PMC realigned again... The transmission was down to 0.70, and the MC was having a hard time trying to autolock.

Have we been running the PMC autolocker lately?  I can't remember, and I also can't find where it might be running.  It's not on megatron, either in the crontab or Q's new /etc/init place.  It's also not on op340m. 

Anyhow, what prompted this was that the PMC transmission has been incredibly fuzzy today.  On the StripTool it looks like it was fine until about -7 hours ago, when it lost lock.  Then Diego relocked it around -3 hours ago, and it's been fuzzy ever since. It was unlocked again for about 15 minutes about 45 minutes ago, and when I relocked it, it was even more fuzzy.

The FSS slow is almost exactly zero, the PMC's PZT is not at the edge of the range, the FSS PC drive RMS has been both high and low, and the PMC fuzz level has just been consistently high.  I was checking the parameters in the PMC phase shifter screen, and looked up the autolocker to see what the nominial values are supposed to be. 

For the RFADJ value, the autolocker sets it to 2.0, and after it locks increases it up to 4.5.  I found the value at 2.0, and the autolocker isn't running, which made me wonder if the autolocker died sometime after it set the value low, but before it could detect lock and reset the value to high.  (Actually, after lock it sets the value to whatever is in the channel C1:PSL-STAT_PMC_NOM_RF_ADJ, which is 4.5).

Anyhow, I manually set the RFADJ value to 4.5, and the PMC transmission immediately improved. 

EDIT, 8pm, JCD:  Rana reminded me that he attached a screenshot back on 30Dec2014 (http://nodus.ligo.caltech.edu:8080/40m/10849), which I had ignored earlier because the parameters weren't written in text.  My bad.  Anyhow, after the New Year's tune-up, the RFADJ should be 6.0.  I have set it so, and also changed the C1:PSL-STAT_PMC_NOM_RF_ADJ chan to be 6.0.

PMC aligned.

PMC Trans increased from 0.740 to 0.782

IMC Trans increased from 16200 to 17100

I wanted to check the status of the ISS. The AOM driver response was measured on Friday night.
The beam path has not been disturbed yet.

- I found the AOM crystal was removed from the beam path. It was left so.

- The AOM crystal has +24V power supply in stead of specified +28V.
  I wanted to check the functionality of the AOM driver.

- I've inserted a 20dB directional coupler between the driver and the crystal.
  To do so, I first turned off the power supply by removing the corresponding fuse block at the side panel of the 1X1 Rack.
  Then ZFDC-20-5-S+ was inserted, the coupled output was connected to a 100MHz oscilloscope with 50Ohm termination.
  Then plugged in the fuse block again to energize the driver box.

  Note that the oscilloscope bandwidth caused reduction the amplitude by a factor of 0.78. In the result, this has already been compensated.

- First, I checked the applied offset from a signal generator (SG) and the actual voltage at the AOM input. The SG OUT
  and the AOM control input are supposed to have an impedance of 50Ohm. However, apparently the voltage seen at the
  AOM in was low. It behaved as if the input impedance of the AOM driver is 25Ohm.
  In any case, we want to use low output impedance source to drive the AOM driver, but we should keep this in mind.

- The first attachment shows the output RF amplitude as a function of the DC offset. The horizontal axis is the DC voltage AT THE AOM INPUT (not at the SG out).
  Above 0.5V offset some non linearity is seen. I wasn't sure if this is related to the lower supply voltage or not. I'd use the nominal DC of 0.5V@AOM.

  The output with the input of 1V does not reach the specified output of 2W (33dBm). I didn't touch the RF output adjustment yet. And again the suppy is not +28V but +24V.

- I decided to measure the frequency response at the offset of 0.53V@AOM, this corresponds to the DC offset of 0.8V. 0.3Vpp oscillation was given.
  i.e. The SG out seen by a high-Z scope is V_SG(t) = 1.59 + 0.3 Sin(2 pi f t) [V]. The AOM drive voltage V_AOM(t) = 0.53 + 0.099 Sin(2 pi f t).
  From the max and min amplitudes observed in the osciiloscope, the response was checked. (Attachment 2)
  The plot shows how much is the modulation depth (0~1) when the amplitude of 1Vpk is applied at the AOM input.
  The value is ~2 [1/V] at DC. This makes sense as the control amplitude is 0.5, the applied voltage swings from 0V-1V and yields 100% modulation.

  At 10MHz the first sign of reduction is seen, then the response starts dropping above 10MHz. The specification says the rise time of the driver is 12nsec.
  If the system has a single pole, there is a relationship between the rise time (t_rise) and the cut-off freq (fc) as fc*t_rise = 0.35 (cf Wikipedia "Rise Time").
  If we beieve this, the specification of fc is 30MHz. That sounds too high compared to the measurement (fc ~15MHz).
  In any case the response is pretty flat up to 3MHz.

This is good news. It means that the driver probably won't limit the response of the loop - I expect we'll get 20-30 deg of phase lag @ 100 kHz just because of the acoustic response of the AOM PZT + crystal.

PMC was locked in a bad state. FSS slow actuator adjust was at ~ -0.7 and PZT voltage at ~45.

So I set these right by moving the appropriate sliders and relocked it. FSS slow actuator adjust brought back to zero and PZT voltage ~115. PMC trans after relock is 0.789.
 

Another thought about the mystery PCDRIVE noise: we've been thinking that it could be some slow death inside the NPRO, but it might also be a broken and intermittent thing in the MC servo or MC REFL PD.

Another possibility is that its frequency noise in the old oscillator used to drive the pre-PMC EOM (which is the Pockel Cell for the FSS). To check this, we should swap in a low noise oscillator for the PMC. I have one for testing which has 36 and 37 MHz outputs.

The laser went off around 11am yesterday. It was turned on

Let's order a pair of 35.5 MHz Wenzel for this guy and package like Rich has done for the WB low noise oscillators.

WE're only sending 6 dBm into it now and its using a 13 dBm mixer. Bad for PMC stability.

Also, if anyone has pix of the servo card, please add them to the DCC page for the PMC.

I just turned off the PSL enclousure lights.
 

The PMC is not happy and the ITMX UL OSEM is moving too much

[Yutaro, Koji]

We found that the PMC LO level was fluctuating in a strage way (it was not stable but had many clitches like an exponential decay), we suspected the infamous PMC LO level decay. In fact, in June 2014 when Rana recalibrated the LO level,  the number on the medm screen (C1:PSL-PMC_LO_CALC) was about 11dBm. However, today it was about 6dBm. So we decided to jump in to the 1X1 rack.

The LO and PC outputs of the PMC Crystal module (D980353) were measured to be 6.2dBm and 13.3dBm. Rana reported in ELOG 10160 that it was measured to be 11.5dBm. So apparently the LO level decayed. Unfortunately, there was no record of the PC output level. In any case, we decided to pull the module for the replacement of ERA-5 chips.

Once we opened the box we found that the board was covered by some greasy material. The ERA-5 chip on the LO chain seemed unreasonably brittle. It was destryed during desoldering. We also replaced the ERA-5 chip in the PC chain, just in case. The board was cleaned by the defluxing liquids.

Taking an advatage of this chance, the SMA  cables around the PMC were checked. By removing some of the heat shrinks, suspicious broken shields of the connectors were found. We provided additional solder to repair them.

After the repair, the LO and PC output levels became icreased to 17.0dBm(!) and 13.8dBm, respectively. (Victory)
This LO level is way too much compared to Rana's value. The MEDM LO power adj has little effect and the adj range was 16dBm~17dBm. Therefore we moved the slider to 10, which yields 16dBm out, and added a 5dB attenuator. The measured LO level after the attenuator was measured to be 11.2dBm.

Locking of the PMC was tried and immediately acquired the lock. However, we noticed that the nomoinal gain of 10dB cause the oscillation of the servo. As we already adjusted the LO level to recover the nominal value, we suspeced that the modulation depth could be larger than before. We left the gain at 0dB that doesn't cause the oscillation. It should be noted that the demodulation phase and the openloop gain were optimized. This should be done in the day time as soon as possible.

When the PMC LO repair was completed, the transmission of the PMC got decreased to 0.700V. The input alignment has been adjusted and the transmission level of 0.739V has been recovered.

The IMC lock stretch is not stable as before yet. Therefore, there would still be the issue somewhere else.

I think the IMC locking was somewhat improved. Still it is not solid as long time before.

Before the PMC fix (attachment 1)
After the PMC fix (attachment 2)

To do
- PMC loop inspection / phase check / spectral measurements
- PMC / IMC interaction
- IMC loop check
 

PMC follow up measurements have been done. The servo circuit was reviewed.

Now the PMC, IMC, X/Y arms are locked and aligned waiting for the IFO work although I still think something is moving (ITMX?)
as the FPMI fringe is quite fast.

The result of the precise inspection for the PMC servo board for the 40m was done.

The record, including the photo of the board, can be found at https://dcc.ligo.org/D1400221-v2

- I found some ceramic 1uF caps are used in the signal path. They have been replaced with film caps by WIMA.

- In later measurements with the openloop TF measurement, it was found that the notch frequency (14.6kHz) was off from the a sharp PZT resonance at 12.2kHz.
I replaced the combined caps of 1220pF to 1742pF. This resulted nice agreement of the notch freq with the PZT resonant freq.

Past related elogs:

SRA-3MH mixer installed in 2009: http://nodus.ligo.caltech.edu:8080/40m/1502

R20 increased for more LO Mon gain: http://nodus.ligo.caltech.edu:8080/40m/10172

I'm still analyzing the open loop TF data. Here I report some nominal settings of the PMC servo

Nominal phase setting: 5.7
Nominal gain setting: 3dB

After the tuning of the notch frequency, I thought I could increase the gain from 5dB to 9dB.
However, after several hours of the modification, the PMC servo gradually started to have oscillation.
This seemed to be mitigated by reducing the gain down to 4dB. This may mean that the notch freq got drifted away
due to themperature rise in the module. PA85 produce significant amount of heat.
(The notch frequency did not change. Just the 22kHz peak was causing the oscillation.)

Summary

The PMC servo error (MIX OUT MON on the panel) and actuation (HV OUT MON) have been calibrated using the swept cavity.

Error signal slope in round-trip displacement: 2.93e9 +/- 0.05e9 [V/m]
HV OUT calibration (round-trip displacement): 5.36e-7 +/- 0.17e-7 [m/V]
PZT calibration (round-trip displacement): 10.8 +/- 0.3 [nm/V] => corresponds to ~2.5 fringes for 0~250V full range => not crazy

Measurement condition

The transmission level: 0.743V (on the PMC MEDM screen)
LO level: ~13dBm (after 3dB attenuation)
Phase setting: 5.7
PMC Servo gain: 7dB during the measurement (nominal 3dB)

Method

- Chose PMC actuation "BLANK" to disable servo
- Connect DS345 function generator to EXT DC input on the panel
- Monitor "MIX OUT MON" and "HV OUT MON" with an oscilloscope
- Inject a triangular wave with ~1Vpp@1 or 2Hz with appropriate offset to see the cavity resonance at about the middle of the sweep.
  The frequency of the sweep was decided considering the LPF corner freq formed by the output impedance and the capacitance of the PZT. (i.e. 11.3Hz, see next entry)

Result

- 4 sweep was taken (one 2Hz seep, three 1Hz sweep)
- The example of the sweep is shown in the attachment.
- The input triangular wave and the PDH slopes were fitted by linear lines.
- Spacing between the sideband zero crossing corresponds to twice of the modulation frequency (2x35.5MHz = 71MHz)
- The error signal slope was calibrated as V/MHz
- FSR of the PMC is given by google https://www.google.com/search?q=LIGO+pmc.m
  => Cavity round trip length is 0.4095m, FSR is 732.2MHz
- Convert frequency into round-trip displacement

- Convert HV OUT MON signal into displacement in the same way.
- The voltage applied to the PZT element is obtained considering the ratio of 49.6 between the actual HV and the HV OUT MON voltage.

The PMC PZT capacitance was measured.
- Turn off the HV supplies. Disconnect HV OUT cable.
- Make sure the cable is discharged.
- Measure the capacity at the cable end with an impedance meter.
=> The PMC PZT capacitance at the cable end was measured to be 222nF
Combined with the output impedance of 63.3kOhm, the LPF pole is at 11.3Hz

Summary

The PMC servo was analysed. OLTF was measured and modeled by ZPK (Attachment 1). The error and actuator signals were calibrated in m/rtHz (Attachment 2)

Measurement methods

OLTF:

- The PMC servo board does not have dedicated summing/monitor points for the OLTF measurement. Moreover the PZT HV output voltage is monitored with 1/49.6 attenuation.
  Therefore we need a bit of consideration.

- The noise injection can be done at EXT DC.
- Quantity (A): Transfer function between HV OUT MON and MIX OUT MON with the injection.
  We can measure the transfer function between the HV OUT (virtual) and the MIX OUT. (HV OUT->MIX OUT). In reality, HV OUT is attenuated by factor of 49.6.
  i.e. A = (HV_OUT->MIX_OUT)*49.6
- Quantity (B): Transfer function between HV OUT MON and MIX OUT MON without the injection.
  This is related to the transfer function between the MIX OUT and HV OUT. In reality, HV OUT is attenuated. 
  i.e. B = 1/((MIX_OUT->HV_OUT)/49.6)

- What we want to know is HV_OUT->MIX_OUT->HV_OUT. i.e. A/B = (HV_OUT->MIX_OUT*49.6)*((MIX_OUT->HV_OUT)/49.6) = HV_OUT->MIX_OUT->HV_OUT

PSD:

- The MIX OUT and HV OUT spectra have been measured. The MIX OUT was calibrated with the calibration factor in the previous entry. This is the inloop stability estimation.
  From the calibrated MIX OUT and HV OUT, the free running stability of the cavity was estimated, by mutiplying with |1-OLTF| and |1-1/(1-OLTF)|, respectively, in order to recover
  the free running motion.

OLTF Modeling

Here is the model function for the open loop TF. The first line comes from the circuit diagram. The overall factor was determined by eye-fit.
The second and third lines are to reproduce the peak/notch feature at 12kHz. The fourth line is to reproduce 28kHz feature.
The LPF right after the mixer was analyzed by a circuit simulation (Circuit Lab). It can be approximated as 150kHz LPF as the second pole
seems to come at 1.5MHz.

The sixth line comes from the LPF formed by the output resistance and the PZT capacitance.

The seventh line is to reproduce the limit by the GBW product of OP27. As the gain is 101 in one of the stages,
it yields the pole freq of ~80kHz. But it is not enough to explain the phase delay at low frequency. Therefore this
discrepancy was compensated by empirical LPF at 30kHz.

function cmpOLTFc = PMC_OLTF_model(freqOLTFc)

cmpOLTFc = -7e5*pole1(freqOLTFc,0.162).*zero1(freqOLTFc,491)... % from the circuit diagram
    .*zero2(freqOLTFc,12.5e3,100)... % eye-fit
    .*pole2(freqOLTFc,12.2e3,6)... % eye-fit
    .*pole2(freqOLTFc,27.8e3, 12)... % eye-fit
    .*pole1(freqOLTFc,150e3)... % Mixer LPF estimated from Circuit Lab Simulation
    .*pole1(freqOLTFc,11.3)... % Output Impedance + PZT LPF
    .*pole1(freqOLTFc,8e6/101)... % GBW OP27
    .*pole1(freqOLTFc,3e4); % Unknown

end

Result

Attachment 1:

The nominal OLTF (Nov 17 data) shows the nominal UGF is ~1.7kHz and the phase margin of ~60deg.

The measured OLTF was compared with the modelled OLTF. In the end they show very sufficient agreement for further calibration.
The servo is about to be instable at 28kHz due to unknown series resonance. Later in the same day, the gain of the PMC loop had to be
reduced from 7dB to 3dB to mitigate servo oscillation. It is likely that this peak caused the oscillation. The notch frequency was measured
next day and it showed no sign of frequncy drift. That's good.

We still have some phase to reduce the high freq peaks by an LPF in order to increase the over all gain.

Attachment 2:

The red curve shows the residual floor displacement of 2~10x10^-15 m/rtHz. Below 4Hz there is a big peak. I suspect that I forgot to close
the PSL shutter and the IMC was locked during the measurement. Then does this mean the measured noise corresponds to the residual laser
freq noise or the PMC cavity displacement? This is interesting to see.

The estimated free running motion from the error and actuation signals agrees very well. This ensures the precision of the caibration in the precious entries.
 

As a final tune of the PMC servo, I've added 1nF cap at the error signal amplification stage. The diagram has been updated and uploaded to DCC. https://dcc.ligo.org/D1400221

It should be noted that this modification yielded the error signal to have 17.5kHz roll off.

The openloop TF after the modification has been measured. (Attachment 1)

With the new nominal gain of 9dB, almost the same gain margin for the 28kHz peak has been realized.
=> We have 6dB (factor of 2) more gain at low frequency. Currently, the feature at 8kHz causes the oscillation when the gain is further increased.
 

Here is the model function for the OLTF.

function cmpOLTFc = PMC_OLTF_model(freqOLTFc)

cmpOLTFc = -9.5e5*pole1(freqOLTFc,0.162).*zero1(freqOLTFc,491)... % from the circuit diagram
    .*pole1(freqOLTFc,17.5e3)... % Newly implemented input filter => GBW pole was replaced with this
    .*zero2(freqOLTFc,12.5e3,100)... % eye-fit
    .*pole2(freqOLTFc,12.2e3,6)... % eye-fit
    .*pole2(freqOLTFc,28.8e3, 12)... % eye-fit
    .*pole1(freqOLTFc,150e3)... % Mixer LPF estimated from Circuit Lab Simulation
    .*pole1(freqOLTFc,11.3)... % Output Impedance + PZT LPF
    .*pole1(freqOLTFc,3e4); % Unknown   
end

The free-running round-trip displacement (roundtrip) / frequency noise is shown in Attachments. There we compare the spectra with and without IMC locked.

i.e. When the IMC is not locked, we are measuring the laser frequency noise with the sensor (PMC cavity) that is noisy due to the PMC displacement.
When the IMC is locked, the laser frequency is further stabilized while the sensor (PMC) noise is not changed.

- Without IMC locked

Can we see the laser freq noise? It seems that it is visible above 100Hz.
The red curve is the measured noise level. The NPRO (although it is LWE NPRO) noise level from S. Nagano's thesis (see our wiki) is shown there.

- With IMC locked

When the IC is locked, we see the increase of the noise between 1~4Hz. It means that the IMC is not only noisier than the laser, but also noisier than the PMC cavity.
Sounds reasonable. And the PMC is capable to handle this motion.

The reduction of the frequency noise is seen from 100Hz to 30kHz.

The interesting point is that we can see the noise increase above 30kHz when the IMC is locked.
I believe that the phase correction EOM is shared with the PMC modulation. i.e. PMC sees the corrected laser frequency.

We expect that the frequency noise is reduced at this frequency. But in reality not.

In addition, there is a sharp peak at ~35kHz. I wonder If this is caused by the IMC servo. It is worse to investigate.

The EOM upstream of the PMC is used as the phase corrector for the FSS/IMC servo. It is also used to apply the 35.5 MHz PDH RF sidebands for the PMC locking. There is a Pomona box which is used to merge the two signals onto a single cable for the EOM.

Does this circuit make sense to anyone?

The mode cleaner was misaligned probably due to the earthquake (the drop in the MC transmitted value slightly after utc 7:38:52 as seen in the second plot). The plots show PMC transmitted and MC sum signals from 10th june 07:10:08 UTC over a duration of 17 hrs. The PMC was realigned at about 4-4:15 pm today by rana. This can be seen in the first plot.

The PMC transmission slow degration or it's input beam is not stable.

There was only one razor blade beam dump labeled for atmospheric use left, but that's all we need. Steve is working on restocking. I placed the modified AOM mount on the PSL table near its intended location (near the AOM where it doesn't block any beams).

Things to keep in mind:

• The laser power needs to be turned down for the installation of the AOM. Current laser settings are: Crystal Temperature: 29.41 C, Diode Current: 2.1 A.
• The AOM driver must not be left unterminated when turned on (which it currently is and will be).
• The HEPA filters are currently running at ~50%. While the PSL enclosure is open for the work we'll set them to 100% and lower them after a job well done.

The setup:

The AOM has a deflection angle of about 20 mrad, which requires about 10cm of path for a separation of 2mm of the two beams. I need to survey closer and confirm, but I hope I can fit the beam dump in before the PMC (this of course also depends on the spot size). Alternatively, the PMC hopefully isn't resonant for anything remotely relevant at 80MHz offset, in which case we can also place the beam dump in its reflection path.

So this is the plan:

• Determine coupling efficiency into PMC for reference
• Turn down laser power
• No signal on AOM driver modulation input
• Mount AOM, place in beam path, and align
• Correct alignment into PMC?
• Secondary beam detectable? Adjust modulation input and laser power until detectable.
• Find a place for beam dump
• Confirm that primary beam is not clipping with PMC
• Turn up laser power
• Determine coupling efficiency with restored power to compare

Any thoughts? Based on the AOMs resting place I assumed that it is supposed to be installed before the PMC, but I'm actually not entirely sure where it was sitting before.

Subham and I have placed the AOM back into the setup right in front of the PMC.

Steps undertaken:

1. The HEPA filters were turned off for some reason. They were turned back on, running at 100% while the enclosure was open.
2. Before the installation, after initial realignment, the PMC TRANSPD read out 748 mV.
3. The laser injection current was dialed down to 0.8 A (just above the threshold, judging by PMC cameras.
4. AOM was attached to the new mount while staying connected to its driver. Put in place, a clamp prevents the cable from moving anywhere near the main beam.
5. Aligned AOM to beam, centering the beam (by eye) on front and back apertures.
6. We then applied an offset to the AOM driver input, eventually increasing it to 0.5 V. A secondary beam became clearly visible below the primary beam.
7. In order to place the razor blade dump (stemming from a box, labeled "cleaned for atm use") before the PMC, where the beam separation was about 3 mm, to make sure we can hit the edged area, we had to place the dump at an angle, facing up.
8. Keeping the 0.5V offset on the driver input, with the lights off, we increased the laser diode current in steps of ~200 mA to its original value of 2.1A, while checking for any IR light scattered from the beam dump. Not a trace.
9. At original current setting, we realigned the beam into the PMC, and obtained 743 mV on the TRANSPD in the locked state.
10. Closed off PSL table, dialed HEPAs down to 50%

Good job Johannes and Subham.

We wanted to re-activate the Heater for the reference cavity today to use it as a testbed for PID autotuning and the new heater driver circuit that Andrew is working on for the coating thermal noise experiment.

Unfortunately, it seems that the large power supply which is used for the heater is dead. Or maybe I don't remember how to use it?

The AC power cord was plugged in to a power strip which seems to work for IO chassis. We also tried swapping power strip ports.

We checked the front panel fuses. The power one was 3 Ohms and the 'bias' one was 55 Ohms. We also checked that the EPICS slider did, in fact, make voltage changes at the bias control input.

Non of the front panel lights come on, but I also don't remember if that is normal.

Have those lights been dead a long time? We also reconnected the heater cable at the reference cavity side.

[yinzi, craig, gautam]

Yinzi had translated the Perl PID script used to implement the discrete-time PID control, and had implemented it with Andrew at the PSL lab. Today afternoon we made some minor edits to make this suitable for the FSS Slow loop (essentially just putting the right channel names into her Python script). I then made an init file to run this script on megatron, and it looks to be working fine over the last half hour of observation or so. I am going to leave things in this state over the weekend to see how it performs.

We have been running with just the MC2 Transmission QPD for angular control of the IMC for a couple of months now because the WFS loops seemed to drag the alignment away from the optimum. We did the following to try and re-engage the WFS feedback:

• Close the PSL shutter, turned off all the lights in the lab and ran the WFS DC offsets script : /opt/rtcds/caltech/c1/scripts/MC/WFS/WFS_DC_offsets
• Locked the IMC, optimized alignment by hand (WFS feedback turned off) /opt/rtcds/caltech/c1/scripts/MC/WFS/WFS_DC_offsets
• Unlocked the IMC, went to the AS table and centered the spots on the WFS
• Ran WFS RF offsets script - this should be done with the IMC unlocked (after good alignment has been established) /opt/rtcds/caltech/c1/scripts/MC/WFS/WFS_DC_offsets
• Re-engaged WFS servo

GV addendum 23Nov2016: The WFS have been working well over the last few days - I've had to periodically (~ once in a day) run the WFS reflief script to keep the outputs to the suspension PIT and YAW DOFs below 50cts, but the WFS aren't dragging the alignment away as we had noticed before. The only thing I did differently is to follow Rana's suggestion and set the RF offsets with the MC unlocked as opposed to locked. I've added a line to the script to remind the user to do so... Also, note that EricQ has recently cleaned up the scripts directory to remove the numerous obsolete scripts in there...

I made a very slighly updated version of Yinzi's script that pulls the channel names and actuator hardstop limits from an external .ini config file. The idea was to avoid having as many versions of the script as there are implimentations of it. Seems like slighly better practice, but maybe I'm wrong. The config files are also easier to read. Its posted on the elog (PSL:1758) with lastest on the 40mSVN .../trunk/CTNLab/current/computing/scripts . 

If you're working off her first implimentation 'RCAV_thermalPID.py' then there is an indent issue after the if statement on line 88: only line 89 should be indended. If you deactivate the debug flag then the script fails to read in PID factors and dies.

Re-aligned the beam going into the PMC today around 5 PM. I noticed that its all in pitch and since I moved both of the mirrors by the same amount it is essentially a vertical translation.

I wonder if the PMC is just moving up and down due to thermal expansion in the mount? How else would we get a pure vertical translation? Need to remember next time if the beam goes up or down, and by how many knob turns, and see how it correlates to the lab temperature.

We are thinking to use the PMC signals to help us in figuring out the feedback / feedforward stuff and making it better.

Today we scoped out the PMC DAQ channels (which were never re-hooked up after the Joe/Jamie CDS upgrade 6 years ago).

There is a 4-pin LEMO connector on the front panel which gives

1. the error signal (after the 4th order, post-mixer lowpass and a OP27 buffer with a 17 kHz low pass)
2. the feedback voltage to the PZT, after a resistive divide by 50

Both of these signals are buffered by the AD620 inst amp configured with a gain of 1. In the green scope trace, you can see that there's a ~110 MHz signal strongly evident there. In the spectrum analyzer screen shot there is a instrument noise trace and then a PMC error point trace. You can see that all the peaks are ony there when I connect to the servo board instead of a Terminator. This RF noise is mainly the higher harmonics of the 35.5 MHz modulation getting there. It seems to be in both the error and control DAQ outputs, and a question is whether or not it is also in the servo electronics.

I also attach a close up of the servo board in the region of the post-mixer LC low pass filtering. I think its supposed to be 4th order cutoff at 1 MHz, but maybe the caps are busted or there's a way for the RF from the mixer to bypass the filters and get into the main servo path?

In the medium term, we probably want to use the new PDH servo that Rich is making. Need to buy/make a HV driver to use, but that should be easy.

The C1IOO frontend machine that resides in 1X1/1X2 has 2 ADCs, ADC0 and ADC1. The latter has 28 out of 32 channels unused at the moment, so I decided to use this for setting up fast channels for the PMC DAQ. On the RTCDS side of things, the PSL namespace block lives in the C1ALS model. I made the following modifications to it:

1. Added channels for the PMC DAQ
2. Added CDS filters for both the newly added PMC DAQ channels and the existing FSS DAQ channels, so that we can calibrate these into physical units
3. Changed the names of the existing FSS channels from FSS_MIXER and FSS_NPRO to FSS_ERR and FSS_CTRL. The latter is still a bit ambiguous, but I felt that FSS_CM_BOARD_CTRL was too long. 
4. Added DQ channels for the new PMC channels. These are recording at 16K at the moment, but since we have the fast testpoints courtesy of the CDS filter modules for diagnostics, perhaps the DQ channels need only be recorded at 2K?

The PSL namespace block in C1ALS looks like this now:

I then tried hooking up the DAQ signals from the PMC servo board to the ADC via the 1U generic ADC interface chassis in 1X2 - this has 4pin LEMO inputs corresponding to 2 differential input channels. I used J6 (corresponding to ADC channels 10 and 11) for the PMC_ERR and PMC_CTRL respectively. I was a little confused about the status of the 4 pin LEMO output on the front panel of the PMC servo board. According to the DCC page for the modified 40m servo board, the DAQ outputs are wired to the backplane connector instead of the 4 pin LEMO. But looking at photographs on the same DCC page, there are wires soldered on the rear-side of the PCB from the 4-pin LEMO to the backplane connector. Also, I believe the measurements made by Rana in the preceeding elog were made via the front panel LEMO. In any case, I decided to use the single pin LEMO monitor points on the front panel as a preliminary test. The uncalibrated spectra with ADC terminated, IMC unlocked and IMC locked look like:

So it looks like at the very least, we want to add some gain to the AD620 instrumentation amplifiers to better match the input range of the ADC. We also want to make the PZT voltage monitor path AC coupled. My plan then is the following:

1. Figure out what is going on with the 4-pin LEMO connector on the front panel - is it connected to the DAQ monitor points or not?
2. Ground pin 5 of U15 (this has already been done by Koji for U14 according to the DCC page)
3. Add a resistor between pins 1 and 8 of U14 and U15 to get some gain. According to the datasheet, a 1k resistor will give a gain of 50, which for U15 will mean that we undo the existing 1/50 attenuation. Of course we need to AC couple this path first by adding a capacitor in series with R14. 
4. Figure out where the RF harmonics are coming from and what is the best way to attenuate them.

I will update with a circuit diagram with proposed changes shortly.

Proposed changes:

1. Cut PCB trace between R14 and R13, install capacitor - what is is correct type of capacitor to use here? I figured installing a series capacitor after the resistive divider, to the input of the instrumentation amplifier avoids the need for a HV capacitor, so we can use a 1uF WIMA capacitor.
2. Add gains to U14 and U15 (error and control signal monitors respectively). Based on the uncalibrated spectra attached, I think we should go for a gain of ~50 for U15 (1kohm between pins 1 and 8), and a gain of ~200 for U14 (250ohms between pins 1 and 8).

The PCB layout is such that I think using components with leads is easier rather than SMD components.

If this sounds like a reasonable plan, I will pull out the servo card from the eurocrate and implement these changes today evening...

As part of the ongoing effort to try and calibrate the PMC DAQ channels into physical units, I tried to get a calibration for the PSL NPRO PZT actuator gain. In order to do this, I selected "Blank" on the PMC servo MEDM screen such that there was no feedback signal to the PMC PZT for length control. Then I used the summing box right before the  PSL PZT to inject a ~1Hz triangular wave, 4Vpp. This was sufficient to sweep the NPRO frequency over 70MHz such that both sidebands and the carrier go through resonances in the PMC cavity. I then simultaneously monitored the applied triangular wave voltage and the PMC error signal (using the single pin LEMO connector on the front panel) on an oscilloscope. Analysis is underway, but a quick look at one measurement suggests a PZT actuator gain of ~1.44MHz/V, which is close to what we expect for the Innolight NPROs. The idea is to use this calibration to convert the DQ channels into physical units. 

Details + plots + error analysis to follow...

What you have drawn looks good to me: the cut should be between TP3 and pin3 of the AD620. This should maintain the DC coupled respons for the single-pin LEMO and backplane EPICS monitors.

We want to use the PMC signal down to low frequencies, so the filter on the input of the AD620 should have a low frequency cutoff, but we should take care not to spoil the noise of the AD620 with a high impedance resistor.

It has a noise of 100 nV/rHz and 1 pA/rHz at 1 Hz. If you use 47 uF and 10 kOhm, you'll get fc = 1/2/pi/R/C ~ 0.3 Hz so that would be OK. 

Summary:

By sweeping the laser frequency and looking at the PMC PDH error signal, I have determined the 2W Mephisto Innolight PZT actuator gain to be 1.47 +/- 0.04 MHz/V. 

Method:

1. Re-aligned the input beam into the PMC to maximize transmission level on the oscilloscope on the PSL table to 0.73V.
2. Disabled control signal from IMC servo to PSL. 
3. Unlocked the PMC and disabled the loop by selecting "BLANK" on the PMC MEDM screen.
4. Connected a 0.381 Hz 5Vpp triangular wave with SR function generator to the "SUM" input of the Fast I/F box just before the PSL PZT input. These params were chosen considering the Pomona box just before the NPRO has a corner at 2.9Hz, and also to sweep the voltage to the NPRO PZT over the full 150V permitted by the Thorlabs HV amplifier unit. Monitored the voltage to the Thorlabs HV amp from the "AFTER SUM" monitor point on the same box. Monitored the PMC PDH error signal using the single-pin LEMO monitor point on the PMC servo board (call this Vmon). Both of these signals were monitored using a Tektronix digital O'scope.
5. Downloaded the data using ethernet.
6. Fit a line to the voltage applied to the NPRO PZT - I assumed the actual voltage being applied to the PZT is 15*Vmon, the pre-factor being what the Thorlabs HV amplifier outputs. The zero crossings of the sideband resonances in the PDH error signal are separated by 2*fmod (separated by fmod from the carrier resonance, fmod = 35.5MHz assumed). With this information, the x-axis of the sweeps can be converted to Hz, from which we get the PZT actuator gain in MHz/V. 

An example of the data used to calculate the actuator gain (left), and the spread of the calculated actuator gain (right - error bars calculated assuming 5e-4 s uncertainty in the sideband zero-crossing interval, and using the error in the slope of the linear fit to the sweep voltage):

This will now allow calibration of the PMC DAQ channels into Hz.

GV 4 April - The y-axis of the lower plot in Attachment #1 has mis-labelled units. It should be [V], not [MHz/V].

I made some changes to the DAQ path on the PMC servo board, as per the plan posted earlier in this thread. Summary of changes:

1. AC coupling PMC control signal path using 2 x 47uF metal film capacitors (in parallel)
2. Grounding pin 5 of U15
3. Adding gain to U14 (gain of ~500) and U15 (gain of ~50)

Details + photos + calibration of DAQ channels to follow. The PMC and IMC both seem to remain stably locked after this work.

good cal. I wonder if this data also gives us a good measurement of the cavity pole or if the photo-thermal self-locking effect ruins it. You should look at the data for the positive sweeps and negative sweeps and see if they give the same answer for the cavity poles. Also, maybe we can estimate the PMC cavity pole using the sidebands as well as the carrier and see if they give the same answer? 

As discussed at the Wednesday meeting last week, I tried moving the demodulation of the PMC error signal off the PMC servo board, by using some minicircuits components. This is just a quick summary elog, more details to follow tomorrow.

• I used the Mini Circuits ZAD-6+. This is a level 7 mixer, and the LO board puts out ~16dBm, so I replaced the existing 3dB attenuator between the LO board and the input to the PMC servo board with a 9dB attenuator.
• On the RF side, I retained the 35.5 MHz bandpass filter on the PD input.
• On the IF output, I used an in-line 50ohm terminator in series with a minicircuits BLP1.9+ low pass filter
• The mixer output was routed to the FP1 test input of the servo board
• After some twiddling with the demod phase MEDM screen, I was able to lock the PMC. I've not done a thorough characterization of the loop with the current configuration, this will be done tomorrow. But the PMC and IMC have been stably locked for the last couple of hours...

During the course of this work, I noticed that there was a 35.5MHz line (at ~-55dBm) in the 4-pin LEMO DAQ outputs even when all other inputs to the servo board were terminated. So it seems like this pickup is not coming from the RFPD or demod path. The LO board has a shield enclosure similar to what we have on the LSC demod boards, but perhaps this shield does not enclose the full RF path, and there is some residual pickup between the two cards in close proximity in the Eurocrate?

On the bright side, with this demod setup, the higher harmonic peaks seem to be significantly suppressed.

In particular, the 3x35.5 MHz peak which was very prominent when I looked at these spectra with the nominal demod setup, seems to be much suppressed. 

I'm leaving the PMC servo in this configuration (off servo board demodulation using minicircuits parts) overnight.

Here is a more detailed comparison of the spectra of the signals at the front panel DAQ LEMO output, measured with the Agilent analyzer. I've left the scale linear, it looks like when the demodulation is done on the servo board, the 1x, 3x and 5x harmonics of the 35.5MHz modulation are clearly visible. I also plut in a plot of the spectra when both the PD and LO inputs to the servo board are terminated (and so the PMC is unlocked), but with the HV In and OUT of the servo board still connected. In this case, the higher harmonics vanish, but a 35.5MHz peak of ~-50dBm remains. Since this is present with no input to the servo board, this must be direct pickup from the nearby LO board? 

In any case, it looks like many of the harmonics that are present with the nominal demod setup either vanish or are much more suppressed when the error signal demodulation is done off the servo board .

Further down the signal chain, I had noticed sometime last week that the ADC signals for the PMC DAQ channels I set up seemed to saturate around 4000 counts. Rana mentioned that the ADC interface box with LEMO connectors on the front is powered with +/-5V. Valera and co. had simply increased the suppy voltage sometime ago to get around this problem, so I did something similar, and increased the supply voltage to +/- 15V. I then confirmed that the ADC doesn't get saturated by driving the input with a +/-5V signal. So now the amplified AD620 signals from the PMC servo board are better matched to the ADC range. 

Here is an uncalibrated spectrum (taken with IMC locked), compared to the current ADC noise and signal levels before the AD620s were given gain.

I now need to think a little about what exactly the control scheme would be if the PMC is used as a reference for the IMC over some frequency range...

Quick entry, details to follow in the AM tomorrow.

• I calibrated the PMC DAQ channels into physical units - there now exists in the filter modules  cts2m and cts2Hz filter modules, though of course only one must be used at a time
• Finally measured the PMC OLTF, after moving the PMC PDH error signal demodulation off the servo board - I used the same procedure as Koji when he made the modifications to the PMC servo board, I will put up the algebra here tomorrow. Turns out the previously nominal servo gain of +10dB on the MEDM sliders was a little low, the new nominal gain is +20dB, and has been updated on the MEDM screen.

ToDo:

• Put up the modified schematic on the 40m DCC tree Done April 18 10pm
• Check calibration by comparing inferred PMC cavity displacement from error point and control point spectra, using the measured OLTF
• Finish up looking at multicoherence with MCL and various witness channel combinations

Here are the details:

1. PMC OLTF:
   • the procedure used was identical to what Koji describes in this entry.
   • I used the SR785 to take the measurement.
   • MEDM gain slider was at +20dB 
   • I used the two single pin LEMO front panel monitor points to make the measurement. 
   • Mix_out_mon was CH2A, HV_out_mon was CH1A on the SR785
   • A = CH2A/CH1A with the SR785 excitation applied to the EXT_DC single pin LEMO input on the front panel. I used an excitation amplitude of 15mV
   • B = CH2A/CH1A without any excitation
   • Couple of lines of loop algebra tells us that the OLTF is given by the ratio A/B. The plot below lines up fairly well with what Koji measured here, UGF is ~3.3kHz with a phase margin of ~60degrees, and comparable gain margin at ~28kHz. As noted by Koji, the feature at ~8kHz prevents further increase of the servo gain. I've updated the nominal gain on the PMC MEDM screen accordingly... I couldn't figure out how to easily extract Koji's modelled OLTF so I didn't overlay that here... Overlaid is the model OLTF. No great care was taken in analyzing the goodness of the agreement with the model and measurement by looking at residuals etc, except that the feature that was previously at 28.8kHz now seems to have migrated to about 33.5 kHz. I'm not sure what to make of that. 
     
2. PMC DAQ calibration:
   • The calibration was done using the swept cavity, the procedure is basically the same as described by Koji in this elog.
   • The procedure was slightly complicated by the fact that I added gain to the AD620 buffers that provide the DAQ signals. So simply sweeping the cavity saturates the AD620 very quickly.
   • To workaround this, I first hooked up the un-amplified single pin LEMO front panel monitor points to the DAQ channels using some of the available BNC-LEMO patch cables.
   • I then did the swept cavity measurement, and recorded the error and control signals fron the single pin LEMO front panel monitor points. Sweep signal was applied to EXT_DC input on front panel.
   • In the nominal DAQ setup however, we have the amplification on the AD620. I measured this amplification factor by hooking up the single pin LEMO monitor point, along with its corresponding AD620 amplified counterpart, to an SR785 and measuring the transfer function. For the PMC_ERR channel, the AD620 gain is ~53.7dB (i.e. approx 484x). For the PMC_CTRL channel, the AD620 gain is ~33.6dB (i.e. approx 48x). These numbers match up well with what I would expect given the resistors I installed on the PMC board between pins 1 and 8 of the AD620. These gains are digitally undone in the corresponding filter modules, FM1.
   • To calibrate the time axis into frequency, I located the zero crossings of the sidebands and equated the interval to 2 x fmod. For the PMC servo, fmod = 35.5MHz. I used ~1Hz triangle wave, 2Vpp to do the sweep. The resulting slope was 1.7026 GHz/s.
   • The linear part of the PDH error signal for the carrier resonance was fitted with a line. It had a slope of 1.5*10^6 cts/s.
   • The round trip length of the PMC cavity was assumed to be 0.4095m as per Koji's previous entry. This allows us to calibrate the swept cavity motion from Hz to m. The number is 1.4534 * 10^-15 m/Hz. I guess we could confirm this by sweeping the cavity with the DC bias slider through the full range of 0-250V, but we only have a slow readback of the PMC reflection (and no readback of the PMC transmission).
   • Putting the last three numbers together, I get the PMC_ERR signal calibration as 1.6496 pm/ct. This is the number in the "cts2m" filter module (FM10).
   • An analogous procedure was done to calibrate the control signal slope: from the sweep, I got 4617 cts/s, which corresponds to 2.7117*10^-6 cts/Hz. Using the FSR to convert into cts/m, I get for PMC_CTRL, 535.96 pm/ct. This is the number in the "cts2m" filter module (FM10).
   • For convenience, I also added "cts2Hz" calibration filters in FM9 in the corresponding filter modules. 

The updated schematic with changes made, along with some pictures, have been uploaded to the DCC page...

What's the reasoning behind setting the the gain to this new value? i.e. why do these 'margins' determine what the gain should be?

I used a one hour stretch of data from last night to look at coherence between the PMC control signal and MCL, to see if the former can be used as a witness channel in some frequency band for MCL stabilization. Here is a plot of the predicted subtraction and coherence, made using EricQs pynoisesub code. I had thought about adopting the greedy channel ranking algorithm that Eric has been developing for noise subtraction in site data, but since I am just considering 3 witness channels, I figured this straight up comparison between different sets of witness channels was adequate. Looks like we get some additional coherence with MCL by adding the PMC control signal to the list of witness channels, there is about a factor of a few improvement in in the 1-2Hz band...  

Summary:

At around 10:30AM today morning, the PSL mysteriously shut off. Steve and I confirmed that the NPRO controller had the RED "OFF" LED lit up. It is unknown why this happened. We manually turned the NPRO back on and hte PMC has been stably locked for the last hour or so.

Details:

There are so many changes to lab hardware/software that have been happening recently, it's not entirely clear to me what exactly was the problem here. But here are the observations:

1. Yesterday, when I came into the lab, the MC REFL trace on the wall StripTool was 0 for the full 8 hour history - since we don't have data records, I can't go back further than this. I remember the PMC TRANS and REFL cameras looked normal, but there was no MC REFL spot on the CCD monitors. This is consistent with the PSL operating normally, the PMC being locked, and the PSL shutter being closed. Isn't the emergency vacuum interlock also responsible for automatically closing the PSL shutter? Perhaps if the turbo controller failure happened prior to Jamie/me coming in yesterday, maybe this was just the interlock doing its job. On Friday evening, the PSL shutter was certainly open and the MC REFL spot was visible on the camera. I also confirmed with Jamie that he didn't close the shutter.
2. Attachment #1 shows the wall StripTool traces from earlier this morning. It looks like ~7.40AM, the MC REFL level went back up. Steve says he didn't manually open the shutter, and in any case, this was before the turbo pump controller failure was diagnosed. So why did the shutter open again? 
3. When I came in at ~10AM, the CCD monitor showed that the PMC was locked, and the MC REFL spot was visible. 
4. Also on attachment #1, there is a ~10min dip in the MC REFL level. This corresponds to ~10:30AM this morning. Both Steve and I were sitting in the control room at this time. We noticed that the PMC TRANS and REFL CCDs were dark. When we went in to check on the laser, we saw that it was indeed off. There was no one inside the lab area at this time to our knowledge, and as far as I know, the only direct emergency shutoff for the PSL is on the North-West corner of the PSL enclosure. So it is unclear why the laser just suddenly went off.

Steve says that this kind of behaviour is characteristic of a power glitch/surge, but nothing else seems to have been affected (I confirmed that the X and Y end lasers are ON). 

The last entry I found relating to ref cavity was 2011 Aug 19

The PSL HEPA was running noisy at 100V   The bearing is wearing out. I turned it down to 30V It is quiet there.

I moved the axuiliary NPRO to the PSL table today and started setting up the optics.

The Faraday Isolator was showing a pretty unclean mode at the output so I took the polarizers off to take a look through them, and found that the front polarizer is either out of place or damaged (there is a straight edge visible right in the middle of the aperture, but the way the polarizer is packaged prevents me from inspecting it closer). I proceeded without it but left space so an FI can be added in the future. The same goes for the broadband EOM.

There are two spare AOMs (ISOMET and Intraaction, both resonant at 40MHz) available before we have to resort to the one currently installed in the PSL.

I installed the Intraaction AOM first and looked at the switching speed of its first order diffracted beam using both its commercial driver and a combination of minicircuits components. Both show similar behavior. The fall time of the initial step is ~110ns in both cases, but it doesn't decay rapidly no light but a slower exponential. Need to check the 0 order beam and also the other AOM.

Intraaction Driver

Mini Circuits Drive

I don't understand why the 1st order diffracted beam doesn't go to zero when you shut off the drive. My guess is that the standing acoustic wave in the AO crystal needs some time to decay: f = 40 MHz, tau = 1 usec... Q ~ 100. Perhaps, the crystal is damped by the PZT and ther output impedance of the mini-circuits switch is different from the AO driver.

In any case, if you need a faster shut off, or want something that more cleanly goes to zero, there is a large (~1 cm) aperture Pockels cell that Frank Siefert was using for making pulses to damage photo diodes. There is a DEI Pulser unit near the entrance to the QIL in Bridge which can drive it.

I'll look for it tomorrow, but I haven't given up on the AOMs yet. I swapped in the ISOMET modulator today and saw the same behavior, both in 0th and 1st order. The fall time is pretty much identical. Gautam saw no such thing in the PSL AOM using the same photodetector.

1st order diffracted                                                          0th order

In the meantime I prepared the fiber mode-matching but realized in the process that I had mixed up some lenses. As a result the beam did not have a waist at the AOM location and thus didn't have the intended size, although I doubt that this would cause the slower decay. I'll fix it tomorrow, along with setting up the fiber injection, beat note with the PSL, and routing the fiber if possible.

nominal changed from 22 to 23 dB to minimize PC drive RMS

previous loop gain measurement is sort of bogus (made on SR785); need some 4395 loop measurements and checking of crossover and error point spectrum