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.
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.
I've started a spreadsheet for the BHD optics specifications and populated it with my best initial guesses. There are a few open questions we still need to resolve, mostly related to mode-matching:
The spreadsheet is editable by anyone. If you can contribute any information, please do!
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.
- No more SR 560, upgraded to LB1005 P-I controller. Because: Elog 14687. Schematic of new setup shown in Attachment 1.
- For this, the Marconi was moved to the other (east) side of the PSL table and a power supply was also placed in the enclosure.
I think that the RF power at the mixer in this new configuration is 0 dBm (since the spectrum analyzer read ~ -20 dBm)
- Turned up the HEPA to 100%, closed the PSL shutter, misaligned the ITMX, connected the LB1005 to the PZT. [The PZT has been reconnected to the X arm PDH servo, HEPA back to 20-30%]
- Tried to look for the PSL+X beat, but it was not there. Gautam identified the flipmount in the path which sorted it out (eventually), but there was no elog about it.
- After much trial, the loop seemed to lock with PI corner 1 kHz, gain ~2.9 (as read on knob), LFGL set to 90 dB. The beat note looked quite stable on the oscilloscope, but the error signal had an rms of ~100 mV (Rana pointed out that it could be the laser noise) and the lock lasted for ~1 min each time.
The parameters were similar to that in elog 14687. Why do we require such a high PI corner frequency and LFGL?
The RF transimpedance of the PMC PDH RFPD was measured, and found to be 1.03 kV/A.
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:
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).
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.
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.
The burt snapshotting is still not so reliable - for whatever reason, the number of snapshot files that actually get written looks random. For example, the 14:19 backup today got all the snaps, but 15:19 did not. There are no obvious red flags in either the cron job logs or the autoburt log files. I also don't see any clues when I run the script in a shell. It'll be good if someone can take a look at this.
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.
I will pull the board and effect the change later today.
The mixer + LPF combo used to demodulate the PMC PDH error signal seems to work as advertised.
Measurement setup --- Attachment #1. The IF signal was monitored using the scope in High-Z mode.
Results --- Attachment #2.
upgrade was done
cronjob testing wasn't one by one 😢
burt snapshots were gone
i brought them back home 🏠
Megatron is now running Ubuntu 18.04 LTS.
wiped and install Debian 10 on rossa today
still to be done: config it as CDS workstation
please don't try to "fix" it in the meantime
Gautam and I debugged a communications problem with TP3 that was causing its python service to fail. We traced the problem back to the querying of the pump controller for its operational parameters (speed, voltage, temp). Some small percentage of the time (~5%, indeterministically), the pump controller is returning an invalid response which causes the service to shut itself down and signal a NO COMM error.
As a temporary fix, I wrapped the failing query in an exception handler to continue past this particular error. However, we suspect the microprocessor in the TP3 controller may be beginning to fail. There is a spare controller sitting right next to it in the vacuum rack. We will ask Chub to install the replacement in the near future.
gautam: this pump is responsible for pumping the annular volume under normal operations. while this problem is being resolved, the annular volume is valved off (as it has been since July 2019 anyway which is when this problem first manifested).
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.
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.
I looked into this a little more today.
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...
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 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.
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.
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.
Note that for all the alignment work, only the two steering mirrors immediately upstream of the PMC cavity were touched.
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.
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.
The measurements are consistent with the specifications, and there is no evidence of compression at any of the power levels we expect to supply to this box (<0dBm).
These "gain blocks" were acquired for the purpose of amplifying the IR ALS beat signals before transmission to the LSC rack for demodulation. The existing ZHL-3A amplifiers have a little too much gain, since our revamp to use IR light to generate the ALS beat.
Attachment #4: Setups used to measure transfer functions and noise.
For the transfer function measurement, I chose to send the output of the amplifier to a coupler, and measured the coupled port (output port of the coupler was terminated with 50 ohms). This was to avoid saturating the input of the AG4395. The "THRU" calibration feature of the AG4395 was used to remove the effect of cabling, coupler etc, so that the measurement is a true reflection of the transfer function of OUT/IN of this box. Yet, there are some periodic ripples present in the measured gain, though the size of these ripples is smaller than the spec-ed gain flatness of <0.6dB.
For the noise measurement, the plots I've presented in Attachment #3 are scaled by a factor of sqrt(2) since the noise of the ZFL-500-HLN+ and the ZHL-1010+ are nearly identical according to the specification. Note that the output noise measured was divided by the (measured) gain of the ZFL-500-HLN+ and the ZFL-1010+ to get the input referred noise. The trace labelled "Measurement noise floor" was measured with the input to the ZFL-500-HLN+ terminated with 50ohms, while for the other two traces, the inputs of the ZHL-1010+ were terminated with 50ohms.
Raw data in Attachment #5.
I will install these at the next opportunity, so that we can get rid of the many attenuators in this path (the main difficulty will be sourcing the required +12V DC for operation, we only have +15V available near the PSL table).
With Gautam's help today the PLL managed to be be locked for a few brief moments. Turns out the signal power of the beat was an issue.
What was changed prior to/ during the experiment:
1. The PSL shutter was closed so not light goes into the input mode cleaner.
2. HEPA turned up (will be turned back down to ~30%)
3. AOM driver offset voltage decreased from 1V to ~100 mV (this will be reverted to 1V by the end of today). This increases the beat signal by deflecting the zeroth order beam to create the beat.
4. Output of servo SR 560 sent to the PZT of the X NPRO laser (the cable was disconnected from the pomona box at the X end)
5. The SR560, mixer, LPF and cables required for connections were moved into the PSL enclosure.
6. The error and control signals were hooked up to the oscilloscope where the beat outputs were visible (the setup has been reverted back to the original).
Elog 14687 has a detailed description of the conditions that provide a stable lock. I was told that the PI controller (LB1005) may be a better servo than the SR560, but today it was not used.
1) Parameters during the more successful attempts:
LPF: 5 MHz, Mixer: ZP-3+
Gain set at SR560: varied, but generally 200
Filter at SR560: 1 Hz low pass (single pole? at least by the label)
2) The LO had to be very close (<2 MHz) to the beat frequency in order to achieve a lock for ~30s
For a Unity Gain Frequency (UGF) of 1 kHz, assumed PZT response of 1 MHz/V, Mixer response of 25 mV/ rad, the required gain of the amplifier is
G ~ 0.8
- Measured the mixer response
- PSL laser temperature was adjusted so that beat frequency was roughly 25 MHz and the amplitude was found to be roughly -30dBm.
- At the RF port instead of the beat signal, a signal of 25 MHz + few kHz at -30 dBm was inputted. The LO was a 25 MHz signal was sent from the Marconi at 7 dBm.
- The mixer output was measured, with setup as in Attachment 1 Figure (A), on an oscilloscope. The slope near the small angle region of the sine curve would be the gain (in V/rad) and was found to be: rad
- Since from the above calculations it seemed like an amplifer gain of 1 should work for the PLL, I rearranged the set up as in Figure (B) of Attachment 1 to actuate the X end NPRO PZT, I adjusted the PSL temperature (slow control) to try and match the frequency to 25 MHz, but couldn't lock the loop. I was monitoring the error signal after amplification (50 ohm output of the SR 560) which showed oscillations when the beat frequency was near 25 MHz and nothing significant otherwise.
- I used a 20 dB attenuator at the amplifier output and saw the beat note oscillate for longer, but maybe because it was a 50 ohm component in a high impedance channel it did not work either (?). I tried other attenuator combinations with no better luck.
- Is there a better location to add the attenuator? Should I pursue amplifying the beat signal instead?
- Also, it seemed like the beat note drift was higher than earlier. Could it be because the PMC was unlocke
Per Yehonathan's request, I removed one PDA10CF from a pickoff of REFL on the AS table (it was being used for the mode spectroscopy project). I placed a razor beam dump where the PD used to be, so that when the PRM is aligned, this pickoff is dumped. This is so that team ringdowns can use a fast PD.
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.
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.
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.
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.
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.
Single arm locking using POX and POY has been restored. After running the dither alignment servos, the TRX/TRY levels are ~0.7. This is consistent with the IMC transmission being ~11000 counts with the AOM 1st order diffracted beam (c.f. 15000 counts with the undiffracted beam).
Tomorrow, I'll check the single-arm locking and the ALS system.
Every new year (on Dec 31 or Jan 1), all of the realtime models will report a "0x4000" error. This happens due to an offset to the GPStime driver not being updated. Here is how this can be fixed (slightly modified version of what was done at LASTI).
Steps to fix the DC errors:
/* 2019 had 365 days and no leap seconds */
pHardware->gpsOffset += 31536000;
/* 2019 had 365 days and no leap seconds */
pHardware->gpsOffset += 31536000;
sudo make install
sudo systemctl daqd_* stop
sudo modprobe -r symmetricom
sudo modprobe symmetricom
sudo service daqd_* start
Independent of this, there is a 1 second offset between the gpstimes reported by /proc/gps and gpstime. However, this doesn't seem to drift. We had effected a static offset to correct for this in the daqd config files, and it looks like these do not need to be updated on a yearly basis. All the daqd indicators are now green, see Attachment #1.
There was no light entering the IFO. I worked on a few things to bring the interferometer to a somewhat usable state. The goal is to get back to PRFPMI locking ASAP.
Problem: All fast models report a "0x4000" DC error. See Attachment #1.
Solution: I think this is a "known" issue that happened last new year too. The fix was to add a hard-coded 1 second offset to the daqd config files. However, incrementing/decreasing this offset by +/- 1 second did not fix the errors for me today. I'll reach out to JH for more troubleshooting tips.
Update 15 Jan 2020 830am: The problem is now fixed. See here.
Problem: c1susaux and c1auxey were unresponsive.
Solution: Keyed c1auxey. Rebooted c1susaux and as usual, manually started the eth0/eth1 subnets. The Acromag crate did not have to be power-cycled. ITMY got stuck in this process - I released it using the usual bias jiggling. Why did c1susaux fail? When did it fail? Was there some un-elogged cable jiggling in that part of the lab?
Problem: IMC autolocker and FSS slow processes aren't running on megatron after the upgrade.
Solution: Since no one bothered to do this, I setup systemd infrastructure for doing this on megatron. To run these, you do:
and to check their status, use:
The systemd setup is currently done in a naive way (using the bash executable to run a series of commands rather than using the systemd infrastructure itself to setup variables etc) but it works. I confirmed that the autolocker can re-acquire IMC lock, and that the FSS loop only runs when the IMC is locked. I also removed the obsolete messages printed to megatron's console (by editing /etc/motd) on ssh-login, advising the usage of initctl - the updated message reflects the above instructions.
In order to do the IMC locking, I changed the DC voltage to the AOM to +1V DC (it was +0.8 V DC). In this setting, the IMC refl level is ~3.6 V DC. When using the undiffracted AOM beam, we had more like +5.6 V DC (so now we have ~65% of the nominal level) from the IMC REFL PD when the IMC was unlocked. IIRC, the diffraction efficiency of the AOM should be somewhat better, at ~85%. Needs investigation, or better yet, let's just go back to the old configuration of using the undiffracted beam.
There was also an UN-ELOGGED change of the nominal value of the PMC servo gain to 12.8, and no transfer function measurement. There needs to be a proper characterization of this loop done to decide what the new nominal value should be.
I'm going to leave the PSL shutter open and let the IMC stay locked for stability investigations. Tomorrow, I'll check the single-arm locking and the ALS system.
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.
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.
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.
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.
[Yehonathan, Jon, Shruti]
Since the PMC would not lock, we initially burt-restored the c1psl machine to the last available shapshot (Dec 10th 2019), but it still would not lock.
Then, it was burt-restored to midnight of Dec 1st, 2019, after which it could be locked.
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.
PSL controls on the sitemap went blank. Rebooted c1psl. PSL screens seem normal again.
I've assembled a new SuperMicro rackmount machine to replace c1psl. It is currently set up on the electronics bench.
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.
RTFE. Where did the spares go?
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.
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.
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.
PSL wiring spreadsheet is ready. (But the link was stripped. Koji)
Link to a wiki page with the link to the wiring spreadsheet (Yehonathan)
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.
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.
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.
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.
Final (hopefully) PSL channel list is attached with allocated Acromag channels. Wiring spreadsheet coming soon.
Current Acromag count:
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
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)
- Also, it seemed like the beat note drift was higher than earlier. Could it be because the PMC was unlocked?