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).
In order to allow other individuals besides myself to consider the proposed design of the ISS, I have created a publicly available CircuitLab drawing, which can be found here: CircuitLab Drawing. For simplicity, I have used ideal op-amps without voltage rails or their associated power supplies. In the actual implementation of the ISS, we will most likely also have trim resistors to ensure a zero offset for each op-amp. We interpret the PD as a voltage source for simplicity and I will use an actual summing amplifier in place of the summing junction used in the diagram.
The diagram linked above is simply a naive copy of a design by Rich Abbott so there are most likely mistakes and/or unnecessary elements, but it is a work in progress. I began discussing, with Jamie, the relative use of the first few filter stages in the servo. As far as my understanding goes, the first 'stage' was part of cascade of op-amps that served to convert a differential input from the PD into a single DC signal referenced to ground. Indeed, the first stage of my diagram (U1) is simply a unity-gain low-pass filter with f~5 MHz. Additionally, the second filter 'stage', U2, is also a unity-gain low-pass filter although it introduces a phase shift of 180 deg as the input to the second stage is on the inverting input of the op-amp. These characteristics were determined using LISO and examining the transfer function.
Noise analysis was also performed for the above circuit. The noise from various elements is examined at the output of the servo (labeled as 'outU6' in my LISO file). In the attached diagram, we see the voltage noise at the output from each op-amp as well as the sum of all the various noises, which includes resistor noise and current noise from the inputs of each op-amp. These are LISO's standard considerations and it is also worthwhile to note that the result is not referred to the circuit input, but as we have the transfer function of the whole servo, referring the noise to the input is trivial.
I have also included the following output for the sake of completeness.
from 1 Hz onwards noise by OP:I+ (U3) dominates.
from 38.6812 Hz onwards noise by R(R24) dominates.
from 115.478 Hz onwards noise by R(R11) dominates.
Attached are both the circuit diagram and the liso formatted *.fil for the main branch of the ISS, as well as the resulting transfer function when analyzed. Unfortunately, as noted in the file, not all of the elements are possible to analyze in liso, such as any type of op-amp with more than two inputs and one output (AD602 used in this design has 16 pins with two distinct amplifiers contained within).
I have begun prototyping this circuit on a breadboard.
## ISS Main Branch
## All circuit elements are named according to the circuit diagram
## "D020241-D2.pdf" by R. Abbott.
# Stages are separated by empty lines and elements between stages are
# also separated by empty lines for easy file navigation
# Before the first stage there is a 'fully differentiable' op-amp
# that I believe serves to isolate the device from the power supply
# However, liso does not have the capability to analyze such an op-amp,
Unfortunately, as noted in the file, not all of the elements are possible to analyze in liso, such as any type of op-amp with more than two inputs and one output (AD602 used in this design has 16 pins with two distinct amplifiers contained within).
Typically, you can still find a way to model the important parts of the stages that are not as simply added. In the case of the differential input stage, in particular, it is important to include it because it will usually set the input noise level of the circuit. In this case, the noise is the same as the second stage (U5) and it has a gain of 1, so there is essentially no difference (up to factors of sqrt(2) or 2).
You can edit the opamp.lib file and add in custom components. For the input stage, you can just pretend it is a simple non-inverting amplifier with the specified noise characteristics from the datasheet: un = 1.3n, uc = 50 Hz (see below).
For dual op amps, you can usually just model each part separately. For example, the OPA2604 is a dual op amp that is included in the opamp.lib and can be treated as a single one in a model.
After spending a good deal of time learning how to use the SR785, I was able to characterize my prototype circuit. The transfer function from a swept sine measurement looks very similar to the theoretically calculated transfer function (both of which are attached). The frequency response of the circuit was considered over the range 10 Hz - 10 kHz, which contains the eventual working range of the ISS (at least to my knowledge).
Note that OP27 op-amps were used instead of the high-speed AD829 op-amps that will be implemented in the actual design. This was done as a result of the limitations and inherent noise characteristics of the breadboard on which the prototype was built.
Unfortunately, I saved the wrong dataset (i.e. phase of the transfer function, not magnitude) and thus the presented function here is image generated by the SR785.
RXA: One must learn to use the python-GPIB interface to not lose data in the future.
We took relative intensity noise (RIN) spectra of the ISS error point and the monitor PD (attm1).
In-loop RIN is the sensor PD and "Out of the loop RIN" is the monitor PD.
The ISS gain slider was at 8dB in this measurement.
It looks normal.
An open loop transfer function of the ISS loop was measured (attm2). The UGF was 22kHz with the phase margin of ~22deg.
We should increase the UGF up to ~60kHz
When we increase the gain up to 14dB, the CS saturation warning comes up in the EPICS screen.
We confirmed this by monitoring the CS drive signal with an oscilloscope.
It is the output of an AD602J, which has +/-3V output range.
By increasing the gain of AD602J, we saw that the output signal hits the rail.
There seems to be a lot of high frequency (100kHz - a few MHz) noise, out of the control band.
We also observed that AD602J itself oscillates at about 10MHz (don't remember the exact number) when the gain is increased.
(We saw this even when the loop is off. There is no such an oscillation in the input to the AD602J).
When we took wide band spectra of the CS drive signal, we saw many large harmonics of ~180kHz. We believe these peaks are limiting
our ISS gain now (causing the CS saturation). The harmonics persisted even when we disconnected the PDs. So it is not coming from the light.
We saw the same harmonics in the power lines. They may be the switching noise of the Sorensens.
We took spectra of those harmonics, but the netgpibdata.py somehow did not save the data from AG4395A correctly. I have to debug this.
Stefan removed DC offsets from the AD829s (many of them are used in the ISS board) by turning the pots for offset adjustment.
This eliminated the problem of getting a large DC CS feedback (observable in C1:PSL-ISS_CSDRIVE_MEAN) when the gain is increased.
During the investigation, I noticed that increasing the PMC gain too much (~22dB) caused an oscillation of the PMC loop and consequently made
the ISS saturate. When the ISS is behaving bad, we should check the PMC gain.
Currently, the ISS is running OK with the gain = 8dB. I modified the mcup script to set the ISS gain to 8dB when the MC is locked.
Wait for Peter's answer about spare ISS boards.
Power line filtering.
Find the cause of AD602J oscillation (Well this is the one mounted upright. So just mounting it normally might solve the problem :-).
This plot shows the RIN as measured by the ISS. Its ~2 x 10^-7, whereas its supposed to be more like 3 x 10^-8.
The ISS has DC coupled RIN channels (with a _F suffix) and AC coupled RIN channels (with a _FW suffix). By using a swept sine, Rob determined that the AC coupled channels have an AC coupling pole at ~80 Hz. The attached plot uses this and then has the overall gain adjusted to match with the _F channels below 10 Hz.
The _F channels can be converted directly into RIN by just dividing the spectra by the mean value of the time series. The dark offset of these channels is small and so this only introduces a ~5-10% calibration error.
Question #1: Why is the RIN so bad? According to the MEDM screen, the photocurrent on the MON/SENS PDs is 1.9/1.3 mA. That's sort of low, but should still allow us to get 5x10^-8 in RIN.
Question #2: Does it make an effect on the current DC Readout work? IF so, should we try to fix up the ISS in a temporary way? Since the in-loop and out-of-loop detectors are completely coherent, all of the noise is likely just unsuppressed noise from the laser. We are unable to increase the gain because of the high frequency noise from the NPRO.
Let's remember to replace this ISS with a new one that can drive an AOM. Need a volunteer to get us a new ISS.
Right near the end of summer, I had an ISS board that was nominally working, but had a few problems I couldn't really sort out. Since I've been back, I've spent a lot of time just replacing parts, trying different circuit topologies and generally attempting to make the board function as I hoped it might in all those design stages. Below is a brief list of some of the problems I've been fixing as well as the first good characterization of the board transfer function that I've been able to get.
We'll start with some of the simple problems and proceed to more complicated ones.
The above list encompasses all the issues I've had in making the ISS board function correctly. No other major problems exist to my knowledge.
I was able to measure both the open- and closed-loop transfer functions of the servo with the SR785. The results are shown below.
The transfer function with the boosts on caps at a particular value set by op-amp railing, i.e. below 100 Hz, the op-amps are already putting out their max voltage. This is the usual physical limitation when measuring the transfer function of an integrator. We can also see that the measured phase follows the simulated phase above ~300 Hz. The 'phase matching' at low frequency is again do to the op-amp railing in the servo output..
The closed-loop gain is shown below,
The measured closed-loop gain with the boosts on again matches the LISO simulation quite well except at low frequency where we are limited by op-amp railing. We compare the measured closed-loop transfer function to the desired noise suppression stipulated in my previous elog 9331,
And we might hopefully conclude that my servo functions as desired. One should note that the op-amp railing seen in these measurements is not indicative of limitations we might face in some application of the ISS for the following reason. These transfer functions were measured with a 100 mV excitation signal (it is necessary to keep this signal amplitude large enough so that the inherent signal-to-noise ratio of the excitation source is large enough for accurate measurement) which leads to somewhat prompt railing of the op-amps. When the ISS operates to actually stabilize a laser, the input error signal will be much smaller (on the order of a few 10's of mV or less) and will decrease significantly assuming correct operation of the ISS. This means we won't see the same type of gain limitations.
What now, you ask?
Aside from the problem with the AD8436 chip, the ISS board seems to be functioning correctly. The transfer functions we have measured are correct to within the component tolerances and all of the various subsystems are behaving as they were designed to. Moving toward the goal of having this system work in situ for the CTN experiment, I need to do the following things,
So close, or so I say all the time
I wanted to look into the ISS situation. Some weeks ago, I found the PD that was previously used as the in-loop photodiode. I wanted to use this and measure the open-loop RIN at a few places (to see if there's any variation and also to check its functionality). However, I didn't get very far tonight - for a start, the PD height is 3" (while our beam height is 4" everywhere outside the vacuum), and I needed to put together a circuit to supply the 5V bias and +/- 15 V since the transimpedance is done on the head. I was only able to do a low-level functionality test tonight, checked that the DC voltage output varied linearly with the incident power (calibrated against an NF1611 photodiode, data will be put up later). I didn't get to measuring any noise performance - is an incandescent light bulb still shot noise limited at ~10 Hz < f < 10kHz? Some notes:
Unconnected to this work - this problem reared its ugly head again (i noticed it yesterday morning already actually). I don't have the energy to embark on a fix tonight, Koji is going to be in the lab all day tomorrow and so he will fix it.
that little PD in the black mount was never very good. The AD829 is not a good opamp for transimpedance and especially not good for low frequencies. Stefan Ballmer and I were able to get 2e-8 out of these (@100 Hz) many years ago.
I wonder if we have some of Zach's M2ISS photodetectors around, perhaps in QIL or Cryo. I doubt that any of them are in use now. Those had good performance nad BNC output.
Ok I was using the PD in the black mount because Rana recommended it a few weeks ago.
Regarding the M2ISS, I acquired the hardware from QIL some months ago, including a circuit board, and 2 PDs. These had LEMO outputs though (not BNC), and the mounts are not 4". These photodiodes are what I'm using as the airBHD DCPDs right now, and some photos are here - are these the photodiodes you mentioned? Or are there yet more M2ISS photodiodes? I remember Johannes had some custom mounts extruded to make them 4" high, do you mean those? Can I retrieve them his Cryo setup?
BTW, my elog scraping shows only one spectra from Stefan in the ATF elog, and the performance there is more like 1e-7/rtHz @ 100 Hz, and that’s using a dedicated high BW servo circuit, not the SR560. Am I just missing the measurement of 2e-8/rtHz?
I would like to measure the switching time of the AOM. So I have disconnected the modulation input to the AOM that comes from the ISS. I have also turned OFF the SR560's and the AWG that belong to ISS.
Pics and cable connections of the state in which the ISS setup was left at, will be updated soon.
I noticed that the ISS Mean Value and CS Saturation were both RED and unhappy. (The alarms were going off, and they were both red on the MEDM screen). None of the MEDM settings seemed off kilter, so we went out to take a look at the PSL table.
Rob checked that light is indeed going to both of the ISS photodiodes (Morag and Siobhan). Next we checked that all the cables were good, and that the power to the ISS box was plugged in. In this process, Rob wiggled all the cables to check that they were plugged in. Just after doing this, the Mean Value and CS Sat were happy again. Rob thinks the current shunt connection might be bad, but we don't really know which one it was since all of the cables were jiggled between our checking the screens.
Right now, everything is happy again, but as with all bad-cabling-problems, we'll probably see this one again.
I don't know why in particular the connection decided to spaz out this afternoon...I don't think anyone opened the PSL table before Rob and I went to investigate. I was working on the PMC servo (checking the LO levels...to be posted in a couple minutes), but didn't have anything to do with the ISS. After I was done, I put everything back, and locked the PMC and the MC, and everything was good, until some time later when the ISS started flipping out.
I was working on the ISS excitation to take TFs.
I used ISS IL excitation, stealing from a small box on the floor for the OMC.
All the configuration was restored except that the HEPA is on.
Yoichi and me found that the transfar function of the current shunt changed with the current of PA.
We changed PA current and fixed the unstability of ISS.
Now, laser power is stabilized finely, with band of about 1 Hz.
Yoich will post the stabilized noise spectrum.
There looks to be some non-linear relation between PA current and the TF of current shunt.
It had changed from the TF which we measured yesterday, so it might change again.
I try to write scripts to sweep PA current and measure the laser power and its rms automatically.
It will be apply for auto-adjustment of PA current.
Attached files are the transfar function of the current shunt with changing PA.
They have difference in lower frequency.
I measured the output noise of eache stage of ISS servo, and calcurated the noise ratio between input and
output of each stage.
Generaly, each noise ratio corresponds to their transfar function. This means servo filter works well, not
adding extra noise.
I attache example of them.
For 2nd stage, the noise ratio is smaller than transfar function with a few factor. This is because the
input noise is coverd by analyser's noise and ratio between output and input looks small.
This means the input noise of 2nd stage was enough small and all stage before 2nd stage work well
I attache the transfar function of ISS servo.
The 4th stage and variable gain amplifier has alomost same transfar function, so their lines pile up.
We found that one OP-amp used in ISS servo oscillated in 10 MHz, 100mV.
Moreover, we found another OP-amp had big noise.
We guess that these oscilation or noise cause saturation in high frequency, and they effect to lower frequency to cause
Attached files are open loop transfar function of ISS.
The blue points are open loop TF, and the green line is product of TF of ISS servo filter and TF of current shunt TF of servo filter.
This two must be same in principle, but They have difference f<2Hz and f>5kHz.
Back in November, Nic and Evan turned on an SR560 based ISS. It uses the PMC TRANS PD as the error signal and makes an AC coupled loop with 2 SR560's and then it drives the RF amplifier which drives the AOM upstream of the PMC.
This was the saturating SR560 under the PSL table that Steve found this week*. Tonight I found that the +24 V rack fuse for this was blown. I replaced the previous 2A fuse with a new 2A fuse (turned off the +/24 V Sorensens during this operation). I think all of the servo settings are basically the same as before, except that I'm using a gain of 10000 instead of 50000 on the first SR560. It was saturating otherwise. My guess is that the fuse blew many months ago and no one has noticed...
I checked the out of loop performance in MC_TRANS and in the IFO REFL_DC and there's some high frequency improvement with the loops on.
The main improvement, however, was in lowering the HEPA fan speed. This should only be turned up to Hurricane when you are working on the table. Similarly, those of us trying to lock at night, can't really trust that the HEPA is set to its nominal low setting of 20%. The whole difference in the MC_TRANS from 5-50 Hz is from this however, so we can use this ISS reference .xml as a way to see if the HEPA is up too high.
If we want to do better for RIN from 100-1000 Hz for improving the REFL_DC/CARM noise, we would have to think of how to improve the PMC_TRANS PD RIN.
* Steve gets +1 point for finding this, but then -3 points for not elogging.
I re-centered the ITMX & ITMY Optical lever beams today since they were off. First I aligned the beam into the vacuum so that it went through the center of the on table optics and then tweaked the receiver optics alignment.
There are several bad practices on these which probably makes them drift:
According to the datasheets, the laser has a beam diameter of 0.6 mm and a divergence angle of 1.3/2 mrad. So we can just calculate the right lens positions next time and not have to experiment with the whole visible laser lens kit.
For next Wednesday's cleanup, someone should volunteer to make the mounts more stable for the ITMs.
I went to see what was wrong with the ITMs and found that people have been working on them and have left them in a broken state with no elog entry.
Whoever is working on these should post into the elog what the Oplev layout plan is, have someone check it, and ONLY THEN get to work on it.
The layout as it looks tonight is too complicated. With too many optics we will not have a low noise optical lever setup. The new layout should use a bare minimum number of optics and only use very stable mounts.
I first set the bias sliders to 0 on the MEDM screen (after checking that the nominal values were stored), then shut down the watchdogs, and then pulled out the boards for inspection + photo-taking.
I've uploaded high-res photos + marked up schematics to the same DCC page linked in the previous page. I've noted the S/Ns of the ITM, BS and SRM boards on the page, I think it makes sense to collect everything on one page, and I guess eventually we will unify everything to a one or two versions.
To take the photos, I tried to reproduce the "LED light painting" technique reported here. I mounted the Canon EOS Rebel T3i on a tripod, and used some A3 sheets of paper to make a white background against which the board to be photographed was placed. I also used the new Macro lens we recently got. I then played around with the aperture and exposure time till I got what I judged to be good photos. The room lights were turned off, and I used the LED on my phone to do the "painting", from ~a metre away. I think the photos have turned out pretty well, the component values are readable.
Computar 75-12.5 zooms were installed for closer look at the resonant spots. Their alignment and focus needs more loving adjustment.
Atm 1, ITMX ( it was 10-40 mm Tamron before )
Atm 2, ITMY ( it was 12mm wide angle showing the towers before )
I focused these lenses so that we could get a clean image of the mirrors and the OSEMs.
Our goal is to have an image where the optic diameter almost fills the entire monitor. We want the focus to be adjusted for the YAG beam (which is almost the same as focusing for the OSEMs). This will NOT produce a nice image of the cage using visible light, but that is just fine.
When Justin Garofoli was here he found a nice lens combo that did the job, so if anyone can find his old email or elog, lets just go back to that.
For now, we do not need a camera/lens system to focus very tightly on the center of the optic.
This graph shows 5 hours data in minute trend for ITMX and ETMX from 5am to 10 am today. ITM pitch drift is 3 times lager than ETM pitch if the OSEM sensitivity is assumed to be the same.
This graph is last 1 hour data of above graph in second trend.
It is clealy seen that ITM yaw is jumping between two stages. I guess ITM is something wrong, touching magnets or earthquake stops?
Two ITM spares (ITMU01/ITMU02) and five new PR3 mirrors (E1800089 Rev 7-1~Rev7-5) were transported to Downs for phasemap measurement
GariLynn worked on the measurement of E1800089 mirrros.
The result of the data analysis, as well as the data and the codes, have been summarized here:
ITMU01 / ITMU02 as well as the five E1800089 mirrors came back to the 40m. Instead, the two ETM spares (ETMU06 / ETMU08) were delivered to GariLynn.
Jordan worked on transportation.
Note that the E1800089 mirrors are together with the ITM container in the precious optics cabinet.
In preparation for locking tonight, I re-centered the spots on the Oplev QPDs for the ITMs, BS and PRM after locking and running the dither alignment for the arms and also the PRMI carrier. In the past, DC coupling the ITM Oplevs helped the angular stability a bit, let's see if it still does.
Steve and Koji (Friday, Apr 02)
Installation of ITMs are going on. Two new ITMs were placed on the optical table in the vacuum chambers. ITM for the south arm was put at the right place in accordance to the CAD drawing. ITM for the east arm is still at a temporaly place.
Tower placement (10:30-11:30)
- Put the tower on the table at a temporary place such that we can easily work on the OSEMs.
ITM (South arm) (14:00-16:30)
- Put the tower on the table at a temporary place such that we can easily work on the OSEMs.
- Leveled the table approximately.
- Released the EQ stops
- Removed anchors for the OSEM cables as it was too short. The wire distribution will be changed later.
- Put the OSEMs. Adjust the insertion to the middle of the OSEM ranges.
- Clamped the EQ stops again
- Placed the tower to the right place according to the CAD drawing.
- Released the EQ stops again.
- Check the OSEM values. The LL sensor showed small value (~0.5). Needs to be adjusted.
ITM (South) damping adjustment
- Found the signs for the facing magnets are reversed.
- Otherwise it damps very well.
The cabling on the seismic stack was rerouted so it could reach the south edge of the table: the cables were removed from the viton padded clamps and repositioned this morning.
ITM-south tower's earthquake screw viton tips could be a little bit larger. They do not stay in their screw hole after a hard clamping action.
4-40 earthquake screws under the test mass:viton tips can fall out without action, the treads are cross threaded so the screws are wobbling
The two ITM spares and two ETM spares are together stored in the optic storage (B110) at Downs. c/o Liyuan and GariLynn
While trying to set up the SIS-FFT to use our new ITM phase maps, I noticed that the surface of our ITMs looks pretty good actually (even compared to the aLIGO pathfinder optic map on the AIC wiki). I'm attaching it here for your viewing pleasure.
The code to plot it has been added to the SVN in the PhaseMaps/mat directory.