My goal was to do some further characterization of the IR ALS system tonight. With POX as an OOL sensor, I measured an RMS displacement noise of 8 pm with the arm under ALS control. I calculated the CARM linewidth to be 77 Hz (=10.3 pm) for the 40m parameters, assuming 30ppm arm loss. Fuurthermore, this number is 3x better than the 24 pm RMS quoted in the Izumi et. al. paper. Of course I am quoting the best results from my efforts tonight. Conclusions:
Since the stability and noise seemed quite good, I decided to collect some arm scan data to give to our modeSpec SURFs to practice fitting (which is the short dip in TRX in Attachment #4). Although after the discussion with Rana today, I think it may be that we want to do this measurement in reflection and not transmission, and look for a zero crossing in the PDH signal. In any case, I was able to scan 7 FSRs without any issues. I will upload the data to some git repo. GPS start time is 1208850775, sweep was 3mins long.
I think the next step here is to noise-budget this curve. At least the DFD noises
These spectra were taken with the arm cavity length locked to the PSL frequency using POX as an error signal, and the EX laser frequency locked to the XARM cavity length by the analog PDH servo at EX, so there is no feedback control with the ALS beat signal as an error signal.
I borrowed the Olympus and forgot to leave a note - I should have it for at most a day. dmassey@ligo if you need it urgently
The script is moving forward and we feel we are close, however we still have a couple of issues, which are:
1) some python misbehaviour between the system environment and the anaconda one; currently we call bash commands within the python script in order to avoid using the ezca library, which is the one complaining;
2) the fine scan is somewhat not so robust yet, need to investigate more; the main suspects are the wavelet parameters given to the algorithm, and the Offset and Ramp parameters used to perform the scan.
Here is an example of a best case scenario, with 20s ramp and 500 points:
Tonight I tried some more tests on the script; it seems to work better, with both performance and robustness improved, although the Xarm behaved badly almost all the time. I did not perform all the tests I wanted because the ALS lock was pretty unstable tonight (not only because of the X arm), with more than a few lock losses; after the last lock loss, however, I couldn't restore the Xarm. I'll do some more tests as soon I can recover it, or post the result of the first batch of tests.
In addition, I encountered the following error multiple times, but I have no idea about what could it be:
Thu Nov 06 02:00:13 PST 2014
medmCAExceptionHandlerCb: Channel Access Exception:
Channel Name: Unavailable
Native Type: Unavailable
Native Count: 0
Message: Virtual circuit disconnect
Requested Type: TYPENOTCONN
Requested Count: 0
Source File: ../cac.cpp
Line number: 1214
EDIT on X arm: I found different settings in C1SUS_ITMX, with respect to ETMX, ITMY and ETMY (namely LSC/DAMP is OFF and LSC/BIAS is ON); I don't know if this is intended or for some reason ITMX was not recovered properly after the lock loss, so I didn't change anything, but it may be worth looking into that.
Still no luck in recovering the X arm, I am giving up for tonight; honestly I didn't try many things, as I don't know well the system and didn't want to mess things up.
Preliminary results so far:
I confirm that the best settings for the ramp of the ALS scan are 20s and 500 points; this causes however the script to be fairly slow (80s for the scan/data collection, 7s for the coarse peak finding, 17s for the fine peak finding, total ~2 min); in the best cases the TR*_OUT obtained is around 0.90, as shown in the first plot (early in the evening, all the following plots are in chronological order, if that can help finding the reason for the X arm misbehaviour...):
However, after a few minutes somehow the TR*_OUT went down a bit, without any kind of intervention; also, it is visible the instability of the X arm:
Even when X arm was somewhat stable, its performance and robustness were (far) worse than the Y arm ones:
The following plot shows (about the Y arm only) that there is still some margin, as the maximum value of TRY_OUT is not completely kept at the end of the procedure:
Finally the last plot I managed to obtain, before the X arm went completely crazy...
The next step, after obviously figuring out the X arm situation, is to try some averaging during the fine scan, I don' t know if this will improve the situation, however it shouldn't impact on the execution time. Tomorrow I'll post something more detailed on the script itself and the wavelet implementation.
Yesterday I did some more tests with a modifies script; the main difference is that scipy's default wavelet implementation is quite rigid, and it allows only very few choices on the wavelet. The main issue is that our signal is a real, always positive symmetrical signal, while wavelets are defined as 0-integral functions, and can be both real or complex, depending on the wavelet; I found a different wavelet implementation, and I combined it with some modified code from the scipy source, in order to be able to select different wavelets. The result is the wavelet_custom.py module, which lives in the same ALS script directory and it is called by the script. In both the script and the module there the references I used while writing them. It is now possible to select almost any wavelet included in this custom module; "almost" means that the scipy code that calls the find_peaks_cwt routine is picky on the input parameters of the wavelet function, I may dig into that later. For the last tests, instead of using a Ricker wavelet (aka Mexican hat, or Derivative of Gaussian Order 2), I used a DOG(6), as it also has two lesser positive lobes, which can help in finding the resonance; the presence of negative lobes is, as I said, unavoidable. I attach an example of the wavelet forms that are possible, and in my opinion, excluding the asymmetric and/or complex ones, the DOG(6) seems the best choice, and it has provided slightly better results. There are other wavelet around, but they are not included in the module so I should implement them myself, I will first see if they seem fitting our case before starting writing them into the module. However, the problem of not finding the perfect working point (the "overshoot-like" plot in my previous elog) is not completely solved. Eric had a good idea about that: during the fine scan, the the PO*11_ERR_DQ signals should be in their linear range, so I could also use them and check their zero crossing to find the optimal working. I will be working on that.
Tonight I started testing a new method for the fine scan:
The other night (before the holidays), I tried ALS offset tuning with IR POX/POY signals and it worked pretty good.
I didn't need to tune the offset after the scanning script stopped.
Once we are at the foot hill of the main resonance, I ran something like
ezcaservo -r C1:LSC-POX11_I_MON C1:LSC-ALSX_OFFSET -g -0.003 &
ezcaservo -r C1:LSC-POY11_I_MON C1:LSC-ALSY_OFFSET -g -0.003 &
(... I am writing this with my memory. I could be wrong but conceptually the commands looked like these)
[Jenne and Kiwamu]
This time we aligned the vertical angle (not the translation) of the IR beam so that the transmitted light from BS shoots the center of ETMY.
The idea is to use ETMY as a beam pointing reference instead using IP_ANG, assuming the translation is not so bad.
As a result it looks like we are wining. A quick A2L test on ITMX_PITCH showed a small off-centering at sub-milimeter level.
We are concluding that the initial beam after PZT2 had been pointing downward somehow.
Before doing this whole job, we checked the spot shape on IP_POS to see if the beam is clipped or not. It was a round shape, which means no clipping around MMT.
But on the other hand, the spot on IP_ANG had been clipped more than half of its bottom as Suresh reported on his elog (see here).
I found that this clipping is able to be fixed by moving the beam angle upward. I guess the clipping happened at one of the steering mirror in the ETMY chamber.
According to these information, we imagined that the beam was somehow pointing downward after PZT2.
So we started aligning the beam by touching only PZT2 for vertical direction. Then we found a beam spot on ETMY's suspension frame, and brought it to the center.
Then we aligned BS and X arm for this new beam axis. The it resulted a small off-centering on pitch.
Once the MC fully gets back, we will examine the TRX degradation with this configuration.
I tried aligning the IR beam axis for the X arm to have good beam centering on ITMX and ETMX.
As a first attempt, I started translating the beam upward by steering PZT1 and PZT2, since the pitch was quite off from the center on ITMX.
As a result I could decrease the pitch off-centering down to about 0.5 mm on ITMY, but on the other hand TRX decreased a lot (by a factor of 4).
I am worrying if something in the central part of IFO might be clipping the beam.
When I was touching PZT1 and PZT2, I payed attention on IP_ANG so that I don't lose a beam spot on IP_ANG.
As long as the beam is on the IP_ANG QPD, the angle of the beam should not be so much different.
Each time after I touched the PZTs, I realigned ITMX and ETMX to maximize the transmitted light.
In this way I proceeded the alignment by changing the PZT offsets little by little while keeping the X arm locked always.
At the beginning, all the PZT offsets were zero. And at the end of this work they became:
C1:LSC-PZT1_Y = 1.880
C1:LSC-PZT2_Y = -1.699
But during this alignment work TRX gradually decreased eventually down to 0.25, which had been 1 at the beginning (TRX is calibrated by dividing it by its maximum power).
Along with this TRX reduction, I found that the optical gain also decreased by a factor of about 5.
This fact has been confirmed by intentionally increasing the filter gain such that the servo oscillates at the UGF.
The amounts of the X arm's beam off-centering have been measured by the A2L technique.
PIT = -1.61 mm
YAW = -0.918 mm
PIT = -3.76 mm
YAW = -2.24 mm
The IR beam was found on the PRM surface, some CCDs, and in the X arm. The TTs are not aligned well yet.
I'm leaving the IFO with the following state.
ITMY/ETMY - aligned to the given green beam. GTRY (no PSL green) 1.0~1.1
ITMX/ETMX - aligned to the green beam. The end PZT for the green beam was steered to have maximum GTRX (0.76 without PSL green)
TT1/TT2 - unknown alignment, TT1/TT2 are related such that the spot is on the POP CCD
PRM - aligned to the given IR beam (i.e. PRM spot on the REFL CCD)
BS - aligned to the given IR beam (i.e. ITMX spot on the AS CCD, The X arm is flashing)
- ITMX was stuck in the suspension. it was caused by the EQ.
- When the X arm was aligned to the green beam, there was no green hitting on the GTRX PD. That's why the end PZT was adjusted.
- In order to earn more range for TT1, C1:IOO-TT1_YAW_GAIN and C1:IOO-TT1_PIT_GAIN were increased to 300 (100 nominal) and the limiter (at 500) were removed.
- The HeNe laser for BS/PRM does not emit the beam even with the driver turned on. Is there a hidden shutter or something? ==> Jenne
- Find the Y arm fringe by moving TT1 and TT2 without loosing the PRM/AS/POP spots.
Yuta is going to bring this up at the 40m meeting so it can be argued over, but we (I) want a permanent IR beat setup at the PSL table. This isn't a novel idea or anything, I just think it will save us time if we can quickly re-acquire the beat signal, so I'm bringing it up again. Eventually, as Koji suggested to me, we can make the IR beat part of a servo, so that the green beat is always within the bandwidth of the green beat PD. But for Phase 1, it's enough to just see the Ir beat on a ~1GHz PD. Suresh tells me most of the bits and pieces are around, we just have to gather them all in one place.
There has been some discussion here and there of using fiber coupled IR beats for ALS. A few weeks ago, and again today with Eric G, I poked around a bit with the fiber box Manasa set up for the FOL scheme.
Somehow, the IR beatnote is ~1000 times smaller than expected, both with the Thorlabs fiber coupled PD and a fiber coupled NF 1611.
In essence, after the fiber combiner, there is on the order of hundreds of uW each of PSL and AUX X IR light. The output of the fiber from each source looks nice and gaussian. The DC output of the 1611 indicates that it is seeing the right level of light. The green beatnote exists with good SNR at twice the IR beat frequency, so we know that the IR beat isn't some junky modes beating.
For the 1611, we would expect an RF signal of ~1mW*0.9A/W*700V/A -> .6V / +8dBm. Instead we see ~2mV / -40dBm.
Incidentally, there is some 20mV / -20dBm signal at ~400kHz, presumably from the green PDH modulation at ~200k.
(The level out of the thorlabs PD is similarly tiny; it doesn't have a DC output though, so we don't know the DC power that the active surface really sees. Not that I expect it to be much different, but the NF just makes it easier to estimate.)
The only things that should be able to cause the beat to be smaller than expected from the power levels are mode matching and polarization matching. All the fibers are single mode, so mode matching should be effectively 100%. Maybe somthing fishy is happening with the polarizations, but they'd have to be really maliciously close to orthogonal to cause this level of mismatch.
Maybe we just don't understand the splitter/combiners. Mysterious.
Maybe we just don't understand the splitter/combiners.
After an email from Eric G, I think this is the case.
If you read the text at Thorlabs about Fiber-Based Polarization Beam Combiners/Splitters, it suggests that these things take input beams both aligned to their slow axes, and outputs one field along the slow, and one orthogonal to it on the fast axis. Which is exactly what we don't want for a beat.
From the AFW website about our product, the POBC-64-C-1-7-2-25dB:
port1 slow axis -> port3 slow axis
port2 slow axis -> port3 fast axis
I was thinking that the "FOSC" product line (which is called a "coupler" instead of a "splitter/combiner") was what we wanted.
Koji brought to my attention that the 90/10 splitters we already have are of this line. So, I rigged a few up to shine a hopefully beating pair of fields on the fiber coupled thorlabs PD.
I was able to get ~80uW each of PSL and AUX X light on the PD, which produced a -10dBm beatnote! Thus, I think these FOSC splitters are indeed what we want.
I then threw this IR beatnote at our ALS signal chain. The beatnote was too big to throw through our ~+27dB RF amps, so I just sent the -10dBm over to the LSC rack.
The IR beat spectrum is somwhat noisier from 10-100Hz, but, more interesting, is that the sub-4Hz noise is identical in the two beats, and very coherent. This excludes ALS noise arising from anything happening in the green beat optics on the PSL table.
Obviously, the high frequency noise is largely the same and coherent too, but also coherent with the AUX X PDH control signal, so it is understood.
Single mode coupler, 2x2, 1064nm +/-20nm, 50/50 ratio, 900micron loose tube jacket, Hi1060flex fiber, 1m fiber length, FC/APC connectors
Four of these items ordered yesterday from http://afwtechnologies.com.au/sm_coupler.html
Yuta retrieved the IR card that had fallen to the bottom of the IOO chamber, just before we put on the access connector yesterday. The clean "pickle picker" long grabber tool worked wonders.
Sorry to say but MC1, MC2, MC3 and PRM face OSEMS are having the same problem of leaking IR into the sensors
The PMC was not locked for 11 minutes on this plot.
The PRM sensors are no longer effected by IR. What changed? The MC still does.
I came in today to check up on the StripTool and burn the Ubuntu LTS (new CDS OS) DVD. I was pretty excited to see the PRM flashes on Mon1.
I waggled the PRM/BS alignments and got a good contrast MICH and then bright flashes in the PRM that totally overload Mon1's CCD.
Now I can see flashes of some IR junk in the X Arm; its way off on the left edge of the mirror, but there's a beam.
For the short term, we can hook up the IO PZTs to some old EPICS channels (like one of the AUX guys in the LSC area), but eventually it has to get hooked up to the new ASC or ASS. We have to bug Joe to see where this shows up in his master diagram.
**Note: if you get lost sometimes when doing the alignments, remember that you can use time_machine_conlog.
rossa:general>./time_machine_conlog 2010/12/31,11:00:00 PDT C1:SUS-ITMX_PIT_COMM
Issuing the conlog command:
/cvs/cds/caltech/conlog/bin/conlog +epics -interp at 2010/12/31,11:00:00 PDT "C1:SUS-ITMX_PIT_COMM"
LIGO controls: values at 2010 12/31 11:00:00 pst
Attached is the last 8 days of Vacuum Pressure trend which includes the pumpdown.
[Gautam, Lydia, Johannes]
The next step is the tip tilt fine alignment of the IR into the arm, using TRY, from which we removed the ND filter for the time being.
Looks like we might have a problem with the IRIG-B output of the GPS receiver.
Rolf came over this morning to help debug the strange symmetricom driver behavior on fb1 with the new Spectracom card. We restarted the machine againt and this time when we loaded the drive rit was clocking at a normal rate (second/second). However, the overall GPS time was still wrong, showing a time in October from this year.
The IRIG-B122 output is supposed to encode the time of year via amplitude modulation of a 1kHz carrier. The current time of year is:
controls@fb1:~ 0$ TZ=utc date +'%j day, %T'
162 day, 18:57:35
The absolute year is not encoded, though, so the symmetricon driver has the year offset hard coded into the driver (yuck), to which it adds the time of year from the IRIG-B signal to get the correct GPS time.
However, loading the symmetricom module shows the following:
[ 1601.607403] Spectracom GPS card on bus 1; device 0
[ 1601.607408] TSYNC PIC BASE 0 address = fb500000
[ 1601.607429] Remapped 0xffffc90017012000
[ 1606.606164] TSYNC NOT receiving YEAR info, defaulting to by year patch
[ 1606.606168] date = 299 days 18:28:1161455320
[ 1606.606169] bcd time = 1161455320 sec 959 milliseconds 398 microseconds 959398630 nanosec
[ 1606.606171] Board sync = 1
[ 1606.616076] TSYNC NOT receiving YEAR info, defaulting to by year patch
[ 1606.616079] date = 299 days 18:28:1161455320
[ 1606.616080] bcd time = 1161455320 sec 969 milliseconds 331 microseconds 969331350 nanosec
[ 1606.616081] Board sync = 1
Apparently the symmetricom driver thinks it's the 299nth day of the year, which of course corresponds to some time in october, which jives with the GPS time the driver is spitting out.
Rolf then noticed that the timing module in the VME crate in the adjacent rack, which also receives an IRIG-B signal from the distribution box, was also showing day 299 on it's front panel display. We checked and confirmed that the symmetricom card and the VME timing module both agree on the wrong time of year, strongly suggesting that the GPS receiver is outputing bogus data on it's IRIG-B output, even though it's showing the correct time on it's front panel. We played around with setting in the GPS receiver to no avail. Finally we rebooted the GPS receiver, but it seemed to come up with the same bogus IRIG-B output (again both symmetricom driver and VME timing module agree on the wrong day).
So maybe our GPS receiver is busted? Not sure what to try now.
It appears that the timing slave in the c1iscex IO chassis is dead. It's front "link" lights are dark, although there appears to be power to the board (other on-board leds are lit). These front lights should either be on and blinking steadily if the board is talking to the timing system, or blinking fast if there is no connection to the timing distribution box. This likely indicates that the board has had some sort of internal failure.
Unfortunately Downs has no spare timing slave boards lying around at the moment; they're all stuffed in IO chassis awaiting shipping. I'm going to email Rolf about stealing one, and if he agrees we'll work with Todd Etzel to pull one out for a transplant
The c1iscex IO chassis seems to be working again, and the iscex front-end is running again.
However, I can't say that I actually fixed the problem.
Originally I thought the timing slave board had died by the fact that the front LED indicators next to the fiber IO were out. I didn't initially consider this a power supply problem since there were other leds on the board that were lit. I finally managed to track down Rolf to give downs the OK to pull the timing boards out of a spare IO chassis for us to use. However, when I replaced the timing boards in the chassis with the new ones, they showed the exact same behavior.
I then checked the power to the timing boards, which comes off a 2-pin connector from the backplane board in the back of the IO chassis. Apparently it's supposed to be 12V, but it was only showing ~2.75V. Since it was showing the same behavior for both timing boards, I assumed that the issue was on the IO chassis backplane.
I (with the help of Todd Etzel) started pulling cards out of the IO chassis (while power cycling appropriately, of course) to see if that changed anything. After pulling out both the ADC and DAC cards, the timing system then came up fine, with full power. The weird part is that everything then stayed fine after we started plugging all the cards back in. We eventually got back to the fully assembled configuration with everything working. But, nothing was changed, other than just re-seating all the cards.
Clearly there's some sort of flaky connection on the IO chassis board. Something is prone to shorting, or something, that overloads the power supply and causes the voltage supply to the timing card to drop.
All I can do at this point is keep an eye on it and go through another round of debugging if it happens again.
If it does happen again, I ask that everyone please not touch the IO chassis and let me look at it first. I want to try to poke around before anyone giggles any cables so I can track down where the issue might be.
I clicked on the FE status screen, just to check on things, and everything on the c1iscex section was red (the IOP and c1scx). Upon deciding that was probably a bad thing, I did a soft reboot from the control room. Now the IOP says "NO SYNC", and the c1scx status thing is totally frozen.
I have sent Jamie a whiny email. He promises to be here soon to fix it.
We discovered that the left network cable is not rigidly connected to the back of the ISCY FE computer. You can easily pull it out a mm disconnecting it. It should click rigidly in place. Not clear if it's the cable or the network card.
Here I have included the full schematic (so far) of the proposed ISS. There are two sheets: the first schematic details the filter stages and their accompanying circuitry while the second schematic details the RMS threshold detection and subsequent triggering.
The first schematic is fairly self explanatory as to what different portions do, and I have annotated much of the second schematic as there are some non-traditional components etc.
I have not yet included some mechanism to adjust the threshold voltage in real time or any of the power regulation, but these should follow fairly quickly.
I have made significant changes to the ISS schematic, mostly in the form of adding necessary subsystems.
Some changes I have made:
On the front page, all inputs and outputs are currently BNC ports, although this is most likely not the final design that will be used. For instance, the ports ENABLE, INPUT GND and INVERT are supposed to be logic inputs for a MAX333a switch. These will most likely be front panel switches that either connect the switch's logic pin to GND (Logic 0) or something like a +5 V supply (Logic 1).
I also have not included power regulation for my board although I have some of the actual D1000217 Chasis Power Regulator boards and I'll incorporate those in my design soon.
More changes that I've made:
After many, many moons of getting to know exactly how frustrating Altium can be, I have completed the PCB layout for my ISS board (final page of ISS_v3.pdf).
Before I get into detail about the PCB, there is one significant schematic change to note: the comparator circuit was changed (with significant help from Koji) so that the voltage reference for boost triggering is established in a more logical way. Instead of the somewhat convoluted topology I had before, now there are only two feedback resistors, R82 and R83. Because their resistances (500k and 50k respectively) are so much larger than the total resistance of the 1k potentiometer (used to establish a tunable threshold voltage), the current flowing through the feedback loop is negligible compared to the 5 mA current flowing through the potentiometer (the pot is rated for 2 W and with 5 mA -> 25 mW dissapation). This allows one to set the threshold voltage for my schmitt trigger, at pin 2 of both the pot and the comparator, entirely with the pot. This trigger also has hysteresis given by the relation deltaV ~ (R83/R82) * (Voh - Vol) where deltaV is the separation between threshold voltages, Voh is the high-level comparator ouput and Vol is the low-level comparator output. Koji simulated this using CircuitLab and I plan to verify the behavior by making a quick prototype circuit.
Now, on to the PCB. The board itself is of a 'standard' LIGO size (11" x 6") has 3 routing layers and 3 internal planes, one for +15 V, one for -15 V and one for GND. In the attached pdf, red is the top routing layer, blue is the bottom layer and brown is the middle routing layer (used for ±5 V exclusively). The grey circles are pads and vias (drilled through) and anything in black is silkscreen overlay. I placed each component and track by hand, attempting to minimize the signal path and following the general rules below,
Sections of the board have been partitioned and labeled with silkscreen overlay to help in both signal pathway recognition as well as eventual troubleshooting.
On the board, I have also included holes so that it can be mounted inside of an enclosure. There is a DCC number printed as well as a 'barcode' (TrueType font: IDAutomationC39S), although they both contain filler asterisks as I haven't published this to the DCC and thus do not have a number.
AOM driving from DAC:
I found that the DAC channels for TT3 and TT4 are connected up in the simulink model, but we aren't using them, since we don't actually have those tip tilts installed. So, we hooked up the TT4 LR DAC output, which is channel 8 on the 2nd set of SMA outputs. We put our AOM excitations into TT4_LR_EXC.
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.
We installed the ISS AOM in the PSL. The AOM was placed right after the EOM. The beam diameter is ~600 um at the AOM. The AOM aperture is 3 mm.
We monitored the beam size by scanning the leakage beam through the turning mirror after the AOM. The beam diameter changed from 525 um to 515 um at a fixed point. We decided that the AOM thermal lensing is not large enough to require a new scan of the mode going into the PMC and we can proceed with PMC mode matching using the scan that was taken without the AOM (to be posted).