be ~0.064 GV/m. This is ~50 times lower than what is reported here. Perhaps this is a signature of the infamous ERA decay, needs more investigation.

Details:

• Calibration of the error and control points were done using 1 Hz triangle wave injection to the "EXT DC" input of the PMC servo. Two such sweeps are shown in Attachment #1 (measured data as points, fits as solid lines). For the control signal monitor, I've multiplied the signal obtained on the scope by 49.6, which is the voltage divider implemented for this monitor point.
• The PDH discrimiannt was calibrated into physical units knowing the modulation frequency of the PMC, which is 35.5 MHz. The error in this technique due to the free-running NPRO frequency noise is expected to be small since the entire fringe is crossed in <30 ms, in which time the laser frequency is expected to change by < 5 kHz.
• The drive to the PZT was calibrated into physical units using the same technique. This number is within a factor of 2 of the number reported here
• Attachment #2 shows the loop OLTF measured using the usual IN1/IN2 prescription (with an SR560). In fact, the 8kHz feature makes the loop unstable. For convenience, I've overlaid the OLTF from March 2017, when things were running smoothly. It is not clear to me why even though the optical gain is now lower, a smaller servo gain results in a larger UGF.

The light level hasn't changed by a factor of 50, leading me to suspect the modulation depth. Recall that the demodulation of the PMC is now done off the servo board using a minicircuits mixer (hence, the "C1:PSL-PMC_LODET" channel isn't a reliable readback of the LO signal strength over time). Although there is a C1:PSL-PMC_MODET channel which looks like it comes from the crystal reference card, and so should still work - this, however, shows no degradation over 1 year.

Somebody had removed the BLP-1.9 that I installed at the I/F output of the mixer to remove the sum frequency component in the demodulated signal, I reinstalled this. I find that there are oscillations in the error signal if the PMC servo gain is increased above 14.5 on the MEDM slider.

Attachment 1: PMCsweep.pdf
Attachment 2: OLTFmeas.pdf
15138   Wed Jan 22 11:00:21 2020 gautamUpdatePSLPMC REFL ghost beam

I looked into this a little more today.

1. The steering optic used to route PMC REFL to the RFPD is in fact a window (labelled W1-PW-1025-UV-1064-45P), not a High-T beamsplitter.
2. With the PMC unlocked, I measured ~10.70 mW in the stronger of the two beams, 5.39 mW in the weaker one.
• The window spec is Tp > 97%. Since we have ~1.3 W incident on the PMC, the primary reflection corresponds to T=99.2%, which is consistent with the spec.
• There is no spec given for the coating on the back side of this window. But from the measured values, it seems to be R = 100* 5.39e-3 / (1.3*T^2) ~ 0.4%. Seems reasonable.

Currently, the iris is set up such that the stronger beam makes it to the PMC RFPD, while the weaker one is blocked by the iris. As usual, this isn't a new issue - was noted last in 2014, but who knows whether the new window was intalled...

 Quote: Today I noticed that the beam reflected from the PMC into the RFPD has a ghost (attachment) due to reflection from the back of the high transmission beam splitter that stirs the beam into the RFPD.
15139   Wed Jan 22 11:22:39 2020 gautamUpdatePSLPMC modulation depth measurement

Summary:

I estimate the PMC servo modulation depth to be approximately 50 mrad. This is only 15% lower than what was measured in Jan 2018, and cannot explain the ~x50 reduction of optical gain measured earlier in this thread. Later in the day, I also confirmed that the LO input to the ZAD-6 mixer is +7 dBm. So the crystal is not to blame.

Details:

1. PSL frequency is locked to the IMC length.
2. Arm lengths are locked to the PSL frequency using POX/POY.
3. EX green laser locked to the X arm length using end PDH servo. GTRX was ~0.4 in this measurement, which is the nominal value.
4. The 20dB coupled port of the beat between the EX and PSL lasers was monitored using the AG4395A in "Spectrum" units.
5. The beat was set at ~90 MHz, and a spectrum was taken for ~100 MHz span centered at the beat frequency.
6. The modulation depth is estimated by considering the ratio of power at the beat frequency relative to that 35.5 MHz away. See Attachment #1.

Assuming a finesse of 700 for the PMC, we expect an optical gain of 2*Pin*J0(50e-3)*J1(50e-3)/fp  ~ 1.2e-7 W/Hz (=0.089 GW/m). I can't find a measurement of the PMC RFPD transimpedance to map this onto a V/Hz value.

Attachment 1: modDepth.pdf
15143   Wed Jan 22 20:12:36 2020 gautamUpdatePSLPMC demodulator electrical characterization

Summary:

The mixer + LPF combo used to demodulate the PMC PDH error signal seems to work as advertised.

Details:

Measurement setup --- Attachment #1. The IF signal was monitored using the scope in High-Z mode.

Results --- Attachment #2.

So the next step is to characterize the RF transimpedance of the PMC RFPD.

Attachment 1: demodChar.pdf
Attachment 2: mixerChar.pdf
15144   Thu Jan 23 14:37:05 2020 gautamUpdatePSLPMC VGA chip damaged?

[jordan, gautam]

Summary:

The AD602 chip which implements the overall servo gain for the PMC seems to be damaged. We should switch this out at the next opportunity.

Details:

1. According to the PSL cross connect wiring diagram, the VME DAC that provides the control voltage to the VGA stage goes to pins 7/8 of cross connect J16.
• Jordan and I verified that the voltage at this point [Vout], is related to the PMC_GAIN EPICS slider [dB] value according to the following relationship: $V_{\mathrm{out}} = (10-\mathrm{dB})/2$.
2. On the PMC servo board, this voltage is scaled by a factor of -1/10.
• This was confirmed by peeking at this voltage using a DMM (I clipped onto R31) while the gain slider was varied.
• This corresponds to +/- 1000 mV reaching the AD602.
• However, the AD602 is rated to work with a control voltage varying between +/- 625 mV.
• What this means is that the EPICS slider value is not the gain of the AD602 stage. The latter is given by the relation $G [\mathrm{dB}] = 32 \times V_{\mathrm{G}} + 10$.
• @team PSL upgrade: this should be fixed in the database file for the new c1psl machine.
3. Using TP1 and TP2 connected to the SR785, I measured the transfer function of the AD602 for various values of the EPICS slider.
• Result is shown in Attachment #1.
• I did this measurement with the PMC locked, so I'm using the in-loop error signal to infer the gain of the VGA stage.
• As expected, the absolute value of the gain does not match that of the EPICS slider (note that the AD602 has an input impedance of 100 ohms. So the 499 ohm series resistor between TP1 and the input of AD602 makes a 1/5 voltage divider, so the gain seen between TP1 and TP2 has this factor folded in).
• Moreover, the relative scaling of the gain for various slider values also doesn't appear to be liner.
• For the highest gain setting of +15 dB, the servo began oscillating, so I think the apparent non-flatness of the gain as a function of frequency is an artefact of the measurement.
• Nevertheless, my conclusion is that the IC should be changed.

I will pull the board and effect the change later today.

I pulled the board out at 345pm after dialling down all the HV supplies in 1X1. I will reinstall it after running some tests.

Attachment 1: VGAchar.pdf
15146   Thu Jan 23 16:37:14 2020 ranaUpdatePSLPMC VGA chip damaged?

doesn't seem so anomolous to me; we're getting ~25 dB of gain range and the ideal range would be 40 dB. My guess is that even thought this is not perfect, the real problem is elsewhere.

15147   Thu Jan 23 18:52:31 2020 gautamUpdatePSLPMC RFPD characterization

Summary:

The RF transimpedance of the PMC PDH RFPD was measured, and found to be 1.03 kV/A

Details:

With the new fiber coupled PDFR system, it was very easy to measure the response of this PD in-situ 🎉 . The usual transfer function measurement scheme was used, with the AG4395 RF out modulating the pump current of the diode laser, and the measured transfer function being the ratio of the response of the test PD to the reference PD.

I assume that the amount of light incident on the reference NF1611 photodiode and the test photodiode were equal - I don't know what the DC transimpedance of the PMC REFL photodiode is (can't find a schematic), but the DC voltage at the DC monitor point was 16.4 mV (c.f. -2.04 V for the NF1611). The assumption shouldn't be too crazy because assuming the reference PD has an RF transimpedance of 700 V/A (flat in the frequency range scanned), we get a reasonable shape for the PMC REFL photodiode's transimpedance.

The fitted parameters are overlaid in Attachment #1. The 2f notch is slightly mistuned it would appear, the ratio of transimpedance at f1/2*f1 is only ~10. The source files have been uploaded to the wiki.

Knowing this, the measured PDH discriminant of 0.064 GV/m is quite reasonable:

• expected optical gain from modulation depth assuming a critically coupled cavity is 0.089 GW/m.
• Assume 0.7 A/W responsivity for InGaAs.
• Account for the fact that only 0.8 % of the reflected light reaches the PMC photodiode because of the pickoff window.
• Account for a conversion loss of 4.5 dB in the mixer.
• Account for the voltage division by a factor of 2 at the output of the BLP-1.9 filter due to the parallel 50 ohm termination.
• Then, the expected PDH discriminant is 0.089e9 W/m * 0.7 A/W * 0.8e-2 * 1.03kV/A * 10^(-4.5/20) * 0.5 ~ 0.15 GV/m. This is now within a factor of ~2 of the measured value, and I assume the total errors in all the above assumed parameters (plus the cable transmission loss from the photodiode to the 1X1 rack) can easily add up to this.

So why is this value so different from what Koji measured in 2015? Because the monitor point is different. I am monitoring the discriminant immediately after the mixer, whereas Koji was using the front panel monitor. The latter already amplifies the signal by a factor of x101 (see U2 in schematic).

Conclusion:

I still haven't found anything that is obviously wrong in this system (apart from the slight nonlinearity in the VGA stage gain steps), which would explain why the PMC servo gain has to be lower now than 2018 in order to realize the same loop UGF.

 So the next step is to characterize the RF transimpedance of the PMC RFPD.
Attachment 1: PDresp.pdf
15149   Thu Jan 23 22:10:01 2020 gautamUpdatePSLPMC servo pulled out

While I have the board out, I'll try and do a thorough investigation of TFs and noise of the various stages. There is no light into the IFO until this is done.

 I pulled the board out at 345pm after dialling down all the HV supplies in 1X1. I will reinstall it after running some tests.
15150   Thu Jan 23 23:07:04 2020 JonConfigurationPSLc1psl breakout board wiring

To facilitate wiring the c1psl chassis and scripting loopback tests, I've compiled a distilled spreadsheet with the Acromag-to-breakout board wiring, broken down by connector. This information is extractable from the master spreadsheet, but not easily. There were also a few apparent typos which are fixed here.

The wiring assignments at the time of writing are attached below. Here is the link to the latest spreadsheet.

Attachment 1: c1psl_feedthrough_wiring.pdf
15152   Fri Jan 24 15:42:08 2020 gautamUpdatePSLPMC servo restored

The PMC servo was re-installed at ~345pm. HV supplies were re-energized to their nominal values. I will update the results of the investigation shortly. The new nominal PMC servo gain is +9dB.

Quote:

While I have the board out, I'll try and do a thorough investigation of TFs and noise of the various stages. There is no light into the IFO until this is done.

 I pulled the board out at 345pm after dialling down all the HV supplies in 1X1. I will reinstall it after running some tests.
15154   Sat Jan 25 11:54:42 2020 YehonathanUpdatePSLRingdown measurements

Zero order beam PMC ringdown

On Wednesday I installed 3 PDs (see attached photos) measuring:

1. The input light to the PMC. Flip-mirror was installed (sorry Shruti) on the beam path to the fiber coupler.

2. Reflected light from the PMC.

3. PMC transmitted light.

I connected the three PDs to the oscilloscope and the AOM driver to a function generator. I drive the AOM with a square wave going from 1V to 0V.

I slowly increased the square wave frequency. The PMC servo doesn't seem to care. I reach 100KHz - it seems excessive but still works. In any case, I get the same results doing a single shut-down from a DC level.

I download the traces. I normalize the traces but I don't rescale them (Attachment 4) so that the small extinction can be investigated.

I notice now that the PDs show the same extinction. It probably means I should have taken dark currents data for the PDs.

Also, I forgot to take the reflected data when the PMC is out of resonance with the laser which could have helped us determine the PMC transmission.

Again, the shutdown is not as sharp as I want. There is a noticeable smoothening in the transition around t = 0 which makes the fit to an exponential difficult. I suspect that the function generator is the limiting device now. I hooked up the function generator to the oscilloscope which showed similar distortion (didn't save the trace)

I try to fit the transmission PD trace to a double exponential and to Zucker model (Attachment 5).

The two exponentials model, being much less restrictive, gives a better fit but the best fit gives two identical time constants of 92ns.

The Zucker model gives a time constant of 88ns. Both of these results are consistent with more or less with the linewidth measurement but this measurement is still ridden with systematics which hopefully will become minimized IMC ringdowns.

Attachment 1: Input_beam_path.jpg
Attachment 2: Reflected_Beam_Path.jpg
Attachment 3: Transmitted_Beam_Path.jpg
Attachment 4: PMCRingdownNormalizedRawdata.pdf
Attachment 5: TransPDFits.pdf
15156   Sun Jan 26 13:47:00 2020 gautamUpdatePSLPMC servo characterization

Summary:

1. I investigated the stage-by-stage transfer functions of the PMC servo up till the HV stage. See Attachment #1. There were no unexpected features.
2. I replaced the AD602 used to implement the VGA capability. After the replacement, the gain of the VGA stage had the desired performance, see Attachment #2, Attachment #3.
3. The servo board was re-installed and the OLTF of the PMC loop was measured. See Attachment #4.

​To avoid driving the PA85 without the HV rails connected, I removed R23. This was re-installed after my characterization.

Input stage:

Since we do the demodulation of the PMC PDH signal off this servo board, the I/F mixer output is connected to the "FP1test" front panel LEMO input.

• A DG190 is used to enable/disable this path.
• Initially I tried checking the enable/disable functionality by measuring the resistance across the IC's I/O pins. However, this method does not work - the resistance read off from a DMM varied from ~23 ohms in the "ON" state to ~123 ohms in the "OFF" state. While the former value is consistent with the spec, the latter is confusing.
• But I confirmed that the switch does indeed isolate the input in the "OFF" state by injecting a signal with a function generator (100 Hz sine wave, 100mVpp) and monitoring the output on an oscilloscope.

Electronic TFs:

Using some Pomona mini-grabbers, I measured the electronic TFs between various points on the circuit. There were no unexpected features, the TFs all have the expected shape as per the annotations on the DCC schematic. I did not measure down to 0.1 Hz to confirm the low frequency pole implemented by U6, and I also didn't measure the RF low pass filter at the input stage (expected corner frequency is 1 MHz).

VGA characterization:

After replacing the IC, I measured the transfer function between TP1 and TP2 for various values of the control voltage applied to pin 4A on the P1 connector, varying between +/- 5 V DC.

• Pin 9A on the P1 connector has to be grounded for the signal to be allowed to pass through the VGA.
• Note that there is an overall gain of -1/10 applied to the control voltage between pin 4A and pin #1 of the AD602, which is what actually sets the gain.
• Furthermore, the input impedance of the AD602 is spec-ed to be 100 ohms. Because of the series resistance of 500 ohms from TP1 to the input of the AD602 (so that the upstream OP27 isn't overdrawn for current), the relation between the control voltage applied to Pin 4A and gain (measured between TP1 and TP2) is modified to G [dB] = 32*(-0.1 * V_pin4A) - 6.
• The gain behavior after the IC swap is as expected, both in terms of absolute gain, and the linearity w.r.t. the control voltage.
• Note that in Attachment #2, each color corresponds to a different control voltage to the AD602, varying from -5V DC to +5V DC in 1V steps.

PZT Capacitance measurement

I confirmed that the PZT capacitance is 225 nF. The measurement was made using an LCR meter connected to the BNC cable delivering the HV to the PZT, at the 1X1 rack end.

OLTF measurement

After re-soldering R23, I put the board back into its Eurocrate, and was able to lock the PMC. For subsequent measurements, the PSL shutter was closed.

• I measured the OLTF using the usual IN1/IN2 prescription, implemented with the help of an SR560.
• At the original PMC Servo gain of +12dB, I found that the feature at ~8kHz results in an OLTF with multiple unity gain crossings.
• So I lowered it to +9dB. This yields an OLTF with ~60deg phase margin, ~2.3 kHz UGF.
• The feature that sets the gain margin is actually not any of the peaks fit by LISO, but is one of the high frequency features at ~40 kHz. At the new setting of +9dB gain, the gain margin is ~10 dB.
• The measured TF (dots in Attachment #5) was fit with LISO (solid lines in Attachment #5) to allow inferring the out-of-loop servo noise by monitoring the in-loop noise (that plot to follow).
Attachment 1: elecTFs.pdf
Attachment 2: VGAchar_postFix.pdf
Attachment 3: VGAlinearity_postFix.pdf
Attachment 4: newOLTFs.pdf
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